Do you send OncotypeDx testing in breast cancers with mucinous histology?

I don't send Oncotype in pure mucinous tumors routinely. NCCN doesn't recommend adjuvant chemotherapy for node negative favorable histology tumors due to their good prognosis. Retrospective studies (Turashvilli et al., PMID 28702894, Wang et al., PMID 30619463) show the majority score low to intermediate. Based on SEER data in the Wang study, all mucinous tumors had low event rates. If the tumor is node + or ductal with mucinous features then you can consider ordering it.

The PlanB data suggested that Oncotype was rarely high in post-menopausal women with low grade, low Ki-67, strongly ER/PR positive tumors, which mucinous cancers often are! I would typically take into account the histology along with those other risk factors (e.g. Ki-67, grade, ER/PR status). If something seemed a little discordant (e.g. PR- and mucinous) or as Dr. @Dr. First Last notes, the cancer was mixed histology or node +, I'd consider sending an Oncotype if otherwise appropriate.

Agree. Most mucinous tumors are low grade and strongly hormone positive, I typically do not send an Oncotype unless the patient has nodal involvement or mixed tumor.