Morphea of the breast is an autoimmune reaction of the subcutaneous connective tissue which can be triggered by exposure to ionizing radiation. The literature suggests incidence rates of 1:500 to 1:3,000 which, however, do not seem to match the very small number of cases reported.

The aim of the present study was to determine the incidence of morphea following irradiation of the breast in order to generate more evidence about the frequency of this serious and mutilating complication.

Retrospective analysis of patient data who underwent adjuvant radiotherapy in the period 2009-2018 following breast-conserving surgery and who made use of the recommended radiooncology follow-up examinations in 2018. Analysis was done by descriptive statistics.

Of a total of 5,129 patients who had undergone radiotherapy over a 10-year period, follow-up data were available in 2,268 patients. In 2,236 patients (98.6%) the breast had been irradiated using conventional fractionation schemes with a total dose of 50-50.4 Gy; 32 (1.4%) were given a total dose of 40.05 Gy in 15 fractions. During the observation period, 6 patients were diagnosed with morphea (4 unilateral and 2 bilateral) by punch biopsies resulting in a cumulative incidence proportion of 0.26% (95% CI: 0.24-0.28), translating into 1 case for every 378 irradiated patients.

In the case studies reported to date, morphea is described as a very rare complication. In contrast, our data suggest a cumulative incidence of 1:378, which is higher than other authors have estimated. This leads us to suspect that in a large number of patients, morphea is incorrectly diagnosed as an infection (in the early stages) and radiation-induced fibrosis (in later stages).