Presentation Effects of beam interruption time on tumor control probability of non-small cell lung cancer for single fractionated carbon-ion radiotherapy

Inaniwa, Taku  ,  Kanematsu, Nobuyuki  ,  Suzuki, Masao  ,  B. Hawkins, Roland

Purpose: Treatment plans of carbon-ion radiotherapy have been made on the assumption that the beams are delivered instantaneously irrespective to the dose delivery time as well as the interruption time. The single fractionated irradiation against non-small cell lung cancer (NSCLC) requires more time to deliver a fractional dose than the conventional multi-fractionated ones, since four beams at different directions are delivered in one fraction with a patient re-setup among them. The purpose of this study is to investigate the effects of the beam interruption time for the patient re-setup on tumor control probability (TCP) of NSCLC for single fractionated carbon-ion radiotherapy.Methods: The rate equations defined in the microdosimetric kinetic model (MKM) for primary lesions caused in DNA were applied to the single fractionated irradiation of NSCLC. The rate constant characterizing the time of the primary non-lethal lesions to repair was experimentally determined for human salivary gland tumor cells. Treatment plans were made for a NSCLC case for various fraction sizes ranging from 25 to 50 Gy (RBE), on the assumption that the beams are delivered instantaneously. These plans were recalculated by varying the interruption time ranging from 3 to 60 min for continuous irradiations of 3 min per beam based on the MKM with the determined repair rate constant. The curative dose which gives TCP of 95% was investigated for respective interruption times.Results: The rate constant for non-lethal lesion to repair is 2.19±0.40 h-1. The biological effectiveness in the target decreases with the interruption time τ compared to the planned one due to the repair of non-lethal lesions during the interruption. The curative dose for planned instantaneous irradiation was 33.8 Gy (RBE), while it increased to 35.8, 37.9, 39.5 and 41.1 Gy (RBE) for τ=3, 15, 30 and 60 min.Conclusions: We investigated the effects of beam interruption time on TCP of NSCLC for single fractionated carbon-ion radiotherapy. The biological effectiveness of therapeutic carbon-ion beam decreases with the interruption time. The curative dose can increase by 20% or more as compared to the planned dose if the interruption time prolongs to 30 min or longer. These effects can be considered in designing the carbon-ion treatment planning.
The 15th International Congress of Radiation Research (ICRR 2015)

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