A translational couch technique for total body irradiation

The authors have devised a computer controlled translational couch to administer total body irradiation accurately and safely. In this technique, patients comfortably rest on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient´s dimensions, beam geometry and machine dose rate. In the authors´ design, the couch velocity is continuously updated for possible machine dose rate fluctuations to eliminate dose non-uniformity within the patient. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents special challenges for dosimetry calculations. A dosimetry parameter is introduced that converts moving beam output to fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been implemented that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Comprehensive dose measurements within phantoms confirmed the validity of the calculations method. Further improvements in dose uniformity within the patient by modulating couch velocity with patient thickness and shielding the organs at risk are discussed