Page 236 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 13 Radiation Oncology 215

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responding tissues because of the shortened overall treatment machines or linear accelerators. Because megavoltage radiation
time in SRT. Work is ongoing to establish SRT constraints for has excellent tissue-penetrating capabilities, RT can be performed
91,92
acutely responding tissues.
on deeply seated tumors for which orthovoltage therapy would

VetBooks.ir not be an option.
Orthovoltage x-rays, which have low energy, distribute maxi-
PALLIATIVE RADIATION THERAPY
mum doses to the skin surface. Acute effects to the skin can be
Palliative RT (PRT) is an important tool in cancer treatment, and quite severe, causing discomfort to the patient, and late effects
its use in veterinary medicine for a wide variety of tumor types to the skin and subcutaneous tissues can be dose limiting. Mega-
is increasing. PRT is not ideally intended to provide long-term voltage radiation has a higher energy than orthovoltage, and the
or definitive tumor control; rather, it is intended to relieve pain photons must interact with tissues, allowing the dose to build up
or improve function or quality of life in patients in which other before the maximum dose can be achieved. The skin therefore can
factors (e.g., advanced metastatic disease, comorbidities) are likely receive a significantly lower dose than the underlying tumor. This
to lead to early demise. Because curing the patient is no longer skin-sparing effect of megavoltage radiation allows the optimal
the goal, the treatment should not negatively affect quality of dose to be administered to a more deeply seated tumor without
life by causing acute radiation effects. Radiation protocols used causing severe reactions to the skin. When the tumor involves or is
for palliation are far less aggressive and more flexible than when in close proximity to the skin, megavoltage radiation can be used
treating with curative intent. PRT is most often hypofractionated successfully by placing a sheet of tissue-equivalent material, called
compared with curative-intent protocols, often administered in a bolus, over the tumor. This allows the dose buildup to occur
6 to 10 Gy per fraction, in one to six total fractions once or twice before reaching the skin so that the skin and associated tumor can
weekly. Conversely, PRT can be delivered in conventional or mod- receive the maximum dose of radiation.
estly hypofractionated schedules once or twice daily for a shorter The absorption of megavoltage radiation, unlike that of ortho-
treatment regimen. Most publications have focused on metastatic voltage radiation, is minimally dependent on the composition of
or primary bone tumors, principally canine OSA (see Chapter the tissue. This characteristic permits even distribution of the dose
25), but it is also reported for nasal tumors, hemangiosarcoma, throughout the tissues in the field. Orthovoltage radiation is pref-
soft tissue sarcoma, mast cell tumors, GI lymphomas, and oral erentially absorbed by bone. 119 If a tumor adjacent to or overlying
tumors. 114–117 Palliative RT is more convenient and less expensive bone is administered a meaningful dose of orthovoltage radiation,
for the owner than fractionated RT or SRT with curative intent. the probability that late effects to the bone (bone necrosis) will
It is, however, important to remember that palliative RT is not a develop is quite high. 119,120 Treatment with orthovoltage should
substitute for curative-intent protocols despite the convenience. be limited to small, superficial tumors, such as nasal planum
Some palliative RT protocols may have an increased probability of tumors, or to superficial tumor beds after surgical excision.
causing late radiation effects based on biologic effective dose calcu- Interaction of megavoltage radiation with tissues is quite pre-
lations 102,103 ; however, because they are prescribed to patients that dictable; this has allowed the development of accurate computer-
have a poor long-term prognosis, these late effects generally do not ized treatment planning systems. These planning systems allow
have time to manifest. Curative-intent RT protocols require strict treatment of the tumor with multiple beams administered from
adherence to radiation biologic principles; palliative RT protocols, different angles. The goal of computerized treatment planning is
on the other hand, are far more flexible. As in human hospital set- to ensure a desired minimum tumor dose to a region specified
tings, the protocols may vary dramatically between centers and, by the radiation oncologist and to spare normal tissue structures
regardless of the protocol, most palliative RT patients receive some when possible. Three-dimensional conformal RT uses CT or MRI
level of benefit. Because the duration of response after palliation images, which are imported or contoured using a tablet. The sum-
is limited, some owners elect pursuing a second course. 118 PRT is mation of multiple beams coming from different directions pro-
not restricted to specific modalities of radiation administration. vides a higher dose to the tumor than surrounding tissues.
SRT is also an option for palliative treatment. It may provide In addition to photons, radiation treatments in veterinary
quicker and more durable response for the primary or metastatic medicine also include electron-beam therapy. Various energies of
lesion being treated and ideally maintain quality of life until death electrons are commonly incorporated in the capabilities of mod-
from disseminated disease. ern linear accelerators. Electron beam therapy is most favorable
for superficial targets, because electrons do not penetrate to nearly
Radiation Therapy Equipment the depth of a photon beam, which can allow for sparing of deep
structures.
Ionizing radiation can be administered by an external source Although not yet commonly utilized in veterinary medicine
(teletherapy) through placement of radioactive isotopes intersti- due to the technical demands and cost, proton beam radiation
tially (brachytherapy) or by systemic or cavitary injection of radio- equipment is becoming widely available in human treatment facili-
isotopes, such as iodine-131 ( 131 I). Teletherapy, also referred to ties and offers dosimetric benefits that photons cannot. A machine
as external beam RT, is the most commonly used method of RT called a cyclotron speeds up protons, imparting high energy that
in veterinary medicine. External beam RT usually is classified as determines the travel depth of the proton. Unlike photon therapy,
orthovoltage or megavoltage radiotherapy, based on the energy protons deposit radiation dose in the targeted range and minimal
of the photon. Orthovoltage machines produce x-rays with an dose exits the tumor. 42
energy of 150 to 500 kVp; megavoltage radiation emits pho-
tons with an average energy greater than 1 million electron volts Radiation Planning and Delivery Techniques
(1 MeV). Although some veterinary radiation oncology cen-
ters continue to treat with orthovoltage machines, megavoltage Advances in treatment planning and imaging over the past
radiation is the most commonly used type of external beam RT. decade have led to the development of image-based, 3D confor-
Megavoltage radiation for therapy can be obtained from cobalt mal RT (3D-CRT), which permits better conformity between