Page 300 - Canine Lameness

272 17 Neoplastic Conditions of the Thoracic Limb

tumors is consistent with other regions. Of tumors affecting the digits, the majority are malignant,
with squamous cell carcinoma being the most common type followed by malignant melanoma
(Marino et al. 1995; Henry et al. 2005; Wobeser et al. 2007). Secondary infections may accompany
digital tumors and lead to increased pain and lameness. On examination, the digit may be swollen
or malformed or may have been sloughed, and discharge may be present. These signs are easily
mistaken for a primary infectious paronychia.
Digital squamous cell carcinoma affects the thoracic limb twice as often as the pelvic limb. Most
squamous cell carcinomas arise from the subungual epithelium, which typically will cause bone
lysis of the third phalanx (Figure 17.1), whereas only 5% of melanomas cause radiographically
evident bone lysis (Marino et al. 1995). Squamous cell carcinoma arising from the subungual epi-
thelium has a better prognosis than other locations on the digit. Dogs with multiple squamous cell
carcinomas of the digits are typically large-breeds with black skin and haircoat. Breeds at higher
risk to get squamous cell carcinoma are standard Poodles, Labradors, Giant Schnauzers, Gordon
Setters, Rottweilers, Dachshunds, Flat‐Coated Retrievers, Beaucerons, and Briards. Dogs with digi-
tal malignant melanoma are overrepresented by Scottish Terriers, and in females more often than
males. Both digital squamous cell carcinoma and malignant melanoma can affect multiple digits;
however, this is uncommon.
Osteosarcoma accounts for 2–6% of digital tumors (Henry et al. 2005; Wobeser et al. 2007).
Other malignant tumors of the digit include mast cell tumor, undifferentiated sarcoma, fibro -
sarcoma, synovial cell sarcoma, plasma cell tumor, hemangiosarcoma, adenocarcinoma, and

(A) (B) (C)

Figure 17.1 Digital neoplasia: (A–C) radiographs of the distal limb of three patients affected by digital
neoplasia, and white arrows indicate the affected distal phalanx and evident bone lysis. The most likely
differential diagnoses would be squamous cell carcinoma, although malignant melanoma and osteomyelitis
should be considered as differential diagnoses (but are less likely to cause the depicted degree of bone
lysis). (C) Oblique views can be helpful to clearly outline radiographic abnormalities.