A case study on optimizing helical tomotherapy parameters for small cell lung cancer with extensive pleural metastasis

The study aimed to evaluate the impact of various parameters on treatment plan quality and delivery time in helical tomotherapy (HT) for small cell lung cancer patients with extensive pleura metastasis (SCLC-EPM) and to identify the optimal parameter combination. Four SLCL-EPM patients were included. For each patient, the planning target volume (PTV) and organs at risk (OARs), including the left lung, right lung, total lung, heart, esophagus, spinal cord, were delineated. A total of 27 treatment plans per patient (108 total) were generated by varying three parameters: Field Width (FW), Pitch Factor (PF), Modulation Factor (MF). FW defined treatment field length at isocenter. Smaller FWs boosted precision but extend beam-on time. PF was couch movement per rotation relative to FW; tighter PFs reduced dose fluctuations; MF, the max-to-average leaf-opening ratio, improved modulation at the cost of longer delivery. The reference plan used default values (FW = 2.5 cm, PF = 0.287, MF = 3.6), assumed to yield the best dose distribution and treatment time. Plan quality was assessed using PTV metrics (Homogeneity index (HI), conformity index (CI), D_95%, D_90%, D_2%), OAR metrics (Dmean, Dmax, V_5Gy, V_20Gy, V_25Gy, V_30Gy), and treatment time. FW significantly influenced dose distribution (particularly homogeneity, right lung, liver, and esophagus) and was the most critical parameter for reducing treatment time (average reduction > 50%). Tighter PF slightly decreased OARs dose (left lung, total lung, and live) without prolonging treatment time. Higher MF moderately increased treatment time and affected lung dose but showed no significant differences in CI or HI. Clinically, three patients achieved partial response, and one had stable disease at 6 months post-thoracic radiation, with no severe lung toxicities observed. Balancing plan quality and efficiency, the optimal HT parameters for SCLC-EPM were FW:2.5 cm, PF: 0.287, MF: 3. Radiotherapy for SCLC-EPM proved effective and safe after first-line chemotherapy with or without immunotherapy.