The application of gradient dose segmented analysis of in-vivo EPID images for patients undergoing VMAT in a...

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.