Room: Exhibit Hall
Purpose: Repainting is an effective strategy to mitigate the interplay effect between proton beam scanning and intra-fractional target motion. In this study, we provide a comprehensive theoretical framework for predicting its effectiveness and point out some potential pitfalls of its clinical application.
Methods: For a non-gated proton patient with regular periodic breathing, we can express the relative dose variance of an anatomical point of interest in terms of its dose rate calculated on CT images, uniformly sampled over a representative respiratory cycle of the patient. The relative variance is a good measure of the interplay effect between the radiotherapy delivery and the patientâ€™s respiratory motion. Therefore, we can quantitatively study the effectiveness of various repainting approaches to gain insight of the reason why they are successful. Then we will introduce a more realistic patient respiratory model that includes irregularity in patientâ€™s breathing pattern and re-examine the effectiveness of the repainting proposals in a proton treatment delivery.
Results: 1) For a single stationary proton field, we provide a rigorous proof of why stretching its delivery duration to multiple breathing cycles can be very beneficial. 2) In the case of repainting the same proton target multiple times, we provide the optimal time interval between adjacent deliveries to reduce the relative dose variance for a patient with a regular breathing pattern. 3) For a patient with irregular breathing, we provide evidence that the above suggestions are still optimal solutions to this proton dose variance reduction problem. Except for some very isolated situations, the irregular motion in a patientâ€™s respiratory motion provides a damping effect that will reduce the interplay effect.
Conclusion: Our study provides new insights in dose repainting strategies for proton delivery with target respiratory motion. More studies are needed to further improve the robustness of proton radiotherapy delivery for actual patients.