USING PROTON BEAM FOR CALIBRATION OF NEUTRON DOSIMETERS IN PROTON THERAPY

Using Proton Beam for Calibration of Neutron Dosimeters in Proton therapy

Literature Review

The development of hospital-based proton facilities represents a major step forward in radiotherapy, in part because of excellent dose distributions around the tumor [1], and in part because of the potentially lower whole-body dose compared with photon radiotherapy [2]. In the context of the whole-body dose, however, the issue of secondary neutrons produced by the scattering components of passively scattered radiotherapeutic proton beams has recently attracted much research and considerable discussion [3- 20]. Whilst there has been a justifiable focus on establishing the neutron doses involved [7-20], there is still no agreement about whether these scattered neutrons really represent a clinically relevant issue. To briefly summarize the issue: for most practical proton radiotherapy, it is necessary to spread out the narrow pencil beam produced by a proton accelerator, in order to provide uniform coverage over the target. This can be done, as in most current proton radiotherapy facilities, by inserting scattering material into the beam (passive scattering, or by using deflecting magnets to sweep the beam across the tumor (active scanning.

Using Proton Beam for Calibration of Neutron Dosimeters in Proton therapy

Tumor control and normal tissue-complication probabilities are steep functions of absorbed dose, the determination of which must therefore be accurate and reproducible. Dosimetry measurements at any facility must be consistent with those made at other facilities if clinical data are to be compared. A relative accuracy of ±3% is desirable, although ±5% is often accepted, while relative reproducibility of ±2 % is required. Comparisons of dose measurements between different centers are important for establishing uniform standards and for verifying the integrity of the dosimetry procedures. These are especially important for new facilities with exotic beams such as neutrons and protons, and it is recommended that all such facilities undertake dosimetry comparisons with existing similar facilities before commencing the clinical program. The requirements for a dosimeter depend on the accuracy of the absorbed dose determination required, the sensitivity of the measuring system, the energy dependence of the dosimeter response, and the spatial resolution required. In principle, calorimeters give the smallest uncertainty in the determination of absorbed dose in any intense radiation field. These instruments give a direct determination of the energy imparted (dose) in a sensing element as indicated by a temperature rise. Assuming that all (or a known amount) of the deposited energy is converted to heat, absorbed dose may be absolutely determined. However, calorimeters are cumbersome devices and not practical for routine clinical use. Ionization chambers are more practical but provide an indirect determination of absorbed dose since calibration factors determined in standard radiation fields, as well as correction factors that require knowledge of various physical data and constants, have to be applied. Several dosimetry protocols for both neutron [1-6] and proton [6-11] therapy dosimetry have been published. These protocols all recommend that, in the absence of a calorimeter, reference absorbed dose measurements in the clinical situation ...