Improvements in technique for whole-brain irradiation

Abstract

1539 Background: Brain metastases are commonly addressed with whole-brain irradiation, using unblocked opposed lateral fields, delivering 30 Gy in 10 fractions via 6 MV photons prescribed to midplane, with the isocenter set in the center of the cranial contents. A frequent result in long term survivors is permanent partial epilation in a strip of skin along the skull apex, a so-called “reverse Mohawk”. The object of our study was to determine if simple measures would be able to decrease radiation dose in this region at risk. We also investigated the dose delivered to the lens. Methods: A 6-cm wide, 4-mm thick strip of scalp on the top of the head, and bilateral lenses were contoured using the ADAC treatment planning system. Data from five patients undergoing CT simulation for brain irradiation were used for dose measurements. We investigated the dosimetry with an open-field whole-brain set up (WB), with a 1-cm inferior margin below all brain tissue, whole-brain set up with a scalp block drawn 1-cm external to the inner table of the skull (WBB), whole-brain set up with half beam (HB), with the isocenter at the inferior border and whole-brain set up with half beam with the same scalp block (HBB). To verify relative doses to regions of the scalp, dose was measured at representative points in a phantom, using thermoluminescent detectors (TLD) placed under a 3-mm slab of bolus to match the depth of hair follicles. Results: The percent of the scalp strip receiving > 90% of the prescribed dose for each set up was WB 16.4%, WBB 9.5%, HB 12.8% and HBB 4.8%. Dose to the whole brain was not affected. TLD measurements with the phantom confirmed that the top of the scalp received 22% more dose than the sides of the head. The percentage of the total dose delivered to the lenses with each technique was WB 78.6%, WBB 20.5%, HB 80% and HBB 15.1%. Conclusions: Dose to the strip of scalp at risk for permanent epilation from whole-brain irradiation is reduced by using a half-beam technique, and conformal blocking 1-cm external to the inner table of the skull. Both of these changes in technique also reduce lens dose while ensuring that the whole brain is adequately irradiated. This approach prevents development of the “reverse Mohawk” and can improve quality of life. No significant financial relationships to disclose.