Abstract PO5-04-12: Comparison of chemotherapy efficacy in metastatic lobular vs. ductal breast cancer

Abstract

Abstract Comparison of chemotherapy efficacy in metastatic lobular vs. ductal breast cancer Background: Invasive lobular carcinomas (ILC) are thought to be less chemo-sensitive than invasive ductal carcinomas (IDC), as reflected by lower rates of pathological complete response following neoadjuvant therapy. In the metastatic setting, evidence regarding chemotherapy (Cx) efficacy is limited. Methods: A retrospective review of prospectively collected data for consecutive patients (pts) with metastatic ILC (mILC) treated at a single institution between 2000 and 2023 was included. Pts with mILC were matched on a 1:2 ratio with a cohort of pts with metastatic IDC (mIDC) by age, era of diagnosis and initial metastatic burden. Primary outcome was efficacy of Cx in mILC vs. mIDC pts as measured by time to next treatment - TTNTc (months (m) between start date of first & second Cx). Key secondary endpoints included differences in efficacy of endocrine therapy between the two groups (as measured by TTNT and outcomes in pts with endocrine resistance (EnR) as defined by ESMO consensus guideline) and overall survival (time from diagnosis of metastatic breast cancer (BC) to death or last follow-up [OS]). Results: 376 pts were included, mILC (122) and mIDC (254) with median age 64y and 62y, respectively. A significantly higher proportion of mILC pts had hormone receptor positive/HER2 negative or lower proportion of triple negative disease (p< 0.0001). Compared to mIDC, mILC pts were more likely to have de novo disease (29% vs. 18%; p=0.02), lower grade (p< 0.0001), received endocrine therapy only (p=0.03) and were less likely to have visceral disease at diagnosis or any time after metastatic diagnosis (20% vs. 45% p< 0.0001, 35% vs. 51% < 0.0004, respectively). Median time to commencing chemotherapy from metastatic diagnosis was longer in mILC pts (5.8 v 2.0 m, p=0.03). There was no significant differences in TTNTc for patients with mILC and mIDC except for those with visceral disease at diagnosis (Table 1). There was no significant difference in TTNTc by histological subtype of mILC (p=0.46) or by choice of first line chemotherapy used (p=0.91). In the entire population, no significant OS difference observed in mILC and mIDC (Table 1). BC outcome was assessed in pts receiving endocrine therapy as first-line treatment (mILC 65, mIDC 102). There was no difference in TTNT (mILC 14.6m vs. mIDC 14.8m). There were no differences in incidence of EnR between the groups. OS was shorter in mILC pts with primary EnR. The presence of primary, secondary or no resistance within each histological group was significantly associated with OS (Table 1). Conclusion: Pts with mILC have a longer time to commencing chemotherapy, but efficacy of chemotherapy is similar to pts with mIDC; irrespective of lobular histologic subtype, first line chemotherapy regimen used and sites of disease, except for those mILC with visceral disease. There was no significant difference in OS between mILC and mIDC pts treated in this single centre study. Efficacy of endocrine treatment when given as first line treatment was similar in both groups with no significant difference when pts were evaluated by presence or absence of endocrine resistance, except for primary EnR, but pt numbers were small in this group. Our results demonstrate equivalent chemotherapy efficacy in pts with mILC as compared to mIDC.