Abstract
In volumetric‐modulated arc therapy (VMAT) treatment delivery perfect beam symmetry is assumed by the planning system. This study aims to test this assumption and present a method of measuring time‐resolved beam symmetry measurement during a VMAT delivery that includes extreme variations of dose rate and gantry speed. The Sun Nuclear IC Profiler in gantry mount was used to measure time‐resolved in‐plane and cross‐plane profiles during plan delivery from which symmetry could be determined. Time‐resolved symmetry measurements were performed throughout static field exposures at cardinal gantry angles, conformal arcs with constant dose rate and gantry speed, and during a VMAT test plan with gantry speed and dose rate modulation. Measurements were performed for both clockwise and counterclockwise gantry rotation and across four Varian 21iX linacs. The symmetry was found to be generally constant throughout the static field exposures to within 0.3% with an exception on one linac of up to 0.7%. Agreement in symmetry between cardinal angles was always within 1.0% and typically within 0.6%. During conformal arcs the results for clockwise and counterclockwise rotation were in agreement to within 0.3%. Both clockwise and counterclockwise tended to vary in similar manner by up to 0.5% during arc consistent with the cardinal gantry angle static field results. During the VMAT test plan the symmetry generally was in agreement with the conformal arc results. Greater variation in symmetry was observed in the low‐dose‐rate regions by up to 1.75%. All results were within clinically acceptable levels using the tolerances of NCS Report 24 (2015).PACS number(s): 87.55.Qr
Highlights
RapidArc is the Varian version (Varian Medical Systems, Palo Alto, CA) of volumetric-modulated arc therapy (VMAT), which was first proposed by Otto in 2008.(1) VMAT is an extension of IMRT where, as well as dynamic MLC motion and dose rate modulation, the treatment is delivered as a rotating gantry arc with potential for gantry speed modulation
The IC profiler has been used to measure time-resolved symmetry for a series of exposures of increasing complexity ranging from a simple static gantry open field, through a conformal arc with constant gantry speed and dose rate, to a VMAT test plan with variable gantry speed and dose rate
Symmetry was always measured to be within the 2% tolerance of Netherlands Commission on Radiation Dosimetry (NCS) Report 24,(8) and this study provides results and a practical test to meet the requirements of this report
Summary
RapidArc is the Varian version (Varian Medical Systems, Palo Alto, CA) of VMAT, which was first proposed by Otto in 2008.(1) VMAT is an extension of IMRT where, as well as dynamic MLC motion and dose rate modulation, the treatment is delivered as a rotating gantry arc with potential for gantry speed modulation. The symmetry of a linear accelerator (linac) photon beam is dependent on the position and angle with which the incident electron beam strikes the target in relation to the flattening filter. When the beam is turned on, steering coils direct the electron beam onto the target at the correct angle and position so that the resulting photon beam impacts the flattening filter correctly. The recommendation of AAPM Task Group 142(4) is that, at annual QA testing, the X-ray symmetry change from baseline be within ± 1%
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