Oncology and Fibroids
A patient is receiving external beam radiation for treatment of metastatic endometrial cancer. The treatment field includes the entire pelvis. Which of the following tissues within this radiation field is the most radiosensitive? (AIIMS Nov 2011)
1. Different tissues tolerate different doses of radiation, but the ovaries are by far the most radiosensitive.
2. They tolerate up to 2,500 rads, while the other tissues listed tolerate between 5,000 and 20,000 rads.
3. Acute evidence of excessive radiation exposure includes tissue necrosis and inflammation, resulting in enteritis, cystitis, vulvitis, proctosigmoiditis, and possible bone marrow suppression. Chronic effects of excessive radiation exposure are manifest months to years after therapy, and include vasculitis, fibrosis, and deficient cellular regrowth; these can result in proctitis, cystitis, fistulas, scarring, and stenosis.
4. Successful radiation depends on
a. the greater sensitivity of the cancer cell compared with normal tissue and
b. the greater ability of normal tissue to repair itself after irradiation,
5. The maximal resistance to ionizing radiation depends on an intact circulation and adequate cellular oxygenation.
6. Resistance also depends on total dose, number of portions, and time intervals. The relative resistance of normal tissue (cervix and vagina) in cervical cancer allows high surface doses approaching 15,000 to 20,000 rads to be delivered to the tumor with intracavitary devices, and, because of the inverse square law, significantly lower doses of radiation reach the bladder and rectum. The greater the fractionalization (number of portions the total dose is broken into), the better the normal tissue tolerance of that radiation dose; hence 5,000 rads of pelvic radiation is usually given in daily fractions over 5 weeks, with approximately 200 rads being administered each day