Room: Track 5
Purpose: To design an Ocular Option on the ProBeam pencil-beam-scanning system for the treatment of ocular melanoma, and to compare its dosimetric properties to a traditional proton eyeline.
Methods: The Ocular Option (OO) consists of a range shifter and field-specific collimator, and uses standard spot-scanning dose delivery. In the design the following parameters were optimized: 1) range-shifter material, thickness, and position, 2) collimator air gap, and 3) spot-map spacing and size. The lateral penumbra and distal fall-off were evaluated as function of these design parameters. The RayStation Monte Carlo dose calculation algorithm was commissioned and validated against measurement. Ultimately, treatment plans were developed for 10 melanoma cases allowing the evaluation of target coverage and dose to critical organs for promising Ocular Option configurations. Both single-anterior-field as well as three-field, inversely-optimized plans were created for the OO. These were compared to treatment plans developed for a dedicated eyeline.
Results: The lateral penumbra was improved by increasing the isocenter-to-range-shifter distance, decreasing the aperture air gap, and minimizing the spot-map size. In a design in which a 3.5-cm Lucite range shifter is placed downstream of the nozzle’s ionization chambers, the collimator air gap is 7 cm, and the spot map is 3x3 cm2, the lateral penumbra in air is found to be 2.4 mm slightly larger than the 1.8 mm of the dedicated eyeline. Compared to dedicated-eyline plans, the three-field OO plans allowed for a better sparing of the anterior segment of the eye although did worse in cases were a critical structure abutted the target.
Conclusion: With limited modifications to the ProBeam system, an Ocular Option can be installed whose dosimetric properties are not as good as a dedicated eyeline but that can deliver eye treatments of acceptable quality. This Ocular Option opens the possibility of multi-field treatments and inverse planning.