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SHOOT-THROUGH PROTON FLASH FOR NEUROLOGICAL TUMORS LIMITS UNCERTAINTIES IN (BIOLOGICAL EFFECTIVE) DOSE
Abstract
Background and Aims
In conventional proton therapy, Bragg peaks are positioned inside the tumor and in a margin surrounding it. This causes uncertainties in LET, RBE and range, endangering critical structures distal from the tumor. To combat this, shoot-through flash beams position the Bragg peaks behind the patient. While potentially losing the dosimetric advantage of Bragg peak plans, the FLASH protective effect may compensate for this. A planning study was performed on 5 neuro cases to evaluate dose-averaged LETD and robustness for both planning strategies.
Methods
5 neuro cases were planned using four 227 MeV Pencil Beam Scanning proton beams. LETD was calculated for the clinical plan and the shoot-through plan, applying a 2Gy dose threshold (RayStation 10A & 9AR-IonPG). A FLASH protective factor of 1.5 was used for tissues outside the CTV. Robust evaluation was performed considering movement (0.1 cm, 14 directions) and density uncertainty (±3% throughout entire volume).
Results
Clinical plans showed large LETD variations compared to shoot-through plans. For shoot-through plans, LETD merely reflects stopping power ratio differences (Fig. 1,2) and the maximum LETD in OAR is 2–6 times lower. Although less conformal, shoot-through plans met the same clinical goals as the clinical plans. The shoot-through plans are almost entirely robust to density uncertainties (Fig 3a). Considering both spatial and density uncertainties, the target coverage is less robust while the OAR D2% is in general more robust compared to clinical plans (Fig 3b).
Conclusions
Proton shoot-through flash beams avoid LETD and range uncertainties, provide adequate target coverage, meet planning constraints and are robust.