Presenter of 1 Presentation
CONFORMITY OF EQUIVALENT DOSE TO TARGET IN FRACTIONATED STEREOTACTIC FLASH PT WITH SHOOT-THROUGH BEAMS OF EARLY-STAGE LUNG CANCER
Abstract
Background and Aims
In PT, the highest dose rate is achieved at 250MeV. Such beams shoot through the patient at the expense of conformity. Morever, healthy-tissue fraction doses above the FLASH theshold of 8-10Gy require single beams per fraction, compromising conformity of equivalent dose (EQD2). We investigate for which FLASH enhancement ratio (FER), this is outweighed by the FLASH effect in lung SBPT.
Methods
Assuming a/b=10Gy for the target, isoeffective plans to 54Gy/3, 65.5Gy/5, 73.7Gy/7 and 80.0Gy/9 were optimized for 12 patients in in-house developed software. CTV delineations were available. A 5mm CTV-PTV margin was applied. Only small PTVs (range: 4.4-10.1cc) were included. Equiangular co-planar arrangements of 244MeV shoot-through beams, avoiding serial OARs, were used. To enable single-beam per fraction delivery, the beam number was equal to the fraction number. Dose was prescribed per beam: D95%,PTV = 100%Dpres/#beams. A constant FER was assumed and applied to healthy-tissue voxel fraction doses: dj → dj/FER for all voxels j. Voxel EQD2s for FLASH-enhanced single-beam per fraction delivery and conventional multi-beam treatments were calculated. The conformity index (CI) of EQD2, with a/b=3Gy for healthy tissue, i.e., CI=VEQD2,pres/VPTV, was evaluated.
Results
Median CIs were 7.1 (range: 4.7-8.2), 5.7 (3.9-6.7), 5.2 (3.6-5.9) and 4.6 (3.4-5.5) for conventional 3, 5, 7 and 9 beam treatments. CIs for FLASH-enhanced treatment are shown in Fig. 1. Break-even FERs were 1.33 (1.30-1.36), 1.23 (1.22-1.24), 1.18 (1.15-1.20) and 1.15 (1.10-1.15) with 3, 5, 7 and 9 beams.
Conclusions
If the FLASH enhancement ratio exceeds 1.3, FLASH outweighs EQD2 conformity loss in single-beam per fraction shoot-through lung SBPT.
Author Of 1 Presentation
CONFORMITY OF EQUIVALENT DOSE TO TARGET IN FRACTIONATED STEREOTACTIC FLASH PT WITH SHOOT-THROUGH BEAMS OF EARLY-STAGE LUNG CANCER
Abstract
Background and Aims
In PT, the highest dose rate is achieved at 250MeV. Such beams shoot through the patient at the expense of conformity. Morever, healthy-tissue fraction doses above the FLASH theshold of 8-10Gy require single beams per fraction, compromising conformity of equivalent dose (EQD2). We investigate for which FLASH enhancement ratio (FER), this is outweighed by the FLASH effect in lung SBPT.
Methods
Assuming a/b=10Gy for the target, isoeffective plans to 54Gy/3, 65.5Gy/5, 73.7Gy/7 and 80.0Gy/9 were optimized for 12 patients in in-house developed software. CTV delineations were available. A 5mm CTV-PTV margin was applied. Only small PTVs (range: 4.4-10.1cc) were included. Equiangular co-planar arrangements of 244MeV shoot-through beams, avoiding serial OARs, were used. To enable single-beam per fraction delivery, the beam number was equal to the fraction number. Dose was prescribed per beam: D95%,PTV = 100%Dpres/#beams. A constant FER was assumed and applied to healthy-tissue voxel fraction doses: dj → dj/FER for all voxels j. Voxel EQD2s for FLASH-enhanced single-beam per fraction delivery and conventional multi-beam treatments were calculated. The conformity index (CI) of EQD2, with a/b=3Gy for healthy tissue, i.e., CI=VEQD2,pres/VPTV, was evaluated.
Results
Median CIs were 7.1 (range: 4.7-8.2), 5.7 (3.9-6.7), 5.2 (3.6-5.9) and 4.6 (3.4-5.5) for conventional 3, 5, 7 and 9 beam treatments. CIs for FLASH-enhanced treatment are shown in Fig. 1. Break-even FERs were 1.33 (1.30-1.36), 1.23 (1.22-1.24), 1.18 (1.15-1.20) and 1.15 (1.10-1.15) with 3, 5, 7 and 9 beams.
Conclusions
If the FLASH enhancement ratio exceeds 1.3, FLASH outweighs EQD2 conformity loss in single-beam per fraction shoot-through lung SBPT.