Craniospinal irradiation in the supine position: a dosimetric analysis.

Abstract

Background: The prone has been considered a standard position for treatment in craniospinal irradiation (CSI). However, in some circumstances, it is unfeasible, necessitating a supine position. Objective: To analyze the distribution of radiation dose of CSI in the supine position, using a conventional technique, and to compare the radiation dose at the junction of cranial fields and upper spine field among different techniques. Materials and methods: Fifteen patients underwent computed tomography (CT) simulation in supine position for CSI. A total dose of 36 Gy was prescribed to the entire neuraxis. Treatment plan for conventional CSI (technique A; skin gap 10 mm without collimator-rotation of cranial fields) was calculated for median dose to the brain, spinal axis target volume, and normal organs. Three other techniques with collimator-rotation of whole brain fields (technique B, C, and D with skin gap of 2, 5, and 10 mm) were applied and compared for the radiation doses at the cervical junction. Results: Conventional CSI (Technique A) resulted in the median radiation dose (D50%) to the brain and spinal axis of 3,691 (2.5% overdose), and 3,812 cGy (6.0% overdose), respectively. The radiation doses to normal surrounding organs were within tolerable dose ranges. However, thyroid gland received substantial median dose, 2,725 cGy. Concerning the D50% at the cervical junction, techniques A, B, C, and D resulted in 107.7%, 112.9%, 103.1%, and 91.5% of the prescribed dose, respectively. The minimum radiation dose (D99%) were 3,285, 3,392, 3,275 and 2,994 cGy with techniques A, B, C, and D, while the maximum radiation dose (D1%) were 4,252, 4,556, 4,248, and 3,775 cGy, respectively. Conclusion: The supine position for CSI, using our conventional radiation technique, provided an acceptable median dose to the brain, spinal axis, and normal organs. Conventional CSI and technique C showed a better isodose distribution.