WHICH VETERINARY CANCERS CAUSE PAIN?
It seems obvious that primary bone tumors, the most common being appendicular osteosarcoma, will cause a marked degree of
pain (Figures 2A & 2B). However, it is important to understand that any tumor type can be associated with pain. Pain can be due to invasion and
destruction of surrounding tissues, including nerves and bones. It can also result from regional or distant metastasis to
sites such as bone, the body cavity (serosal surfaces), or the meninges.
Figure 2A. This 4-year-old Labrador retriever has obvious signs of pain, including a nonweightbearing lameness of the left
hindlimb, associated with a large mass of the distal femur.
Additionally, tumors may cause pain by stretching the capsule of certain visceral organs or by causing flow obstruction (in
the biliary, urinary, or gastrointestinal tract). Finally, a tumor can become painful because of inflammation or secondary
infection. Table 1 lists examples of pain caused by various tumors in veterinary patients.
Figure 2B. A lateral radiograph confirms a destructive lesion of the distal left femoral metaphysis, with both lytic and blastic
changes, suggestive of a primary bone tumor.
WHICH DIAGNOSTIC OR THERAPEUTIC PROCEDURES CAUSE PAIN?
Any mildly invasive diagnostic or therapeutic procedure can cause pain in veterinary patients with cancer. Some routinely
performed staging procedures, such as soft tissue biopsies (Tru-Cut, punch, wedge), bone marrow aspiration or biopsy, and
bone biopsies (Figure 3), predictably result in mild to moderate pain. This pain can be treated with analgesics and local blocks preemptively and
a nonopioid analgesic postoperatively. More aggressive surgeries usually require more aggressive preemptive analgesia, such
as regional blocks, epidural or interpleural analgesia, and the use of postoperative opioids, in addition to nonopioid drugs.
Such surgeries include amputation, hemipelvectomy, thoracotomy, radical mastectomy, large en bloc tumor resection with dorsal
spinous processes, mandibulectomy, and maxillectomy (Figure 4). These more aggressive pain control measures should be standard rather than be given on an as-needed basis. With a good
pain control program, it is common to observe that patients appear more comfortable after radical surgeries to remove painful
Figure 3. Obtaining a bone biopsy sample of a Rottweiler's distal tibia by using a Jamshidi biopsy needle. This procedure
is moderately to severely painful, and a preemptive multimodal analgesic protocol should be used.
While chemotherapy is not thought to be painful per se, it occasionally results in painful side effects. Examples include
colitis from doxorubicin, constipation from vincristine in cats, sterile hemorrhagic cystitis from cyclophosphamide, pancreatitis
from various agents, and a burning sensation at the catheter site during dacarbazine infusion (low pH).7,12,18,19 In people, painful peripheral neuropathies are known to occur at a certain frequency with drugs such as vincristine, cisplatin,
and the taxanes, but this has not been well-described in cats and dogs.1,2 Perivenous extravasation of certain chemotherapy agents, including vincristine, doxorubicin, vinblastine, mechlorethamine,
and dactinomycin, causes painful tissue destruction, sometimes severe enough to necessitate surgical débridement.1,2,7,12,18 Such complications should be avoided by placing clean-stick catheters in undamaged veins immediately before administering
these known vesicants.
Figure 4. Less than 24 hours after a segmental maxillectomy for an oral osteosarcoma, this German shepherd appears relatively
pain-free, thanks to preemptive multimodal and postoperative analgesia.