SU‐F‐J‐61: The Effect of CBCT Volume Cutoff On Dose Calculations for Adaptive Planning

Abstract

Purpose:In adaptive re‐planning, the CBCT is used to recalculate the dose and adjust the treatment plan accordingly. The limited field of view of a CBCT can cause external volume cutoff of the patient, which may cause the dose in re‐planning to be miscalculated. This research determined how often this volume cutoff occurs, and how much the dose calculations are affected by this cutoff.Methods:30 consecutive prostate patients treated on the Synergy S (Elekta AB, Stockholm, Sweden) and 28 treated on the Edge (Varian, Palo Alto, CA) linacs were included. For three patients, external contours were added to the original CTs. The external contours added included the true external volume and five others created to mimic the cutoff seen in the CBCTs. The original treatment plans were recalculated using these cutoff contours.Results:30% of Synergy patients were affected by volume cutoff in their CBCTs, in which 75% of their CBCTs had volume cutoff that was less than 700cc, or 6% of the total patient volume imaged. At this 6% change in volume the target dose increases by <2% although there is up to a 13% increase in organ at risk dose. 11% of Edge patients were affected by volume cutoff, all of these CBCTs were affected by a volume cutoff less than 300cc, or 3% of the total imaged volume. At this 3% change in volume the target dose increases by <1% and there is up to a 7% increase in organ at risk dose. The Edge CBCT field of view diameter is 46 cm, whereas the Synergy is only 41 cm, which explains this discrepancy.Conclusion:There is no cutoff observed for 79% of patients studied. For 76% of the patients with cutoff, the target dose increases by <2%. Godley receives research grant from Elekta