It can be estimated that more than half of tumor-bearing pets suffer cancer-related pain. Quality of life and key physiological
functions are negatively impacted by pain, and treating it is a priority. Whether cancer pain is confirmed, suspected, or
expected to occur, efforts should be spent to treat it effectively. While some patients are relieved adequately and rapidly
by treating the underlying malignancy, a vast majority requires symptomatic therapy to achieve improved quality of life, the
main objective in veterinary oncology. For cancer pain to be adequately managed, it must be recognized early, properly assessed,
and frequently reassessed by both the clinician and the pet owner. Many barriers remain for the appropriate treatment of cancer
pain in animals, including its poor recognition with many neoplastic conditions, difficulty to objectively assess the degree
of pain in veterinary patients, inappropriate reassessment once therapy is implemented, fear to use or lack of knowledge regarding
the analgesic therapeutic arsenal, and suboptimal communication with the caretaker.
TREATING THE UNDERLYING CANCER
The ideal way to alleviate cancer pain, if possible, is to treat the underlying tumor. The option of excising a tumor, or
achieving local control with radiation therapy or a remission with chemotherapy, should be weighed. Still, standard analgesics
are often needed for proper pain relief while waiting for the anticancer therapy to abrogate the source of pain. Occasionally
with resistant, refractory, recurring, or terminal cancers, only supportive and symptomatic therapy can truly benefit the
patient (concept of "best supportive care"). Purely palliative therapy does not intend to alter the course of the disease
but rather aims to improve the patient's overall quality of life using various analgesic therapies and supportive care, and
frequently combining it with traditional anticancer therapeutic modalities.
The complete surgical removal of a painful tumor should be performed whenever possible and indicated. With suitable preemptive
and multimodal analgesia, most patients with painful tumors will be more comfortable following radical surgeries. Surgery
can rapidly remove the source of pain, and various procedures can be performed depending on the tumor type, local extent,
and anatomic site. Tumors treated and possibly cured with surgery alone include canine oral squamous cell carcinoma and sarcomas,
cutaneous mast cell tumors, mammary tumors, soft-tissue sarcomas, splenic or hepatic tumors with painful capsular distension,
nailbed tumors, synovial cell sarcoma, and tumors of the ear canal. Occasionally, patients may benefit from palliative surgeries,
where the removal of painful tumors may not impact the survival. A classic example is amputation for appendicular osteosarcoma
(OSA) where metastases ultimately determine survival time. In select cases, surgical removal of pulmonary metastases from
OSA may provide rapid relief from painful paraneoplastic hypertrophic osteopathy (within 24 hours in a case series). Finally,
and though such salvage techniques have yet to be described for the control of cancer pain in pets, peripheral and central
neuroablative surgeries are valuable in terminal human cancer patients with poorly responsive pain.
Radiation therapy (RT) is invaluable in the treatment of many veterinary cancers. Fractionated protocols (a.k.a. "curative
protocols" - multiple small daily doses) are used for radiosensitive tumors where a clinical benefit and survival advantage
can be expected. Examples of painful macroscopic tumors that are treated this way include sinonasal tumors, certain oral tumors,
solitary osseous plasma cell tumor, certain forms of lymphoma (mediastinal, CNS), and thyroid carcinomas. Certain painful
tumors may not respond as well to fractionated protocols, but alleviation of pain is still possible when a small number of
large fractions of radiation (typically weekly) are administerd. This is often called palliative RT. Examples of painful
tumors treated this way include canine oral malignant melanoma, feline oral squamous cell carcinoma, OSA (appendicular or
axial, primary or metastatic), unresectable mast cell tumors (MCT), unresectable SQ or IM hemangiosarcoma, and any bone metastases.
Painful early side effects of radiation therapy, especially oral mucositis, acute moist dermatitis, colitis, and conjunctivitis,
are more commonly encountered with daily fractionated protocols and need to be addressed. Newer radiation therapy techniques,
called stereotactic radiosurgery (gamma knife and CyberKnife®), permit the precise delivery of a very large dose of radiation.
These newer techniques permit the delivery of a "curative dose" of irradiation in one or two fractions, while sparing normal
tissues, but the availability is currently limited to a handful of sites in the U.S.
Cytotoxic chemotherapy is used for chemosensitive tumors or cancers, if a clinical remission may result. Lymphoma tends not
to cause obvious pain, but certain anatomic sites (CNS, bone) may lead to moderate to severe pain. In addition, patients with
marked hepato-splenic involvement can have visceral pain from capsular stretch. Multiple myeloma (MM) is often painful, with
multifocal bone lysis that may result in pathologic fractures, and compression of the meninges and spinal cord. Other therapies
(RT, bisphosphonates) can be co-administered to MM patients and alleviate their pain, but chemotherapy is crucial for quality
of life and survival improvement. Other examples of cancers that may be cause moderate to severe pain and may respond to chemotherapy
include unresectable or disseminated MCT, carcinomatosis (intracavitary chemotherapy), and transmissible veneral tumors (TVT)
which can cause pain and discomfort of various mucosal surfaces in adventurous sexually-intact dogs. A cure is obtained in
a majority of TVT-bearing dogs treated with vincristine monotherapy.