Dose escalation in prostate cancer using intensity modulated neutron radiotherapy

Background and purpose: Initial promising results of 3D conformal neutron radiotherapy (3D-CNRT) were subsequently limited by high normal tissue toxicities. It is now possible to deliver intensity modulated neutron radiotherapy (IMNRT). The present work compares photon IMRT, 3D-CNRT and IMNRT for three prostate patients to quantify the benefits of IMNRT. Materials and methods: We compare updated 3D-CNRT plans, IMNRT plans, and conventional IMRT plans by translating neutron DVHs into effective photon DVHs using the dose dependent radiobiological effectiveness (RBE) for each structure. RBE curves are parameterized for a range of normal tissue and prostate tumor values. Generalized equivalent uniform dose (gEUD) and gEUD in 2 Gy fractions (gEUD2) is calculated for each structure, plan, and parameterization. Rectal sparing and dose to prostate-GTV are compared for 3D-CNRT, IMNRT, and IMRT as a function of normal tissue and prostate RBE. Results: The closer the RBE values of prostate tumor and normal tissue, the greater the advantage of IMNRT over 3D-CNRT. The rectal sparing achieved using IMNRT ranged from ∼5% to 13% depending upon the choice of RBE for rectum and the α/β value of prostate tumor. IMNRT may provide a theoretical dose advantage over photon IMRT if the α/β value of prostate is 1.5 and the RBEs of prostate and rectum differ by more than 5%. For higher values of prostate α/β any advantages of IMNRT over IMRT could require that the RBEs of prostate and rectum differ by as much as 20%. Conclusions: IMNRT provides a clear normal tissue sparing advantage over 3D-CNRT. The advantage increases when the RBEs of the target structure and the normal tissue are similar. This RBE translation method could help identify clinical sites where the dose sparing advantages of IMNRT would allow for the exploitation of the radiobiological advantages of high-LET neutron radiotherapy.