Voxel model in BNCT treatment planning: performance analysis and improvements

SARA GONZÁLEZ; DANIEL CARANDO; GUSTAVO SANTA CRUZ; ROBERT ZAMENHOF

PHYSICS IN MEDICINE AND BIOLOGY

Institute of Physics Publishing

Lugar: Bristol; Año: 2005 vol. 50 p. 441 - 458

0031-9155

During the last years, much effort have been made to study the performance of treatment planning systems in deriving an accurate dosimetry of the complex radiation fields involved in Boron Neutron Capture Therapy. The technique used to generate the computational model of the patient’s anatomy is one of the main features involved in this subject. This work presents a detailed analysis of the performance of the 1cm-based voxel reconstruction approach. Based on previous works, the performances of the 1cm-based voxel methods used in the MacNCTPlan and NCTPlan treatment planning systems for BNCT are compared by standard simulation tests. In addition, the NCTPlan voxel model is benchmarked against in-phantom physical dosimetry of the RA-6 reactor. This investigation shows the accuracy of the 1 cm resolution for all reported tests, even in the extreme cases such us a parallelepiped phantom irradiated through one of its sharp edges.  This accuracy is degraded at very shallow depths in which, to improve the estimates, the anatomy images need be positioned in a suitable way. Since the skin is an organ of relevance in BNCT, the performance of the voxel technique is deeply analyzed in these shallow regions. A theoretical analysis is carried out to assess the distortion caused by homogenization and material percentage rounding processes. Then, a new strategy for the treatment of surface voxels is proposed and tested using the two different irradiation problems. For a parallelepiped phantom perpendicularly irradiated with a 5 keV neutron source, the large thermal neutron fluence deviation present at shallow depths (from 54% at 0 mm-depth to 6% at 4 mm-depth) is reduced to 2% on average. Reassigning fluence values in the case of this phantom in angular position produced the maximum deviation in the thermal fluence to decrease from 140% to 23% at the surface of the phantom. Since these results substantially improve the performance of the 1cm-based voxel model in surface boundary regions, the proposed strategy will be implemented in future versions of the NCTPlan code.