(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Radiosurgery indices, useful in evaluating SRS/SRT/SBRT plan quality are derived from a treatment plan based on definition. However some indices may vary based on the method of derivation. We illustrate this using the High Dose Spillage (HDS) that is required in several RTOG protocols, e.g., RTOG 0813/ 0915
Methods: We utilized clinically approved treatment plans of eight previously treated lung SBRT patients. For each plan, we computed HDS, defined as the volume of tissue outside the PTV, expressed as a percentage of the PTV volume, irradiated by at least 105% of the prescribed dose (1.05Rx). We used 2 techniques (T1, T2) to compute HDS in each patient case; T1: HDS = (All Tissue Volume receiving at least 1.05Rx) MINUS (PTV Volume receiving at least 1.05Rx). T2: First we used the treatment planning system (TPS) to derive a new contour, namely the external contour MINUS the PTV (External-MINUS-PTV), then HDS = Volume of External-MINUS-PTV contour receiving at least 1.05Rx
Results: The derived HDS per T1 and T2 were consistently different among all 8 cases. On average, we observed a 90% ± 13% discrepancy between T1 and T2. Furthermore, depending on the method used in deriving the new contour in T2, the resulting HDS calculated can be different. This can be attributed to volume round-off errors which may not be significant with respect to the final derived volumes but does have an effect in the calculated HDS.In our clinic, we use ClearCheck (RadFormation Inc, NY) to compute HDS for our SBRT plans. ClearCheck results agree with those calculated in T1.
Conclusion: This work shows that important quality indices used in evaluating SRS/SRT/SBRT plans may be different depending on the method of derivation. Therefore when reporting or requesting these indices, it is important to clearly state how the indices are derived.
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