Adaptation requirements due to anatomical changes in free-breathing and deep-inspiration breath-hold for standard and dose-escalated radiotherapy of lung cancer patients.

BACKGROUND Radiotherapy of lung cancer patients is subject to uncertainties related to heterogeneities, anatomical changes and breathing motion. Use of deep-inspiration breath-hold (DIBH) can reduce the treated volume, potentially enabling dose-escalated (DE) treatments. This study was designed to investigate the need for adaptation due to anatomical changes, for both standard (ST) and DE plans in free-breathing (FB) and DIBH. MATERIAL AND METHODS The effect of tumor shrinkage (TS), pleural effusion (PE) and atelectasis was investigated for patients and for a CIRS thorax phantom. Sixteen patients were computed tomography (CT) imaged both in FB and DIBH. Anatomical changes were simulated by CT information editing and re-calculations, of both ST and DE plans, in the treatment planning system. PE was systematically simulated by adding fluid in the dorsal region of the lung and TS by reduction of the tumor volume. RESULTS Phantom simulations resulted in maximum deviations in mean dose to the GTV-T (GTV-T) of -1% for 3 cm PE and centrally located tumor, and + 3% for TS from 5 cm to 1 cm diameter for an anterior tumor location. For the majority of the patients, simulated PE resulted in a decreasing GTV-T with increasing amount of fluid and increasing GTV-T for decreasing tumor volume. Maximum change in GTV-T of -3% (3 cm PE in FB for both ST and DE plans) and + 10% (2 cm TS in FB for DE plan) was observed. Large atelectasis reduction increased the GTV-T with 2% for FB and had no effect for DIBH. CONCLUSION Phantom simulations provided potential adaptation action levels for PE and TS. For the more complex patient geometry, individual assessment of the dosimetric impact is recommended for both ST and DE plans in DIBH as well as in FB. However, DIBH was found to be superior over FB for DE plans, regarding robustness of GTV-T to TS.