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Serum samples were taken at baseline, three, and 12\u00a0months. E2 and FSH were measured with routine chemiluminescent immunoassays. E2 and E1 were analyzed after trial completion with a highly sensitive liquid chromatography-tandem mass spectrometry method (LC\u2013MS\/MS) with lower limits of quantification (LLOQ) of 5\u00a0pmol\/L. QoL was measured at baseline and at 12\u00a0months with the EORTC QLQ-C30 and QLQ-BR23 and the Women\u2019s Health questionnaires, and menopause-related symptoms with the modified Kupperman Index.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>Of 100 screened patients 90 completed the trial. Baseline mean LC\u2013MS\/MS E2 and E1 were 12\u00a0pmol\/L (range\u2009&lt;\u20095\u201357) and 66\u00a0pmol\/L (&lt;\u20095\u2013226), respectively. E2 levels measured by immunoassay and LC\u2013MS\/MS showed no correlation. E2 and E1 were completely suppressed by letrozole except for one occasion (E1 11\u00a0pmol\/L at 3\u00a0months). Pain, side effects of systemic therapy, vasomotor symptoms, joint and muscle aches, and vaginal dryness increased during letrozole treatment. A high baseline E2 was significantly associated with increased aching joints and muscles, but not with the other side effects.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusions<\/jats:title>\n                    <jats:p>Letrozole supresses E2 and E1 completely below the LLOQ of the LC\u2013MS\/MS in postmenopausal women. High pre-treatment E2 levels were associated with more joint and muscle pain during letrozole. Automated immunoassays are unsuitable for E2 monitoring during letrozole therapy due to poor sensitivity.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1007\/s10549-023-07054-3","type":"journal-article","created":{"date-parts":[[2023,7,25]],"date-time":"2023-07-25T13:02:02Z","timestamp":1690290122000},"page":"425-435","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":8,"title":["Effects of letrozole on serum estradiol and estrone in postmenopausal breast cancer patients and tolerability of treatment: a prospective trial using a highly sensitive LC\u2013MS\/MS (liquid chromatography\u2013tandem mass spectrometry) method for estrogen measurement"],"prefix":"10.1007","volume":"201","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0749-2955","authenticated-orcid":false,"given":"M\u00e1ria","family":"Faltinov\u00e1","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Leena","family":"Vehmanen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Heli","family":"Lyytinen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mikko","family":"Haanp\u00e4\u00e4","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Esa","family":"H\u00e4m\u00e4l\u00e4inen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Aila","family":"Tiitinen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Carl","family":"Blomqvist","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Johanna","family":"Mattson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2023,7,25]]},"reference":[{"issue":"5","key":"7054_CR1","doi-asserted-by":"publisher","first-page":"423","DOI":"10.1200\/jco.18.01160","volume":"37","author":"HJ Burstein","year":"2019","unstructured":"Burstein HJ, Lacchetti C, Anderson H, Buchholz TA, Davidson NE, Gelmon KA, Giordano SH, Hudis CA, Solky AJ, Stearns V et al (2019) Adjuvant endocrine therapy for women with hormone receptor-positive breast cancer: ASCO clinical practice guideline focused update. 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