Radiotherapy

Question 1

1. Why do the MU’s increase when using IMRT?

Answer 1

• MU’s (amount of time the linear accelerator is producing radiation)
• More mu’s = machine is on for more time
• In IMRT, treatment is divided into segments so delivering dose to the PTV takes much longer.

Question 2

2. Why is 6MV used and not 10MV in IMRT/VMAT?

Answer 2

Build up region, dose build up and dmax is reached
2 reasons:
1. Higher energy=wider lateral penumbra

• IMRT/VMAT uses segments to administer the dose across small sections of the PTV.
• These segments must be as precise as possible so the sum of all segments add up to the prescribed dose of the PTV.
• Therefore, these segments must be as sharp as possible.
• Therefore, they benefit from low energy beams that provide less penumbra (blurring at the edge of the beam)
• It would be more difficult to be precise if a blurring of one segment goes on another segment.
• So segments deliver dose in a sharp way-we want to have matching segments

2. Neutron production (the production of secondary neutrons from the head of the linac)
   - 10MV or more = production of neutrons
   - There might be neutron leakage from the head of the linac which can create a problem in radiation protection as the room design might need to be adjusted as gamma rays are generated from neutrons (neutron door with borated paraffin, more materials for shielding, longer mazes).
   - Energies react with the head of the gantry and will eject neutrons and can activate a normal isotope into a radioactive isotope but then they decay.

Question 3

3. What are the advantages of using 10MV?

Answer 3

• Higher energies reduce the lower dose to the OAR that are shallowly located relative to the tumour target due to greater penetrating power and larger dose buildup at 10MV
• BUT this does not compensate for the problem of production of neutrons
• Higher energy will reduce the number of MU’s needed as with less MU’s dose at depth can be achieved.

Question 4

4. What is the role of the FFF and what are its advantages?

Answer 4

Role

• To make the photon beam dose distribution uniform at reference depth within the allowed variations.
• In the centre of the beam, we have more photons than in the edges
• Different intensities in the beam profile-oar sparing

Advantages

• Advantages of FFF- intensity of the beam in all areas more photons come out of the machine per unit of time-segment delivered faster.
• We can use FFF in IMRT as we’re already going to deliver different segments with different amounts of intensities.
• We don’t need our beam to be flat/ homogenous
• The amount of time we need to deliver the segment/dose is much lower so time is reduced
• Used because even though we don’t have a flat beam profile we can still plan each of the segments with the intended intensity
• Increased dose rate=less leakage radiation- photons are going to the patient so no scatter- reduce dose to the patient – faster treatment delivery
• Disadvantages?
• The Flattening Filter removes some low energy photons – with FFF (without FF), more photons with low energy come out, increasing skin dose  but IMRT/VMAT distributes the dose so much around the patient that skin side-effects do not increase. Therefore, this disadvantage is negligible

Question 5

5. Why do we need to be accurate in-patient position?

Answer 5

• We image everyday as VMAT is very conformal to the PTV. THE 95 % isodose line/therapeutic dose takes the shape of the ptv so if we are off we won’t be treating the treatment site completely and might treat an OAR instead which will cause side-effects.
• We also need to be accurate even with 3DCRT since errors result in PTV less treated and normal tissues irradiated – however, this is more severe to IMRT/VMAT because the 95% dose is often much closer to the PTV

Question 6

6. Why does the frequency of imaging effect our margin?

Answer 6

• If we have frequent imaging, we reduce setup errors as we see the tumour inside the PTV therefore reducing organ motion and setup errors. Therefore, we can correct and apply shifts. If we have less setup-errors we can reduce the margin between the CTV and PTV.
• N.B. MARGINS BETWEEN CTV AND PTV HAVE NOTHING TO DO WITH THE TREATMENT TECHNIQUES BUT WITH SETUP ERRORS AND INTERNAL MOTION. Therefore, imaging affects the margin between the CTV and PTV and not the rt technique.

Question 7

7. Why do we need higher accuracy imaging verification when using IMRT/VMAT?

Answer 7

1. Conformity of dose
   - If we miss the target- more healthy tissue is irradiated, and we miss the tumour
2. Heterogeneity of tissues
   - If the patient is in different positions, the beams may pass through different tissues with different densities, changing the dose distribution-may be a corner or a small part of the area.

Question 8

8. When we increase the tissues receiving low doses what late side effects are increased?

Answer 8

• Secondary tumours (Stochastic effects) can occur in all tissues especially sensitive tissue such as thyroid, lung.
• Stochastic=probabilistic

Question 9

9. What is IMRT?

Answer 9

• Irradiation of the target using beams with a heterogeneous intensity.
• With IMRT we can play with the intensity of the beam from different angles which allows us to avoid OAR and have a more conformal dose to the tumour.

Question 10

10. How are different intensities created in the beam?

Answer 10

• Beam divided into segments with each segment giving varying intensities
• MLC will open for a certain amount of time and closes when the desired radiation is given
• MLC will close fast when low intensity is required, and MLC will stay open longer when high intensity is required such as at the tumour site
• Sliding window is used in the department- radiation is on whilst the MLC’s are moving.
• Step and shoot is used on synergy

Question 11

11. Why does IMRT give more skin dose than 3DCRT?

Answer 11

• Larger amount of skin but lower doses!
• More skin dose as more beams- more beams, means less dose to the skin because the dose is spread around the patient-However, more beams means more skin irradiated with lower doses
• FFF -more photons with low energy
• But since the beams are distributed around the patient, so the doses are lower. Unlike 3dcrt where dose is distributed in one place

Question 12

12. What can be used in 3DCRT to make up for differences in depth

Answer 12

Wedges

Question 13

Why does IMRT/VMAT improve the conformity but increase the volume of normal tissues with low doses, when compared to 3DCRT?

Answer 13

• The more beams = more conformal 95% to tumour
• Because of the higher number of beams spread around the patient there is a larger amount of normal tissues irradiated, compared with 3DCRT

Question 14

14. Why are odd number of beams recommended in IMRT?

Answer 14

• To irradiate different corners of PTV- we don’t need parallel opposed beams in IMRT-still have an equal 360-degree distribution around the tumour
• We’’ll be irradiating the same angle but from different sides and beams are equidistant from each other
• Even beams are parallel opposed.
• Short answer: to avoid parallel opposed beams
• parallel opposed mean that we would be applying intensity modulation in the same angle twice, just coming from opposite sides – this is not beneficial to achieve a better dose distribution

Question 15

What is VMAT?

Answer 15

• VMAT is a rotational IMRT technique where the intensity of the radiation beams is modulated using simultaneous variation in gantry rotation speed, dose rate, and MLC leaf positions.
• The treatment machine continuously reshapes and changes the intensity of the radiation beam as it rotates around the patient
• Arc or multiple arcs are used instead of a fixed number of beams like IMRT
• MLC’s - MLC adapts to allow variation of the intensity of the beam
• Unlike IMRT dose is delivered whilst gantry is rotating
• Intensity of the beam can be changed in 3 ways
  1. MLC’s change shape between degrees- to give dose in different areas of the PTV.
  2. Dose rate will change in different areas- certain areas will receive a high dose others will receive a low dose.
  3. Gantry speed- faster in certain areas-low dose, slower in certain areas-high dose

Question 16

Aim of change in intensity?

Answer 16

Normal tissues receive the lowest possible dose to reduce side effects and tumour tissues receive the highest possible dose.

Question 17

17. What is the benefit of compensating for surface irregularities?

Answer 17

• Dose to the tumour is more conformal

Question 18

What is deep inspiration breath hold?

Answer 18

• Deep inspiration breath hold (DIBH) is a radiation therapy technique where patients take a deep breath during treatment and hold this breath while the radiation is delivered. By taking a deep breath in, your lungs fill with air and the heart will move away from the chest.
• Therefore, this prevents irradiation of the heart which can cause side effects such as pericarditis-inflammation of the tissue surrounding the heart
• Breathe is kept for 20 seconds so patient must be capable of holding their breath for so long.

Question 19

1. What is 3DCRT?

Answer 19

• Uses kv planar 3d imaging to adjust the shape of the individual radiation beams
• Beam conformed to the PTV due to the MLC
• Done with MLCs
• Shape of beam=shape of the target

Question 20

20. Why is the gantry extension removed?

Answer 20

Mostly done in head and neck patients.

1. Removed to prevent any collision especially when there is a couch rotation
2. Extension has metal clips which will attenuate the beam during the image. Metal appears as a bright area on an X-ray, blocking visibility of underlying structures.
3. Main couch and extension have different thicknesses the attenuation will be different from the main couch and the extension and during planning doses are calculated with the main couch during planning.

Question 21

21. Why do we override?

Answer 21

• The machine won’t let us treat without override

- Previous treatment would have been done with the override and image would have been fine so its ok to override.

Question 22

24. Why should everything be at zero before treating

Answer 22

• Crosswires and lasers match

- SSD at G0 is accurate

Question 23

25. Why does cream have to be applied up to 2 hrs before treatment?

Answer 23

• Cream done exactly before treatment will increase the thickness and increase skin dose as the beam will not penetrate as deep- it’s like having a bolus- therefore there is an increase in side-effects.

Question 24

26. Stochastic vs Deterministic side effects

Answer 24

Deterministic
- Requires a specific level of exposure to ionizing radiation before it can occur is called a deterministic effect.
- The severity of a deterministic effect increases as the dose of exposure increases and considers a minimum threshold, below which no detectable clinical effects occur.
- This type of effect is predictable and reproducible
Causes
- Severe cell damage or death. Individuals who experience the physical effects of this cell death do so when it is large enough to cause significant tissue or organ impairment.

• Deterministic effects are short-term, adverse tissue reactions resulting from a dose that is significantly high enough to damage living tissues.
• The severity of a deterministic effect increases with radiation dose above a threshold, below which the detectable tissue reactions are not observed.
• Deterministic effects are usually predictable and reproducible.
• For example, localized doses to certain parts of the body at increasing levels will result in well-understood biological effects.

Stochastic
- Probabilistic effects that occur by chance.
- The probability of occurrence is typically proportional to the dose received.
- Stochastic effects after exposure to radiation occur many years later (the latent period).
- The severity is independent of the dose originally received.
Examples of stochastic effects include:

• Cancer
• Heritable or genetic changes

Question 25

28. Staging vs Grading

Answer 25

Staging
-	size of primary tumour
-	how far cancer has spread
-	tnm, 4 tier model
-	“stages of life”
Grading
-	Appearance of tumour- differentiation-poorly differentiated (different but poorly not well)- looks different from normal cells
-	Aggressiveness of tumour
-	Gives an idea of how quickly the cancer can grow.

Question 26

What is dmax

Answer 26

Dmax is the maximum dose point within the PTV and the organ at risk.

Question 27

31. What is the dose build up region?

Answer 27

• The region between the skin and the depth at dose maximum (Dmax) is called the build-up region.
• This region between the surface and depth d is referred to as the dose build-up region in megavoltage beams, and it results from the kinetic energy deposited in the patient by secondary charged particles (which have relatively long ranges) released inside the patient by photon interactions (photoelectric effect, Compton effect, pair production).

Question 28

32. Percentage depth dose?

Answer 28

• The ratio (in percent) of the dose absorbed at a predefined depth (Dxto Dmax (the dose maximum) for a predefined SSD and field size is termed the percentage depth dose (PDD or DD%)

Question 29

33. What are the 5 r’s of radiotherapy?

Answer 29

1. Repopulation
2. Redistribution
3. Repair
4. Reoxygenation
5. Radiosensitivity