Technical note: dosimetry in the moving gap region in craniospinal irradiation

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.