Author: Sameera Kumar, Chang Ming Charlie Ma, Robert A. Price 👨🔬
Affiliation: Fox Chase Cancer Center 🌍
Purpose:
Difficulty assessing lung limits during treatment planning in the retreatment environment can be compounded as limits are volume-based (ie. VxGy<37%) and require a critical volume with a maximum allowable dose (1500cc:max xGy) be maintained. Available TPS metrics such as min, mean and max dose are difficult to utilize. In addition, the fractionation is often different in the initial vs. subsequent treatment courses. In this work we present a method to maintain volume-based limits to determine available lung that can be safely irradiated during retreatment.
Methods: Determine the lung dose limit for the course2 fractionation. Convert from physical dose to EQD2 using an α/β of 3Gy (late effects). Determine the prior physical dose corresponding to this EQD2 and convert this dose volume to a structure. Fuse the initial and current CT scans and subtract the associated target volumes from the total lung on the latter. Subtract the union of this lung volume and EQD2 dose structure to get the baseline total lung volume. The physical dose volume for the course2 fractionation should not exceed the given limit expressed as a percentage of the baseline. Evaluate the associated DVH at the specified critical volume to determine the maximum dose delivered and convert this dose to a structure. Subtract this structure from the baseline to get the critical volume location. The physical dose available for planning is the max limit minus this dose.
Results: Course1 prescription 60Gy in 30fxs. Course2 prescription 45Gy in 15fxs. Lung volume limit (15fx) is 37% at 18Gy (15.12Gy EQD2). 15.12Gy EQD2 equates to 20.52Gy physical dose for 30fx delivery. The initial dose to the critical volume is 1.3Gy max leaving a 15.2Gy planning limit.
Conclusion: The available lung volume for re-irradiation can be determined and aid in plan generation using this simplified geometric method.