RBE and weighting of absorbed dose in ion-beam therapy.

Absorbed dose is the fundamental quantity used to quantify the exposure of any biological system to ionizing radiation. However, the relationship between dose and biological effect is not unique but varies with fractionation and time factor(s), radiation quality and irradiation conditions. In radiation therapy, weighting factors are used to correlate absorbed dose and clinical effects when altering irradiation conditions, or for combining or comparing different technical modalities. For some well established therapy modalities (e.g. fractionated photon beam therapy), a general agreement on weighting factors has been reached: it is based on the linear-quadratic model. For neutron, proton or ion therapy, the differences in radiation quality are currently accounted for using a diversity of methods (almost hospital specific). This paper reviews the current approaches used for evaluating or selecting the weighting factors and their application in clinical practice. The weighting factors take account of the RBE and other factors, such as fractionation when needed. Harmonization of these approaches will facilitate the exchange of information within the radiation oncology community and between centres using different technical modalities. In any case, when reporting the treatments, absorbed dose and irradiation conditions should always be specified in addition to the weighting factor and the weighted dose

Dose prescription in boron neutron capture therapy

Purpose: The purpose of this paper is to address some aspects of the many considerations that need to go into a dose prescription in boron neutron capture therapy (BNCT) for brain tumors; and to describe some methods to incorporate knowledge from animal studies and other experiments into the process of dose prescription. Materials and Methods: Previously, an algorithm to estimate the normal tissue tolerance to mixed high and low linear energy transfer (LET) radiations in BNCT was proposed. We have developed mathematical formulations and computational methods to represent this algorithm. Generalized models to fit the central axis dose rate components for an epithermal neutron field were also developed. These formulations and beam fitting models were programmed into spreadsheets to simulate two treatment techniques which are expected to be used in BNCT: a two-field bilateral scheme and a single-field treatment scheme. Parameters in these spreadsheets can be varied to represent the fractionation scheme used, the B-10 microdistribution in normal tissue, and the ratio of B-10 in tumor to normal tissue. Most of these factors have to be determined for a given neutron field and B-10 compound combination from large animal studies. The spreadsheets have been programmed to integrate all of the treatment-related information and calculate the location along the central axis where the normal tissue tolerance is exceeded first. This information is then used to compute the maximum treatment time allowable and the maximum tumor dose that may be delivered for a given BNCT treatment. Results and Conclusion: The effect of different treatment variables on the treatment time and tumor dose has been shown to be very significant. It has also been shown that the location of D-max shifts significantly, depending on some of the treatment variables-mainly the fractionation scheme used. These results further emphasize the fact that dose prescription in BNCT is very complicated and nonintuitive. The physician prescribing the dose would need to rely on some method, like the one developed here, to come up with an appropriate dose prescription

BNCT: a promising area of research?

The renewed interest in boron neutron capture therapy (BNCT) is driven mainly by the disappointing progress in the treatment of brain tumors by other modalities over the last decades. Even though molecular biology newer drugs and strategies may promise better results in the future, BNCT is an attractive approach. Brain tumors kill by local growth and not be metastases. Boron can be delivered to the tumor while normal brain is protected by the blood brain barrier, which can be disrupted to the degree desired. Tumor selectivity can be obtained not only by improved drug barrier, which can be disrupted to the degree desired. Tumor selectivity can be obtained not only by improved drug delivery but also by restricting the capture reaction to the region of interest by targeted radiation. Both boron drug and thermal neutrons alone are to some extent innocuous to tumor and normal tissues in this binary form of therapy. The pattern of treatment failure from uncontrolled primary tumor, the blood brain barrier protection of normal surrounding tissue and the limited range of epithermal neutrons explain why brain tumors are the main focus of BNCT research