Analyzing the performance of accelerators in BNCT: evaluation of the therapeutic potential of the proposed facility and its comparison with global benchmark clinical beams

MARÍA S HERRERA; SARA J. GONZÁLEZ; WILLIAM S. KIGER III; HIROAKI KUMADA; ANDRÉS J. KREINER

Helsinki

Congreso; 16th International Congress on Neutron Capture Therapy; 2014

Introduction: The aim of this work is to contribute to the development and optimization of accelerator based-Boron Neutron Capture Therapy (AB-BNCT) beam designs and the consolidation of this innovative treatment modality in the field of radiation therapy. This study is part of a broader program of the National Atomic Energy Commission of Argentina (CNEA) that includes the construction and installation of a particle accelerator of protons or deuterons of low energy (~2.3 or 1.5 MeV, respectively) and high current (~30 mA) for BNCT in a specialized cancer institute in Argentina. The capability of a beam shaping assembly (BSA) design developed for this accelerator to treat various disease sites was assessed in the clinical context of patient treatment plans, comparing its calculated performance with that obtained with existing high quality reactor-based neutron beams used in BNCT treatments. Material and Methods: The first part of this work comprised the design of a new BSA based on previous work, together with the development of the methodology and software necessary for accelerator-based BNCT treatment planning. Well-characterized neutron beams from nuclear reactors used in the treatment of brain tumors (MIT FCB, USA and JRR-4, Japan) and cutaneous melanoma of the extremities (RA-6, Argentina) were used to assess the proposed design in a clinical scenario. We compared the quality of in-air beam parameters (flux, current, specific doses, etc.), both outside and inside the exit port. Also, dosimetric comparisons were made using clinical cases for two different disease sites: two glioblastoma multiforme cases involved in the clinical BNCT protocol of Harvard-MIT, and two nodular malignant melanoma cases treated with the B1 beam from the RA-6 reactor in the context of BNCT clinical studies in Argentina. In all cases, the comparisons were made under identical simulation conditions. To avoid introducing any bias, the definition of the calculated quantities (tallies), the composition of materials, voxel size, among others, were standardized. The NCTPlan treatment planning system and the MCNP5 transport code were used. Results: Calculation of in-air parameters showed that the proposed accelerator beam design provides a neutron flux considered suitable for the therapy (> 1.2×109 n/cm2 s1, with 82% in the epithermal energy range) with a relatively low contamination of fast neutrons and photons (specific doses of 5.5×10-13 and 2.9×10-13 Gy cm2/n, respectively). In addition, the radial neutron and gamma fluxes decline rapidly outside the exit port, thus yielding low peripheral dose. Simulating an analytical whole-body phantom confirmed the low peripheral doses, finding that mean doses computed in 15 organs are similar to those achieved with the epithermal beam mode of the JRR-4 reactor. Regarding the clinical cases, the calculated dose distribution to tumors and normal tissues are comparable to those obtained with the real beams considered in the study. Conclusion: The international collaboration between the different BNCT clinical institutions allowed assessing the performance of an accelerator-based source design using existing neutron beams, the MIT FCB (USA), JRR-4 (Japan), RA-6 (Argentina) reactors, and treatment plans for real BNCT patients. In the intercomparison, in-air and in-patient figures of merit were considered. The calculations show that the proposed accelerator-based neutron beam yields good dosimetric performance, comparing favorably with existing epithermal neutron beams. Also, it has been shown that an accelerator-based facility may be used in the treatment of both superficial and deep-seated tumors as long as different strategies for treatment planning are employed.