Use of a non-coplanar half-beam block on the lower spinal field to decrease the maximum bowel and cumulative dose in craniospinal irradiation.

Abstract

2039 Background: To develop and compare a non-coplanar half beam block technique that can be used in both prone and supine treatment positions with conventional beam matching for craniospinal irradiation (CSI) in order to decrease the maximum cumulative dose and dose to the bowel, while maintaining the therapeutic dose to the spinal axis. Methods: Ten treatment plans from five patients who underwent CSI were analyzed. The bowel was contoured en bloc for each patient on their simulation cat scan. Two different geometric techniques for each patient were planned and analyzed. The first technique consisted of the conventional method for CSI utilizing two coplanar beams to cover the entire spinal axis. The other technique used a non-coplanar half beam block on the lower spinal beam to exactly match the upper spinal beam’s divergence. Four “featherings” between the two spinal beams for each technique were still necessary to minimize under and overdosing which occur at abutting beam fields. Maximum doses for the plan and the bowel were compared between the two techniques on the same patient. Results: The maximum bowel dose was decreased between 10 to 35 percent when the non-coplanar half beam block was used. The maximum doses for the conventional technique were 5 to 35 percent higher than the plans using a non-coplanar half beam block. The homogeneity of the dose to the spinal axis was not altered with the use of the non-coplanar half beam block. Conclusions: Use of a non-coplanar half beam block to match the two spinal fields in craniospinal irradiation significantly reduces the maximum dose to the bowel and of the entire plan possibly resulting in reduced gastrointestinal toxicity while maintaining therapeutic dose to the spinal axis.