Spatially Fractionated (GRID) Radiation Therapy using Proton Pencil Beam Scanning (PBS): Feasibility Study and Clinical Implementation.

PurposeGRID therapy is an effective treatment for bulky tumors. Linear accelerator (Linac)-produced photon beams collimated through blocks or multileaf collimators (MLCs) are the most common methods used to deliver this therapy. Utilizing the newest proton delivery method of pencil beam scanning (PBS) can further improve the efficacy of GRID therapy. In this study, we developed a method of delivering GRID therapy using proton PBS, evaluated the dosimetry of this novel technique and applied this method in two clinical cases. Materials/MethodsIn the feasibility study phase, a single PBS proton beam was optimized to heterogeneously irradiate a shallow 20x20x12cm(3) target volume centered at a 6cm depth in a water phantom. The beam was constrained to have an identical spot pattern in all layers, creating a beamlet at each spot position. Another GRID treatment using PBS was also performed on a deep 15x15x8cm(3) target volume centered at a 14cm depth in a water phantom. Dosimetric parameters of both PBS dose distributions were compared with typical photon GRID dose distributions. In the next phase, four patients have been treated at our center with this proton GRID technique. The planning, dosimetry, and measurements for two representative patients are reported. ResultsFor the shallow phantom target, the depth-dose curve of the PBS plan was uniform within the target (variation <5%) and dropped quickly beyond the target (50% at 12.9cm and 0.5% at 14cm). The lateral profiles of the PBS plan were comparable to those of photon GRID in terms of valley-to-peak ratios. For the deep phantom target, the PBS plan provided smaller valley-to-peak ratios than the photon GRID technique. Pretreatment dose verification QA showed close agreement between the measurements and the plan (pass rate >95% with a gamma index criterion of 3%/3mm). Patients tolerated the treatment well without significant skin toxicity (radiation dermatitis grade 1). ConclusionsProton GRID therapy using a PBS delivery method was successfully developed and implemented clinically. Proton GRID therapy offers many advantages over photon GRID techniques. The use of protons provides a more uniform beamlet dose within the tumor and spares normal tissues located beyond the tumor. This new PBS method will also reduce the dose to proximal organs when treating a deep-seated tumor.