[Determination of the field size for a telegamma therapy apparatus]

Proceeding from an analysis of radiation field sizes (by a 50% dose) measured on gamma-beam units under various conditions of irradiation by means of the LSC-2 precision linear dosimetric scanner (Terados production, Sweden), the author recommends verifying the conformity of the sizes of these fields at a preset distance from a source with the readings of digital indication on the unit but not with the sizes of light fields. The divergence of the sizes of radiation fields measured at a preset distance from a source has been shown in the air, in the water at the depth of maximum ionization and in the water at 15 cm depth. In calibration of the diaphragm scale by the geometric field, i.e. by opening up its fringes , unavoidable non-correspondence of the actual sizes of radiation fields to the readings of digital indication is marked; the bigger the field and a source-surface distance the greater this non-correspondence. When measuring in the water at 15 cm depth, this divergence is greater than that in the water at the depth of maximum ionization, and the latter is greater than that measured in the air. It is found more appropriate to calibrate the diaphragm scale by the sizes of radiation fields measured in the air. The author assesses a maximum error in the determination of a field size by the manual shifting of a detector and an error of a dose meter. Basing on the above assessment it is found appropriate to use the precision automatic shifting of a detector in measuring a dose distribution pattern in the beam cross-section.