Page 237 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

216 PART III Therapeutic Modalities for the Cancer Patient

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A B
• Fig. 13.3 (A) Teeth being placed in preformed acrylic bite block that is inserted into a carbon fiber indexed
frame. Neck is resting on a vacuum-lock bag. (B) An acrylic face mask being placed.

the irradiated high-dose volume and the geometric shape of the provides a quantitative method of evaluating treatment plans
tumor. 3D-CRT requires importation of CT images into a treat- and enhances quality assurance.
ment planning system. The animal must be positioned for the IMRT and related modalities, such as TomoTherapy, allow
imaging in a fashion that can be closely replicated on a day-to- even greater sculpting of the radiation dose. IMRT requires a
day basis for treatment. Acrylic face masks and Vac-Lok Cushions treatment planning system that uses inverse treatment planning.
are commonly used for positioning (Fig. 13.3). The radiation Inverse planning requires that the various tumor structures (GTV,
oncologist identifies important normal tissue structures known CTV, and PTV) as well as critical normal tissue structures be
as organs at risk, gross tumor volume (GTV), clinical target vol- identified and contoured into the planning system. Optimization
ume (CTV), and planning target volume (PTV) on these images. objectives for each structure are entered, and a sophisticated algo-
By definition, the GTV only includes gross tumor whereas the rithm attempts to meet all objectives. This is standard of care for
CTV includes the GTV plus an expansion based on the known the treatment of prostate tumors, head and neck cancers, verte-
clinical behavior of the specific tumor to account for regional bral cancers, some brain cancers, and pelvic cancers in humans.
microscopic disease. 121 For example, the CTV expansions for a A major benefit associated with IMRT is that dose to adjacent
sarcoma are generally larger than for a carcinoma. If the patient normal tissue structures can be minimized, dramatically reducing
has had cytoreductive surgery and only microscopic tumor acute effects. Patients are more comfortable and require less pain
remains, there is no GTV and the CTV is based on the scar, medication. 122 The tumor dose can be increased without exceed-
regions of surgical disruption, and an expansion for microscopic ing normal tissue tolerance, presumably leading to improved
disease beyond the surgical site. 121 In addition to the GTV and tumor control. Fractionation schedules and other factors, such
CTV, the PTV includes expansion for an internal margin that as immobilization, margins, and CTV expansions, are similar to
accounts for variations in size and shape relative to anatomic 3D-CRT. TomoTherapy, a form of IMRT that uses a helical deliv-
landmarks (e.g., filling of bladder, respiratory movements) and ery system to sculpt the beam, and volumetric-modulated arc RT
set-up margin (SM). The SM accounts for uncertainties in are also being used in veterinary medicine. 11,123 IMRT is proving
patient positioning and alignment during planning imaging and useful for the treatment of tumors or tumor beds with complex
subsequent treatments. 121 The better the immobilization device, geometry located near important normal tissues, such as nasal
the smaller the SM expansion can be. SM expansions will vary tumors, oral tumors, and urogenital tumors. 123,124
based on the location of the tumor because some sites, such as SRT is a treatment technique made possible by highly confor-
the head, are more amenable to rigid immobilization devices, mal radiation treatment techniques, such as IMRT and proton
such as bite blocks, which provide better replicability. Decreas- therapy, in combination with precise positioning devices and
ing the PTV expansion by using good immobilization and avail- localization methods, such as cone-beam computed tomography
able imaging affects the volume of normal tissue treated, is a (CBCT). In SRT, the usual wide margins (CTV) to account for
key component to successful 3D-CRT, and is critical to other microscopic disease extension are not applied, and PTV expan-
advanced treatment modalities. PTV expansions are both facil- sion may be decreased to spare important adjacent normal tissues
ity and region specific and ideally are determined with input by (Fig. 13.4). Highly conformal radiation is applied to a specific
both the medical radiation physicist and the radiation oncolo- gross (tumor) target with rapid dose fall-off within a small mar-
gist. Somewhat more sophisticated beam shaping is performed gin. This technique minimizes the radiation dose reaching nor-
by taking advantage of fixed multileaf collimators. The major mal structures adjacent to the treated target. Table 13.2 shows
advantage of 3D-CRT is that dose-volume histograms can a comparison of characteristics of traditionally fractionated RT
be obtained for the tumor and normal tissue structures. This compared with SRT.