Optimization of a cord shielding technique for electrons

Large anterior electron fields are sometimes used to irradiate the neck when treating head & neck tumors. To offer a degree of spinal cord shielding, wax bolus, approximately the width of the vertebral bodies, is placed on the immobilization shell. The thickness of the bolus is adjusted so that the radiological depth of the anterior edge of the vertebral bodies is equal to the R80 depth for the energy used. This approach ignores electron scattering. Using a CT study of a thyroid cancer patient, neck contours were generated at 0.5 cm intervals and entered into the Alberta Treatment Planning system. Internal contours for the trachea and vertebral bodies were added and CT information was used for treatment planning purposes. The bolus outline was added as described above, and the dose calculated using a 3D implementation of the M.D. Anderson (Hogstrom) algorithm. The calculation shows that the simple bolus technique described above is inappropriate. The spinal cord is adequately shielded, but the target volume is not covered by the 80% isodose line. Qualitatively, the results can be explained by the lateral scatter non-equilibrium introduced by the bolus. By iteratively adjusting the shape and thickness of the wax bolus and recalculating the dose distribution, we were able to better fulfill the dose prescription. Comparison with measured data shows reasonable, but not perfect agreement. In conclusion, electron beam treatments must be examined closely to ensure that the treatment goals are met. In some cases, treatment integrity may be compromised by incorrect assumptions regarding the nature of the electron transport and dose deposition.