Validation of the first VMAT implementation in East and Central Africa using AAPM TG 119 datasets




IMRT, QA, gamma evaluation


Background. VMAT is considered a complex radiotherapy modality due to the higher number of variables than classical modalities that need to be controlled for the delivered treatment to be a close reproduction of the treatment plan. For this reason, preclinical validation of new VMAT implementations, following established recommendations, is a vital part of the system’s commissioning.

Aim. This study aimed to validate the commissioning of a radiotherapy system upgraded to be VMAT capable and to establish initial local confidence limits.

Method. VMAT treatment plans were created on TG-119 structure sets registered to a homogenous RW3 phantom. Dosimetric verification was performed using a calibrated ionisation chamber in absolute dose mode, a detector array, and an independent dose calculation software. Dose-difference ratios between measured and planned ‘point doses’ in the phantom and the associated confidence limits were derived. Gamma assessment of planned versus measured and independently-calculated dose distributions was evaluated against criteria of 3%/3mm, 3%/2mm, and 2%/2mm at 10% threshold. 

Results. The point dose-difference averaged over measurement locations in high-dose regions was 0.019 (0.015) for a confidence limit of 0.048. In the low dose regions, the average dose-difference was 0.012 (0.011) and confidence limit (0.034). The combined 2D gamma passing rates for the MatriXX for 3%/3mm, 3%/2mm, and 2%/2mm criteria were 99.6% (0.24), 98.4% (0.82), and 95.3% (2.01), with confidence limits 0.8, 3.2, and 8.6, respectively. Combined 3D global gamma passing rates for the independent calculation software were 99.9% (0.08), 99.8% (0.25), and 98.3% (2.20). The confidence limits were 0.2, 0.7, and 6.0, respectively.

Conclusion. Dosimetric evaluation yielded passing rates and confidence limits comparable or superior to those of TG 119 and reports of similar studies available in the literature. The dose-difference ratios were also within tolerance limits recommended by TG 218. As such, the commissioning of our VMAT system was validated, and initial local confidence limits were established.

Author Biography

Jonathan Mbewe, Cancer Treatment Centre The Nairobi Hospital

Cancer Treatment Centre

Medical Physicist






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