Characterization and Performance Evaluation of the First-Proton Therapy Facility in India.

Purpose: The purpose of this study is to evaluate the performance characteristic of volumetric image-guided dedicated-nozzle pencil beam-scanning proton therapy (PT) system. Materials and Methods: PT system was characterized for electromechanical, image quality, and registration accuracy. Proton beam of 70.2-226.2 MeV was characterized for short- and long-term reproducibility in integrated depth dose; spot profile characteristics at different air gap and gantry angle; positioning accuracy of single and pattern of spot; dose linearity, reproducibility and consistency. All measurements were carried out using various X-ray and proton-beam specific detectors following standard protocols. Results: All electro-mechanical, imaging, and safety parameters performed well within the specified tolerance limit. The image registration errors along three translation and three rotational axes were <= 0.5 mm and <= 0.2 degrees for both point-based and intensity-based auto-registration. Distal range (R90) and distal dose fall-off (DDF) of 70.2-226.2 MeV proton beams were within 1 mm of calculated values based on the international commission on radiation units and measurements 49 and 0.0156x R90, respectively. The R90and DDF were reproducible within a standard deviation of 0.05 g/cm2 during the first 8 months. Dose were linear from 18.5 (0.011 MU/spot) to 8405 (5 MU/spot) MU, reproducible within 0.5% in 5 consecutive days and consistent within 0.8% for full rotation. The cGy/MU for 70.2-226.2MeV was consistent within 0.5%. In-air X(Y) spot-sigma at isocenter varies from 2.96 (3.00) mm to 6.68 (6.52) mm for 70.2-226.2 MeV. Maximum variation of spot-sigma with air-gap of +/- 20 cm was +/- 0.36 mm (5.28%) and +/- 0.82 mm (+/- 12.5%) along X- and Y-direction and 3.56% for full rotation. Relative spot positions were accurate within +/- 0.6 mm. The planned and delivered spot pattern of known complex geometry agreed with (gamma%<= 1) for 1% @ 1 mm >98% for representative five-proton energies at four gantry angle. Conclusion: The PT-system performed well within the expected accuracy level and consistent over a period of 8 months. The methodology and data presented here may help upcoming modern PT center during their crucial phase of commissioning.