Page 231 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

210 PART III Therapeutic Modalities for the Cancer Patient

TABLE 13.1 Acronyms Commonly Used in Radiation Oncology

VetBooks.ir GTV Gross tumor volume The extent of gross disease as determined by palpation, direct visualization, or indirectly through
imaging techniques. (GTV cannot be defined after a tumor has been surgically excised.)
The region of suspected subclinical tumor involvement, including normal tissue with presumed
Clinical target volume
CTV
microscopic extension. Additional volumes such as lymph nodes may be included in the CTV.
PTV Planning target volume The volume including the CTV and an additional margin to account for variations in size, shape, and
position of the CTV during therapy. The PTV margin also accounts for setup and physical uncertain-
ties.
SIB Simultaneous integrated boost A portion of the tumor, generally central, that is prescribed a higher dose than the rest of the tumor.
Allows for high-dose delivery to the central core of the tumor and a dose fall-off toward the periph-
ery.
OAR Organ at risk Normal organs/tissues in proximity to a target structure that may influence prescribed dose or
treatment plan parameters.
MLC Multileaf collimator A beam-shaping device made up of individual independently moving “leaves” of shielding material
(usually tungsten).
OBI On-board imaging An imaging system and tool on a radiation delivery device used to confirm the match of treatment
setup to the planned setup.
PRT Palliative radiation therapy A radiation treatment course delivered with the intent to minimize tumor-related symptoms and
improve quality of life with few short-term side effects.
DRT Definitive radiation therapy A radiation treatment course delivered with the intent of achieving long-term tumor control or durable
tumor response.
3DCRT Three-dimensional conformal Radiation planning technique utilizing cross-sectional imaging (i.e., computed tomography scan) and
radiation therapy treatment-planning software to optimize radiation beam angles, blocks, and beam-shaping devices
(i.e., wedges) with the goal of maximizing doses to targets and minimizing doses to organs at risk.
Typically involves static blocks and gantry angles.
IMRT Intensity modulated radiation Radiation planning technique and delivery technique by modulating fluence within each of multiple
therapy radiation fields. Requires “inverse” treatment planning.
SRT Stereotactic radiation therapy Radiation-planning technique involving minimal margins applied to the target and maximizing dose
fall-off outside of target and near OARs. Most commonly used in high dose-per-fraction protocols.
SRS Stereotactic radiosurgery Generally implies a single fraction, high-dose radiation treatment course involving minimal margins
applied to the target and maximizing dose fall-off outside of target and near OARs.
VMAT Volumetric modulated arc A radiation delivery technique involving the radiation gantry rotating about the patient while delivering
therapy IMRT.
QA Quality assurance Processes that insure all aspects of radiation treatment quality and safety. These processes frequently
involve independent measurements that verify radiation dose delivered for comparison to expected
values or values calculated by a treatment planning system.

populations (e.g., epithelial stem cells), although slowly dividing Oxygen Effects
and nondividing cells (e.g., bone and cells of the nervous system)
are also affected by radiation and often limit the dose deliver- Because of their rapid growth and abnormal vasculature, tumors
able during RT. 3,33 Tumor cells are also capable of DNA repair, often become hypoxic. 35,36 Oxygen is a critical factor in the
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although DNA mutations may alter repair capabilities. response to irradiation, because reactive oxygen species gener-
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ate much of the damage from radiation. As a result, normoxic
Cell Cycle Effects cells are up to three-fold more sensitive to radiation than hypoxic
cells. Additionally, tumor hypoxia is associated with the upregu-
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The period of the cell cycle in which DNA undergoes synthesis is lation of hypoxia-inducible proteins that may prepare the tumor
known as S-phase. Before and after S-phase are periods without cells to handle stresses and can also affect tumor metabolism and
overt activity by the DNA; these periods are called the G1 phase metastatic potential. 36,37
and the G2 phase. The G2 phase is followed by mitosis (M phase).
Cells are distributed throughout the cell cycle in a tumor or tis- Relative Biologic Effectiveness
sue at a given time. Individual cell sensitivity to irradiation varies
depending on the phase of the cell cycle at the time of irradiation. Although this chapter primarily discusses the effects of photons
Cells in late S-phase are most resistant to irradiation, and cells in (x-rays and gamma rays) and electrons, other forms of radiation
late G2 or M phase are most sensitive. 29,30 Cells with a long G1 can be used for treatment, including protons, neutrons, and car-
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period may be resistant early in that phase. bon particles. The relative biologic effectiveness (RBE) is not a