When considering breast cancer and breastfeeding, the discussion must be categorized into history of breast cancer versus breast cancer diagnosed during lactation. The open access Academy of Breastfeeding Medicine Protocol #34: Breastfeeding and Breast Cancer provides a comprehensive review of breast cancer and lactation. For more detailed information and references on specific medications, refer to LactMed, e-lactancia, Infant Risk, or Mother to Baby.

Modern breast cancer treatment involves a multidisciplinary approach to care, including chemotherapy, surgery, radiation, and endocrine therapy.   Young survivors may have undergone some or all of these therapies prior to pregnancy and lactation.

Previous chemotherapy does not preclude future breastfeeding.  However, chemotherapy intended to concentrate in breast tissue to treat breast cancer may result in long-lasting effects on lactational ability.¹ Patients should be referred for support and guidance during pregnancy and in the postpartum period.

A history of a total mastectomy, skin sparing total mastectomy, or nipple sparing total mastectomy precludes breastfeeding on the affected side.  It should be noted that a mastectomy removes 95% or greater of breast parenchyma.² However, due to variation in mastectomy technique, some patients may have more residual functional breast tissue than others.  This may hypertrophy during pregnancy and give the appearance of functionality.  However, no normal lactation should be expected from this breast. A history of partial mastectomy (“lumpectomy” or “segmental mastectomy”) does not preclude breastfeeding in itself.  However, a patient with a history of partial mastectomy for cancer very likely would have undergo post-operative adjuvant radiation therapy which would preclude breastfeeding from that breast (see below).

Radiation therapy affects the functional ability of the breast, the elasticity of the skin, and its effects can continue for years after treatment.³ Limited lactational ability has been reported from a treated breast³ so the more efficacious approach to lactation may be to augment the unaffected breast capacity.

Endocrine therapies may be recommended adjuvantly.  Breastfeeding is contraindicated with use of aromatase inhibitors (AI’s), as they transfer readily into breastmilk and may suppress estrogen in the infant per Infant Risk. As above, SERM’s are also contraindicated with breastfeeding.  The Pregnancy Outcome and Safety of Interrupting Therapy for women with endocrine responsive breast cancer (POSITIVE) trial, which is evaluating the interruption of up to two years of endocrine therapy, will help inform discussions regarding potential for safe periods of interruption for breastfeeding.⁴

As in patients with a history of breast cancer, those women who present with a new diagnosis of breast cancer while lactating in the postpartum period will be managed by a multidisciplinary team.  Because many cancers that present in the postpartum period are biologically aggressive and locally advanced at presentation,⁵ many patients will undergo chemotherapy prior to surgery.

Due to long half lives and high transfer of cytotoxic agents into breastmilk, breastfeeding is contraindicated with chemotherapy for breast cancer.⁶ Some patients may seek to express and discard milk.   However, pumping and discarding milk for 4-6 months of breast cancer chemotherapy regimens carries a high risk of mastitis and other complications during treatment.⁷ Patients should also consider the fact their infant may not want to resume breastfeeding after many months of not breastfeeding and that breastmilk volumes may be decreased by chemotherapy. Breastfeeding intermittently at certain points during chemotherapy may be possible depending on the agent used. Patients should be engaged in shared decision making about weaning prior to initiating chemotherapy for breast cancer.

A patient may undergo upfront surgery and wish to continue breastfeeding from the affected breast. This is not possible in the setting of total mastectomy, skin sparing total mastectomy, or nipple sparing total mastectomy. It is possible in the setting of partial mastectomy; wound complication rates in lactating patients has been reported consistent with those of non-lactating patient.⁸ However, as described above, a patient who undergoes partial mastectomy very likely will require post-operative adjuvant radiation therapy.

As above, we recommend weaning the affected breast and augmenting supply on the contralateral side.

Endocrine therapies may be recommended adjuvantly. Breastfeeding is contraindicated with use of aromatase inhibitors (AI’s), as they transfer readily into breastmilk and may suppress estrogen in the infant per Infant Risk. As above, SERM’s are also contraindicated with breastfeeding.  Regarding adjuvant endocrine therapy during lactation and the POSITIVE trial, see above.