30. A patient with breast cancer was given a dose of radiation along with chemotherapy and was apparently cured of the tumor. After five years, a tumor was noticed in the patient’s lungs, but the doctors confirmed that it was derived from cells of the mammary gland.
    The following possibilities were suggested by the doctor.
    A. Bacterial infection, after radiation, led to development of the tumors in the lungs.
    B. Migration of residual chemo-resistant cells from the mammary gland resulted in tumors in the lungs.
    C. Epithelial-to-mesenchymal transition had occurred in the lungs.
    D. Cells in the lungs were induced to become a tumor after chemotherapy and from factors secreted by mammary cells:
    Which of the following is correct?
    (1) B and D (2) Only B
    (3) A and B (4) A and C

The correct explanation for the patient’s lung tumor derived from mammary gland cells is:

(2) Only B

Explanation:

• The lung tumor originated from mammary gland cells, indicating a metastasis from the breast cancer to the lung.

• Metastasis occurs mainly through the migration of residual chemo-resistant cancer cells that survive initial treatment and relocate to secondary sites such as the lungs.​

• Bacterial infection after radiation (A) is not a cause of metastatic tumor development.

• Epithelial-to-mesenchymal transition (EMT) (C) is a process cancer cells undergo to gain migratory capacity, but EMT happens in the primary tumor cells, not induced in the lungs independently.

• Induction of tumors in the lungs by factors secreted by mammary cells (D) is not a recognized mechanism of metastasis.

The primary mechanism supported by scientific evidence is that residual chemo-resistant breast cancer cells migrated to and colonized the lungs, causing metastatic tumors.​