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brachy > Brachy > Flashcards

Brachy Flashcards

1
Q

safety device that all staff should have during a treatment

A

personal dosimeter

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2
Q

what does hitting interrupt do?

A

retracts source

done if for example patient moves

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3
Q

what does E stop do?

A

retracts source using a more powerful motor

do this if patient falls off bed or interrupt fails

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4
Q

what is done if estop fails?

A

physics goes in and turns the crank
If the source fails to retract, then the RO removes the applicator from the patient and places it in the pig (physicist may assist with this). Survey everyone (staff and patient) after leaving the treatment room.

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5
Q

what happens if error code 58 comes on??

A
  • source disconnected from cable
  • don’t bother with crank- RO removes applicator from patient and everyone exits the room
  • If low radiation levels, then disconnect channels at indexer, withdraw patient from room, perform survey (therapist does this).
  • If high radiation levels, RO removes sutures/applicators from the patient – do not disconnect from indexer (although inactive channels may be removed from the indexer – physicist does this). Physicist helps RO put applicators in storage container. Remove patient to maze, perform survey.
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6
Q

what tells you if the source is truly retracted or not?

A

radiation detector indicator lights

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7
Q

series of considerations when estop fails

A

 Therapist should start stop watch once source fails to retract to time how long the emergency lasts.
 Therapist should take survey meter into the room to determine if radiation levels are low (<1 mSv/hour; indicative of source somewhere inside the afterloader but not completely back in the safe position) or high (>1 mSv/hour; indicative of source completely outside of the afterloader). Normal background is <0.01 mSv/hour in the treatment room when the source is completely in the safe.
 If the radiation levels are high, physicist goes into room and turn the gold crank to retract the source manually. Stand BEHIND the afterloader for optimal shielding.
• If source can be retracted, then therapist can stop the stopwatch. Disconnect channels at the indexer, move patient to the maze and survey them in the maze (therapist does this).
• If the source won’t crank in, RO must manually remove sutures/applicators – do not disconnect at indexer. Physicist helps place applicators in emergency storage container. If it does not then just drop it on the ground and evacuate. Use remote handling tools if possible.
o Also avoid cutting the transfer tube/applicator corresponding to the channel that contains the source at all costs (since this will result in the source being loose, no longer tethered to the afterloader). Can cut other channels if needed to help remove the applicator from the patient. This is why you need to make note of which channel it is.
o Remove patient from the room and perform survey of all personnel.

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8
Q

global background radiation

A

• Normal background in the world is ~3 mSv/year (natural sources contribute ~2.4 mSv while artificial sources contribute ~0.6 mSv per year), which corresponds to 0.00034 mSv/h

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9
Q

what do you do if patient is radioactive during survey?

A

RO removes all equipment from patients and throws the equipment into the Tx room

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10
Q

after the emergency is over, what should be surveyed?

A

everybody including self immediately after leaving the room.

check radiation detector indicators to verify source is contained

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11
Q

3 levels of security for the source

A

door to the treatment room

afterloader is chained/locked to the wall machine itself stores source in a locked safe

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12
Q

permanent implant seeds

A

I-125
Pd-103
Au-198

I-125 and Pf-103 are popular in prostate permanent implants

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13
Q

eye plaque seeds

A

I-125

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14
Q

intra-vascular brachy seeds

A

Sr-90

beta emitter

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15
Q

liver TARE seeds

A

Y-90

beta emitter

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16
Q

Where is I-131 used?

A

treatment of thyroid cancer and thyroid disorders. Half life: 8 days; beta max: 606 keV; gammas: 364-723 keV; used as an unsealed source

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17
Q

where is Ra-223 used?

A

alpha emitter used for castration-resistant prostate bone metastases (uptake in bones is similar to calcium).

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18
Q

what seeds are used for permanent breast implants?

A

Pd-103

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19
Q

what is used for HDR implants in gyne, prostate, skin

A

Ir-192

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20
Q

where is Cs-137 used?

A

LDR gyne implants

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21
Q

what replaced Ra-226 for temporary LDR treatments?

A

Cs-137

  • higher activity (shorter 1/2 life)
  • Rn-222 (alpha emitter) is also potentially hazardous decay product of Ra-226
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22
Q

where is Co-60 used?

A

. Used in the form of pellets in a remote afterloading device (with very HDR dose rate e.g., 180 Gy/h at point A) or tubes. Replaced Ra-226. HDR temporary implants may be used for gyne.
-high specific activity

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23
Q

where is Au-98 used?

A

used to be used for eye plaques, various interstitial treatments. Replaced by I-125 which has a longer half life and lower photon energy

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24
Q

where was Rn-222 (encapsulated gas) used?

A

seeds used to be used for permanent implants, but were discontinued because of brems arising due to beta emission, which may be carcinogenic. Rn-222 is an alpha emitter.

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25
Q

what is gamma?

A

air kerma rate constant

Gamma gives air kerma rate if activity is known

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26
Q

Co-60 average photon energy (MeV), half life, HVL in lead (mm), TVL in lead, gamma (micro Gray meters^2/Gigabequeral hour)

A

1.25, 5.26 years, 11, 40, 309

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27
Q

Cs-137 average photon energy (MeV), half life, HVL in lead (mm), TVL, gamma (micro Gray meters^2/Gigabequeral hour)

A

0.66, 30 years. 6.5, 21, 77.3

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28
Q

Au-198 average photon energy (MeV), half life, HVL in lead (mm), gamma (micro Gray meters^2/Gigabequeral hour)

A

0.41, 2.7 d, 2.5, 56.2

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29
Q

Ir-192 average photon energy (MeV), half life, HVL in lead (mm), TVL in lead (mm), gamma (micro Gray meters^2/Gigabequeral hour), ACTIVITY

A

0.38, 73.8, 3, 12,108

10 Ci at source change

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30
Q

I-125 average photon energy (MeV), half life, HVL in lead (mm), TVL, gamma (uGy m^2/GBqh)

activity 0.3-0.6 mCi per seed at insertion

A

0.028, 60 d, 0.02, 0.07, 34.3

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31
Q

Pd-103 average photon energy (MeV), half life, HVL in lead (mm), TVL, gamma (uGy m^2/GBqh)

A

0.021, 17 d, 0.01, 0.04, 17.6

1-1.4 mCi at insertion

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32
Q

Ra-226 average photon energy (MeV), half life, HVL in lead (mm), gamma (micro Gray meters^2/Gigabequeral hour)

A

0.83, 1600 yr, 8, 8.5

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33
Q

Rn-222 average photon energy (MeV), half life, HVL in lead (mm), gamma (micro Gray meters^2/Gigabequeral hour)

A

0.83, 3.83 d, 8, 10.15

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34
Q

How does FDG work?

A

It is a glucose analog with the positron-emitting radionuclide F-18 substituted for a hydroxyl group in the regular glucose molecule.
taken up by high glucose using cells such as cancer cells (also, e.g., kidneys) and is not released again from the cell, once it has been absorbed due to the missing hydroxyl group (prevents further glycolysis).

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35
Q

How is F-18 produced?

A

bombardment of Ne-20 with deuterons (= nucleus of deuterium = one proton and one neutron)
OR (more commonly) by bombarding O-18 enriched water with protons to create a (p,n) reaction

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36
Q

F-18 half-life

A

o Half-life = 110 minutes; decays by positron emission (beta plus decay) 97% of the time (electron capture 3% of the time)
both modes of decay yield stable O-18.

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37
Q

Tc-99 m half life and energy, decay process

A
  • 140 keV gamma rays
  • decays by gamma emission 88% of time, 12 % of time decayse by internal converson. IC contributes to dose but doesn’t add info

6 hour half life, 1 day biological half life

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38
Q

how is Tc-99 m produced?

A
  • U-235 fission yields Mo-99

- Mo-99 is parent of Tc-99m (1/2 life of 2.75 days)

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39
Q

how to convert units from cGy/min to Gy/h

A

multiply cGy/min by 0.6 to get Gy/h; multiply Gy/h by 1.666 to get cGy/min

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40
Q

Dose rate classification at 1 cm

A

o LDR: 0.4 to 2 Gy/h
o MDR: 2 to 12 Gy/h
o HDR: > 12 Gy/h

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41
Q

remote afterloader classification

A

o LDR: 20-30 mCi
o HDR: 3-12 Ci
o PDR: 1-2 Ci (~10% of source strength of Ir-192 HDR unit)
o LDR seed typical activities: Pd-103: 1-1.4 mCi (1.3-1.8 uGy h-1 m2) (higher activity due to shorter half-life); I-125: 0.3-0.4 mCi (0.4-0.5 uGy h-1 m2)
 Typical I-125 dose: 145 Gy and Pd-103: 125 Gy.

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42
Q

what is PDR?

A

modality that combines physical advantages of high-dose-rate (HDR-BT) technology (isodose optimization, radiation safety) with the radiobiological advantages of low-dose-rate (LDR-BT) brachytherapy.

consists of using stronger radiation source than for LDR-BT and producing series of short exposures of 10 to 30 minutes in every hour to approximately the same total dose in the same overall time as with the LDR-BT.

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43
Q

• Relationship between units

A

o 1 Ci = 3.7×10^10 Bq = 37 GBq = activity of 1 g of Ra-226
o 1 Bq = 1 count per second (cps)
o 1 Gy = 100 rad; 1 cGy = 1 rad
o 1 Sv = 100 rem; 1 cSv = 1 rem
o 1 R [Roentgen] = 2.58 x 10-4 C/kg [units of exposure]

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44
Q

Pros and cons of the capsule

A

contains radioactivity
proides rigidity
absorbs low energy radiation that don’t penetrate deeply enough to be useful

-possibly makes brems

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45
Q

how do you check the integrity of the capsule?

A

wipe test

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46
Q

what are electronic brachy sources?

A

miniaturized x-ray vacuum tubes, typically 50 kVp, ~10 cGy/min at 1 cm, water cooled, fully disposable. Benefit of these is that they don’t decay and represent a minimal radiation safety concern (since they emit no radiation when they are off). They do still need to be replaced periodically since various components wear over time.

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47
Q

• Dose fall-off rules of thumb

A

o Inverse square is usually a larger factor than tissue attenuation (G falls off faster than g)
o For high energy sources (not I-125 or Pd-103), attenuation and scatter approximately cancel out (g ~ 1)

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48
Q

what are interstitial implants?

A

needles are implanted directly in the target area, often requiring surgery

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49
Q

what are radio-opaque dummy markers used for?

A

may be used to identify first source dwell position. For needles, no dummy markers are used, and first dwell position is determined based on measurement of free length, and knowledge of total length; dead area at tip of needle must be known so that physician knows how far to push in the needle

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50
Q

how is dwell position/time calculated?

A
  • inverse optimization

- graphical adjustment of isodose lines, or manual adjustment of dwell times

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51
Q

Manchester system

A

dose specification of cancer of the cervix
dose in cervix cancer is specified according to four points: point A, point B, a bladder point, and a rectum point. Duration of implant is determined based on dose rate calculated at point A. Dose to other points is used to evaluate the treatment plan

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52
Q

where is point A in manchester system?

A

2 cm superior to the external cervical os (outer part of cervix), and 2 cm lateral to (centre of) the cervical canal

o Point A represents where the uterine vessels cross the ureter (connects kidney to bladder) – it is believed that the tolerance of these structures is the main limiting factor in irradiation of cervix. Point A may end up being inside or outside the cervix depending on patient anatomy.

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53
Q

where is point B in manchester system?

A

3 cm lateral to point A

o Point B represents pelvic wall lymph node dose.

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54
Q

what happens to point A and B if the tandem displaces the central canal?

A

point A moves with the canal, but point B remains fixed at 5 cm from midline

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55
Q

downside to manchester definition

A

cervix sizes and tumour sizes vary such that could end up with either under- or over-dosage

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56
Q

where is bladder point on frontal radiograph?

A

centre of the foley balloon

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57
Q

where is bladder point on lateral radiograph?

A

the bladder point is on the surface of the balloon, where it is most posterior (closest to the sources)

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58
Q

where is rectum point on frontal radiograph?

A

midpoint of the ovoids

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59
Q

where is rectum point on lateral radiograph?

A

5mm behind vaginal wall

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60
Q

what is the american brachy society recommendation for dose to point A (D90)

A

EQD2 of 85-90 Gy total (typically 45-50 Gy EBRT (e.g., 25 x 1.8 Gy) plus 40-45 Gy LDR boost or 5 x 6 Gy HDR boost (Rx for boosts to point A)

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61
Q

typical vault Rx at NSHA

A

21/3 for monotherapy; 15/3 when combined with EBRT (45/25)

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62
Q

when is brachy monotherapy used?

A

low risk cases, when lymph nodes are not involved

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63
Q

ultrasound depth of penetration vs resolution

A

• Ultrasound resolution is improved with higher frequency. However, depth of penetration is reduced with higher frequency.

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64
Q

what is PTV used for in prostate brachy?

A

help with needle placement since the software will not allow you to place needles outside of the target, which makes it hard to get dose on the periphery unless you add a margin around the prostate

asymmetric margin with 3 mm everywhere except where it is 0 mm at bladder (superior) and rectum (posterior) interfaces.

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65
Q

do we care about PTV coverage in brachy?

A

No, only CTV

PTV just a tool used to cover CTV

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66
Q

• Plan objectives for prostate 15 Gy in one fraction (combined with 3750 cGy in 15 EBRT)

A 
o	Prostate V100% = 95-99%, 
V90% = 99-100%, 
V150% < 35%, 
V200% < 11%
o	Urethra D10% < 118%, Dmax < 125%
o	Rectum V80 < 0.5 cc = 500 mm3
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67
Q

how many seeds do you check for permanent implant brachy?

A
  • 10%

- mean should agree with vendor calibration within 3% and seeds should agree with mean within 5 %

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68
Q

how to integrate to get total dose?

A

-integrate exponential decay of dose rate over time from 0 to infinity
total dose = (initial dose rate) x (half life) / ln(2) = 1.44 x (initial dose rate) x (half life) = (initial dose rate) / (decay constant)

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69
Q

issue with prostate swelling due to brachy

A

yields uncertainty in calcs based on CT scan taken previously

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70
Q

give example of unsealed source

A

I-131 used to treat thyroid cancer: half life is 8 days. Emits gamma rays 360-720 keV plus 250-800 keV betas

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71
Q

what is TARE

A

transarterial radioembolization (TARE) for treatment of liver cancers (hepatocellular carcinoma) via hepatic artery (which supplies most of blood flow to liver tumours)

Embolization is a procedure that injects substances directly into an artery in the liver to block or reduce the blood flow to a tumor in the liver. The liver is special in that it has 2 blood supplies. Most normal liver cells are fed by the portal vein, whereas a cancer in the liver is mainly fed by the hepatic artery

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72
Q

What is used for TARE?

A

Y-90
Half life ~3 days; average energy of betas emitted = 930 keV; max energy = 2.28 MeV.
o Post implant SPECT used to assess location of spheres

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73
Q

what is benefit of beta emitters?

A

they deliver dose within a well defined range (useful for sparing normal tissues)

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74
Q

3 methods for source calibration

A

o Using a well-type ion chamber
o In-air measurement with ion chamber
o In-phantom measurement with ion chamber

should agree with manufacturer within 3%)

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75
Q

example of well chamber construction

A

walls of ion chamber surround the source (re-entrant chamber)
aluminum wall ion chamber filled with argon gas under high pressure

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76
Q

why is there energy dependence in well chamber?

A

intrinsic energy dependence due to absorption and scattering of photons/electrons in the chamber walls/gas (hence calibration must be based on same source type/design), as well as dependence on source position within the chamber (hence must find “sweet spot” of the chamber where the reading is maximum). To clarify, chamber response depends not only on the particular isotope used, but also on the particular source construction/source model, and its position within the chamber.

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77
Q

what does calibration coefficient do?

A

converts corrected chamber reading to air kerma strength

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78
Q

How does NRC obtain the well chamber air kerma calibration?

A
  • use spherical graphite ion chamber
  • For IR-192 source, use arithmetic mean of the calibration coefficients for 250 kV x-rays and Cs-137 gamma radiation (since irradiation using an Ir-192 HDR source is not practical)
  • This approach assumes flat response of the spherical graphite chamber to Co-60, Cs-137 and 250 kV x-rays
  • well chamber is then irradiated using Ir-192 source
  • The calibration coefficient is then calculated as the ratio of the air kerma strength of the source at the time of calibration (as determined by the spherical graphite chamber) divided by the corrected well chamber reading
  • potentially include correction factor for any non-zero reading (B) in the absence of radiation: P_bkgd = 1 – B/Mraw which has the effect of replacing Mraw with Mraw – B
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79
Q

how can the constancy of the well chamber be checked?

A

Cs-137 check source (or other source with a long half-life) However, a special source holder is needed to ensure consistent position and orientation of the source since chamber response is very dependent on position/orientation. A linac beam aimed toward the opening of the well chamber may also be used.

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80
Q

corrections to raw readings of well chamber

A 
temperature, pressure
electrometer not reading true coulombs
ion recombination
polarity effects
-background radiation
A correction factor correcting for attenuation in the applicator/catheter is also required except if the same applicator is used during the calibration procedure.
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81
Q

what is the raw reading for the well type chamber?

A

integrated charge measured per unit time (i.e., use current mode on electrometer to avoid the source transit effect i.e. nC would collect during the time the souce is in transit, better to measure current not nC)

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82
Q

What is NRC/NIST primary standard for measuring source strength

A

either a large volume free-air chamber (usually a spherical thimble chamber) with the source ~1 m away OR using a wide-angle free-air chamber (WAFAC)

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83
Q

what is ideal chamber for weaker, lower energy sources like I-125?

A

WAFAC
has larger collecting volume
subtends larger solid angle (~8 degrees cone half angle compared to «8 degrees for a spherical chamber and ~4pi radians for a well chamber)
therefore will measure a larger signal than a thimble chamber

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84
Q

how are directional dependencies handled for in-air calibration?

A

-the source is rotated to average out dependecies

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85
Q

advantage of WAFAC over point air kerma strength measurement

A

point air kerma strength measurements are more sensitive to small changes in internal source geometry and source alignment compared to the WAFAC, which averages photon fluence over a cone with half angle = 7.6 degrees

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86
Q

considerations for in-air measurement geometry

A

-apparatus should be as far as possible from scattering surfaces
-source to detector distance ~ 1 m so Sk is independent of distance (acts like a point source)
AND so that there is less fluence gradient across the sensitive volume of the chamber (the inverse square law is a relatively small effect)

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87
Q

what is Sk?

A

-air kerma rate in vaccuum multiplied bu distance squared

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88
Q

chamber volume size to achieve acceptable SNR for brachy sources

A

> 100 mL

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89
Q

W/e

A

33.97 J/C

used to convert exposure rate (C/kg) to dose

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90
Q

Do we use open air geometry and thimble chamber to calibrate Ir-192?

A

No
Ir-192 is higher energy
because the effects of scatter with this higher energy (compared to e.g., Pd-103 or I-125) make it difficult and time-consuming to achieve a “good geometry”
o However, in-air measurement can be carried out using a farmer chamber calibrated for orthovoltage energies, with the source irradiating the chamber on both sides at close range (~10 cm). Must irradiate from both sides to reduce the dose gradient across the chamber (make the fluence across the chamber more uniform)

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91
Q

what is needed to ensure electronic equilibrium for higher energy sources?

A

build-up cap

also filters out secondary electron contamination due to source encapsulation or catheter/applicator

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92
Q

correction factors required for in-air measurements?

A

P, T, polarity, ion recombination, volume averaging, attenuation and scatter in buildup cap and chamber wall, scatter & attenuation in the air, scatter & attenuation in the room, attenuation in the applicator/catheter

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93
Q

when do you use in-phantom measurements in brachy?

A
  • high energy sources
  • Ir-192, Cs-137, Co-60\
  • lower energy sources will be attenuated too much
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94
Q

Pros and cons of in-phantom measurements

A
  • only good for high energy
  • more easily reproducible
  • ion chambers used in RO department can be used
  • need calibration coefficient for gamma energy of the radionuclide considered (Gy/C)
  • room scatter is smaller effect since it is mostly attenuated before reaching the detector
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95
Q

how to deal with source positioning uncertainties for in-phantom measurements?

A

average over several equally spaced surrounding points to reduce the effect of positioning uncertainties

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96
Q

correction factors required for in-phantom measurements

A 
T
P
polarity
ion recombination
volume averaging
scatter and attenuation in chamber wall
attenuation in applicator/catheter
	A perturbation correction factor accounting for differences [in fluence spectrum] associated with using phantom material in the surroundings instead of water are necessary if calibration coefficient is for absorbed dose to water; another correction factor accounting for the additional absorption and scattering in the phantom material compared to air is also required since in the end we want air kerma strength
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97
Q

how to convert to air kerma strength?

A

need uen/p air to water and 1/(1-g) to convert uen/p air to utr/p air

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98
Q

do we need uen/p if using a calibration coefficient for air kerma with in-phantom method?

A

No, because calibration coefficient is for air kerma already

-however must use correction factors to account for phantom not being air (attenuation and scatter will differ)

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99
Q

what is g?

A

fraction of energy transffered to the medium that is subsequently re-irridiated as bremsstrahlung

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100
Q

what is radiation yield?

A

fraction of charged particle kinetic energy that goes into x-ray production as particle slows to stop in a thick target. Radiation yield = integral from 0 to initial kinetic energy E of radiative stopping power divided by total stopping power divided by initial kinetic energy E.

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101
Q

are uncertainties in souce position along axis off applicator or uncertainties in source to detector distance more significant?

A
  • source-to-detector is more significant
  • effect is worse when source=to-detector distance is shorter
  • source-to-detector distance error is 4 % error for uncertainty of 1 mm when source-to-detector distance is 5 cm
  • error is 1 % for uncertainty of 5 mm when source to detector distance is 5 cm for uncertainties in source position in applicator
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102
Q

can one use same well-type chamber for LDR and HDR sources?

A

Not typically as LDR source chamber will have a very large sensitive volume - too high a sensitivity for HDR sources

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103
Q

6 ways to apply brachy sources

A 
external applicators (skin cancer)
interstitial application (prostate, breast)
intracavitary (cervix, uterus, vagina)
intraluminal (bronchus, esophagus)
intraoperative (sources implanted during surgery)
intravascular (source placed into arteries)
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104
Q

advantage of brachy compared to EBRT

A

improved localized delivery of dose to the target volume of interest. However, brachytherapy requires that the tumour be well localized and relatively small, generally requires more staff, and may require surgery (is a more invasive procedure).

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105
Q

common cervix cancer dose

A

60 Gy

although ABS recommends EQD2 of 85-90 Gy

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106
Q

what does ICRU report 38 do?

A

recommends list of data needed for reporting intracavitary therapy in gynecology

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107
Q

What is the list of data required per ICRU 38?

A

o Dose at reference points: bladder, rectum, pelvic wall, lymphatic trapezoid
o Description of technique: applicator type, source type…
o The “time dose pattern”
o Description of reference volume: the dimensions of the isodose surface that just surrounds the target volume, what is the value of the isodose
o Total reference air kerma = total air kerma strength of sources times the implant duration

108
Q

How is ICRU 38 reporting different from Manchester system in gyne?

A

ICRU: dose distribution to target volume

manchester- dose to specific point A, B, baldder, rectum

109
Q

What does ICRU report 58 do?

A

-recommends reporting data for interstitial treatments

110
Q

what data reporting are recommended by ICRU 58?

A

o Description of clinical target volumes
o The sources, technique (what applicators were used), and implant time
o The total reference air kerma = total air kerma strength of sources times the implant duration
o Description of the dose: what is the dose level and how is it prescribed? To a point or surface? Mean central (representative of plateau dose region inside the target volume) and peripheral dose (relevant for tumour control), as well as high dose and low dose regions should be reported.
o Dose uniformity indices
o DVHs

111
Q

List the different types of decay

A 
alpha
beta minus (also get anti-neutrino)
electron capture (also eject neutrino)
gamma decay (isomeric transition)
internal conversion
nuclear activation
neutron capture
112
Q

explain internal conversion

A

nuclear excitation energy is transferred to k-shell orbital electron which is ejected with a kinetic energy = excitation energy – binding energy
 Vacancy is filled with higher level orbital electron; transition energy emitted as characteristic photons or Auger electrons.

113
Q

explain gamma decay

A

nucleus in excited state decays to ground state

114
Q

pros and cons of Ir-192 for HDR

A

-higher specific activity than Co-60 or Cs-137 (can use smaller source)
lower photon energy (less shielding)
however has shorter half life so has to be replaced every 3-4 months

115
Q

mean photon energy for Ir-192, Cs-137, Co-60

A

380 keV Ir-192
660 keV Cs-137
1.25 MeV Co-60

116
Q

half lives of Ir-192, Cs-137, Co-60

A

74 days, Ir-192
30 years, Cs-137
5.3 years, Co-60

117
Q

how to produce Ir-192

A

 Ir-191 and Ir-193 exist in nature with 37% and 63% abundance
Ir-192 is created by neutron bombardment (activation) of Ir-191 in a nuclear reactor

118
Q

how to produce I-125?

A

Xe-124 is bombarded with thermal neutrons in a nuclear reactor to yield metastable Xe-125 via neutron capture which decays to I-125 via beta plus decay

119
Q

max leakage at 10 cm distance per Nuclear Regulatory Commission

A

0.01 mSv/h when source is shielded

120
Q

describe generally how physician places sources in

A
  • The patient is under general or local anesthesia and the physician places the implant devices in the patient using some form of guidance: e.g., ultrasound guidance for prostate, palpation and visual inspection with gynecologic applicators, endobronchial tube with bronchoscopy guidance. Next, typically, orthogonal radiographs are obtained to localize the applicators. Radio-opaque marker wires may be inserted into the applicators to help visualize the first, most distal dwell position.
  • Source position check ruler can be used to determine the total length of the catheter required for source travel.
121
Q

simple method for checking HDR computer calculations

A

valid if distance from point of calc to the source centres is at least twice the active length of the source so that ISL can be measured
dose rate = (dose rate constant in cGy h-1 U-1) * (air kerma strength in U = cGy cm2 h-1 = µGy m2 h-1) * r02 / r2. Multiply this dose rate by the corresponding dwell time to get the dose contribution at each point
-r0 is 1 cm

122
Q

is dose rate constant fixed for a given seed model?

A

Yes, but air kerma strength varies from seed to seed

123
Q

TG-43 equation for dose rate

A

dose rate = air kerma strength Sk * dose rate constant A * IS * radial function * anisotropy function
-can be simplified to Sk * IS for point dose

124
Q

treatment options for prostate cancer

A

radical prostatectomy (better for younger patients who can withstand general anesthesia; want to avoid risk of secondary cancer)
external photon beam irradiation (better for older patients)
brachytherapy (permanent or temporary implant)
watchful waiting
hormone therapy (androgen deprivation)

125
Q

describe androgen deprivation therapy

A

Also known as hormone therapy. Can be used in combination with radiation therapy. Prostate cancer cells need androgen hormone (such as testosterone) to grow. Androgen therapy drugs block androgen receptors and/or inhibit or suppress androgen production

126
Q

what to do if pubic arch interference, issue with prostate size

A

patient may need hormonal therapy for a few months to shrink the gland to allow for an adequate implant. There is also a threshold size above which prostate brachytherapy is contraindicated. Large prostate size often corresponds with pubic arch interference, although if the prostate is large, brachytherapy is contraindicated even if there is no pubic arch interference. Patient must be able to undergo general anaesthesia (or at least sedation) and must be able to be in dorsal lithotomy position

127
Q

brachy options for prostate

A

permanent I-125 or Pd-103 implants

temporary Ir-192 implants with EBRT

128
Q

space between holes in prostate template

A

5 mm

129
Q

overall treatment time limit for gyne

A

< 55 days

130
Q

what is PDR

A

pulsed dose rate brachy

  • HDR level dose rate delivered for a short time in many fractions over the same total length of time that LDR insertion would take (ex. deliver dose for a few minutes every hour for 5-6 days)
  • allowed for sub-lethal damage repair while not segragating the patient for as long as the continuous LDR
  • ties up machine for the same patient for several days
131
Q

PDR brachy souce strenth compared to HDR

A

10% of HDR

132
Q

historical context of PDR

A

before HDR, only LDR was available, and brachytherapy patients (e.g., gyne, prostate temporary implants) had to stay bedridden in the hospital for days/weeks. When HDR was first introduced, there was no clinical evidence to suggest that hypofractionation with HDR would be as effective as the LDR treatment, and allowing for normal tissue repair was a concern with use of HDR. So, at first, HDR treatments were delivered in a PDR approach such that entire duration of treatment was the same as for the LDR treatment (with the patient bedridden in the hospital for days/weeks). Eventually, people started trying out hypofractionated treatment regimes, which are today’s standard treatment approach, allowing for high patient throughput.

133
Q

what can be used to increase rectal sparing?

A

rectal retractor or packing

134
Q

why is real-time planning popular?

A

difficult to set the patient exactly in the same position for the actual implant as they were in during the pre-implant volumetric study. Also, the shape of the prostate changes as the needles are inserted or removed (i.e., edema), which causes the pre-planned theoretical seed distribution to be difficult to be reproduced precisely. Also the patient would have to come to the hospital twice.

135
Q

why use differential DVH in brahy?

A

• (Cumulative) DVHs for brachytherapy are difficult to interpret due to the inverse-square falloff of dose around the sources, which results in an inherent inhomogeneity, and which obscures regions of underdosage or overdosage. Differential DVHs make inhomogeneities more obvious.

136
Q

Paris system reference isodose, treated volume, hyperdose volume

A 
The reference isodose
has a value equal to 85% of the basal dose.
This value is based upon clinical
experience.
• Treated Volume - The volume surrounded
by the 85% isodose surface.
• Hyperdose volume - The volume of the
170% of the basal dose (twice the reference
isodose).
137
Q

in Paris sytem, how to determine number of source planes to use?

A

The number of source planes to be used
depends upon the thickness. If the
thickness exceeds 12 mm, there must be 2
source planes

138
Q

what is Paris system?

A

complete dosimetric system which greatly
facilitates brachytherapy, using iridium 192 sources at low, pulsed or high doserate
particular mode of dose specification within the implanted volume, and
the fixed value of the Reference Isodose (RI) equal to 85% of the Basal Doserate (BD), representative of the arithmetic mean of the minimal dose-rates in
the central region of the implant. The method to calculate the treatment time is
given. Simple relationships which can be used to predict the minimal dimensions
of the treated volume (volume encompassed by the RI) at the very moment of
the implant are presented

139
Q

hot loading

A

applicator is preloaded with radioactive sources at the time of placement into the patient.

140
Q

after-loading

A

the applicator is placed first into the target position and the radioactive sources are loaded later, either by hand (manual afterloading) or by machine (automatic remote afterloading).

141
Q

• Consideration for appropriate choice of source

A

o Penetration depth into tissue (depends on energy spectrum) – want to balance ability to cover target, dose distribution homogeneity with normal tissue sparing beyond the target.
 Note that lower energy sources are typically used for permanent implant brachytherapy due to the fact that patients have radiation within their body after the procedure, and may pose a risk to people that they interact with.
o HVL for shielding requirements (depends on maximum energy in spectrum)
o Half-life (ideally long for temporary implant so that the source doesn’t need to be replaced very often; however, if it is too long, then dose rate will be too low and treatment times will be too long)
 For permanent implant, if half-life is too long, then dose rate may be too low to provide tumour control if tumour cells proliferate quickly.
o Specific activity, source strength (ideally high specific activity; how big does the source need to be?)
o Dose fall-off away from the source (due to scatter, attenuation, ISL).

142
Q

what happens to radial dose function g above 300 keV

A

attenuation by tissue is approximately compensated for by scatter buildup so that the radial dose function g ~ 1

143
Q

typical shielding requirement for low and high energy brachy sources

A
  • high: several mm lead

- low: < 0.1 mm lead

144
Q

forms of brachy sources

A 
needles
tubes
seeds
pellets
wires
145
Q

opturators during CTSim

A

remove them

146
Q

eye placque brachy

A

gold shells with low energy photon seeds (I-125, Pd-103) or solid beta placques (Sr-90)
seeds are inside the placque
gold shields rest of body from radiation
plaque is applied externally to the surface over the tumour base. Different plaque diameters are available. The prescription point is defined as the tumour apex (most distant from source) if the apical height exceeds 5 mm, and 5 mm depth from the interior sclera (the white outer part of the eyeball) if the apex is < 5 mm high

147
Q

image fix enabler function in Oncentra

A

gives flexibility in case couch is tilted from CVSim images

148
Q

purpose of ECS

A

in Oncentra
useful because brachy needles don’t follow along the cardinal axes
allows you to reconstruct in arbitrary planes so you can align along the needles

149
Q

indexer length

A

from applicator to first dwell position

150
Q

needle tip to 1st dwell position for our metal prostate and interstitial needles

A

10.5 mm

151
Q

does treatment unit adjust dwell times from Oncentra based on source decay?

A

Yes

152
Q

what is Oncentra IPSA?

A

inverse planning simulated annealing

153
Q

what do you set a margin for in Oncentra?

A

For how far outside of the target you can be (if this is too small, some needles will be prevented from firing off)

154
Q

gyne 1500/3 bladder and rectum constraints

A

bladder 620 cGy
rectum 420 cGy

<120% of prescription dose

155
Q

normalization vs optimization feature in Oncentra

A

normalization - gives you constant dwell times that will get you different doses
opimization = different dwell times but similar dose

156
Q

simplify function in Oncentra

A

to smooth contours

157
Q

compare perscription points for prostate vs. uterus vs vaginal vault in this clinic

A
  • prostate- prescribe to volume
  • vault- prescribe to points 5 mm beyond cylinder
  • cervix/uterus- prescribe to point A (the other 3 points of the Manchester system are used for evaluation; our clinic also considers points 5 mm from the outside of the ring/ovoids for normalization purposes)
158
Q

Is CT-based or radiography treatment planning preferred for brachy?

A

CT provides a 3D image set, which makes catheter reconstruction easier. However, CT dose is generally higher than two orthogonal x-rays. Also, in some centres, the patient may have to be transported elsewhere in the hospital in order to obtain a CT, which is inconvenient, especially is the patient is anaesthetized.

159
Q

lip/oral cavity cancer dose

A

o 60-70 Gy when used alone (early stage; LDR at 0.8 to 1 Gy/h)
o OR if used 2-4 weeks after 45-54 Gy EBRT, then LDR brachy boost is 15-30 Gy.
o Typically Ir-192 is used.

160
Q

APBI brachy dose

A 
o	HDR balloon brachytherapy dose: 34 Gy b.i.d. x 5 days 
o	 OARs (skin, lung, heart) must be far enough away; with small breast this may be difficult to achieve.
161
Q

esophagus dose

A

chemo + EBRT 50/25 + HDR 5 Gy x 3 or LDR 20 Gy x 1

162
Q

episcleral plaque brachy dose

A

tumour apex (most distal from plaque) prescribed 40 Gy; base receives 100-200 Gy

163
Q

emergency scenario: wipe test of old source cable showing > 2 X background reading

A
  • ask engineer to stay onsite
  • double check wipe test-close down treatment room, ensure signs to stop people from entering
  • cancel/delay treatments
  • lock room to door, remove keys etc to stop acess
  • contact RSO, hopsital admin, CNSC, vendor
  • • Check previous wipe test result to confirm that the leak occurred sometime since the last source change (to ensure it wasn’t already leaking when it was installed).
  • • Begin investigating areas of potential contamination using a survey meter: transfer tubes, applicators, catheters, the afterloader itself, the place where equipment is sterilized, the place where equipment is stored. Also survey people who may have been contaminated: physicists, ROs, therapists, nursing staff, people who do the sterilization/cleaning of the equipment. Patients who were treated will need to return to the hospital to be surveyed again. Equipment such as transfer tubes may need to be replaced. Applicators that are used less often may not need to be replaced depending on results of survey.
164
Q

two materials have different half-lives, and we want the two sources to have the same activity. How must the # atoms in each sample differ?

A

• Use the fact that A = Nλ = dN/dt. So N1λ1 = N2λ2 where λ = ln(2) / (half-life). Therefore, if the half-life of #1 is greater than #2, then λ2 > λ1 (#2 decays more quickly). So, need N2 < N1 so that N1λ1 = N2λ2 holds true. You need less of the shorter half-life/larger λ/higher activity per atom source.

165
Q

two materials have different half-lives, and we want the same dose at a particular point. How must the activities of the sources differ?

A
  • We assume that the question wants to know how the initial activities of the sources must differ, and we assume that “same dose” at a point means same total dose from time 0 to infinity.
  • We also assume that the energy deposition per decay is the same for both. Ignoring also any radiobiological effects (e.g., if energies differ, then LET and therefore RBE will differ; different dose rates have different effectiveness depending on tumour proliferation rate)
  • To get the same total dose, we want the same initial number of atoms in each source since all atoms will decay eventually:
  • A = Nλ so N1 = N2 thus A1/λ1 = A2/λ2. Assume λ2 > λ1 (ie source 2 has shorter half life)
  • Therefore, we need A2 > A1. We need initial activity of the shorter half length/larger λ/higher activity source to be larger.
166
Q

better to use less high activity seeds or more lower activity seeds?

A

lower activity seeds will yield more homogeneous distribution

Not sure about this…

167
Q

how many seeds do you check activity for?

A

10%

should be within +/- 5 %

168
Q

how does Oncentra calculate dwell time?

A

Uses optimized weights and decay corrected activity to determine dwell times on a given day (can even change source between fractions)

169
Q

what happens if anasthesized patient moves during treatment

A

pause treatment

re-image with US to see if substantial changes occurred

170
Q

what is bigger effect on dose closer to source, IS or inhomogeneity?

A

IS

171
Q

why do you use 46/23 for some EBRT/HDR patients?

A

if lymph node involvement, decrease dose/fx to protect the bowel

172
Q

HDR monotherapy prostate experiments dose levels

A

19 Gy x 1 fx
13.5 Gy x 2 fractions

19 x1 was not effective

173
Q

how much does Ir-192 source decay per day?

A

~ 1 %

174
Q

why isn’t radium used anymore?

A

wide energy spectrum leading to high dose close to the source and still high dose around the patient - shielding difficult
Radon, the daughter product of radium, is a noble gas which is very difficult to contain - contamination risk
The long half life means disposal is very difficult

175
Q

where would you use Pd-103 instead of I-125?

A

Pd-103 delivers high intial dose rate, useful for fast growing high grade tumours

176
Q

permanent implant dose delivered

A

D = Dotau

Do is initial dose rate
tau is 1.44 * half life

177
Q

prescription dose for eye brachy

A

100 Gy

178
Q

radium equivalent of a source of known activity

A

〖𝑅𝑎〗_𝑒𝑞=𝐴×Γ/Γ_𝑅𝑎

179
Q

number of equivalent milligram hours

A

〖𝑅𝑎 “milligram hours equivalent” =Raeq x 1.44 x half life

180
Q

physical states of radionuclides used in bracy

A

solids, liquids, gases

181
Q

disdvantages of HDR vs LDR

A

investment
safety (larger consequence if there is an issue)
radiobiology- as dose rate increases, radiosensitivity of the normal tissue increases faster

182
Q

safety features and operational interloacks of HDR afterloader

A 
A/V system
radiation monitors and treatment on indicator
door interlock
emergency shut-off
emergency crank
backup battery
183
Q

Why are I-125 and Pd-103 used for LDR?

A

low average photon energy to treat only the tumor, and relatively short half lives

184
Q

when can prostate LDR patients be released?

A

when exposure from patient would not exceed 5 mSv

i.e exposure rate from patient at 1 m is < 1 mR/h

185
Q

initial dose rate of a permanent implant

A

= prescription dose/ average life of the source
average life is 1.44X the half-life

  • Pd-103- 21.3 cGy/h
  • I-125: 7 cGy/h
186
Q

melanoma eye dose

A

70-100 Gy to tumor apex (point of maximal thickness)

187
Q

retinoblastoma (EYE CANCER THAT BEGINS IN RETINA) eye dose

A

40-50 Gy to tumor apex in 3-5 days

If chemo, reduce dose to 20-25 Gy or wait several months after chemo

usually brachy is not first choice for these; after cryotherarpy or chemo failed

Retinablastoma most common in young children

188
Q

what sources were the machester and quimby systems designed for?

A

Ra-226

can be used for Ir-192 but not low energy sources like I-125

189
Q

pros and cons of Cs-137 vs Ir-192

A

Cs-137 is higher energy = improved PDD but more shielding required
Cs-137 has longer half life and won’t need as many replacements
-Cs-137 has lower specfic activity- requires larger source and larger catheters

190
Q

rule of thumb for error introduced due to pt source approximation

A

t the distance
between the chamber centre and the centre of the source must be at least 10 times the length
of the source in order to ensure that the error introduced due to the point source approximation is less than 0.1%

191
Q

multiple distance method for scatter correction factor

A

obtain measurements at multiple distances
correct for IS
assume rest of the difference is due to scatter

192
Q

scatter correction factor with shadow shield method

A

cone of hgh Z is placed between source and chambr to prevent primary photons from reaching chamber. Difference in measuremnets with cone and without are used to correct for scatter

193
Q

non-uniformity correction factor for brachy calibrations

A

used because brachy source doesn’t uniformly radiatie chamber the way a collimator EBRT source would

194
Q

why not use free air chambers for high energy sources?

A

range of secondary electrons becomes so large that size of air chamber would be really large

195
Q

prescrption point for ocular brachy

A

tumor apex or point of maximal thickness

prescription isodose line should encompass entire tumour

196
Q

dose rate for ocular brachy

A

not less than 0.6 Gy/h

197
Q

uveal melanoma treatment duration

A

5-7 days

could be as short as 3

placque is put in under anasthesis and taken out under anasthesia again

198
Q

main advantage of ocular brachy over proton beam (60Gy/4)

A

low energy brachy sources have less radiobiologic effect on OAR structures compared with proton beam

protons spare tissue posterior and lateral to proton beam but require anterior entry

i.e. brachy = fewer anterior segment complications. Both have posterior segment complications

since eye placques are sewn to the eye wall beneath their target volume, the placque moves with the eye

brachy is less expensive than protons

199
Q

typical eye tumour thickness

A

3-10 mm

basal diameter is 10-16 mm

200
Q

what is uvea

A

middle layer of wall of eye

retina is inner layer
sclera is outer layer

201
Q

when is androgen deprivation therapy typically used?

A
  • no benefit for low risk patients
  • improvements for high risk patients and those with suboptimal dosimetry
  • androgen deprvation could have potential detriment on overall survival especially in older men or those with cardio problems
202
Q

brachy role in breast

A

APBI
conserve the breast instead of masectomy
used in women who undergo breast-conserving surgery- RT helps prevent reoccurence

brachy APBI (like EBRT APBI) lets women get treated within a week- less time than standard breast RT

-also can do brachy boost after WBI

203
Q

requirements for APBI brachy

A

tumor size < 3 cm
patient must be node negative
>50 years old
-surgical margins are negative
-all invasive subtypes and DCIS are included
-estrogen receptor can be positive or negative

also should consider prior radiation to breast/chest, pregnancy, connective tissue disease, calcifications, family history

204
Q

types of breast brachy

A
  • ballon based (MammoSite)

- interstitial

205
Q

APBI brachy prescription

A

34 Gy in 10 fx twice daily

vs EBRT of 38.5 Gy in 10 fx twice daily

206
Q

dosimetric guidelines for APBI brachy

A

V150 < 70 cc for interstitial, 50 cc for balloon
V200 < 20 cc for interstitial, 10 cc for balloon
dose homogeneity index > 0.75
skin max < 100% interstitial and < 145 % balloon

207
Q

What is non-invasive image guided breast brachy?

A

mammo-guided target delineation

  • Ir-192
  • special surface applicators
  • gives hghly collimated photon emissions
208
Q

when can brachy be used for penile cancers?

A

inbvasive T1, T2 and selected T3 penile cancers
typical < 4 cm max diameter
lesions confined to the glans are ideal

209
Q

penile brachy margins

A

10 mm or greater in all directions

210
Q

penile brachy technique

A

Ir-192 LDR or PDR
Paris system
12-18 mm spacing between template positions for LDR
10-12 mm for HDR (helps to get better dose distribution- HDR can be optimized more easily than LDR)

211
Q

how can you get dose to cover end of penis without piecing the skin?

A

use bolus and place a needle in the bolus

212
Q

LDR penile dose

A

60 Gy at 0.5-0.6 Gy/h with treatment in 5 days
-Ir 192

For PDR, pulses equivalent to the hourly dose rate of an LDR implant are delivered every hour

213
Q

HDR penile dose

A

54 Gy/18 fx- 2 per day

38.4 Gy/12 fx- 2 per day

intervak between fractions has to be at least 6 h

constraints are:
V125< 40%
V150 < 20%
Urethrea V115 < 10%
Urethra V90 < 95%

skin dose < 125%

214
Q

what is contoured in HDR penile

A 
CT based
GTV
CTV
skin
urethra
215
Q

most promising technologies for skin cancer imaging

A

optical coherence tomography
confocal microscopy
-both use infrared light

216
Q

margins for skin cancer brachy

A

4 mm for low risk
6 mm for high risk

For interstitial, PTV = CTV
For superficial,
CTV to PTV margin of 5 mm

select applicator based on depth of PTV/CTV and dimension

217
Q

contraindications to brachy skin treatment

A
  • genetic diseases
  • invasion of bone

-brachy for skin is usually limited to patients > 50 yo

218
Q

standard prescription depth for surface applicators in skin brachy

A

3 mm

-max is 4-5 mm as deeper than this would yield very high skin dose

219
Q

standard brachy skin prescriptions for surface applicators

A

Leipzig, Valencia, and eBT
40Gy/8
42Gy/6

achieves BED of 60-71.4 Gy

treatment usually delivered every other day

prescription are prescription points are all over map, some at surface, some 3-5 mm from skin surface

-most common is 3-4 mm prescription depth for Leipzig, valencia, and eBT applicators

~10%/mm gradient (PDD)- so skin surface if prescrbing to 3 mm depth is 130-150%

220
Q

standard brachy skin interstitial treatment dose

A

30 Gy/10fx

used for lesions more than 5 mm deep (surface brachy would give unacceptable high skin dose)

221
Q

standard brachy skin dose for flaps/custem molds

A

40 Gy/10 to 42Gy/6, for BED of 65 to 70 Gy

treatment usually delivered every other day

prescription are prescription points are all over map, some at surface, some 5-10 mm from applicator surface

222
Q

where are active dwell positions in custom molds for skin brachy?

A
  • active dwell positions are typically 2-5 mm from skin

- catheters are 10 mm or less apart

223
Q

radionuclide - based skin applicators like Leipzig and Valencia

A

cup-shaped applicators- designed to help improve dose distribution and limit normal tissue exposure

  • limited to flat surfaces and lesions
  • SSD of 13-15 mm
  • plastic end cap at skin surface reduces skin surface dose and prevents tissue from protruding into cone
  • Valencia has FF between source and skin surface = increased treatment time than Leipzig but better dose profile
  • typically made of high Z material (thus can be artifacts if CT is used for imaging)

Have to use Monte Carlo because TG43 doesn’t model the metal

224
Q

energy of electronic brachy devices

A

50-69.9 kV

225
Q

does skin applicator have to be flush to skin?

A

yes, otherwise there would be an air gap
-dose fall off for air gap of 1 mm can be 10%

however too much pressure on the applicator can cause tissue compression resulting in overdosage or hypoxic change to the target tissue

226
Q

what depth must interstitial skin brachy catheters be?

A

at least 2.5-5 mm from skin surface or else will get skin toxicity

227
Q

what US frequency should be used for skin brachy?

A

> 18 MHz

gives better detail of skin surface

228
Q

superficial applicator dose rates

A

0.2 - 4 Gy/min

229
Q

what should dose to the skin surface be limited to for skin brachy?

A

125% for flaps and 140% for custom molds

230
Q

what should QA of applicators include?

A

end-to-end testing

231
Q

where is brachy used for sarcomas?

A

enhance local control in patients undergoing limb-sparing sarcoma resection

radiation therapy is either pre-operative EBRT or post-operative EBRT or BT, or combination

So far no comparisons of BT vs EBRT

both LDR and HDR

232
Q

typical sarcoma margin in brachy

A

2 cm craniocaudally

1-2 cm radially

233
Q

for sarcoma brachy- how close to the skin can the catheter be?

A

> 0.5 cm to limit skin toxicity

234
Q

sarcoma brachy dose rate

A

~0.5 Gy/h

235
Q

sarcoma prescription dose

A

typically 45-50 Gy EBRT plus 12-25 Gy brachy

or 45-50 Gy brachy only

236
Q

are permanent seeds used in sarcoma brachy?

A

yes, especially for small targets such as head and neck

237
Q

breast and prostate LDR- why type of loading?

A

hot loading

needles are pre-loaded with sources

238
Q

breast interstitial template

A

2- one on each side of breast

239
Q

permanent seeds- safety considerations

A

transport
inventory tracking
disposal

240
Q

If someone asks about implementing a LDR program, what are key big picture points to mention?

A

assemble multi-disciplinary team
refer to relevant AAPM report, CPQR, CNSC
radiation safety considerations

241
Q

advantages and disdvantage of electronic brachy capsule

A
  • can be turned on and off
  • larger capsule size
  • may require repair
  • low energy (doesn’t penetrate deeply)
242
Q

why use brachy for skin vs kV or electrons?

A

skin brachy is useful if the skin has a complicated surface

kV and electrons- need flat surface. With skin, can use freiburg flap, moulds

skin brachy can be interstitial- less dose to skin surface compared to kV. Also less dose to organs past skin with brachy compared to EBRT

Valencia/Leipzig can be tough for skin surface as PTV has to fit within 3 cm diameter device and need flat surface- however, faster dose fall-off than ortho and will “stick” to target

freiburg flap-can get more heterogeneous dose distribution compared to using kV or electrons

no brachy near eyes (skin too fragile)- skin in general is risky-can disfigure face

243
Q

penile brachy applicators

A

needles
freiburg flap
3D printed

244
Q

big issue with penile brachy

A

set-up

hard to keep penis still, gaps between applicator and penis etc

245
Q

bronchi applicators

A

through nose

palliative

246
Q

esophagus brachy

A

1-2 fractions of 8 Gy each
palliative

thourhg mouth or nose

have to be careful of perforation
-need endoscopist

-patient awake

247
Q

common head and neck brachy site

A

base of tongue

need surgery to access base of tongue

248
Q

how many dwell positions does mammosite have?

A

single one

-unless it has several lumens

249
Q

breast permanent implants

A

can use templates around breast

needles are pre-loaded (imaging and workflow is similar to prostate permanent)

250
Q

what does ocular plaqe look like?

A

gold shield

has concentric circle of sources

251
Q

advantages of HDR vs LDR

A

-no exposure to staff
-optimization: can adjust dwell times for each channel- better control of dose distribution
stability- movement of applicators is minimized
-can reduce dose to normal tissue by physically displacing it
-small HDR sources allows use of smaller applicators

252
Q

what is anisotropy

A

directional dependence of fluence from a source due to location of radioacive material in the source and differences in wall thickness and construction

253
Q

HDR and LDR source current reading range

A

LDR: 10 ^-11 A
HDR: 10^-7 A

254
Q

what do eye plaque patients wear when nursing is around?

A

leaded glasses to reduce radiation exposure levels by 5-6

255
Q

standard of care for gyne brachy planning

A

MRI

256
Q

can you use PET-CT bunker for HDR suite?

A

No, because PET-CT is shielded for 0.5 MV photons whereas Ir-192 spectrum includes photons of higher energy than 0.5 MV

257
Q

mammosite vs EBRT performance

A

mammosite had increased rates of complications vs EBRT

258
Q

is 60 cc rule in prostate more relevant for HDR or LDR?

A

LDR - with HDR it is easier to spare OARs with optimization

259
Q

you cannot see anterior part of prostate- what can you do?

A

decrease US frequency to get increased penetration at expense of resolution

260
Q

decay mode for Ir-192

A
  • decays 95% of time to 192 Pt through beta emission
  • for remaining 5 % of time, decays through electron capture to 192 Os. Gamma photon with average energy of 0.38 MeV (max 1.06 Mev) is released in the process
261
Q

decay mode for I-125

A

100% electron capture, releases gammas

-IC and auger electrons cause little damage outside the cell which contains the isotope atom

262
Q

decay mode for Pd-103

A

electron capture, releases characteristic x-rays

263
Q

decay mode for Co-60

A

beta decay, releasing 1.17 and 1.33 MeV photons

264
Q

decay mode for Cs-137

A

beta decay, releases 0.66 MeV photons

265
Q

building new brachy program- major consideration?

A

what source to use

266
Q

How does G(r) fall off for Ir-192 vs I-125?

A

Ir-192 as 1/r^2

I-125 faster than this

brachy (17 decks)

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