SU‐FF‐J‐110: Comparison Between Free Breathing, Slow and Respiratory Correlated CT in Radiation Treatment Planning for Lung Cancer Patients

Abstract

Purpose: Radiotherapy for lung cancer requires a margin for internal movement of the tumor. Reduction of this margin and thus dose escalation may be possible by using a slow or a respiratory correlated CT (RC‐CT) during radiation treatment planning. In this study these two novel methods are compared to the use of a conventional free breathing CT (FB‐CT). Method and Materials: On a modified Siemens Sensation 10 an FB‐CT and an RC‐CT scan were acquired in nine patients. The RC‐CT scan was used to measure tumor motion and to reconstruct a CT at 10 respiratory phases (10p‐CT). One of these was termed the mid‐ventilation CT (MV‐CT) and by averaging all CTs, a slow‐CT could be reconstructed. Four treatments were planned based on the FB‐CT (with conventional internal margin); the slow CT; the 10p‐CT, and the MV‐CT (with an internal margin of (motion amplitude)/4). Mean tumor dose was calculated for these four plans for a fixed mean lung dose of 15Gy. The possibility of coldspots (<95%) was investigated by determining minimum tumor dose that occurred during respiration. Results: The mean tumor dose in the FB‐CT based plan was 74Gy, for the MV‐CT 100Gy, for the 10p‐CT 95Gy, and for the slow‐CT 101Gy (p=0.0001 for FB‐CT vs. other; p=0.12 for MV‐CT vs. 10p‐CT). Coldspots occurred occurred in the patient with the largest tumor motion (15mm amplitude) in the case of a plan based on a slow‐CT scan. Conclusion: Since radiation treatment planning based on a slow‐CT can lead to underdosage in tumors moving with a large amplitude and the delineation of the 10p‐CT is rather cumbersome, we conclude that using an RC‐CT scan to reconstruct a mid‐ventilation CT and applying a margin of one quarter the tumor motion amplitude, is the most suitable method for radiation treatment planning in lung cancer patients.