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Variable RBE and Incidence of Radiation Pneumonitis in Lung Patients Treated with Proton Therapy

D Mirkovic

D Mirkovic*, U Titt, C Peeler, L Perles, Z Liao, R Mohan, UT MD Anderson Cancer Center, Houston, TX

TU-G-108-8 Tuesday 4:30PM - 6:00PM Room: 108

Purpose: To determine the correlation between the Relative Biological Effectiveness (RBE)-weighted dose and radiation pneumonitis (RP) incidence in cancer patients treated with proton therapy.

Methods: The Monte Carlo proton dose calculation system MC2 based on MCNPX was used to recalculate treatment plans taken from the clinical treatment planning system (TPS) and compute the dose-averaged LET in each voxel of the CT based patient model. The linear scaling model (LSM) for proton RBE was used with biological coefficients for RP to compute the variable RBE-weighted dose distributions. These were compared with the constant RBE-weighted doses computed by the TPS and currently used in our clinical practice. A restaging FDG PET/CT was registered to the treatment planning CT and the standard uptake values (SUV) were used as a measure of radiation dose response.

Results: The variable RBE model results in higher biological dose at the distal ends of proton beams and the corresponding lung volumes receiving elevated doses can be significant if the beam is terminating inside the low density lung tissue. Preliminary results for the first two patients in the study show that the regions of elevated variable RBE-weighted doses inside the lung are well correlated to the high SUV values in the restaging PET. Both patients were diagnosed with grade 3 RP after completing the treatment.

Conclusion: The preliminary results indicate that variable RBE-weighted doses may provide additional insight into toxicity risk for patients treated with proton radiotherapy. The complete study will include the analysis of many more patients diagnosed with grade 3 RP or higher, treated with protons in our institution.

Funding Support, Disclosures, and Conflict of Interest: NCI grant P01CA021239

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