The effect of an extended source-to-skin distance in the treatment of the spinal field in children receiving craniospinal irradiation.

In this study, we compared a single extended source-to-skin difference (SSD) spinal field and the alternative 2-field gapped approach at 100 SSD on dose to surrounding normal tissues. Five female patients ranging in age from 3 to 20 years underwent computed tomography (CT) simulation for treatment planning of the craniospinal axis, which was treated in its entirety to 36 Gy. For each slice, the clinical target volume (thecal sac and contents), mandible, thyroid gland, esophagus, heart, lungs, liver, and ovaries were contoured. Technique A employed the use of a single posterior spinal field delivered at SSD = 140 cm (140 SSD), and Technique B employed the use of 2 gapped spinal fields using a traditional SSD of 100 cm (100 SSD). Dose-volume histograms (DVHs) were obtained for each organ contoured and for each technique used. In all patients, the average mean dose to all surrounding structures was increased with the use of a single extended SSD (Technique A) when compared to the 2 spinal fields prescribed at 100 SSD (Technique B). The average mean doses to the mandible, thyroid gland, esophagus, and heart were 78%, 19%, 6%, and 16%, respectively, higher with Technique A as compared to Technique B. In addition, the average mean doses to the lungs, liver, and ovaries were 53%, 33% and 69%, respectively, higher as compared to Technique B. However, the clinical target volume at the spinal junction site received a less homogenous dose with Technique B as compared to Technique A. We conclude that although the use of a single-field extended SSD delivered a more homogenous dose to the spine, a higher dose to the ovaries, thyroid gland, mandible, lungs, liver, and heart was seen.