DOSIMETRIC EVALUATION OF TOTAL SCALP IRRADIATION USING A LATERAL ELECTRON-PHOTON TECHNIQUE

Purpose: To evaluate the radiation dosimetry of a new technique for to tal scalp irradiation. Methods and Materials: A treatment technique de scribed 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 th e midsagittal plane is irradiated by parallel-opposed lateral photon f ields. A wax bolus is used to build up skin dose and to protect the br ain from electron dose. The dose distribution and dose-volume histogra ms were evaluated for different field arrangements using a 3-dimension al treatment planning system. After modifying the technique, in-vivo t hermoluminescent dosimetry were used to evaluate the dose distribution s for the first two patients. Results: To compensate for the lack of d ose from the opposed photon field at the junction, the technique was m odified using overlapped fields instead of abutting fields. A field ov erlap of 3 to 4 mm between the electron and photon fields was found op timal. 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 prescribed dose in the region of abutment. Results of the 3-d imensional dose calculation were supported by in-vivo thermoluminescen t dosimetry on two patients. Conclusion: On the basis of computer dose calculations and in-vivo dosimetry, Akazawa's technique for scalp irr adiation can be improved by using a 3 to 4 mm overlap of electron and photon fields. This modified technique is practical and produces clini cally acceptable dosimetry.