••• Chapter 49^ Radiation Therapy, Chemotherapy, Immunotherapy, and Tumor Markers^485
❍ What is a cell survival curve?
It is the relationship between the fraction of cells surviving and the dose of radiation delivered.
❍ In the treatment of malignancy, how is radiation therapy delivered?
Radiation therapy is usually delivered in three ways:
(1) Teletherapy (external beam).
(2) Brachytherapy in which the source is placed within or close to the organ being treated. (interstitial or
intracavitary treatment).
(3) Intracavitary radioisotopes (radioactive chromic phosphate 32P).
❍ Define the term “Growth Delay.”
Growth delay refers to the amount of time following irradiation during which the tumor regrows to the size it was
before it was exposed to radiation.
❍ At which point in the cell cycle are cells most sensitive to radiation?
Cells are more sensitive to radiation during late G 1 and late G 2 phases and more resistant during early G 1 and late S.
❍ What defines “radiosensitivity”?
The response both in terms of degree and speed of regression of the tumor to irradiation.
❍ What four variables influence the differences in the radiosensitivity of tumors?
(1) The ability of cells to repair radiation damage (repair), the degree of hypoxia a cell can tolerate (reoxygenation),
the proportion of clonogenic cells (repopulation), and cellular (redistribution).
(2) The quality of radiation.
(3) The temperature of tissues.
(4) The presence of various drugs.
❍ Do higher doses of irradiation produce better tumor control?
Yes. Higher doses of irradiation do produce better tumor control as demonstrated by numerous published dose-
response curves. For every increment of radiation delivered, a certain fraction of cells will be killed. However,
higher doses do result in greater toxicity to normal tissues.
❍ What radiation dose is typically used for subclinical disease in squamous cell carcinoma or adenocarcinoma
of the cervix or endometrium?
Doses of 4500 to 5000 cGy will result in control of local disease in over 90% of patients.
❍ What doses of radiation are required for microscopic disease of the cervix and endometrium?
Doses in the range of 6000 to 6500 cGy for epithelial tumors.
❍ What is meant by “subclinical disease?”
Subclinical disease refers to deposits of tumor cells that are not microscopically detectable but which can, if
untreated, progress to clinical disease.