Room: Exhibit Hall | Forum 3
Purpose: Use of parallel-opposed proton beams may increase the efficacy of GRID therapy since the proton beam has no exit dose, hence eliminating the overlapping of entrance and exit dose from the beams. In this study, we investigated the dosimetry of GRID therapy using parallel-opposed pencil-beam-scanning (PBS) proton beams.
Methods: 15 patients have received proton-based GRID therapy at our center. One patient who had been treated with a single PBS beam was selected and re-planned with two opposed PBS beams. Two opposing techniques were investigated: matched and offset. In â€œmatchedâ€? technique, the same hexagonal pattern was used in both beams, so that their beamlets are overlapping but from opposite directions. In â€œoffsetâ€? technique, a square pattern was used and their patterns were offset by half a grid spacing in both in-plane and cross-plane directions, so that their beamlets are opposed but non-overlapping.
Results: The prescription is 20 Gy. The mean tumor dose is 9.5 Gy, 10.1 Gy and 14.8 Gy in the single-beam, matched-opposed and offset-opposed plans respectively. The tumor volume that received at least 10 Gy is 36.5%, 39.0% and 98.4% respectively. The peak-to-valley ratio at tumor center is 3.48, 3.81, and 1.86 respectively. The maximum skin dose is 18.3 Gy, 10.3 Gy, 17.3 Gy and the minimum skin dose in the treated area is 2.4 Gy, 1.5 Gy, 0.9 Gy in the single-beam, matched-opposed and offset-opposed plans respectively. The peak-to-valley ratio at the skin surface is 5.83, 6.15, and 21.33 in these three plans respectively.
Conclusion: Compared with single proton beam approach, when opposed proton beams with matched pattern is used, skin dose is reduced to nearly half while keeping similar tumor dose; When opposed proton beams with offset pattern is used, tumor dose is significantly increased and skin sparing is greatly improved with similar maximum skin dose.