Purpose: Respiration-gated radiotherapy (RGRT) can decrease the toxicity of chemo-radiotherapy (CT-RT) by allowing use of smaller treatment fields. RGRT requires a predictable relationship between tumor position and external surrogate, which must be verified during treatment. Time-integrated electronic portal imaging (TI-EPI) identifies mean intra-fractional positions of moving structures, and was used to study reproducibility of anatomy during RGRT for lung tumors.
Materials and methods: TI-EPIs were acquired using an amorphous silicon-based electronic portal imaging system (EPID, aS500) in continuous image acquisition mode in 11 patients treated with audio-coached RGRT at end-inspiration. The Varian Real-time Position Management (RPM) system was used for 4DCT imaging and RGRT delivery. All TI-EPI portals were co-registered to corresponding digitally reconstructed radiographs (DRR) of the planning 4DCT using the spinal column. Displacements in tumor position or that of an adjacent bronchus during RGRT was measured relative to the reference structure on the DRR.
Results: Vertebra-matched portals revealed systematic (Sigma) and random (sigma) errors of 1.8 and 1.3mm in medial-lateral direction and 1.7 and 1.7 mm in cranial-caudal direction, indicating a reproducible tumor/bronchus position during the RPM-triggered gates.
Conclusions: RGRT delivery at end-inspiration can achieve reproducible internal anatomy in 'gated' fields delivered with audio-coaching.