A comparison of VMAT dosimetric verifications between fixed and rotating gantry positions

Abstract

This study comparatively analysed dose distributions between the fixed and rotating gantry positions of volumetric-modulated arc therapy (VMAT) plans measured using different dosimetric techniques with the intent to provide pre-treatment quality assurance (QA). A total of 12 VMAT plans for the treatment of anatomical sites of various complexities were chosen. An ion chamber was used to measure the absolute central point doses, while EPID, Seven29, Matrixx and Delta4 were used to measure the dose distributions. With the exception of Delta4, all detectors were used in one of two different settings: the gantry was either fixed at 0°, or the gantry was rotating. The results were analysed using the γ-evaluation method. Regarding absolute central point doses, the ion chamber results were within 3% of the treatment planning system (TPS) calculated results. For the dose distributions measured by detectors and calculated by TPS, the γ pass rates, with 3% maximum dose and 3 mm γ criteria, were above 96% when the gantry was fixed at 0°. When the gantry was rotating, the pass rates decreased slightly but were still above 90%. The results obtained from the comparison between the measured and calculated doses demonstrated the reliability of four detectors associated with VMAT. However, the treatment delivery and detector response may impact the results when the gantry is rotating.

Study of impacts of different evaluation criteria on gamma pass rates in VMAT QA using MatriXX and EPID

“…Several groups have investigated the applicability of pre treatment verification of the IMRT and VMAT techniques with MatriXX and EPID. Zhu et al [10] compared the gamma pass rates of the VMAT QA using EPID and MatriXX and reported that the gamma pass rates were greater than 98% when the 3%/3 mm gamma criterion and a threshold of 10% were applied. They also noted a reduction in gamma pass rates when the 2%/2 mm criterion was employed for variable gantry positions.…”

“…The diode-detector-based MapCHECK (Sun Nuclear Corporation, US) [7], the ion-chamber based Seven29 2D array (PTW, Germany) [8,9], and MatriXX (IBA Dosimetry, Germany) [10][11][12][13][14] are commonly used commercial 2D detector arrays. The MatriXX array has a linear response with dose and it is independent of energy [12].…”

“…Another tool used for VMAT QA is the electronic portal imaging device (EPID) [10,19]. EPID as a QA tool is preferred for its large detector density, high contrast, linear response to the dose, and excellent online capabilities.…”

“…The advantage of EPID over MatriXX is that the resolution limit of the amorphous silicon flat-panel detector of EPID is significantly better than that of the ion chamber detector of MatriXX [21]. However, because the detector in EPID is mounted to a rotating gantry, there is a risk of angledependent detector sag due to the gravitational force, therefore EPID obtains different results for VMAT plans performed using fixed and rotating gantry configurations [10,20]. Despite their many merits, both MatriXX and EPID are unable to measure large-dimension radiation fields because of the smaller sensitive area and lower spatial resolution of both systems compared to those of film dosimetry.…”

Quality Assurance of Radiotherapy Dose Calculations

“…53 This method uses both dose difference and distance to agreement criteria to evaluate the difference between planned and measured doses on a point-by-point basis. [56][57][58][59] If using the standard gamma index method many institutions use a 3%/3 mm criterion or even 5%/5 mm; however, it has been shown that this may not detect significant errors. Dose difference is used for the low gradient areas, and distance to agreement is used in high gradient areas.…”