The effect of breathing and set-up errors on the cumulative dose to a lungtumor

Background and purpose: To assess the impact of both set-up errors and resp iration-induced tumor motion on the cumulative dose delivered to a clinical target volume (CTV) in lung, for an irradiation based on current clinicall y applied field sizes. Materials and methods: A cork phantom, having a 50 mm spherically shaped po lystyrene insertion to simulate a gross tumor volume (GTV) located centrall y in a lung was irradiated with two parallel opposed beams. The planned 95% isodose surface was conformed to the planning target volume (PTV) using a multi leaf collimator. The resulting margin between the CTV and the field e dge was 16 mm in beam's eye view. A dose of 70 Gy was prescribed. Dose area histograms (DAHs) of the central plane of the CTV (GTV + 5 mm) were determ ined using radiographic film for different combinations of set-up errors an d respiration-induced tumor motion. The DAHs were evaluated using the popul ation averaged tumor control probability (TCPpop) and the equivalent unifor m dose (EUD) model. Results: Compared with dose volume histograms of the entire CTV, DAHs overe stimate the impact of tumor motion on tumor control. Due to the choice of f ield sizes a large part of the PTV will receive a too low dose resulting in an EUD of the central plane of the CTV of 68.9 Gy for the static case. The EUD drops to 68.2, 66.1 and 51.1 Gy for systematic set-up errors of 5, 10 and 15 mm, respectively. For random set-up errors of 5, 10 and 15 mm (1 SD) , the EUD decreases to 68.7, 67.4 and 64.9 Gy, respectively. For similar am plitudes of respiration-induced motion, the EUD decreases to 68.8, 68.5 and 67.7 Gy, respectively. For a clinically relevant scenario of 7.5 mm system atic set-up error, 3 mm random set-up error and 5 mm amplitude of breathing motion, the EUD is 66.7 Gy. This corresponds with a tumor control probabil ity TCPpop of 41.7%, compared with 50.0% for homogeneous irradiation of the CTV to 70 Gy. Conclusion: Systematic set-up errors have a dominant effect on the cumulati ve dose to the CTV. The effect of breathing motion and random set-up errors is smaller. Therefore the gain of controlling breathing motion during irra diation is expected to be small and efforts should rather focus on minimizi ng systematic errors. For the current clinically applied field sizes and a clinically relevant combination of set up errors and breathing motion, the EUD of the central plane of the CTV is reduced by 3.3 Gy, at maximum, relat ive to homogeneous irradiation of the CTV to 70 Gy, for our worst case scen ario. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.