Dosimetric evaluation of total scalp irradiation using a lateral electron-photon technique

Purpose: To evaluate the radiation dosimetry of a new technique for total scalp irradiation. Methods and Materials: A treatment technique described by Akazawa (1989) has been studied. During each fraction, two electron and two photon fields are treated. While most of the lateral scalp is treated with the electron fields, a rind of scalp close to the midsagittal plane is irradiated by parallel-opposed lateral photon fields. A wax bolus is used to build up skin dose and to protect the brain from electron dose. The dose distribution and dose-volume histograms were evaluated for different field arrangements using a 3-dimensional treatment planning system. After modifying the technique, in-vivo thermoluminescent dosimetry were used to evaluate the dose distributions for the first two patients. Results: To compensate for the lack of dose from the opposed photon field at the junction, the technique was modified using overlapped fields instead of abutting fields. A field overlap of 3 to 4 mm between the electron and photon fields was found optimal. When used with the field junction shift of 1 cm midway through the treatment, this scheme resulted in a dose uniformity of -5% to +15% of the prescrived dose in the region of abutment. Results of the 3-dimensional dose calculation were supported by in-vivo thermoluminescent dosimetry on two patients. Conclusion: On the basis of computer dose calculations and in-vivo dosimetry, Akazawa's technique for scalp irradiation can be improved by using a 3 to 4 mm overlap of electron and photon fields. This modified technique is practical and produces clinically acceptable dosimetry.