Clinical Experience of Craniospinal Irradiation Using Volumetric Modulated Arc Therapy on the Siddharth-II Linear Accelerator: A Dosimetric Analysis.

AIM: Craniospinal irradiation (CSI) is a technically challenging radiotherapy procedure due to the extended target volume, multiple isocenters, and the need to maintain dose uniformity across junction regions. This study evaluated the feasibility, workflow efficiency, and dosimetric performance of volumetric modulated arc therapy (VMAT)-based CSI delivered using the Siddharth-II O-ring gantry linear accelerator. MATERIALS AND METHODS: A retrospective feasibility study was conducted in two patients with cerebral germinoma, treated with a multi-isocenter VMAT-based CSI technique on the Siddharth-II linear accelerator (Bangalore, India: Panacea Medical Technologies Pvt. Ltd.). Treatment planning was carried out in RayStation 12A (Stockholm, Sweden: RaySearch Laboratories) using 6 MV photon beams with three isocenters covering cranial, upper spinal, and lower spinal regions. Junction optimization was achieved using longitudinal overlap regions. Dosimetric evaluation included target coverage parameters (D98, D50, D2), conformity index (CI), homogeneity index (HI), and organ-at-risk (OAR) doses. Treatment delivery efficiency was assessed using monitor units and beam-on time. Patient-specific quality assurance was performed using a 2D detector array with 3%/3 mm gamma analysis. Cone-beam CT-based image guidance and six-degrees-of-freedom couch corrections were used for setup verification. RESULTS: Both patients achieved satisfactory target coverage with D98 values of 22.56 Gy and 24.29 Gy and D2 values of 24.19 Gy and 25.77 Gy, respectively. CI and HI values were 1.15 and 1.04, demonstrating favorable conformity and homogeneity. Junction regions showed smooth dose transitions without clinically significant hot or cold spots. Mean lung doses remained below 5 Gy, kidney doses below 6 Gy, and lens doses below 10 Gy. Gamma passing rates exceeded 98%, while treatment delivery times were approximately 8-9 min with total monitor units below 940. CONCLUSION: VMAT-based CSI using the Siddharth-II O-ring gantry linear accelerator is feasible and demonstrates favorable dosimetric performance, effective junction management, reliable quality assurance, and efficient treatment delivery. These findings support the clinical applicability of this platform for complex craniospinal irradiation techniques.