Proton Medical Research Center of Tsukuba University employs a scattering method to obtain approoriate prooriate (almost flat dose distribution of 16cm in diameter on the treatment table) irradiation field for the proton beam therapy. A simple estimation shows that only 10% of protons impinging into the scatterer can reach the irradiation field. Collimators as well as other devices placed along course of the beam port interrupt the rest of protons. As is well known, energetic protons generate neutrons when they interact with matters. Some portions of these strongly penetrating neutrons cause an unexpected whole body irradiation of patients during the proton therapy. In this report, the neutron dose per unit proton target dose is estimated about 0.006 Sv/Gy for a typical treatment planning (the specific fluence and average energy of protons at the surface of the patient are about 1.1 x 109cm-2/Gy and 135 MeV respectively), and a strategy for reducing them is proposed.

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It is generally recommended that the ionozation chamber must be calibrated in air using 60Co gamma ray beams. In an ionization chamber having cylindrical shape, the ionization cavity may be surrounded by an adequate thickness of build-up cap for any direction. For some ionization chambers having parallel plate shape, however, the cavity is surrounded by an extra scattering material for back and side directions. The extra scattering as a function of back and side extra volume have been measured for 60Co gamma ray beams. The correction factor for the extra scatter effect was given by an approximate equation with good accuracy.

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Recently, Maximum A Posteriori estimation using Expectation Maximization algorithm ( MAP EM )has been widely studied in the image reconstruction of Emission Computed Tomography (ECT). Green (1990) and Lange (1990) proposed the MAP EM methods using the prior based on the Gibbs distribution which assumed a locally correlated Markov Random Field ( MRF). In the MAP EM method, we have to decide the Gibbs parameter which has an influence on quality of reconstructed images. However, it is hard to decide the optimal Gibbs parameter uniquely for a given acquired data set. In this paper, we made a comparative study for the Gibbs parameter about the number of projections and the number of pixels in reconstructed images using the MAP EM method.

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