Short communication An isocentrically mounted stand for total body irradiation

The purpose of this study was to construct a stand to support a patient for total body photon irradiation and to expedite the set-up and treatment by rotating the stand. As in other isocentric treatments, the midline dose is impacted less by source-to-skin distance variations. The method of immobilizing the patient is described. A 10 mm lucite plate is supported in front of the patient to increase skin dose. A matrix of holes in this plate serves to index the location of blocks used to shield the lungs. The dosimetry of the set-up is described, as is the production of tissue deficit compensators. The results of phantom studies and in vivo thermoluminescent dosimetry measurements are presented. Experience in total body irradiation (TBI) was gained with opposed 60 Co sources, each 3-5 m from the patient, with low midline dose rates and treatments given in a single dose (1). As the procedure became more common, linear accel- erator beams were used and sometimes the treatment was fractionated, with only an antero- posterior (AP) or posteroanterior (PA) field treated at a single session. It is now customary to treat all fields during each fraction and to attempt to obtain a uniform dose (i10%) along the line formed by the intersection of the coronal and sagittal planes. In the apparatus described here, since the patient is rotated on an axis close to the mid coronal plane, there is no need to measure target-to-skin distance in the AP and PA set-up and independently calculate the sum of these to determine the dose at the midplane. In the technique described here, the dose at the midplane is defined by the product of the ''in air'' axis dose and a tissue-air ratio determined by the integration of scatter-air ratios over the coronal section of the patient. To accomplish this, a TBI stand was mounted on a bearing, allowing manual rotation of the patient to obtain the benefits cited.