The application of gradient dose segmented analysis of in‐vivo EPID images for patients undergoing VMAT in a resource‐constrained environment

The gamma analysis metric is a commonly used metric for VMAT plan evaluation. The major drawback of this is the lack of correlation between gamma passing rates and DVH values. The novel GDSA(mean) metric was developed by Steers et al. to quantify changes in the PTV mean dose (D(mean)) for VMAT patients. The aim of this work is to apply the GDSA retrospectively on head‐and‐neck cancer patients treated on the newly acquired Varian Halcyon, to assess changes in GDSA(mean), and to evaluate the cause of day‐to‐day changes in the time‐plot series. In‐vivo EPID transmission images of head‐and‐neck cancer patients treated between August 2019 and July 2020 were analyzed retrospectively. The GDSA(mean) was determined for all patients treated. The changes in patient anatomy and rotational errors were quantified using the daily CBCT images and added to a time‐plot with the daily change in GDSA(mean). Over 97% of the delivered treatment fractions had a GDSA(mean) < 3%. Thirteen of the patients received at least one treatment fraction where the GDSA(mean) > 3%. Most of these deviations occurred for the later fractions of radiotherapy treatment. Additionally, 92% of these patients were treated for malignancies involving the larynx and oropharynx. Notable deviations in the effective separation diameters were observed for 62% of the patients where the change in GDSA(mean) > 3%. For the other five cases with GDSA(mean) < 3%, the mean pitch, roll, and yaw rotational errors were 0.90°, 0.45°, and 0.43°, respectively. A GDSA(mean) > 3% was more likely due to a change in separation, whereas a GDSA(mean) < 3% was likely caused by rotational errors. Pitch errors were shown to be the most dominant. The GDSA(mean) is easily implementable and can aid in scheduling new CT scans for patients before significant deviations in dose delivery occur.