488 Obstetrics and Gynecology Board Review •••
❍ How can radiation exposure to nursing and health-care providers be reduced or eliminated?
Radiation exposure can be greatly reduced by remote afterloading technology.
❍ How do high-dose rate and low-dose rate delivery systems differ from one another?
Dose rates of 40 to 200 cGy/hour are considered low-dose rates that, in order to deliver clinically useful doses of
1000 to 7000 cGy, must be administered to inpatients over 24 to 144 hours. In contrast, dose rates in excess of
1200 cGy/hour are considered high-dose rates and may be given over several minutes as an outpatient procedure.
In general, 2 to 8 high-dose rate fractions must be administered to approximate the therapeutic ratio of a single
low-dose rate implant.
❍ What is the inverse square law?
The absorbed dose at a given point is inversely proportional to the square of the distance from the source of
radiation. This forms the basis for intracavitary treatment whereby a high dose can be delivered to local tissues
(cervix) with the rapid falloff of dose sparing surrounding tissues (bladder and rectum).
❍ Intracavitary brachytherapy for carcinoma of the uterine cervix traditionally employs the use of an
intrauterine tandem and vaginal colpostats. How are they typically positioned?
The tandem should be in the midline equal distance from the lateral pelvic sidewalls and the vaginal colpostats
symmetrically positioned against the cervix. The tandem should be equal distance from the pubis and sacral
promontory.
❍ What determines the total milligram hours that are indicated for intracavitary brachytherapy for carcinoma
of the uterine cervix?
Several factors determine the total number of milligram hours to be delivered:
(1) The tumor stage and volume that in turn determine the total dose in cGy to be delivered at point A.
(2) The strength of sources employed in the tandem and vaginal colpostats.
(3) The number of insertions.
(4) Whether whole pelvic radiation will be employed.
❍ Which two radionuclides are commonly used in brachytherapy for the treatment of cervical carcinoma?
192 Iridium (T1/ 2 = 74 days) and 137 cesium (T1/ 2 = 30 years).
❍ What additional radiotherapy has been advocated to increase the parametrial dose after conventional
external and intracavitary irradiation?
Interstitial implantation transvaginally or transperineally into the parametrium or cervix with metallic needles
containing^226 Ra,^60 Co, or^137 Cs or with Teflon catheters for insertion of^192 Ir wires or seeds.
❍ In treating carcinoma of the endometrium, what three devices are commonly employed for the delivery of
intracavitary brachytherapy?
Heyman-Simon capsules, afterloading tandem, and vaginal colpostats.
❍ What are typical doses of intracavitary brachytherapy for the treatment of carcinoma of the endometrium?
For preoperative therapy, intracavitary doses of 3500 to 4000 mgh with 2000 mgh to the mucosal vaginal surface.
In patients treated with radiation therapy alone, higher doses in the range of 8000 mgh combined with external
radiation are given. In postoperative irradiation, doses of 1800 to 2000 mgh to the vaginal mucosa are given.