“It’s always important to understand how guidelines play out in a real-world setting,” Kurian said. “Clinical trial participants tend to be highly motivated and may be more likely to adhere to treatment recommendations. But we are interested in outcomes for all our patients – that’s where the rubber really meets the road. And in this context, I was surprised to see that the rate of treatment resumption was so low.”

Ransohoff, Kurian and their colleagues used a Stanford Medicine-maintained research database called Oncoshare that integrates medical records from community and academic medical systems in the Bay Area to identify 215 women who were diagnosed with breast cancers from stage 0 to stage 3 between January 2000 and October 2024 and who later became pregnant. The median age at diagnosis was 33.6 years. Of these women, 161, or 75%, were eligible for hormone therapy and 130 were eligible for breast imaging (women who had both breasts removed as part of their initial treatment were ineligible).

Only a third complete treatment

Forty-eight women with hormone receptor-positive breast cancer never initiated endocrine therapy. Among the 113 who began endocrine therapy, only 36 (32%) completed five years or more. Among the 81 who paused their endocrine therapy for pregnancy (with a median duration of 21 months), 36 (32%) resumed the therapy.

Although the study didn’t address the reasons a person might choose to avoid or truncate endocrine therapy, the researchers point to a variety of possibilities.

“This set of drugs can really impact a patient’s quality of life,” Ransohoff said. “They can cause menopausal symptoms like hot flashes, disturbed mood and sleep, as well as joint pain, which can affect sexual health as well. The people in our study are also going through all the hormone changes and stress that go along with being a new parent.”

Among the 130 women eligible for regular breast imaging to screen for recurrence (those who had not undergone bilateral mastectomy), 87 (67%) resumed imaging after delivery.

“It’s common to think, Oh, I’m done with chemotherapy, now endocrine therapy and ongoing imaging is the easy part,” Kurian said. “But in reality, you’ve finished the sprint; you are not done with the treatment marathon.”

The researchers hope that their study serves as a foundation for larger studies focused on understanding barriers to adherence in breast cancer patients of different backgrounds and geographic regions. They hope to develop a prospective study incorporating a patient questionnaire about treatment expectations, challenges and adherence.

“This is a great opportunity for in-depth discussions with our patients to learn how we can better help them,” Ransohoff said.

“Even in older women, the rates of adherence to five years of endocrine therapy are much lower than we would like – somewhere around 50%,” Kurian said. “These are hard drugs. We are asking them to do hard things. But these women are young, with young families, and we want to be particularly intensive in maintaining their wellness, even during the chaos and life changes of a growing family.”