Review of IMRT/VMAT

Question 1

Single Arc vs Dual Arc vs Partial Arc

Answer 1

• single arc (small volume)
• double arc (large and/or complex volume (i.e. bilateral volume))
• partial arc (unilateral volume)

Question 2

Discuss “beam splitting”. Is it necessary to manually create two beams to satisfy QA?

Answer 2

• beam splitting occurs when the beam is larger than the maximum field size (i.e. 14-14.5 x 19 cm)
• if the beam splits, the optimiser creates an additional arc and determines the dose to the arc (and the optimise could favour the dose to one arc over another arc)
• there are clinical studies, departmental procedures and protocols and physics commissioning to satisfy QA when two arcs are created manually (treatment plan will not pass QA if beam splits)

Question 3

Define “collimator kick”

Answer 3

• a collimator kick is the rotation of the collimator to increase optimisation space to (1) allow optimiser to deliver dose superior and inferior to target volume where the maximum field size is limited (increases treatment plan deliverability) and (2) reduce MLC interleaf leakage and MUs (decreases treatment plan complexity)

Question 4

What is the effect of changing final gantry spacing? How does final gantry spacing affect treatment plan quality?

Answer 4

• < final gantry spacing from 4 degrees (i.e. 2 degrees) improves accuracy of calculation (and improve treatment plan quality)
• > final gantry spacing from 4 degrees (i.e. 6 degrees) improves speed of calculation

Question 5

What is the benefit of DCAT vs VMAT?

Answer 5

• DCAT avoids interplay effect where it uses treatment field which encompasses entire target volume (VMAT uses treatment field segments/control points which each encompass a portion of the target volume so there is concern when treating moving target volumes where the interplay between MLC movement, and target volume or OAR motion (i.e. due to respiration), could underdose or overdose target volume or OAR)
• DCAT requires less MUs to deliver the same total dose due to lower modulation (VMAT uses higher modulation)
• DCAT requires less QA and machine tolerance due to constant dose rate (VMAT uses variable dose rate)
• DCAT requires less time for treatment planning and treatment delivery

Question 6

Discuss “complementary collimator angles”. Is it necessary to use complementary collimator angles to satisfy QA?

Answer 6

• 5-45 degrees / 355-315 degrees
• there are clinical studies, departmental procedures and protocols and physics commissioning to satisfy QA when complimentary collimator angles are used

Question 7

Discuss “jaw tracking”

Answer 7

• jaw tracking is a Varian TrueBeam technique where the jaw can track the aperture of the MLCs to reduce MLC interleaf leakage and transmission, and reduce dose to normal tissues and critical structures/OAR surrounding target volume

Question 8

What is the smallest - largest arc size that can be optimised?

Answer 8

• 30-359.8 degrees

Question 9

Define DCAT

Answer 9

• dynamic conformal arc therapy
• modulates aperture (not across PTV), uses constant dose rate (not variable dose rate), uses manual MLC adjustment to partially block target volume (not fluence)