Radiation doses to normal tissues during craniospinal irradiation: Improvement of the dose to the eye and lens, dosimetric study

Introduction: Craniospinal irradiation is a very important element in treating medulloblastoma. Every effort should be made to decrease the radiotherapy related side effects, especially with the currently available 3D conformal radiotherapy planning system. Objective: This dosimetric study is aiming to report the results of the analysis of doses received by target volumes and organs outside the target volumes during the treatment of medulloblastoma patients. And also by comparing the doses reaching the eyes and the lens with the use of different shielding methods. Methodology: Ten children with recent diagnosis of high risk medulloblastoma were included. They were subjected to MRI of the brain and spine together with CSF cytology 3 weeks after surgery. They were subjected to fixation, conventional simulation, followed by CT simulation. The scans will be transferred to the treatment planning system. The brain, spinal canal, and different normal tissue were outlined.For the cranial fields, parallel opposed fields were used with the isocenter of the fields in the mid-plan in the center of the brain, and behind the eyes. Sensitive tissues close to the target volume were shielded using either MLCS or blocks.The dose prescribed was 36 Gy/20 fractions for cranial and spinal regions. The maximum, minimum, and mean doses to each anatomic structure were computed using dose volume histograms. All patients gave informed consent. Results and conclusion: The dose of radiation received by the target volume and the organs at risk with the use of our new treatment planning system is nearly identical to other studies. The use of block shielding shows lower doses to eyes and lenses regardless of the position of the isocenter, on the other hand, with the use of MLCs for shielding, the isocenter should be behind the eye not in the center of the brain as this shows lower doses to eyes and lenses. Keywords: Craniospinal irradiation, Dose to the eyes and lenses, Dosimetric stud