Prog Med Phys.  2014 Jun;25(2):65-71. 10.14316/pmp.2014.25.2.65.

A Comparison Study of Volumetric Modulated Arc Therapy Quality Assurances Using Portal Dosimetry and MapCHECK 2

• Jin H
• Jesseph FB
• Ahmad S

• KMID: 1708172
• DOI: http://doi.org/10.14316/pmp.2014.25.2.65

Abstract

Varian Portal Dosimetry system was compared to an isocentrically mounted MapCHECK 2 diode array for volumetric modulated arc therapy (VMAT) QA. A Varian TrueBeam STx with an aS-1000 digital imaging panel was used to acquire VMAT QA images for 13 plans using four photon energies (6, 8, 10 and 15 MV). The EPID-based QA images were compared to the Portal Dose Image Prediction calculated in the Varian Eclipse treatment planning system (TPS). An isocentrically mounted Sun Nuclear MapCHECK 2 diode array with 5 cm water-equivalent buildup was also used for the VMAT QAs and the measurements were compared to a composite dose plane from the Eclipse TPS. A gamma test was implemented in the Sun Nuclear Patient software with 10% threshold and absolute comparison at 1%/1 mm (dose difference/distance-to-agreement), 2%/2 mm, and 3%/3 mm criteria for both QA methods. The two-tailed paired Student's t-test was employed to analyze the statistical significance at 95% confidence level. The average gamma passing rates were greater than 95% at 3%/3 mm using both methods for all four energies. The differences in the average passing rates between the two methods were within 1.7% and 1.6% of each other when analyzed at 2%/2 mm and 3%/3 mm, respectively. The EPID passing rates were somewhat better than the MapCHECK 2 when analyzed at 1%/1 mm; the difference was lower for 8 MV and 10 MV. However, the differences were not statistically significant for all criteria and energies (p-values >0.05). The EPID-based QA showed large off-axis over-response and dependence of gamma passing rate on energy, while the MapCHECK 2 was susceptible to the MLC tongue-and-groove effect. The two fluence-based QA techniques can be an alternative tool of VMAT QA to each other, if the limitations of each QA method (mechanical sag, detector response, and detector alignment) are carefully considered.

Keyword

Pretreatment QA; VMAT; EPID; Portal dosimetry; MapCHECK 2

MeSH Terms

Humans
Radiotherapy, Intensity-Modulated*
Solar System

Figure

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  Fig. 1. Gamma analysis of a 6 MV VMAT QA plan at 1%/1 mm using the SNC Patient software. (a) EPID-based measurement, (b) Portal Dose Image Prediction, (c) failed gamma points for EPID-based QA, (d) MapCHECK 2 measurement, (e) calculated dose for MapCHECK 2, and (f) failed gamma points for MapCHECK 2 QA.

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  Fig. 2. Gamma passing rate of the 13 patients at 3%/3 mm: (a) EPID-based QA and (b) MapCHECK 2 QA. For plans with small (less than 10 cm) and large field size (greater than 20 cm), relatively low passing rates were observed for the EPID QA; however, the MapCHECK 2 QA did not show any particular dependence on field size.

Reference

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