How Long Does Radiation Work?

Last Modified: April 1, 2007

Dear OncoLink "Ask The Experts,"

After completing 25 external [beam] radiation treatments for a pineal region tumor, how long does the radiation keep working to destroy a tumor in the brain: is it weeks, months, or years? MRI 's have shown [that] my tumor has continued to grow in size, and I was told it could be swelling from the radiation treatment. I understood radiation lasted only as long as the beam was being administered and [that] the tumor should be decreasing in size and not increasing.

Christopher Dolinsky, MD, Assistant Chief Resident in the Department of Radiation Oncology at the Hospital of the University of Pennsylvania, responds:

External radiation therapy performs damage to tumor cells while the beam is turned on. No radiation remains in your body after your last scheduled external beam radiation treatment. However, the way radiation causes tumors to die is by scrambling the DNA in the tumor cells. Once their DNA is damaged, tumor cells can no longer divide and replicate, and thus the tumor often shrinks. Damage to DNA and cellular replication can take a while to occur, up to weeks after the course of radiation is delivered. Thus, even though the beam isn't still "on", the radiation continues to damage tumor cells for a few weeks following the end of the scheduled radiation treatments. This is why doctors often wait a few months after radiation to re-image a tumor with CT scan or MRI.

Unfortunately, unlike tumor cells, normal tissues can show that they were damaged by radiation many months and even years after radiation is delivered. When tumors don't shrink in size, we sometimes wonder whether that is because the tumor continues to survive or because we are simply visualizing scar tissue formation. Radiation therapy can cause swelling of tumors in the short-term (weeks to months), but this generally doesn't persist for years following radiation. Sometimes the only way to know for sure is to biopsy the abnormality. It would be unusual, however, for scar tissue to rapidly increase in size following radiation therapy.