It has long been recognised that incidental irradiation of bone sites with relatively low doses can reduce the likelihood of bone metastases developing in those sites. Examples are seen with older techniques for postoperative breast cancer, which included the internal mammary lymph node chain and resulted in radiation to the sternum and an exit dose to the dorsal spine. A marked reduction in later incidence of dorsal spine metastases was seen in these patients (1,2). Similarly, a more recent study in prostate cancer comparing 54 patients having prostate alone treated with 56 having prostate and regional lymph nodes irradiated demonstrated a significant reduction in the number of pelvic bone metastases in the cohort having nodal irradiation (3). A similar effect has been demonstrated with low-dose prophylactic radiotherapy delivering a single dose of 8 Gy to the pelvic girdle prior to prostate radiotherapy with none of the 32 patients in a pilot study having pelvic bone metastatic relapse (4). More extreme examples are seen after hemibody radiotherapy or systemic strontium (89Sr) administration. Radiation Therapy Oncology Group (RTOG) 8206 was a phase III study evaluating the added benefit of hemibody irradiation added to local radiotherapy for a painful bone metastasis (5). Progression of bone metastases within the hemibody was seen in 19% of those who received hemibody irradiation and 26% of those who did not. In the TransCanada trial of strontium having a similar phase III design randomly comparing a cohort of patients having local radiotherapy for bone pain to receive strontium (6). The probability of being free from new sites of pain in the strontium group was 58.7% compared to 34% in the control group having just local radiotherapy.
