Modification of a superficial X-ray therapy machine for rectal contact therapy. Preliminary results

X-ray therapy of superficial rectal cancers using a hand-held 50 kV contact unit (Philips RT-50) in a technique first described by Papillon had reached a point of widening clinical acceptability when the manufacturer of this equipment discontinued its production. To pursue this endocavitary approach to rectal therapy, technical modifications have to be made to conventional superficial x-ray therapy machines. Advantages over the original Papillon method include remote viewing of the lesion through the proctoscopic cone and a lower radiation exposure for the operator. We have evaluated a Bucky Combination Therapy Unit under conditions in which the operating voltage (65 kV), target skin distance (23.6 cm), and added filtration (0.39 mm Al) were selected in order to match as closely as possible the beam penetration characteristics of the "standard" (Papillon) technique. With this equipment, the thermal characteristics of the tube anode and housing limit the amount of radiation that can be delivered before a "rest period" for the machine is needed. In practice, 3 minutes of irradiation at an exposure rate of 500 R/min can be performed followed by an interval of 3 minutes before irradiation can be resumed.