L. Duan1, W. Qi1, C. Li1, J. Y. Chen1,2, and S. Zhao1; 1Department of Radiation Oncology, Ruijin Hospital, Shanghai Jiaotong University School of Medicine, Shanghai, China, 2Shanghai Key Laboratory of Proton-therapy, Shanghai, China
Purpose/Objective(s): The study aimed to evaluate the impact of various parameters (field width: FW, pitch factor: PF, and modulation factor: MF) on the quality of treatment plans and treatment time, and identify the optimal combination of plan parameters in helical tomotherapy (HT) technology for small cell lung cancer patients with extensive pleura metastasis (SCLC-EPM). Materials/
Methods: A total of four extensive stage SCLC patients with EPM were included for analysis. Each patient’s target volume and organs at risk (OARs) included PTV, left lung, right lung, sum lung, heart, esophagus, spinal cord. For each patient, 27 treatments were created, consisting of different combination of planning parameters (FW = 1, 2.5, 5 cm; PF = 0.215, 0.287, 0.43; MF = 2.4, 3, 3.6). Default values used in the study were FW = 2.5 cm, PF = 0.287 and MF = 3. The reference plan was optimized for the default values, which was assumed to result in the best dose distribution and treatment time. Finally, 108 plans were created. Each plan was analyzed for dose distribution and treatment time. Dose distribution indices included homogeneity index (HI), conformity index (CI), D95%, D90%, D2% for PTV, Dmean, Dmax, V5Gy, V20Gy, V25Gy, V30Gy for OARs. Results: FW could impact the dose distribution for both the target volume and OARs, particularly for homogeneity, right lung, liver, and esophagus. Meanwhile, FW emerged as the most influential parameter affecting treatment time, resulting in a significant average time reduction of more than half. A tighter PF could slightly reduce OARs dose, especially for the left lung, sum of lung, and liver without increasing the treatment time. MF could impact the dose distribution, especially for the lung. But no obvious differences were observed for CI an HI under varied MFs conditions. The treatment time increased gradually with the rise in MFs, but the increase was modest. After thoracic radiation, three patients archived partial response and on patient with stable disease at 6 months. No severe lung toxicities could be observed in these four patients. Conclusion: Considering the balance between plan quality and treatment time, the study suggested that the optimal parameter combination in the HT technique for SCLC-EPM was FW:2.5 cm, PF:0.287, and MF:3. EPM-radiation was efficient and safety for a highly-selected SCLC-EPM patients after first-line chemotherapy with or without immunotherapy.
