Dosimetric Comparison of Two Craniospinal Radiotherapy Techniques for Treatment Optimization

Introduction: Orthogonal radial fields are those in which the central axes are perpendicular to each other. An example of these orthogonal fields is the set of craniospinal orthogonal fields that are used for radiotherapy of medulloblastoma. Craniospinal radial fields consist of two parallel-opposed fields for brain exposure and one or two posterior spinal fields for spinal cord exposure. The main problem in using these combinative fields is the overlap of radial fields, where they adjoin. Therefore, adjusting radial fields in craniospinal radiotherapy is of remarkable significance and can outstandingly affect the reduction of the side effects due to radiotherapy. In doing so, two different setups were used for craniospinal radiotherapy, and by using dosimetry in each adjustment in the junction region between brain and upper spine fields and in organs at risk, the results of the two adjustments were compared. Materials and Methods: Each one of these two setups was separately performed on a Rando phantom. In the first setup, the arrangement of radial fields was performed without the rotation of the treatment bed and the collimators of the brain fields. In the second setup, the arrangement of radial fields was performed using the rotation of the treatment bed and the collimators of brain fields. For dosimetry, GR-200 TLDs were used. For radiotherapy, a varian linac (2100 C/D Model) was used. Results: The results of dosimetry in the brain CTV, junction of brain and upper spine fields, thyroid and heart in the first setup were equal to 105, 168, 46 and 44 cGy, respectively, and in second setup, 106, 140, 48 and 44 cGy, respectively. Absorbed dose to the testes in both setups was negligible.  Discussion and Conclusion: The results of dosimetry in both setups showed that angling the bed and the collimators for the brain fields prevents the overlap of radial fields and reduces the side effects due to radiotherapy.