HN VMAT/IMRT

Question 1

What is SIB?

Answer 1

Simultaneous integrated boost- Delivering different dose per day to different target regions in the same number of overall fractions

Question 2

Why is VMAT preferred over IMRT?

Answer 2

• Faster
• Less QA
• Can alter gantry speed and dose rate
• Similar target coverage and OAR sparing

Disadvantages:
Increased integral dose = Increased risk of secondary malignancy

Question 3

Beam arrangement?

Answer 3

9 Fields, equally spaced not opposing.

Cannot be opposing as this limits optimiser
System will optimise it the same way

Technique
• ( G180, G140, G100, G60, G20, G340, G300, G260, G220)
• Coll = 0 for all beams except G180, G100 and G260:
• G180: Coll rotated to 270 so MLC can shield spinal cord
• G100 and G260: Coll rotated to miss shoulders

Question 4

EUD = equivalent uniform dose

Answer 4

• The dose if given uniformly to a ROI that will give the same biological
response as the planned heterogeneous dose distribution for the ROI

Question 5

Effect of High density artefacts caused by fillings?

Answer 5

• Dose calculation accuracy negatively impacted

* Need to be contoured and density overrides applied

Question 6

ICRU 83 level 2 dose reporting?

Answer 6

Level 2 dose reporting
• D50% (D Median) should be 100% of prescribed dose
• Prescriptive value D95%
• Dmean
• Near Minimum D98% (>95% of prescribed dose)
• Near Maximum: D2% (<107 % of prescribed dose)
• Prescribe to a volume not RP

Question 7

Why would we contour the pharyngeal constrictors and lips?

Answer 7

To dose dumping resulting in side effects

Question 8

What is the purpose of placing the isocentre in the LDPTV?

Answer 8

Minimise shifts and less chance of collision

Question 9

What is the importance of density overrides?

Answer 9

Allows the beam to account the structure and provide accurate dose calculations

Question 10

How do you do a density override?

Answer 10

Create a manual contour and place it over the artefact and assign density override value

Question 11

Why use 6mV

Answer 11

• build up effect (volumes are close to skin surface)

- several OAR + air pockets

Question 12

Purpose of contracted external?

Answer 12

To reduce pushing reference dose into build up region

Question 13

Purpose of NTT?

Answer 13

Avoid dose dumping

Question 14

When not to use a uniform dose?

Answer 14

Overlapping stuctures (when intermediate dose is in lower dose region) - will make it too hot

Question 15

Why would you use a Max EUD on a parotid?

Answer 15

Overlaps with tumour volume. ‘a’ value determines on reducing hot or cold spots.

Question 16

A value less than 1?

Answer 16

Used on targets to DECREASE cold spots

Question 17

A value equal to 1?

Answer 17

Used on parallel OARs (Cold and hot spots considered equal)

Question 18

A value greater than 1?

Answer 18

Serial strucutres (reduce hot spots)

Question 19

Why is there no expansion on the OBJ PTV structures in VMAT?

Answer 19

Reduces conformality

Question 20

Why is there a ring structure on the high dose PTV?

Answer 20

Replaces the expansion on the OBJ as the distance is smaller (no gap)

Question 21

Why are avoid structures made for VMAT?

Answer 21

Reduce dose dumping - technique desposits dose in every aspect (entry dose)