To Reduce Hot Dose Spots in Craniospinal Irradiation: An IMRT Approach with Matching Beam Divergence

Abstract

Purpose: In craniospinal irradiation (CSI), hot/cold dose spots are commonly seen with two overlapped fields to cover the spinal cord due to different beam divergences. The purpose of this study is to develop new techniques to reduce/eliminate the hot/cold spots and achieve more uniform dose coverage in the spinal cord and brain.

Materials and Methods: A new approach to reduce the effect of beam divergence was investigated. In the new method, two IMRT fields with beam-divergence match were employed to compensate the dose inhomogeneity due to different SSD. Based on a phantom torso, plans were created for the new IMRT method and compared with the conventional technique.

Results: Both new techniques can improve the dose homogeneity of spinal cord. When normalizing the mean dose to 180 cGy, the minimum dose is approximately 168 cGy for all three plans; however, the maximal cord doses are quite different: 237, 204 and 201 cGy for the conventional, 4-field and IMRT plans respectively. The maximal body dose is 269, 214 and 216 cGy and the volume received a dose >200 cGy is 128, 78 and 42 cm3, respectively.

Conclusion: The new IMRT technique with matching beamdivergence has been developed for CSI to effectively reduce hot/ cold spots and improve the dose uniformity in the spinal cord. The two-field IMRT technique has the greatest potential and is feasible to be implemented clinically. The remarkable improvement in dose coverage with the beam-divergence matching techniques warrants further studies with more patient data.