Comparative Assessment of Dose Distributions During Proton and Photon Therapy in Patients with Recurrent High-Grade Gliomas

Purpose: To compare treatment plans on the Prometheus proton therapy complex (PTC) and a linac in terms of dose distribution parameters and radiation doses on organs at risk.

Material and methods: The study included 20 adult patients who were treated on the Prometheus PTC in 2019–2020 for recurrent malignant gliomas. Comparative planning was carried out in the XIO radiation therapy planning system with the preparation of 3D-conformal photon radiation therapy plans using intensity modulated radiotherapy technology (IMRT) based on one set of contours of the irradiated volumes.

Results: Dose-volume histograms were constructed for all volumes, dose parameters were taken into account to assess the coverage of target volumes and compliance with safety criteria for organs at risk. The average dose to the entire brain volume during proton therapy ranged from 4.54 to 20.63 Gy, the median was 6.74 Gy. The average dose during photon therapy planning ranged from 5.9 to 32.48 Gy, the median was 21.2 Gy. The average difference in radiation load to the entire brain volume was 15.24 Gy (p < 0.001). The mean maximum dose to the brainstem during proton therapy ranged from 0.01 to 51.35 Gy, median 9.77 Gy. The mean dose when planning photon therapy using the IMRT technique ranged from 1.6 to 55.1 Gy, median 44.37 Gy. The mean difference was 34.6 Gy (p < 0.003). The mean maximum dose to the optic nerve during proton therapy ranged from 0 to 25.19 Gy, median 2.15 Gy. The mean dose in the photon therapy plan was 0 to 51.35 Gy, median 21.05 Gy. The reduction in the mean difference in dose load when using proton therapy was 18.9 Gy (p< 0.001).The average maximum dose to the chiasm during intensity-modulated proton therapy ranged from 0 to 32.9 Gy, median 0.38 Gy. A similar dose when calculating photon therapy doses ranged from 1.4 Gy to 54.3 Gy, median 28.47 Gy. The average difference in the dose load on the optic nerve in favor of proton therapy was 28.09 Gy (p< 0.001). The average value of the homogeneity index of protons was 0.16 (CI 95 % 0.14–0.18), photons – 0.13 (CI 95 % 0.11–0.14), p=0.00158.

Conclusion: Proton therapy during repeated courses of radiation therapy demonstrates a significant reduction in the dose load on risk organs when compared with photon therapy on a linear accelerator. Repeated irradiation of high-grade gliomas using an active scanning proton beam is a promising direction due to the reduction in overall toxicity of treatment and the possibility of delivering radiation doses close to radical ones.