The effect of changing the segment width on the volumetric modulated arc therapy (VMAT) technique for patient-specific quality assurance

The patient-specific quality assurance (PSQA) procedure plays a crucial role in ensuring the accurate and safe delivery of radiotherapy treatments, particularly for complex treatment techniques such as Volumetric Modulated Arc Therapy (VMAT). VMAT dynamically delivers radiation dose to the tumor while simultaneously modulating the intensity and shape of the radiation beam, providing high conformity and sparing healthy tissues. However, the complexity of VMAT treatment plans requires a comprehensive QA procedure to validate the accuracy and precision of the treatment delivery. This study aims to determine the effect of changes in segment width on VMAT. The Octavius 4D phantom was used as a tool for patient-specific QA. Treatment plans were created using the Monaco Treatment Planning System (TPS). The collected data from measurements and TPS were then analyzed using the VeriSoft Patient Plan Verification software that came with the phantom. We assessed the planning quality based on the gamma index. The treatment plan was conducted using the prostate structure from American Association of Physicists in Medicine (AAPM) Task Group (TG) 119. The criteria used for the gamma calculation were 3%/3 mm, 2%/2 mm, and 2%/1 mm. Changes in segment width in VMAT treatment plans can affect the gamma index for each dose difference (DD) and distance to agreement (DTA) criteria. For the 3%/3 mm criteria, changing the segment width from 1 cm to 0.75 cm resulted in a 0.2% increase, while changing the segment width from 0.75 cm to 0.5 cm led to a 4.6% decrease. For the 2%/2 mm criteria, changing the segment width from 1 cm to 0.75 cm resulted in a 0.2% increase, while changing the segment width from 0.75 cm to 0.5 cm led to a 10% decrease. Similarly, for the 2%/1 mm criteria, changing the segment width from 1 cm to 0.75 cm resulted in a 4.3% increase, while changing the segment width from 0.75 cm to 0.5 cm led to a 20.4% decrease. These results indicate that changes in segment width in VMAT treatment plans can affect the gamma index, monitor units, and planning criteria. Reducing the segment width from 1 cm to 0.5 cm improves the accuracy and precision of the treatment delivery, as evidenced by the increase in planning criteria with a decrease in the gamma index.