ESTRO 2022 - Abstract Book

S1445

Abstract book

ESTRO 2022

it is hypothesized that early and maximum local consolidative radiotherapy (MLCR) of all metastatic lesions which do not achieve complete response to systemic therapy, may delay drug resistance development and disease progression. Traditional radiotherapy planning approaches, in which each metastasis is treated to the same dose of radiotherapy every day, are however not suitable with respect to safety and efficacy for targeting multiple metastases across the body. In the context of MLCR for polymetastatic cancer patients, in fact, it is a priori unclear what dose can or should be prescribed to each metastasis. To address this problem, we implemented a radiotherapy planning approach which simultaneously minimizes the total number of surviving tumor cells and preserves organ function. Materials and Methods An exponential cell survival model was assumed according to which the total number of surviving tumor cell is Ʃ m Ʃ i ϵ PTVm c exp(- α d i ) where d i is the dose in voxel i , α is the radiosensitivity parameter, and we sum over all voxels i that belong to any of the metastases m . This was used as an objective function for IMRT planning, while constraining the mean lung dose to 9 Gy and V 20Gy to 10% in a 5-fraction SBRT regimen. Such a radiotherapy planning approach was investigated for three metastatic melanoma patients with a varying number of lung metastases. Results For a patient with 31 lung metastases, Figures 1 and 2 show the dose distribution and the DVHs of the individual lesions, respectively. The method delivers similar minimum doses of approximately 18 Gy to all metastases, where the value of the minimum dose is determined by lung constraints. However, inhomogeneous doses are delivered within each lesion, with the mean dose varying between the individual metastases by up to 10 Gy. The method exploits the fact that for the same increase in lung dose, lesions located in favourable positions can be irradiated to higher radiation doses than others, and thereby increases overall cell kill for a given lung dose constraint compared to a fixed prescribed dose to all metastases.