Room: Exhibit Hall
Purpose: To investigate differences in dose distributions at the periphery of lung tumors based on plans delivered with flattening filter free (FFF) beams and with conventional flattened (FF) beams, as evaluated with Monte Carlo simulations compared to clinical treatment planning algorithms (AAA and Acuros XB).
Methods: Dose was calculated in a phantom comprised of a chest wall, lung parenchyma and a spherical tumor (GTV). GTV diameters were 1, 3 and 5 cm. The dose was normalized to 50 Gy to the GTV center. It was delivered with 7 coplanar, unmodulated 6 MV beams, with all beams being either FFF or flattened beams. Monte Carlo calculations used EGSnrc and phase space files for the TrueBeam accelerator provided by Varian. Voxels sizes were 0.5 mm for the 1-cm GTV, and 1 mm for the larger GTVs. AAA and Acuros XB (AXB) dose calculations were performed in Eclipse 13.6 with a 2.5 mm dose grid (the resolution normally used clinically).
Results: Monte Carlo dose distributions showed that traditional flattened beams underdosed the periphery of the tumor by up to ~ 2 Gy as compared to FFF beams. In all cases the ring was about 5 mm thick surrounding the GTV and extending into the GTV by 2-3 mm. The effect was most pronounced for the smallest targets. The dose preservation achieved with the FFF beams is caused by delta electrons that, in FFF beams, have lower energy and therefore higher stopping power than delta electrons produced by FF beams. Importantly, the under-dosing observed with conventional flattened beams was not captured by the clinical TPS (particularly AAA), indicating that this issue is not captured in clinical practice.
Conclusion: FFF beams overcome the modest dose loss to the GTV surface compared to flattened beams. Commercial dose algorithms at most only partially capture this issue.
Funding Support, Disclosures, and Conflict of Interest: This study was supported by NIH/NCI grant R03 CA211143