Long-term "protective" effect of aromatase inhibitors on the endometrium of postmenopausal breast cancer patients

Background: Decrement of endometrial thickness was recorded following short-term aromatase inhibitor treatment in breast cancer patients previously treated with tamoxifen. It is necessary to verify if long-term aromatase inhibitor treatment can maintain this phenomenon. Methods: Prospective long-term comparison of the last ultrasonographic endometrial thickness measurement taken before discontinuation of long-term tamoxifen treatment in 64 postmenopausal breast cancer patients, with further repeated measurements, performed following administration of aromatase inhibitors. Results: There was a significant decrement of endometrial thickness, following 36.5 ± 15.7 months of tamoxifen treatment, from a mean value of 8.7 ± 5.2 mm, measured at the last ultrasonographic measurement performed before discontinuation of tamoxifen treatment, down to a mean value of 6.2 ± 4.6 mm, measured following 5.3 ± 4.8 months of aromatase inhibitor therapy (P < 0.001). Further ultrasonographic studies revealed the same significant trend. In the first ultrasonographic study performed during aromatase inhibitor treatment, five (7.8%) patients demonstrated a significant increase of endometrial thickness. Hysteroscopy revealed a benign endometrial polyp in three patients and atrophic endometrium in the other 2. In 35 patients (54.7%), endometrial thickness was reduced following the administration of aromatase inhibitors and in 24 patients (37.5%) there was no change in endometrial thickness. With longer duration of aromatase inhibitor therapy, more patients showed decrement of endometrial thickness. Conclusions: Reversal of endometrial thickening induced by long-term tamoxifen treatment in postmenopausal breast cancer patients is maintained throughout long-term aromatase inhibitor treatment.