Physics and Radiation Concepts

Question 1

Definition of ITV, PTV

Answer 1

ITV: internal target volume: an added margin (internal margin) on the CTV to account for internal organ/target motion

PTV: allows for uncertainties in patient setup and treatment delivery. PTV accounts for day to day geometric errors in patient set-up, beam geometry, mechanical uncertainties, dosimetric uncertainties, whereas ITV accounts for physiological organ movement

Question 2

Examples of motion management IE for liver SBRT

Answer 2

1)ABC
2) 4DCT with abdominal compression (note 4DCT is not actually minimizing motion LOL)
3) Gating: tracks abdominal movement, beam on usually in the exhale position

Question 3

compare LDR and HDR radiobiologically

Answer 3

With LDR cells can repair Sub Lethal Damage and there is more reoxygenation. (if they want a third add redistribution).

Question 4

Definition of EUD

Answer 4

Equivalent uniform dose = Absorbed dose that when homogeneously given to a tumor yields same mean cell kill as non-homogeneous given XRT
Designed to make comparisons among alternative tmt plans when rads are non-homog

Question 5

Name 6 things that you could see on orthogonal radiographs which would identify an optimal tandem/colpostat implant for cervical carcinoma

Answer 5

Tandem midline between ovoids on AP film
Tandem bisects ovoids on lateral film
Tandem vertical and not tilted side-ways
Gold seed fiducial markers
Applicators half-way between bladder and sacrum
Packing in place between ovoids and bladder and rectum
Foley balloon in place and pulled down to bladder neck

Question 6

Which decay emission has shorter range, Beta or alpha emitter

Answer 6

Alpha emitters! <0.1mm, beta is like 5mm!

Question 7

EQD2 for 8/1, 20/5, 30/10

Answer 7

8/1: alpha beta 2 = 20Gy
20/5: Alpha beta 2 = 30Gy
30/10: 37.5Gy

Question 8

Cord rate of myelopathy for
1) 50Gy
2) 60 Gy
3) 69Gy

Answer 8

1) 0.2%
2) 6%
3) 50%

Question 9

Imaging: what is
1) bone and muscle on T1/T2 MRI
2) White on T1
3) White on T2
4) The squence for brain MRI where white matter and grey matter actually appear as named

Answer 9

1) bone always black, muscle always grey
2) Fat (think white matter tracts on T1 brain)
3) Liquid (bc T2 is liquid sensitive)
4) T1!!!! Think: Bc CSF bright on T2 which is opposite IRL

Question 10

PET:
1) how is FDG18 made
2) FDG18 half life
3) 5 reasons why a tumour would not show uptake on PET
4) Lymphoma histologies with no pet uptake
5) optimal timing of PET post treatment in H&N ca

Answer 10

1) electrophilic fluorination
2) 2 hours

3) low overall glucose metabolic rate
low levels of Glut-1 and other transporters
high levels of glucose-6-phosphatase
Tumor necrosis
Poor tumor vascularity
Volume of tumor is too small (<1cm)
(Diabetes can increase noise)

4) marginal zone lymphoma, lymphoblastic lymphoma (think: margin is on side/margin of paper, ll are like two vertical lines I know this is dumb lol), cutaneous T cell lymphoma: think: because OBVIOUSLY in the skin LOL

5) 3 months

Question 11

6 pretreatment QA things to do before RT delivery

Answer 11

• Plan eval
• dose calc QA
• deliverability of plan
• Peer review
• Machine QA
• Image guidance

Question 12

3 things a medical physicist does for IMRT QA:

Answer 12

1) plan QA: mu calculation
2) Phantom QA
3) Machine QA

Question 13

4 things medical physics does as part of QA for linac beam quality

Answer 13

1) dosimetry: photon/electron output consistancy
2) mechanical: gantry/collimator angle, field size indicators
3) Safety interlocks: emergency off switches
4) Others; Lasers, AV monitoring

Question 14

3 ways to confirm dose planned will be delivered accurately

Answer 14

1) Independant MU check (radcalc)
2) phantom or TLD check
3) Trial set up to make sure no obstacles or collissions

Question 15

List SIX factors which affect the dose to a patient when using a single direct photon beam. Do not list basic machine characteristics.

Answer 15

Beam energy
Angle of incident beam
Use of beam modifying devices (bolus, wedges)
Composition of tissue through which beam is travelling (lung vs bone vs tissue)
Thickness of the patient
Field size
SSD

Question 16

Electrons: 5 factors that affect PDD

Answer 16

Beam energy (direct)
Field size at surface (Depends on field size)
SSD (direct)
Depth (inverse)
Obliquity (inverse)

Question 17

Define:
1) conformality index
2) selectivity
3) Coverage

Answer 17

1) Treatment volume/PTV (want <1.3)

2) TV+PTV union / TV

3) TV + PTV union / PTV

Question 18

PDD: define, how does it change with
1) increasing SSD
2) increasing field size
3) increasing dose rate

Answer 18

dose at a reference point as a percentage of dmax
1) increase PDD (bc less drop off with inverse square law)
2) Increase field size: (increase in PDD because there will be more lateral electron scatter towards central axis
3) no effect

Question 19

Describe two purposes of a flattening filter

Answer 19

1) flattens the beam at depth
2) Hardens the beam (more penetrating)

Note: a soft beam has lower energy photons in it, decreasing the average photon dose that is recieved.

Question 20

Comparing heavy particle vs. photons, what are 5 things that affect RBE (4)

Answer 20

Dose rate
Biological end point (tissue sensitivity)
Dose/fraction
LET
Total dose

Question 21

What three things affect geometric penumbra

Answer 21

source to collimator, SSD, diameter of source

Question 22

what happens to surface dose with electrons as you increase energy

Answer 22

IT ACTUALLY INCREASES BETCH!! unlike photons!
- Dmax and 50%IDL also increase/deepens

Question 23

1) Define tissue maximum ratio
2) What is TMR for a 10x10 field using 6Mv at dmax

Answer 23

1) The ratio of the dose in a phantom at depth D over the dose at dmax at same point in space, with the same SAD and FS.
Dependent on depth, energy and field size, and independent of SAD

2) 1

Question 24

What is the activity of Ir-192 (in curie)

Answer 24

10Ci

Question 25

Name 6 things that you could see on orthogonal radiographs which would identify an optimal tandem/colpostat implant for cervical carcinoma

Answer 25

• Tandem midline between ovoids on AP film

-Tandem bisects ovoids on lateral film

• Tandem vertical and not tilted side-ways
• Gold seed fiducial markers
• Applicators half-way between bladder and sacrum
• Packing in place between ovoids and bladder and rectum
• Foley balloon in place and pulled down to bladder neck

Question 26

What will happen to quality of CT images with the following:
1) increase mAs of scan
2) kVP
3) antiscatter

Answer 26

1) increased dose/current increase in resolution (think: LDCT worst quality duh)
2) lower voltage improves quality of CT angiogram
3) improved quality LOL

Question 27

1) how many mSv of background radiation a year?
2) dose in CXR
3) dose in CT chest
4) allowable public dose

Answer 27

2.5mSv? LOL
2) 0.1mSV
3) 7mSv
4) 1mSv

Question 28

3 necessary components of cone beam CT

Answer 28

Gantry-mounted kV source
Detector on a gantry
Bow-tie filter

Question 29

Air cavity - what happens to dose? What two parameters associated with this effect?

Answer 29

Decreased dose pre cavity, increased dose post cavity.
Due to attenuation and scatter

Question 30

Acute radiation syndrome
1) LD50, 99
2)List the name, dose, and average time to death for three syndromes after single total body exposure (6 marks)

Answer 30

1) 4Gy, 8Gy
2) Hematopoeitic syndrome, 2.5-5Gy, weeks-2mo

Gastrointestinal syndrome: 5-12Gy, 3-10days

Cerebrovascular syndrome: 100Gy, 24-48hr

Question 31

1) Agency responsible for safety
2) 6 responsibilities of the radiation safety officer

Answer 31

1) canadian nuclear safety commission
2) Identify problems
implement corrective action
provide safety advice
ensure safety of personal
ensure compliance
report non-compliance

Question 32

What are the three ways of limiting exposure to a radioactive source

Answer 32

distance, time, shielding

Question 33

Workload, Use factor, and Occupancy factor with respect to radiation protection

Answer 33

Workload (W) = beam on time, total dose or exposure at 1 meter distance per unit time (typically per week, month or year)

Use factor (U) = fraction of time the primary beam is striking the barrier. The following beam use factors are usually assumed for external beam machines: floor = 1, walls = 0.25, ceiling = 0.5. For all secondary barriers, use factor is always equal to 1, since secondary radiation is always present.

Occupancy factor (T) = fraction of time a location is occupied when the beam is on. Is a factor with which workload is multiplied to account for the degree of occupancy of the area in question. Typical values: work area = 1, corridors = 0.25, waiting rooms = 0.125

Question 34

remeber these in QA

Answer 34

also includes daily IGRT and weekly clinical assessments!

Question 35

Electrons: 5 factors that affect PDD

Answer 35

Beam energy (direct)
Field size at surface (Depends on field size)
SSD (direct)
Depth (inverse)
Obliquity (inverse)

Question 36

3 things that affect PDD, other than energy, for electrons:

Answer 36

Obliquity
Field size
SSD

Question 37

what 3 factors affect geometric penumbra

Answer 37

SDD (source to collimator), SSD, diameter of source