Regression of a metastatic lung mass after receiving whole brain irradiation: Can the abscopal effect cross the blood-brain barrier?

An abscopal effect refers to distant tumor regression after localized irradiation. The mechanism is thought to be via local radiotherapy triggering the immune system which then attacks distant tumor sites. Only several cases of solid tumors with an abscopal effect have been reported in the past few decades, and whether an abscopal effect can cross the blood-brain barrier is still unknown. Herein, we present a female patient who was admitted due to unsteady gait and dyspnea. Chest X-ray (CXR) and chest CT showed a huge lung mass with left lung collapse and multiple liver masses. Brain MRI also showed multiple brain metastases. A bronchoscopic biopsy proved metastatic adenocarcinoma from colorectal cancer based on immunohistochemical staining. She therefore received 30 Gy (10 fractions) for the brain metastases; however, she refused to receive any systemic therapy for her lung mass and asked for hospice care. She was therefore transferred to a nursing home and was lost to follow-up. She returned 2 months later due to right pneumonia with fever and productive cough. Surprisingly, her left lung mass had markedly regressed even though she had not received any additional systemic anticancer therapy. This may be the first case of an abscopal effect after receiving whole brain irradiation for brain metastases.