Poster — Wed Eve—32: Internal Target Volume Dose Coverage Measurement for Respiratory Tumor Motion

Abstract

The purpose of this study was to evaluate the difference between planned and delivered doses within the ITV generated by the co‐alignment of the standard and slow CT scan. A QUASAR™ phantom with a cedar insert (to simulate the lung) and an ionization chamber with buildup cap was scanned twice using modified slow CT scan technique. The respiratory cycle used for CT scan was 4 seconds/cycle and 10.0 mm amplitude along the longitudinal axis of the phantom. The two image sets were co‐aligned using the same reference points. The volume of the ionization chamber was used as a gross tumor volume (GTV) and was contoured on each scan. The ITV was generated by enveloping two GTVs from the two CT scans. The PTV was created by adding a 5mm margin around the ITV. If the chamber movement is within 1 cm, the minimum ITV dose coverage is 98.5%. The ITV dose coverage drops dramatically when the movement is larger than 1cm. The ITV dose coverage drops from 96.3% in 1.1cm motion to 90.7% in 1.5 cm motion. This study evaluated the difference between planned and delivered dose to the GTV using an ITV generated by the co‐alignment of the standard and slow CT scan. Co‐alignment of a slow CT and a standard CT underestimates the amplitude of GTV motion by 6mm. This results in under‐dosing of the ITV by 1.5%. The discrepancy between the TPS and measurement was 0.8%, for a total discrepancy of 2.3 %.