Yoshikazu Tsunashima

Simplified Monte Carlo Dose Calculation for Therapeutic Proton Beams

A simplified Monte Carlo (SMC) method has been developed for dose calculation of therapeutic proton beams. It uses the depth-dose distribution in water measured by a broad proton beam to calculate the energy loss in a material easily and accurately. It employs the water-equivalent model of inhomogeneous materials. In addition, the multiple scattering effect in the materials is also calculated using the water-equivalent thickness. The accuracy of dose calculations by the SMC method is verified by comparison with dose measurements in a heterogeneous phantom. Results of the measured dose distributions agree well with calculations by the SMC method, though those determined by the dose calculation method based on the pencil beam algorithm show a large discrepancy. Therefore, the dose-calculation method by the SMC method will be useful for application to the treatment planning for proton therapy.

Dosimetry of pulsed clinical proton beams by a small ionization chamber

Response of a micro volume (0.01 ml) ionization chamber has been studied with pulsed proton beams which are used for clinical purposes and has been compared with those of some JARP ionization chambers (0.6 ml). All chambers used had been calibrated by standard 60Co beams at the Electrotechnical Laboratory (ETL) and exposure calibration factors, Nx, were obtained on advance. Two methods are used to compensate the general recombination which occurs during pulsed beam irradiations: theoretical correction by a Boags formulation and a modified two-voltage technique. An evaluation of absolute absorbed dose-to-water is performed on the basis of the protocol provided by ICRU report 59. The results imply that, to a first approximation, both chambers indicate the almost same result within 2% when unknown chamber-dependent parameters of the micro chamber are tentatively assumed to be identical to those of the JARP chamber for the calibration with 60Co beams. The about 1.5% discrepancy observed in the response of both chambers is not discussible due to presumably 1-2% uncertainty of the protocol of ICRU report 59 which does not include any chamber-dependent corrections for the perturbation effects in proton beams.

Measurement of depth-dose distribution of protons by an imaging plate

By using an imaging plate (IP), we have developed a novel, simple method for depth-dose distribution measurement of clinical proton beams. When protons are obliquely incident upon the IP, the energy is deposited closer to the surface of the photo-stimulated luminescence (PSL) material than in the case of normal incidence. On the other hand, the spatial distribution of deposited energy varies considerably with variations in proton energy and results in a dependence of the IP response on linear-energy transfer (LET). Through a combination of these factors, for the oblique incidence, IP response to protons may be relatively enhanced around the Bragg peak because the observed PSL intensity depends on the position (depth) at which the energy is deposited. To examine this geometrical enhancement, a simple calculation is performed. The effect is thought to reduce the dependence of IP response on LET so that the response of an IP may approach that of a parallel-plate ionization chamber. Experimental results are given for protons of various angles of incidence (θ) on the IP. The proposed method may be useful for conducting quick checks of depth-dose distributions in proton-therapy facilities.

Conformal irradiation by proton beam scanning and multilayer energy filter

Using a new energy filter, three-dimensional conformal irradiation for proton therapy can be realized by monoenergetic beam scanning. It is a big advantage that the method does not necessitate energy scanning from the viewpoint of shortening the scanning time for cancer treatment of a moving organ. The new filter can yield an irradiation field where the width of the spread-out Bragg peak is adjusted to the target. Present work shows evidence of three-dimensional conformal irradiation by the new filter. Instead of the intensity modulation necessary for conformal irradiation by beam scanning, irradiation-time modulation is performed in scanning using constant beam current. As the result, a conformal field around a step-shaped virtual target in a solid phantom is realized by the filter.

Dose distribution measurement of clinical proton beams with an imaging plate

For an application to the dose-distribution measurement, response of an imaging plate (IP) has been studied with proton beams which are utilized for the radiation therapy. The upper limit of measurable proton dose by the IP system is almost controlled by the readout range of the scanner used. Within this limit, reasonable linear response of an IP to proton dose to water is maintained. Reproducibility of the PSL intensity is fairly good if both the fading characteristics and the lot-dependence of the sensitivity of each IP are taken into account carefully. Stopping power dependence of IP response has been found to be unnegligible for the precise dose evaluation. Following this, in order to examine the resultant radiation damage by proton irradiation, an IP was excessively irradiated by 250 MeV protons. Notable deterioration of PSL signal was found for the fluence more than 5 /spl times/ 10/sup 12/ protons/cm/sup 2/. Next, by using an IP, we have developed a simple and novel method of the 2-dimensional dose-distribution measurement. The effectiveness of this method is demonstrated by a comparison between the measurement and the dose-distribution calculated by a new model.

SU‐FF‐T‐340: Effect of Marker Implants On Dose Distribution in Proton Therapy

Purpose: To determine whether the dose perturbations caused by implanted markers in or near the mobile tumors have significant impact on the dose distributions, particularly by creating hot or cold spots behind the markers. Method and Materials: A preliminary dose‐depth distribution measurement with 200 MeV protons has been performed at the Proton Medical Research Center of the University of Tsukuba. The effective‐source size was approximately 4 cm, the source‐to‐surface distance (SSD) was 250 cm and the Spread Out Bragg Peak (SOBP) modulation range was 5 cm, between 15 cm and 20 cm depths. A cylindrical Gold marker with 0.3 cm length and 0.12 cm radius has been placed, both in horizontal mount and vertical mount, at 14 cm and 17 cm inside the water tank. The doses have been measured using a novel imaging plate technique. Analytical SOBP dose calculations have been performed in the y = 0 plane, using □z depth integration steps of 0.04 cm and □x cross‐field lateral steps of 0.01 cm. Results: Analytical calculations showed that the presence of the markers modify quite significantly the Central Axis (CAX) dose distributions of single Bragg peaks. However, this dose perturbation is washed out almost entirely when employing the SOBP technique using multiple modulated Bragg peaks. In such case, the CAX dose increases behind the markers by less than 1% and decreases by less then 3% at the distal end of the SOBP plateau. The experimental measurement of the CAX dose distribution did not observe a significant effect, which confirms the theoretical prediction. Conclusion: This work indicates that it should be safe to use small Gold markers in the proton therapy of mobile tumors employing SOBP techniques. The investigation will be continued using various marker sizes and materials of clinical interest, different proton energies and an ionization chamber.