The volume of tissue irradiated to a particular dose has been an interesting topic in radiation therapy. However, the shapes of the volume of the isodose surfaces in most of the situations are irregular and hence it is difficult to evaluate their volumes using geometrical methods. The intraluminal application of Ca-oesophagus using caesium-137 pellets presents an isodose surface, the volume of which can be evaluated or computed using basic geometrical principles. The shape of the volume is usually an elongated cylinder with its length larger than its radius and the tip of the cylinder convex in shape. The volume is obtained by a three-step process. In the first step, the ends or the tips of the volume are found. In the second step, the areas of the transverse sections of the slices at regular intervals are computed. In the third step, the areas of the slices are geometrically integrated and the volume enclosed by an isodose surface is obtained. The volume (cm3) of a particular dose rate (DR) evaluated for three source trains containing 7, 8 and 9 sources is given as V - a (DR) p where a and 13 are constants. The relationship obtained in the present study between volume (cm3) and K/D, where K is the total air kerma in mGy per hour at one metre and D is the dose rate in Gy/hr, is given as V - 162.758' (K/D)1.6788'.Thus, a method which gives the absolute volumes enclosed by the isodose surfaces of the intraluminal applications is developed and presented.

Comparison of dose distribution around shielded region using customized concave tray and flat tray with non-divergent and divergent blocks is carried out for a telecobalt and a dual energy linear accelerator. The dose gradient pattern around shielded region with single divergent block planned at central axis and placed on concave tray are similar to that of divergent blocks planned at various off-axis regions. Similar results are also observed for non-divergent block on concave tray with a marginal increase in dose-gradient width compared with divergent block In case of non-divergent block placed on a flat tray, as block moved away from the central axis to off-axis distance, there is a systematic increase in dose-gradient pattern of 80% - 20% at a depth of 5.0cm. The symmetric dose gradient obtained by concave tray not only helps in getting satisfactory dose distribution around shielded region, but also helps in reducing number of divergent blocks.

A three dimensional (3D) treatment planning system was installed at Apollo Cancer Hospital, Chennai, India in 1995. This paper gives a short description of the system including hardware components, calculation algorithm, measured data requirements and specific three dimensional features. The concept and the structure of the system are shortly described. The first impressions along with critical opinions and the experiences gained during the data acquisition are mentioned. Some improvements in the user interface are suggested. It is emphasized that alt5ough a 30 system offers more detailed and accurate dose distributions compared to a 2D system, it also introduces a greatly increased workload for the planning staff.

The accelerator structure and the magnetron of the 4 MV linear accelerator which was installed at the Regional Cancer Centre, Trivandrum became faulty and the unit went out of operation. The unit has been replaced with the same components and the beam parameters were measured with a radiation field analyser. The measured data were analysed for phantom scatter factor, collimator scatter factor, percentage depth doses, beam profiles both for open fields and wedged fields, off-axis ratios, and wedge output factors, and compared with the standard values and the ones which were in use. It was found that the values which were measured were within acceptable limits with the standard values. The values which were in use with the old guide and magnetron and the measured ones with the new guide were found to vary beyond acceptable limits for some parameters.