“How long does it take for radiation to shrink a tumor?”

Depending which type of tumor such as lymphoma. It will shows response within a week. For solid tumor such squamous cell carcinoma within 2 weeks, sarcoma may take several months or may not shrink, but the patient feels better.

Response to radiation depends on the rate of the growth of the cancer cells. Rapidly growing cells are killed faster than slowly growing ones. That is why tumor shrinkage depends on the nature of the cell and the observed initial growth rate. On the average, shrinkage will be notified between weeks to months.

Very high doses of radiation are required to make a malignant tumor shrink. Radiation therapy treatments are usually given 5 days a week for 6 to 9 weeks. Tumor regression usually begins after 20 treatments, and continues for up to 6 months after radiation.

Thank you for your question. It is quite valid, and usually evokes much concern among cancer patients. However, the answer is not simple.The time for tumor to respond to radiation depends on several factors.1) the type/histology of the tumor, which dictates its sensitivity to radiation therapy.2) the size of the tumor and whether or not the treatment guidelines dictate that the tumor burden/size be reduced by surgery prior to radiation.3) the use of chemotherapy in conjunction with radiation, especially if the chemotherapy agent is a radiosensitizer (i.e. helps radiation to be more effective).4).compliance with radiation as prescribed  i.e. completing the recommended course of treatment, which requires avoiding any treatment breaks, unless absolutely necessary. Thus, the time to response varies due to the factors highlighted above.However, it is reassuring to note that radiation therapy continues to work weeks beyond completion of the prescribed radiation treatment course.This is one of the reasons that your doctor may advise tarrying  6 -12 weeks after completion of treatment to obtain the appropriate imaging study (CT scan, PET scan, MRI etc), in order to ascertain an accurate assessment of treatment response. I hope this response amply addresses your question, and clarifies any lingering uncertainties. Sincerely,Dr. EB