A recent analysis of patient data from Boston USA examined the follow up of 81 patients who had undergone breast conserving surgery for good prognosis T1 breast cancers.
In the group, the average size of the cancer was less than 1 cm in diameter, and the patients had no other high risk features (no lymphatic invasion and no extensive DCIS). Not withstanding the perceived good risk features, the was a local (i.e. in the breast itself) relapse rate of 22% in these unirradiated breasts, with a median time to local relapse of 35 months.
The conclusion of this trial is the same as many other previous studies and is that it is only as safe to practice breast conserving surgery for early breast cancer (as performing total mastectomy) if post-operative radiotherapy to the breast is delivered. A subset of patients, for whom it is safe to leave un-irradiated, is hard to find.