LWBK1006-07 LWW-Govindan-Review November 22, 2011 13:54
Chapter 7•Principles of Radiation Oncology 91Answer 7.5. The answer is C.
Repair explains the shoulder of the radiation survival curve showing that
cells can repair some of the radiation damage. The majority of sublethal
damage repair occurs within 6 hours after irradiation. Redistribution
explains differences in cell-cycle radiation sensitivity, with the mitotic
(M) phase being the most sensitive. Cells gradually increase in resistance
as they proceed through the late G1 and S phases. Repopulation refers to
spontaneous repopulation and induced cell proliferation or recruitment
of cells after irradiation. This may occur in some tumors but occurs less
than that in normal tissues. Reoxygenation explains how a proportion
of hypoxic cells present in a tumor return to preradiation level. Some
tumor cells may reoxygenate after radiotherapy, and the proportion of
the hypoxic cells that was present before irradiation may be seen.Answer 7.6. The answer is C.
Acute effects of radiation usually occur in organs dependent on rapid self-
renewal and are mostly caused by radiation-induced cell death during
mitosis. In the case of nausea and fatigue however, these toxicities are
most likely related to the release of cytokines.Answer 7.7. The answer is E.
The responses of nonirradiated cells appear to be cell-type dependent.
This may be mediated by diffusible substances or by cell–cell contact.
The double-stranded breaks are seen after cells are directly irradiated.Answer 7.8. The answer is C.
A modern LINAC can deliver energies up to 25 MeV. The focal point of
the gantry’s rotation is called the isocenter. The^60 Cobalt teletherapy unit
is a megavoltage machine that relies on radioactive cobalt to produce a
photon beam. Unlike the LINAC, the^60 Cobalt unit is always “on” and
must be kept in shielded position until the beam is needed for treatment.Answer 7.9. The answer is E.
Greater doses of radiation are required for cell killing in hypoxic condi-
tions. The oxygen enhancement is seen more readily during the exponen-
tial portion of the survival curve because of the reduced capacity for cells
to repair sublethal damage under hypoxic conditions. Tumor cells grow-
ing in a physiologic hypoxic condition have reduced capacity to repair
sublethal damage. The OER is the ratio of dose required for equivalent
cell killing in the absence of oxygen compared with the dose required in
the presence of oxygen. The range of OER varies from 2.5 to 3.5. Oxy-
gen must be present at the time of irradiation for oxygen enhancement of
radiotherapy to occur. OER is more important for radiation that damages
cells via hydroxyl radical intermediates. Hyperbaric oxygen often does not
show a dramatic increase in curability with standard fractionated radio-
therapy. However, hyperbaric oxygen appears to increase curability in a