Exhibit Hall | Forum 7
Purpose: The small-fields (<2.5 cm) dosimetry of proton spot-scanning beams for ocular (uveal) melanoma treatments (Rx: 50 Gy(RBE) in 5 fractions) could be challenging. The patient-specific quality assurance (psQA) with 2D scintillator-based CCD camera (Lynx PT, 0.5 mm/pixel) for small fields was demonstrated.
Methods: The proton pencil-beam convolution model in Eclipse TPS for small-fields was comprehensively validated. All the plans for patient treatments and QAs were calculated with 1-mm dose grid. The psQA for 4 uveal melanoma patients (total 14 fields, widths range from 1.4 cm to 3.7 cm) were performed with the 2D dose comparisons of Lynx PT measurements and TPS calculations. The conversion factors (CFs, cGy(RBE)/Count) of Lynx PT were acquired with the parallel-plate (PPC05) and compact ionization chamber (Razorᵀᴹ, 0.01 cm³) in 2 scenarios respectively: (1) at 2.65 cm depth (water equivalent thickness, WET) of a 10x10 cm² monoenergetic 150 MeV proton beam, and (2) at the center (WET= 5.65 cm) of a 4x4x3 cm³ calibration target volume (CaTV) delivered by a multi-energetic proton field. Interpolated-free Gamma evaluation with a passing criterion of 5% DD and 1 mm DTA for SRS IMRT-QA was used. The compact chamber was also used to measure the point doses at the depths inside the treatment volumes.
Results: The averaged Gamma passing rates were 83.4% (range: 63.9%-100%) and 98.7% (range: 93.2%-100%) with CFs measured at the depths at entrance (2.65 cm) and CaTV (5.65 cm), respectively. The use of the entrance-dose CF under-estimated the doses by ~8% and caused the centralized failing points. The point dose deviations measured with compact chamber were <4% (range: -3.04%-3.77%) versus TPS calculations.
Conclusion: The proton small-field psQA can be performed using the high-resolution scintillator-based camera calibrated at the depths of the treatment volumes. A Gamma>95% for proton spot-scanning small-field is achievable using our current beam model.