Why is radiation therapy less effective at the center of a large tumor?

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linear accelerator; external beam radiation therapy — radiation therapy External beam radiation therapy (also known as external beam teletherapy, or long-distance therapy) being delivered using a machine known as a linear accelerator.

Radiation therapy is less effective at the center of a large tumor primarily because those areas are often poorly oxygenated, or hypoxic, due to an inadequate blood supply.1

The effectiveness of ionizing radiation relies on its ability to damage or break cellular DNA, which leads to cell death. This process is significantly more efficient in well-oxygenated tissues than in hypoxic ones. To address this challenge, physicians may sometimes use specific drugs designed to enhance the toxic effects of radiation on these hypoxic tumor cells.1