Purpose: To evaluate the impact of automated flash and modeled swelling in IMRT cases where the patient and target structures exhibit interfraction anatomical changes.
Methods: In the first part of the study, a previously-treated vulva patient was selected from our clinical database. Treatment plans were created to deliver clinically acceptable treatments using a baseline of no tissue swelling, and varying flash between 1.0cm and 2.0cm. These plans were copied, swelling of 0.5cm and 1.0cm was then modeled and introduced and the plans were recalculated using the same fluence pattern and MUs. Dose metrics were evaluated. In the second study, an anthropomorphic phantom was scanned, and clinically acceptable plans were created using 1.0cm of tissue swelling as baseline, using 2.0 cm of flash as a standard parameter. These plans were then copied, the modeled swelling was removed and the dose was recalculated using the same fluence pattern and MUs. Dose metrics were evaluated.
Results: It was found that generally, for cases planned with no-swell as a baseline, increases in swelling lead to decreases in minimum doses and D95 while max doses tended to increase for a given amount of flash. It was shown that for increases in swelling, there is a predictable and clinically relevant loss in target coverage, regardless of the amount of flash. For cases planned with swelling as the baseline, there was no significant change in min, max or D95 doses when the modeled swelling was removed. Similar results were seen in Breast cases, and surface dose measurements confirmed predicted planned dose values within an acceptable level.
Conclusion: While the adoption automated flash is indicated to pre-emptively mitigate decreases in plan quality due to swelling, using a baseline of modeled swelling in conjunction with flash may provide a more reproducible dose distribution as a patient’s anatomy changes.