Research on radiation protection in the application of new technologies for proton and heavy ion radiotherapy.

Particle radiotherapy using proton and heavy ion beams has shown improved clinical results and is a promising cancer therapy which is expected to gradually spread in Japan. There are, however, no special regulations for radiotherapy treatment facilities. They have been operated under the same safety regulations as for a research facility using a research accelerator. Significantly high-energy radiation is necessary for particle radiotherapy compared with conventional radiation therapy. The treatment facility, therefore, should have a large accelerator, which is installed in a room with a thick shield wall. Data on radiation protection for such high energy medical facilities is fragmentary and insufficient. In this study, we examined the necessity of other regulations for the safe operation of medical facilities for particle radiotherapy. First, we measured activation levels of the therapeutic devices and of patients. Next the safety level of the medical facility was evaluated from the viewpoint of radiation protection. We have confirmed the facilities can be safely operated by present regulations given in the Law Concerning Prevention from Radiation Hazards due to Radiation Isotopes, etc. or the Law for Health Protection and Medical Care.

Maximum likelihood estimation of proton irradiated field and deposited dose distribution.

In proton therapy, it is important to evaluate the field irradiated with protons and the deposited dose distribution in a patient's body. Positron emitters generated through fragmentation reactions of target nuclei can be used for this purpose. By detecting the annihilation gamma rays from the positron emitters, the annihilation gamma ray distribution can be obtained which has information about the quantities essential to proton therapy. In this study, we performed irradiation experiments with mono-energetic proton beams of 160 MeV and the spread-out Bragg peak beams to three kinds of targets. The annihilation events were detected with a positron camera for 500 s after the irradiation and the annihilation gamma ray distributions were obtained. In order to evaluate the range and the position of distal and proximal edges of the SOBP, the maximum likelihood estimation (MLE) method was applied to the detected distributions. The evaluated values with the MLE method were compared with those estimated from the measured dose distributions. As a result, the ranges were determined with the difference between the MLE range and the experimental range less than 1.0 mm for all targets. For the SOBP beams, the positions of distal edges were determined with the difference less than 1.0 mm. On the other hand, the difference amounted to 7.9 mm for proximal edges.

Experimental determination of particle range and dose distribution in thick targets through fragmentation reactions of stable heavy ions.

In radiation therapy with highly energetic heavy ions, the conformal irradiation of a tumour can be achieved by using their advantageous features such as the good dose localization and the high relative biological effectiveness around their mean range. For effective utilization of such properties, it is necessary to evaluate the range of incident ions and the deposited dose distribution in a patient's body. Several methods have been proposed to derive such physical quantities; one of them uses positron emitters generated through projectile fragmentation reactions of incident ions with target nuclei. We have proposed the application of the maximum likelihood estimation (MLE) method to a detected annihilation gamma-ray distribution for determination of the range of incident ions in a target and we have demonstrated the effectiveness of the method with computer simulations. In this paper, a water, a polyethylene and a polymethyl methacrylate target were each irradiated with stable (12)C, (14)N, (16)O and (20)Ne beams. Except for a few combinations of incident beams and targets, the MLE method could determine the range of incident ions R(MLE) with a difference between R(MLE) and the experimental range of less than 2.0 mm under the circumstance that the measurement of annihilation gamma rays was started just after the irradiation of 61.4 s and lasted for 500 s. In the process of evaluating the range of incident ions with the MLE method, we must calculate many physical quantities such as the fluence and the energy of both primary ions and fragments as a function of depth in a target. Consequently, by using them we can obtain the dose distribution. Thus, when the mean range of incident ions is determined with the MLE method, the annihilation gamma-ray distribution and the deposited dose distribution can be derived simultaneously. The derived dose distributions in water for the mono-energetic heavy-ion beams of four species were compared with those measured with an ionization chamber. The good agreement between the derived and the measured distributions implies that the deposited dose distribution in a target can be estimated from the detected annihilation gamma-ray distribution with a positron camera.

Enhanced efficiency in cell killing at the penetration depths around the Bragg peak of a radioactive 9C-ion beam.

PURPOSE: To evaluate the potential importance of radioactive 9C-ion beam in cancer radiotherapy. METHODS AND MATERIALS: Human salivary gland (HSG) cells were exposed to a double-radiation-source 9C beam at different depths around the Bragg peak. Cell survival fraction was determined by standard clonogenic assay. For comparison, the same experiment was conducted for a therapeutic 12C beam. To determine relative biologic effectiveness (RBE) values, HSG cells were also irradiated with 60Co gamma-rays of fractionation scheme as the reference. RESULTS: The 9C beam was more efficient in cell killing at the depths around its Bragg peak than was the 12C beam, which corresponded to the 9C-ion stopping region and where delayed low-energy particles were emitted. The RBE value at 50% survival level for the 9C beam varied from 1.38 to 4.23. Compared with the 12C beam, the RBE values for the 9C beam were always higher; an increase in RBE by a factor of up to 1.87 has been observed at the depths distal to the Bragg peak. CONCLUSION: The potential advantage of radioactive 9C-ion beam in cancer therapy has been revealed at low dose rate in comparison with a therapeutic 12C beam. This observation, however, remains to be investigated at therapeutic dose rates in the future.

Spatial fragment distribution from a therapeutic pencil-like carbon beam in water.

The latest heavy ion therapy tends to require information about the spatial distribution of the quality of radiation in a patient's body in order to make the best use of any potential advantage of swift heavy ions for the therapeutic treatment of a tumour. The deflection of incident particles is described well by Molière's multiple-scattering theory of primary particles; however, the deflection of projectile fragments is not yet thoroughly understood. This paper reports on our investigation of the spatial distribution of fragments produced from a therapeutic carbon beam through nuclear reactions in thick water. A DeltaE-E counter telescope system, composed of a plastic scintillator, a gas-flow proportional counter and a BGO scintillator, was rotated around a water target in order to measure the spatial distribution of the radiation quality. The results revealed that the observed deflection of fragment particles exceeded the multiple scattering effect estimated by Molière's theory. However, the difference can be sufficiently accounted for by considering one term involved in the multiple-scattering formula; this term corresponds to a lateral 'kick' at the point of production of the fragment. This kick is successfully explained as a transfer of the intra-nucleus Fermi momentum of a projectile to the fragment; the extent of the kick obeys the expectation derived from the Goldhaber model.

Range verification system using positron emitting beams for heavy-ion radiotherapy.

It is desirable to reduce range ambiguities in treatment planning for making full use of the major advantage of heavy-ion radiotherapy, that is, good dose localization. A range verification system using positron emitting beams has been developed to verify the ranges in patients directly. The performance of the system was evaluated in beam experiments to confirm the designed properties. It was shown that a 10C beam could be used as a probing beam for range verification when measuring beam properties. Parametric measurements indicated the beam size and the momentum acceptance and the target volume did not influence range verification significantly. It was found that the range could be measured within an analysis uncertainty of +/-0.3 mm under the condition of 2.7 x 10(5) particle irradiation, corresponding to a peak dose of 96 mGyE (gray-equivalent dose), in a 150 mm diameter spherical polymethyl methacrylate phantom which simulated a human head.

[The examination of optimal positron emitting nuclide in the estimation of attainment depth within the body of RI beams by positron camera].

The (10)C and (11)C beam stop position in a homogeneous phantom was measured using the range verification system in HIMAC. This system was developed to clear uncertainty of beam range within the patient body in heavy ion radiotherapy. In this system, a target is irradiated with RI beams ((11)C or (10)C) and the distribution of the beam end-points are measured by a positron camera. To inspect the precision of the measurement, three experiments were done, simple PMMA phantom irradiation, empirical beam stop position measurements using a range shifter and boundary irradiation using PMMA and lung phantom. Results of the first two experiments were consistent. Consequently, a 0.2 mm standard deviation of statistical error measurement was possible with 250 determinations. For the third experiment, we compared the precision using (10)C and (11)C beams. The boundary of the PMMA and lung phantom was irradiated with both beams to maximize the positron range effect in the beam range measurement. Consequently, no significant difference was observed between the two beams in spite of the different positron range. Thus, we conclude that the (10)C beam was useful for clinical application because of its good statistics owing to the short half-life.