SU‐E‐T‐270: A “Rolling Average” Adaptive Planning Technique to Compensate Target Volume Changes in Prostate Cancer Radiotherapy
Purpose: In external beam radiotherapy of prostate cancer, the inter‐fractional motion can have several forms: changes in position, shape and volume. The changes in position can be corrected by online image guidance, while the shape and volume changes are typically handled through the patient‐population based margins in treatment planning. In this study, we propose a rolling‐average (RA) adaptive replanning method to account for the volume variation of the target. The effectiveness of RA is evaluated by comparing with the standard online image guidance (IG) only strategy and a single replanning hybrid strategy. Methods: Twenty‐eight (28) patients with at least 16 helical CT scans each were included. Treatment of both low‐risk patient (LRP, CTV=prostate) and intermediate‐risk patient (IRP, CTV=prostate+seminal vesicles) were simulated. A new cumulative index of target volume, CITV, was proposed for the comparison of RA and other strategies. Two hypothetical scenarios: Type Ascending and Descending, simulated by sorting the CTV volumes of actual patient data, were also included in the investigation for a better evaluation of the techniques. Results: Modest target volume shrinkages were observed in our patient group. The CITV comparison demonstrates that the RA is superior to the online IG and hybrid techniques. For LRP, RA is beneficial if there is significant time trend in target volume. The optimal number of fractions necessary for the internal target volume construction is 2 for LRP and 3 for IRP. Conclusions: The RA is effective in compensating the target volume changes during the treatment course. However, the magnitude of improvement depends on how significantly and rapidly the changes in the patient population. The RA requires multiple replanning and is significantly more complex than the online only and hybrid strategies. Effectiveness and efficiency should be balanced in the implementation of RA. © 2011, American Association of Physicists in Medicine. All rights reserved.
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