We have constructed a computer controlled translational couch to administer total body irradiation reproducibly and safely. The system has replaced the previous stationary anterior‐posterior technique in our institution and 30 plus patients have been treated with it so far. In this technique, patients comfortably lie on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in an 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 our design, the couch velocity is continuously updated to compensate for machine dose rate fluctuations. 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 a special challenge for dosimetry calculations. A dosimetry parameter is introduced that converts the moving beam output to the fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been developed that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Multiple point dose measurements within a phantom confirmed the validity of the calculation method.PACS number(s): 87.53.–j, 87.66.–a

To minimize the risk of overlap of adjoining orthogonal fields in craniospinal irradiation, a skin gap is usually maintained between the caudal margin of the brain fields and the cephalic margin of the spine field. The moving gap technique (feathering) is commonly used to improve the dose in the gap region. Using Kodak XV ready-pack films and a polystyrene phantom, the dosimetry in the moving gap region at the depth of spinal cord for different gap sizes and feathering step sizes was studied. The dose profiles in the moving gap region with half-beam block set-up were also measured. Our results show that the combination of a small skin gap and a larger feathering step size results in a higher dose in the moving gap compared with that with identical gap and feathering step sizes. For example, with a 0.5 cm gap and 1 cm feathering step size, the dose in the moving gap region at depth of 5 cm ranges from 82 to 88% of the dose to the brain field. The corresponding dose range with 1 cm gap and 1 cm feathering step size is about 68-73%. With no gap and 1 cm feathering step, a dose range of 92-98% is achieved in the moving gap. The slightly steeper penumbra of a half-beam block used in the brain fields does not result in any dosimetric advantage in the moving gap compared with the conventional set-up when feathering is employed.