Page 308 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 16 Supportive Care for the Cancer Patient 287

TABLE 16.1 Tumor Types Most Likely to Be Associated with Pain a
Category Notes
VetBooks.ir Tumors involving bone Primary bone tumors (both of the appendicular and axial skeleton) and metastasis to bone are painful.
Just as in humans, sometimes metastasis to bone can be relatively nonpainful; however, this should
be considered the exception.
Central nervous system tumors Extradural tumors that expand and put pressure on neural tissue are often associated with pain. Tumors
originating from within the neural tissue are often not associated with pain until later in the course of
the disease. In humans with primary brain tumors or metastases to the brain, up to 90% suffer from
headaches; it should be presumed that animals also suffer such headaches.
Gastrointestinal tumors Pain from gastrointestinal tumors may be very difficult to localize and may manifest as vague signs and
behavioral changes. Colonic and rectal pain is often manifested as perineal discomfort.
Inflammatory mammary carcinoma Inflammatory carcinomas can be particularly painful, manifested as reluctance to move and perform activities.
Genitourinary tract tumors Stretching of the renal capsule appears to produce significant pain. Bladder tumors appear to be predict-
ably associated with pain. Tumors of the distal genitourinary tract are often manifested as perineal
pain or pain that appears to be located in the lumbar region.
Prostate tumors Pain may be manifested as lower back or abdominal pain.
Oral and pharyngeal tumors Soft tissue tumors that project from the surface appear to be relatively nonpainful. Tumors involving
bone or that are growing within the tissues of the maxilla or mandible appear to be significantly
more painful. Soft tissue tumors of the pharynx and caudal oral cavity are particularly painful.
Intranasal tumors Pain caused by intranasal tumors usually manifests as a diminished willingness to engage in normal
behaviors.
Invasive soft tissue sarcomas In the authors’ experience, injection-site sarcomas in cats can be particularly painful, and the size of the
lesion does not necessarily correlate with the degree of pain. Other invasive sarcomas in both species
are painful. In the authors’ experience, one form of soft tissue sarcoma, the peripheral nerve sheath
tumor, is often associated with pain, both spontaneous and associated with palpation.
Invasive cutaneous tumors Especially those that are ulcerative.
Liver and biliary tumors Especially those that are expansile, stretching the liver capsule.
Disseminated intrathoracic and intraabdominal The signs associated with such tumors are particularly vague; however, intracavitary analgesia (e.g., an
tumors (e.g., mesothelioma, malignant histio- intraabdominal local anesthetic) often can markedly improve the animal’s demeanor.
cytosis)
Lung tumors Although significant pain is reported in humans with lung cancer, animals often appear to show few
signs of pain. However, even in those animals, provision of an analgesic can often improve demeanor.

Pain after surgical removal of a tumor Chronic postoperative pain has not been documented in animals, but it is a common problem after
oncologic surgery in humans. Phantom pain (e.g., phantom limb pain), a form of neuropathic pain,
does appear to exist in animals.

a Very often it is difficult for the veterinarian to appreciate that pain may be present. However, the administration of an analgesic to animals suffering from these conditions is reported by owners to result
in an improvement in demeanor. In the face of lack of evidence to the contrary, it is suggested that this improvement is due to the alleviation of pain.

commonly observed in dogs than cats, and the most evident pain best recognized potential source of chemotherapy-associated pain
signs include decreased interaction with the surroundings, lame- in dogs, and they have been reported with both conventional
ness, or increased interest in the affected site (e.g., licking, chew- cytotoxic agents (e.g., doxorubicin) and drugs that are generally
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ing). Acute radiation-associated pain (RAP) is poorly responsive regarded as “safer” (e.g., bisphosphonates). 24,25 Chemotherapy
to standard antiinflammatory and analgesic therapies and thus can also represents a significant cause of chronic neuropathic pain
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be quite difficult to treat. Although acute side effects are transient in human cancer patients. The risk of chemotherapy-induced
and self-limiting, the discomfort they cause can have significant peripheral neuropathy (CIPN) varies from patient to patient,
implications for long-term oncologic outcomes, because the dis- depending on the drug agent, treatment protocol, and coexist-
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comfort can result in early termination of treatment regimens, ing neuropathic disorders. In general terms the pathophysiology
with a consequent decrease in radiotherapeutic efficacy. Develop- involves (1) recruitment and activation of immune and glial cells,
ment of more effective RAP therapies is hindered because little is which leads to the production and release of pronociceptive medi-
known about the etiology of RAP. Indeed, the first model of RAP ators in the DRG and spinal cord 28–31 ; (2) oxidative stress, with
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was described only recently. Until the underlying pathophysiol- increased production of reactive oxygen species (associated with
ogy is better understood, treatment of RAP will remain empiric. mitochondrial dysfunction) 32,33 ; and (3) increased activity of both
In humans, pain results from a wide range of chemotherapy- voltage-gated and ligand-gated ion channels (including voltage-
associated complications. Extravasation reactions are perhaps the activated sodium, calcium, and transient receptor potential [TRP]