SU-E-T-782: A Novel Geometric Index to Evaluate Image Guidance Strategies for Prostate Cancer Adaptive Radiotherapy

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Purpose: Margin has been commonly used to evaluate different image guidance (IG) strategies in prostate cancer radiotherapy. However, it becomes cumbersome when applied to adaptive radiotherapy. In this study, a new index, cumulative index of target volume (CITV), was proposed and compared with other indices. Methods: A hypofractionated protocol was simulated with 28 patients each with at least 16 helical CT scans. Both low-risk patient (LRP,CTV=prostate) and intermediate-risk patient (IRP,CTV=prostate+seminal vesicles) were included. The CITV is defined as the quotient of the sum of PTV over the sum of CTV for all treatment fractions. The change of clinical target volume during treatment course is taken into consideration, so is the multiple adaptive planning with different PTVs. For the same target coverage, the smaller value of CITV indicates better strategy. The CITV was calculated and compared for three IG strategies: online IG only protocol, hybrid strategy combining online image guidance and a single offline replanning, and a “rolling average” multiple replanning strategy. In addition, the hybrid and RA strategies were optimized by searching for the optimal number of fractions to determine the patient-specific plan through the use of CITV. Results: For LRP (IRP), the CITVs are 1.78, 1.62 and 1.59 (2.46, 2.07, 2.00) for online IG only, hybrid and RA strategies, respectively. The optimal number of fraction for the ITV construction is 2 for LRP, and 3 for IRP for both the hybrid and RA strategies. Conclusions: We proposed a new concept based on cumulated target volume for the evaluation of different IG strategies. It is geometric in nature and has advantages over the margins. It is only a function of the underlining patient motion uncertainties. Therefore, it can be generalized and easily adaptable to non-uniform margins and different clinics where the treatment planning process can be quite different.