Poster — Wed Eve—33: Respiratory Internal Target Volume Assessment Using a Modified Slow CT Scan and CBCT

Abstract

The purpose of this study was to investigate the volume of the ITV generated by a modified slow CT scan and CBCT using a QUASAR™ phantom. The standard CT scan (CTstandard) was acquired with pitch 1.7 and scan time 1 second followed by the slow CT scan (CTslow) limited to the tumor region with pitch 1 and scan time 3 seconds. The two image sets were co‐aligned using the same reference point, which was independent of bony anatomy changes with respiration. A gross tumor volume was contoured on each scan; Volume A is for CTstandard and Volume B is for CTslow; The ITV was generated from A∪B in Pinnacle TPS. Nine data sets of CTstandard and CTslow plus CBCT were acquired under free‐breathing. The ITV generated from the CTstandard is smaller than that from the CTslow (80% vs. 93% for 3 cm cube); however, the ITV generated from the co‐alignment of two image sets are improved compared to the sing CT alone. The ITV generated from the co‐alignment of CTstandard and CTslow is 98% for the 3cm cube and acceptable as depicted extent of motion. Modified slow CT scan provides complimentary information for the respiratory tumor motion. It compares favorably with the ITV generated with a cone beam CT. Modified slow CT scan and CBCT with tighter margin were used for lung radiosurgery with daily twice CBCT. It is a useful technique for optimizing the ITV resulting in tighter margins which may reduce toxicity and facilitate dose escalation.