How important is adequate radiotherapy for the long-term results of breast-conserving treatment?

The influence of radiotherapy dose parameters on longterm local control was studied in 134 clinical stage I-II breast cancer patients treated by tumorectomy and telecesium therapy during the 1960s. Recurrence in the breast was more frequent in patients receiving 75 Gy or less to the tumor bed (31% versus 15% for higher doses, p less than 0.05), and weekly dose rates of less than 8 Gy/week were associated with increased local-regional failure (30% versus 15% for higher weekly doses, p less than 0.01). The majority of local-regional failures were observed in the 53 patients having "inadequate" total or weekly radiotherapy doses (18/53, 34% versus 9/81, 11%, p less than 0.001). The importance of adequate radiotherapy applied to both older patients, as well as those younger than 40, who appear inherently to have a higher risk of local relapse. The mean interval to recurrence was markedly longer for inadequate radiotherapy (111 months, versus 75 months for adequate treatment), so that significant differences in local control became apparent only after 5 years. These results underscore the importance of radiotherapy technique in breast-conserving treatment, and suggest that an excessive reduction in the intensity of radiotherapy may result in an inordinate number of late local failures.