Biomedical Engineering Reference
In-Depth Information
Fig. 13.18
Cell survival in the presence of dissolved oxygen
(O
2
) and after purging with nitrogen (N
2
).
The most notable of these are sulfhydryl compounds (e.g., cysteine and cystamine),
which scavenge free radicals. Still other chemical modifiers have little effect on cell
killing, but substantially enhance some multistep processes, such as oncogenic cell
transformation. For carcinogenesis or transformation, for example, such biological
promoters can dwarf the effects of physical factors, such as LET and dose rate, on
dose-response relationships.
Chemical radiosensitizers for use in radiation therapy are under investigation.
Some have the potential to specifically affect resistant hypoxic cells, which are com-
mon in tumors. Chemical radioprotectors have been developed for potential mili-
tary use in a nuclear war.
Dose Fractionation and Radiotherapy
The goal of treating a malignant tumor with radiation is to destroy it without dam-
aging normal tissues to an intolerable degree. By and large, normal cells and tumor
cells have comparable resistance to killing by radiation. Thus, other factors must
come into play in radiotherapy. It is found empirically that the most advantageous
results are obtained when the radiation is delivered to a patient in fractions, admin-
istered perhaps over a period of weeks, rather than all at once.
To understand how the fractionation of dose affects tumor cells more adversely
than normal ones in a patient, there are basically four factors to consider at the cel-
lular level: repair, repopulation, redistribution, and reoxygenation. Administering a
dose in fractions with adequate time between applications allows the repair of sub-
lethal damage and the repopulation of tissue cells. These processes generally occur
on different time scales and to different degrees in the normal and tumor cells. The
therapeutic protocol considers optimization of normal-tissue sparing to the detri-
