Japanese Journal of Radiological Technology

Currently, CT images are being used, namely for disclosing internal body structures ; but its great potential for radiation therapy treatment planning needs further exploitation. A new program based on three dimensional display combined with the isodose curve superimposed is presented. A VARIAN V-360 was used as the whole body CT scanner and an NEC THERAC-III system for radiotherapy treatment planning. The new soft-ware provided prommissing applicability since : 1. On the two-dimension image obtained from HARD-COPY image, relevant display of isodose curve along with the irradiation field, filter used, that is, filter wedge number, direction, and presence or absence of block was supplied. 2. Better image was made available by the three dimensional display of the trunk contour. 3. Higher accuracy for radiation therapy as a three dimension isodose curve was provided. 4. Greater facilities contributed by the various optional section cut displayed along with the isodose cuve.

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The more advanced treatment planning system than the previous reported paper (1), (2) for conformation technique were established. The system were expanded the capabilities of the faster calculation for the requirement of day-to-day treatment planning. Dose distributions of the conformation technique were quickly produced and the calculation time was rapidly improved. Relative depth doses along the beam axis and dose profiles in an asymmetrical fields which applied in conformation technique were measured and comparared with the results of computer calculation. The differences between two modalities were about 1% or less in general, but a certain extent differences were observed in an extreme cases of asymmetrical field. Howevrer, these extreme cases may not be encountered in clinical application with only a few exception. The system can be applied to the treatment planning for conformation technique in daily routine work.

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As the result of the survey of leakage X-ray from the treatment room, unusual high leakage value is found at the entrance to the treatment room. The leakage values at the other measurement locations are less than those expected from a shielding calculation. Layout of the treatment room is an ordinary construction, and no constructional defects are found. As a consequence of tracing the origin of this unusual leakage, it is concluded that neutrons produced in Linac leak from the treatment room through that the maze, and that the main contribution to the measured leakage radiation is that from neutron capture γ-ray in materials composing the maze, the door etc.. For the purpose of reducing the unusual leakage, a reconstruction of the room is performed by reducing an open area at the part of the entrance from the main room to the maze. At the entrance to the treatment room, initial measured X-ray dose equivalent rate is 5.8 mrem/h in the condition of the beam direction vertically downward, the fully closed field and output 4.65 Gy/min, but, after the reconstruction, its value reduces to 1.8 merm/h and measured neutron dose equivalent rate is 3.9 mrem/h in the same condition. The X-ray and neutron leakage values in the fully opened field are less than the above values. From X-ray leakage values before and after the reconstruction, it is deduced that initial neutron leakage dose equivalent rate is about 12 mrem/h in the fully closed field. When a treatment room to set up a Linac with the accelerative energy exceeding 10 MV is designed, it is suggested from the above facts that some plans to reduce neutron leakage through the maze must be taken into consideration. Also, in survey of leakage radiation from the treatment room, not only X or γ-ray but neutron survey is demanded.

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