In brain cancer, why radiation and not chemotherapy?

Q: I was diagnosed with lung cancer that spread to other organs. Even though the new anti-PD-1 immunotherapy I am receiving continues to shrink these, I was told I now have a couple of spots growing in my brain and need radiation treatments. Can you explain why I will need radiotherapy and not a new chemotherapy?

A. While "systemic therapies" such as immunotherapy, targeted biologic therapies, and chemotherapy work well in most of the body, they don't often work effectively in the brain. This is due to the blood brain barrier, the natural filtering system that keeps impurities from diffusing from our circulatory system into the brain. This is the reason we rarely get brain infections. However, it allows the brain to be a sanctuary site for cancer cells by keeping systemic cancer therapies out.

Radiation, on the other hand, goes right where it's aimed. Doctors who prescribe therapeutic radiation spend at least five years training after medical school and are called radiation oncologists.

There are, broadly speaking, two strategies for treating brain metastases: treatment of the whole brain typically delivered over a couple of weeks, and "radiosurgery," radiation focused at lesions visible on brain imaging. Whole-brain radiation greatly reduces the risk of new lesions growing elsewhere in the brain, but at the expense of side effects, such as worsening memory. Radiosurgery rarely causes cognitive problems, but there is a higher risk of tumor recurrence in the untreated brain tissue, and a risk of inflammation at the treated site that can cause symptoms and require treatment.

The treatment decision is based on patient-specific factors, including the risk of recurrence in the brain, the likelihood that if a tumor came back it would be fatal, the type of cancer being treated, the number and size of lesions, and the tumor's location in the brain.

Stephanie Weiss is a radiation oncologist at Fox Chase Cancer Center.

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