ACTIVE MINIMIZATION OF RADIATION SCATTER DURING BREAST RADIOTHERAPY -MANAGEMENT IMPLICATIONS FOR YOUNG-PATIENTS WITH GOOD-PROGNOSIS PRIMARY NEOPLASMS

Background and purpose. Radiotherapy is used to reverse or prevent loc al tumour growth but is also a carcinogen in its own right. A recent a udit of post-radiotherapy second malignancies in this institution reve aled a striking preponderance of tumours originating near the outside edge of the treatment field. Since this finding suggests the existence of a critical subtherapeutic dose range predisposing to tumourigenesi s, we attempted to define and reduce this radiation scatter dose. Mate rials and methods. We undertook a dosimetric review of 6 MV scatter fr om a linear accelerator in sites matching the putative tumourigenic re gion, and then extended this analysis to patients and tissue phantoms. Results. A wide range of radiation scatter doses was confirmed for ex ample, doses 3 cm from the field edge varied from 1.7 to 22% of the th erapeutic dose depending upon the field parameters, Scatter doses were then assessed in a sample of eight patients undergoing standard breas t radiotherapy, Contralateral breast sites 4-12 cm from the midline re ceived 4-10% of the therapeutic dose, or 200-500 cGy for a 50 Gy treat ment, approximating historical estimates of the tumourigenic range. Th e deep component of this scatter dose from medial field breast irradia tion was reduced 19% simply by replacing the 15 degrees medial tangent ial field wedge with a 30 degrees lateral wedge, Other manoeuvres whic h reduced contralateral breast dose by up to 46% included making the p osterior field edges co-planar and shielding the breast during medial field irradiation. Conclusions. These results suggest that the risk of radiogenic second malignancies could be significantly decreased by ca reful attention to the treatment details, Greater awareness of these m easures may prove particularly relevant to the conservative management of young patients with good-prognosis breast neoplasms such as ductal carcinoma in situ.