Pierre Lansonneur1, Anthony Magliari2, Lesley Rosa2, Jessica Perez3, Ricky Sharma4, Michael Folkerts5
1Varian Medical Systems, FLASH, Le Plessis Robinson, France; 2Varian, Medical Affairs, Palo Alto, USA; 3Varian, FLASH, Geneva, Switzerland; 4Varian, Medical Affairs, Crawley, United Kingdom; 5Varian, FLASH, Palo Alto, USA
Lung cancer is one of the most common cancers, and the leading cause of cancer deaths in men and women in North America and other developed countries. For patients with co-morbidities, surgical resection of centrally located tumors is often not suitable, and such patients typically have poor clinical outcomes. Ultra-high dose-rate irradiation (FLASH-RT) has the potential to spare normal tissue for such patients; however, technical hurdles exist with current methods to reach ultra-high dose rates for central tumors, which can have an impact on dosimetric plan quality. In this study, we propose a novel algorithm to optimize spot positions for plan quality benefit, while simultaneously adjusting constrained spot weights delivered at ultra-high dose rates for treatment with Intensity Modulated Proton Therapy (IMPT).
Single-energy-layer 250 MeV IMPT plans were created for an ultra-central lung case with a large PTV size (6 x 8 cm, 150 cc) with significant Organ-at-Risk overlap. Consistent with a contemporary treatment protocol for SBRT (RTOG-0813: https://www.nrgoncology.org/Clinical-Trials/Protocol/rtog-0813), each plan was prescribed to deliver 50 Gy in five fractions to the PTV. Spot positions and MU were optimized while enforcing a minimum spot MU to ensure high dose-rates. Dose metrics were compared to conventional (spot weight only) optimization, both used the same base optimization code with the same DVH based user inputs. PBS dose rate (https://doi.org/10.1002/mp.14456) was calculated and evaluated.
Unlike conventional optimization scheme, the plans optimized with this novel algorithm passed the RTOG metrics, omitting Organ-at-Risk metrics impossible due to PTV overlap. The fraction of irradiated volume (dose above 2 Gy) receiving at least 40 Gy/s reached 92% for lungs and 95% for the heart. Overall, this new optimization method resulted in a significant plan quality improvement with similar planning execution time.
This work supports the optimization of spot positions for plan quality benefit using ultra-high dose rate proton therapy. The results will facilitate the future creation of fractionated IMPT plans for large or complex tumors using FLASH-RT.