Whole breast radiotherapy after tumor lumpectomy is based on the premise that that the breast cancer recurrence rate is re-duced through the elimination of residual cancer foci in the remaining tissue immediately adjacent to the lumpectomy site and occult multicentric areas of in situ or infiltrating cancer in remote areas of the breast.The relevance of remote foci to ipsilater-al breast failure rates after breast conserving treatment is debatable,because65%～100%of recurrences develop in the same quadrant as the initial tumor.This has led several investigators to question whether radiotherapy must be administered to the en-tire breast.An experimental approach of partial breast irradiation using an accelerated hypofractionated treatment scheme,has been e-valuated at a number of centers.That is,radiotherapy is directed to the lumpectomy cavity plus a margin(generally1～2cm)of normal breast tissue through the use of intra-operative electrons,intra-operative low energy photons,interstitial brachytherapy,the MammoSite breast catheter,or external beam intensity modulated radiotherapy.In this lecture,clinical trials of accelerated par-tial breast irradiation from the United States and Europe will be reviewed. Whole breast radiotherapy after tumor lumpectomy is based on the premise that that the breast cancer recurrence rate is re-duced through the elimination of residual cancer foci in the remaining tissue immediately adjacent to the lumpectomy site and occult multicentric areas in in situ or infiltrating cancer in remote areas of the breast.The relevance of remote foci to ipsilater-al breast failure rates after breast conserving treatment is debatable, because65% -100% of recurrences develop in the same quadrant as the initial tumor. This has led several investigators to question whether An radiotherapy must be administered to the en-tire breast. Ann experimental approach of partial breast irradiation using an accelerated hypofractionated treatment scheme, has been e-valuated at a number of centers. is radiotherapy is directed to the lumpectomy cavity plus a margin ( generally 1-2 cm) of normal breast tissue through the use of intra-operative electrons, intra-operative low energy photons, i nterstitial brachytherapy, the MammoSite breast catheter, or external beam intensity modulated radiotherapy. This lecture, clinical trials of accelerated par tial breast irradiation from the United States and Europe will be reviewed.