Which form of therapy involves the direct application of radioactive substances in close proximity to cancerous tissues?

Brachytherapy is the form of therapy that involves the direct application of radioactive substances in close proximity to cancerous tissues. This technique allows for high doses of radiation to be delivered directly to the tumor while minimizing exposure to surrounding healthy tissue.

Brachytherapy involves implanting radioactive seeds or sources directly into or near the tumor, which can enhance treatment efficacy and reduce side effects compared to other forms of radiation therapy that might deliver radiation from a distance. It is commonly used for various cancers, including prostate, breast, and cervical cancers, where precise targeting of the tumor is crucial for successful treatment outcomes.

In contrast, radiation therapy generally refers to external beam radiation therapy, which delivers radiation from outside the body and may not provide the same level of localized treatment as brachytherapy. Surgery involves physically removing the tumor and is a different approach altogether. Competent therapy is not a recognized term in the context of cancer treatment. Thus, brachytherapy is distinctly the correct answer.