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Brachytherapy > Intro to Brachy > Flashcards

Intro to Brachy Flashcards

1
Q

What is the definition of Brachytherapy?

A

A ‘near treatment’ - sealed sources placed close to, in contact with, or inside the volume of tissue to be treated
It can also include miniaturised electronic radiation sources

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2
Q

Why is Brachytherapy used?

A

Gives high dose to the target volume with rapid inverse square law fall off
Low integral dose
Can optimise by adjusting source dwell positions and dwell times
Gives the same dose to the CTV whilst reducing the OAR doses
Sources move with the target
No external beams passing through normal tissue
Radiobiology can be better as fewer fractions can be used and there is greater conformality

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3
Q

When is Brachytherapy used?

A

Post surgery to irradiate any remaining clonogenic cells
As a boost for EBRT
Monotherapy where EBRT and surgery aren’t available

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4
Q

What are the disadvantages of using Brachytherapy?

A

Invasive surgery - time consuming, need lots of resources
Geometric misplacement results in poor coverage due to rapid fall off of dose
Large volumes (>5cm) lose the inverse square law advantage
Need specific radiation protection and security requirements

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5
Q

What are the 4 pieces of legislation around Brachytherapy?

A

Sealed sources registration permit
HASS
ARSAC
EURATOM

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6
Q

What is the Sealed Source Registration Permit required by?

A

Environmental Permitting Regulations 2016 - England and Wales
Radioactive Substances Act 1993 - Scotland

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7
Q

What does the Sealed Sources Registration Permit application include?

A

Sources held, how they are stored, how they are used
Need to reapply if adding sources or changing premises
Includes HASS form

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8
Q

What is HASS?

A

High Activity Sealed Sources Directive 2005

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9
Q

What are the aims of HASS?

A

To prevent exposure of workers and the public to radiation arising from inadequate control of high activity sealed radioactive sources
To harmonise controls in place
Set out specific requirements for control

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10
Q

Which sources require a HASS form?

A

Those with an activity at the time of fabrication of > 1/100 x A1 value in IAEA regs for safe transport of radioactive materials

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11
Q

What is needed to get a HASS permit?

A

Prior authorisation for any practice involving HASS
Demonstrate adequate management arrangements
Adequate provision, financially or otherwise, made for safe management of HASS when they become disused
Responsibilities, training, equipment, and emergency procedures
Record keeping by users and competent authorities
Security arrangements in liaison with local counter terrorist security

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12
Q

What is required in a HASS site security plan?

A 
Posts and responsibilities
Specifications of competence (job descriptions)
Details of information and training
Operating procedures
Outline of maintenance
Emergency procedures
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13
Q

What does the level of security at a HASS site depend on?

A

isotope and activity - specified in table produced by National Counter Terrorism Security Office and National Security Advice Centre

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14
Q

What does a level 1 site require?

A

Security plan
Ability and options to upgrade security
Information security plan
Personnel background checks

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15
Q

What does a level 2 site require?

A

Sources protected by 1 physical security measure

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16
Q

What does a level 3 site require?

A

2 physical security measures, intruder alarm with either a local or police response

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17
Q

What does a level 4 site require?

A

The same as level 3 but with the intruder alarm externally monitored and the response must be from the police

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18
Q

What are the requirements for obtaining an ARSAC licence?

A

For an oncologist to prescribe Brachy treatments
Must demonstrate the Oncologist’s qualifications, experience, the source type to be used, and scientific support
Needs to be renewed every 5 years

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19
Q

What is EURATOM?

A

Permission from the Environment Agency to ship sources between EU member states
Requires form detailing the sender and receipt of sources and the quantity per shipment
Renewed every 3 years

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20
Q

What factors are considered when deciding on a Brachytherapy source?

A

Photon energy spectrum - 0.2-0.4MeV ideal
Charged particle emissions - avoid except pure beta emitters
Source size - small
Specific activity - high=small source
Half life - depends on use
Source strength
Decay scheme - are daughter elements radioactive
Physical form

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21
Q

What is the average photon energy from Ra-226?

A

0.83MeV

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22
Q

How was Ra-226 available?

A

Needles loaded into gold cells - 1cm long, 1mm diameter encased in platinum to prevent leakage, prevent alpha/beta particles escaping, provide strength and rigidity

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23
Q

What are the advantages of using Ra-226?

A

Long half life

Lots of experience using it

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24
Q

What are the disadvantages of using Ra-226?

A

Gaseous recay product, Ra-222, which is radioactive
Low specific activity
Bulky source
High energy beta emissions - needs heavy screening to avoid necrosis
High energy photons - needs shielding

25
Q

What are the advantages of using Ir-192?

A

Unencapsulated energy of 0.37MeV - easy radiation protection
Dominant beta emissions at 0.53, 0.67 MeV - 0.1mm Pt filtration adequate
Decays to stable Pt-192, no gaseous products
High specific activity - small sources - used in remote afterloading HDR

26
Q

What is the disadvantage of using Ir-192?

A

Half life of 73.83 days - replaced every 3 months

27
Q

What are the 3 methods of using Ir-192 and how do they work?

A

Flexible wires - not used - wires consisted of 0.1mm radioactive core of Ir, surrounded by 0.1mm Pt, wires were cut to length so closed source - implanted with nylon tubing
Hairpins - sledom used - Pt covered Ir pins - 0.6mm thick, supplied at 60mm long so cut to length, inserted using slotted guides, with one leg longer than the other for ease of insertion - used when site is only accessible from 1 side
HDR remote afterloading - common - narrow source, driven on drive cable to place source in implants - causes less trauma and more flexible

28
Q

What are the advantages of using I-125?

A

High specific activity
Low photon energies - 27.4, 31.4 keV - don’t need much PPE/shielding - low surface dose rates so no limitations on patient after implant
Half life - 59.4 days - suitable for applications and easy to store/transport

29
Q

How is I-125 used?

A

Used for permanent prostate implant and eye implants
For prostates: use string of 10 seeds, 1cm apart on an absorbable braided carrier which is stiffened and sterilised before insertion. Use strands to prevent seed migration and misplacement at the edge of the prostate
For ophthalmic applicator: applicator sewn into place for 7-14 days at the back of the eye for choroidal malignant melinomas - use US to assess the depth of treatment - use activity of 6.2uGym^2/hr in hexagonal arrangement

30
Q

What are the disadvantages of using I-125?

A

Highly anisotropic dose distribution around seeds, non-trivial dosimetry

31
Q

What is the equation for the total treatment time of an opthalmic applicator?

A

tr = -mean lifetime. ln(1 - prescribed dose/dose rate on day of implantation.mean lifetime))

32
Q

What is Palladium - 103 used for?

A

Similar to I-125 but half life 17 days and photon energy 21keV
Used for permanent interstitial implants - the short half life makes it good for aggressive prostate tumours

33
Q

What is Ru-106 used for?

A

Has energy of 3.54MeV
Used for opthalmic applicators - higher energy than Sr-90 so has a max range of 20mm
Can be used for prostate treatments for a faster delivery but the photon fluence is anisotropic and published data is sparse

34
Q

What are the descriptors for Brachy treatments based on?

A

Radionuclide positioning
Duration of irradiation
Source loading techniques
Dose rates

35
Q

What are the source positioning techniques?

A

Interstitial - sources implanted into tissue
Intracavity - Discrete radioactive sealed sources inserted into body through cavities
Intraluminal - bronchus, esophagus
Intravasular - arteries to prevent restenonsis
Surface moulds - sources placed adjacent to superficial lesions, 0.5cm from skin - moulds conform to body surface

36
Q

What are the two durations of treatment used and what are they used for?

A

Temporary - sources used for short length of time, removed once prescribed dose reached - fractionated if HDR, PDR, continuous if LDR. Used for gynae, eyes, interstitial
Permanent - sources left in implant site, dose delivered exponentially to complete decay. Used for prostate

37
Q

What are the source loading techniques available?

A

Preloading - inserted in theater, either individual sources or applicator containing sources
Afterloading - applicator positioned first, source then loaded, can be by hand or machine

38
Q

What are the 3 dose rate definitions?

A

LDR: 0.4-2Gy/hr ~ 10Gy/day
MDR: 2-12Gy/hr ~10Gy/hr
HDR: >12Gy/hr ~0.2Gy/min

39
Q

What are the advantages of using afterloading?

A

Improves radiation protection for staff
Source can be retracted for nursing care
Treatments optimised to individual anatomy by varying source positions and dwell times

40
Q

What is the process of afterloading?

A

Applicator inserted under GA or sedation
Image to assess positioning
Applicators connected to afterloading machine
Send out dummy source first, then actual source
Treatment times: LDR - 12-48 hrs
HDR - 5-15mins

41
Q

What is the source in a HDR Microselectron?

A

Ir-192 stepping source is mechanically driven on a flexible cable
High activity: 370GBq ~40mSv/hr @ 1m
Source is 0.9mm diameter, 3.5mm long - can travel down smaller thinner wires than LDR microselectron

42
Q

What are the source position options for a HDR Microselectron?

A

48 source positions
Dwell positions every 2.5mm or 5mm
Total treatment length of 11.75cm or 23.5cm
30 channels for applicators

43
Q

How is the dose distribution of a treatment using the HDR microselectron achieved?

A

Increase dwell times at either end of the treatment volume - can reduce skin sparing

44
Q

What radiation provisions are needed for used a HDR Microselectron?

A 
Shielding
Interlocks to retract source
Radiation monitors visible
Audible warning lights
CCTV
45
Q

What are the HDR gynae applicator options?

A

Fletcher Williamson applicator - tungsteon shielding for OARs, cervix sleve so only inserted once
Ring applicator

46
Q

What is the Manchester system for Cervix treatments?

A

Gives loading ratio rules for uternine and vaginal sources to give the same dose to point A regardless of size/shape of uterus and vagina
uterine tubes have 3 lengths available, vaginal applicators have a range of diameters

47
Q

What are the two dose specification points in the Manchester System?

A

Point A - represents the paracervical triangle, where the uternine vessels cross the ureter which is the most radiosensitive point. 2cm up from base of uterine tube, 2cm across from uterine canal
Point B - gives indication of lateral spread of dose (pelvic lymph nodes) 2cm up, 5cm across from base of uterine tube, perpendicular to uterine tube

48
Q

How does the ICRU recommend specifying dose?

A

Use dose to volume

49
Q

How are the doses to the rectum and bladder calculated?

A

Use Foley catheter to loacte bladder - it’s at the centre of the ballon on AP view, and on posterior of balloon on lateral view. Rectal point is0.5cm behind posterior vaginal wall on lateral view

50
Q

What does ICRU 89 include?

A 
Evidence base
Tumour delineation on MRI
Reporting standards
Radiobiology
DVH analysis
Dose calculations
Treatment planning
OAR limits
9 case studies
51
Q

How is cervix Brachy performed?

A

Use US in theatre to reduce false passage

Use interdigitation of Brachy and EBRT or put Brachy fractions on the end over 2 days

52
Q

How is an interstitial ring commissioned?

A

Learning/training
Geometry found using x-rays, CT, MRI or autoradiographs
Make standard library plans
Check using in house software or Radcalc

53
Q

How is the dose to the cervical CTV prescribed in Brachy?

A

Draw high and intermediate risk CTV
Outline based on ICRU89 and GEC-ESTRO guidancce
Use adaptive Brachy planning

54
Q

How does the PDR Microselectron work?

A

Use pulses of HDR - treatment delivered for a fraction of each hour with the source returning to the safe in between
Ir-192 stepping source is the same as HDR Microselectron
Cervix treated in one 24 hour session
Restricts HDR room to one patient for 24 hours
Requires 24 hour nursing
Don’t know if it has the same radiobiological effect

55
Q

How is I-125 prostate brachy performed?

A

Use 0.464uGym^2/hr source
US probe in rectum guides placement
Use template to guide needle placement in prostate
Catheter in ureter

56
Q

How is a Mammosite treatment performed?

A

Place uniflated Mammosite balloon in tumour resection cavity
Inflate with saline and contrast agent
Deliver dose through radioactive seed attached to wire
After treatment deflate the balloon and remove

57
Q

How is an Accuboost treatment delivered for a breast treatment?

A

Mammography paddle immobilises the breast
A non-invasive HDR applicator is positioned on either side of the breast
Delivers radiation from all orientations
Lots of dose in position where the fields overlap

58
Q

How is a restenosis treatment performed and why?

A

To stop recoil, remodelling and hyperplasia

Use a beta-catheter system - use a transfer device containing radioactive source and delivery catheter

Brachytherapy (3 decks)

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