Master of Science (MS)
Physics and Astronomy
Purpose: The delivery of post-mastectomy radiotherapy (PMRT) can be challenging for patients with left-sided breast cancer due to the planning target volume (PTV) size and proximity to critical organs. This study investigates the use of protons, both passively scattered (PS) and intensity modulated proton therapy (IMPT), for PMRT in a clinically-representative cohort of patients, and quantitatively compares the predicted outcomes of volumetric modulated photon arc therapy (VMAT) to those of proton therapy to develop an evidence-based rationale for selecting a treatment modality for PMRT patients. Methods: Eight left-sided PMRT patients previously treated at our clinic with VMAT were included in this study. PTVs included the chest wall and regional lymph nodes. PS and IMPT plans were constructed using a commercial proton treatment planning system. The resulting plans were compared to the corresponding VMAT plans on the basis of PTV coverage; dose homogeneity index (DHI) and conformity index (CI); dose to organs at risk (OAR); tumor control probability (TCP), normal tissue complication probability (NTCP) and secondary cancer complication probability (SCCP). The impact of range, set-up errors, and sensitivity of dose-response models was also evaluated. Statistical significance between VMAT and each proton modality was tested using the paired Student’s t-test (p<0.05). Results: All modalities produced clinically acceptable PMRT plans with nearly 100% TCP. The proton treatment plans provided significantly lower NTCP values for the heart and the lung while maintaining significantly better CI and DHI values. At a prescribed dose of 50.4 Gy (RBE) in the PTV, the mean NTCP value for the patients decreased from 0.83%±0.67% to <0.05%±0.05% for the whole heart (cardiac mortality) and from 2.05%±0.23% to <1% for the lungs (radiation pneumonitis) for VMAT and proton plans, respectively. Proton therapy NTCP and SCCP values were consistently lower than those of VMAT regardless of biological parameter values. Target coverage for VMAT and IMPT plans was most sensitive to positional uncertainties. Conclusions: All three techniques (VMAT, PS, and IMPT) provide acceptable PMRT treatment plans for each patient in this study. However, proton therapy showed significant advantages in terms of sparing OARs and lower complication risks when compared to VMAT.
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Hernandez, Margaret, "A Treatment Planning Comparison of Volumetric Modulated Arc Therapy and Proton Therapy for a Sample of Breast Cancer Patients Treated with Post-Mastectomy Radiotherapy" (2014). LSU Master's Theses. 1015.