Abstract
A case of presumed primary muscular lymphoma in an 8-year-old, intact, male Newfoundland dog is reported. The dog was presented for evaluation of an infiltrating ventral cervical mass, respiratory distress, and anorexia of 1-month duration. Fine-needle aspiration of the mass revealed anaplastic large cell lymphoma. Despite chemotherapy, health status declined and the animal was euthanized a few weeks later. At necropsy, the mass infiltrated the cervical muscles and extended ventrally to the left forelimb and cranially to the tongue and laryngeal musculature. Other muscles were infiltrated by the same neoplasm (diaphragm and intercostal, abdominal, and gluteal muscles) indicating a probable multicentric origin. Histological examination confirmed the diagnosis of anaplastic large cell lymphoma, which showed a strong muscular tropism. Immunohistochemical staining revealed neoplastic cell reactivity for cluster of differentiation 3 (CD3) and Ki-67 antigens (70% and 90%, respectively). The neoplastic cells were negative for CD79a. The presumed histological diagnosis in this dog was primary muscular anaplastic large T-cell lymphoma.
Canine extranodal lymphomas are mostly multicentric in origin (84%). They are reported to arise rarely from the gastrointestinal tract (7%) or the cutaneous system (6%), with the other systems representing less than 1 % of cases. 6 Only 1 primary muscular lymphoma has been described in the dog, 6 and there is no report in other veterinary species. In human patients, less than 20 cases of primary muscular lymphomas have been reported in the literature. 1,2,5,7 Usually, muscular involvement occurs secondarily to adjacent nodal or osseous lymphomas, as a consequence of a metastatic spread, or in the case of multicentric origin. Microscopically, neoplastic cells are observed around the myofibers inducing secondary muscular degenerative and necrotizing lesions.
An 8-year-old, intact, male Newfoundland dog presented with a cervical mass and associated anorexia and respiratory distress. These clinical signs appeared progressively for 1 month. The dog previously received corticosteroids daily (prednisolone, 1 mg/kg) without any change of health status. Four years ago, the animal presented with masticatory myositis that was confirmed histologically. Clinical examination revealed an edematous, infiltrative mass (approximately 25 cm in diameter) in the ventral part of the neck. This mass was associated with lymphadenopathy involving the mandibular, axillary, and prescapular lymph nodes. The dog had lethargy, dyspnea, and a moderate spontaneous cough. Cardiac examination revealed a moderate systolic heart murmur. Hematological and biochemical parameters, including the lymphocyte count, were within reference intervals. Blood smear examination did not reveal any alterations in leukocyte morphology.
Cytological analysis of the mass (after fine-needle aspiration) showed large-sized, atypical, lymphoid cells with blastic features and a pale basophilic cytoplasm with microvacuoles (Fig. 1). These morphological features were mainly consistent with a large anaplastic cell lymphoma. Prescapular lymph node aspirates also contained a few of these neoplastic cells (<1 per high power field). These cells were accompanied by numerous immature plasma cells and small lymphocytes.
A chemotherapy protocol was established following the modified Cotter protocol. The dog was given vincristine (0.75 mg/m 2 per week for 4 weeks and then every 3 weeks), cyclophosphamide (250 mg/m2 once a month), and pred-nisolone (1 mg/kg per day every day for 3 weeks and then every 2 days). A few days after the chemotherapy began, the owners reported a marked improvement of the animal's health status with weight gain, improved appetite, and marked decrease of the tumor size (<15 cm in diameter). Six weeks later, the clinical condition of the dog declined with an increased tumor size. The decision was made to euthanize the dog.
A complete necropsy was performed. An infiltrative, multilobulated, 25-cm-diameter mass was present in the musculature of the ventral cervical region with extension to the left forelimb and scapular muscles, as well as to the tongue and the laryngeal musculature cranially. Caudally, the neoplasm showed multicentric involvement of the gluteal (Fig. 2), diaphragmatic, intercostal, and abdominal muscles and the adjacent fat. On cut section, the tumor was pale and firm. A moderate satellite lymphadenopathy (hypertrophy of the prescapular and axillary lymph nodes) was also observed. Extensive subcutaneous edema was present in the forelimbs and in the ventral cervical and thoracic areas. Following several sections, all other organs (including liver, lungs, and spleen) examined at necropsy lacked macroscopic evidence of tumors. The decision was made to histologically evaluate the neoplastic intramuscular mass, lymph nodes, and all other infiltrated skeletal muscles. Tissue specimens were formalin-fixed, processed routinely, sectioned at 4-μm thickness, stained with hematoxylin and eosin, and examined microscopically.
Fine-needle aspirate showing large-sized anaplastic neoplastic lymphocytes. Bar = 10 μm.
Cut section of the gluteal muscle at necropsy showing a multifocal infiltration by neoplastic cells, which appeared as a pale and firm tissue. Bar = 1 cm.
Muscular infiltration around and within the myofibers by neoplastic lymphocytes. These cells appeared round and medium- to large-sized with expanded eosinophilic cytoplasm and a round, irregular, centrally located nucleus. Some myofibers are degenerative or necrotic. Hematoxylin and eosin. Bar = 100 μm.
The neoplastic lymphocytes show positive staining for the cluster of differentiation 3 (CD3) marker. Bar = 50 μm.
Every muscle section had the same histological appearance. The normal architecture of the skeletal muscles was markedly effaced by diffusely infiltrating, poorly demarcated neoplastic masses. These tumors consisted of dense sheets of round cells infiltrating the perimysium, endomysium, and the myofibers themselves (Fig. 3). Neoplastic cells were round and large with a large amount of eosinophilic homogeneous cytoplasm and distinct cell borders. The nucleus was central or paracentral and round to irregular, with finely stippled chromatin and 1 or 2 prominent nucleoli. Anisokaryosis and anisocytosis were marked. Cellular atypia, such as nuclear gigantism and peripheral nuclear hyperchromatism, were noted frequently. The mitotic rate reached 3 mitoses per high power field of view (400X magnification). The residual myofibers were characterized by cytoplasmic hypereosinophilia, fragmentation, and loss of cross-striation indicating muscular degeneration and necrosis. Associated with the neoplastic infiltrates was a slight infiltration of small reactive lymphocytes in the stroma with some hemorrhage and necrosis. The histological findings were consistent with a primary high-grade muscular anaplastic large cell lymphoma. Few neoplastic lymphocytes (<1 neoplastic cell per high power field) were present in the subcapsular sinuses of the draining lymph nodes. A marked generalized lymphoid depletion was noted and considered to be associated with the chemotherapy.
Immunostaining using classic avidin—biotin complex method protocols was performed on formalin-fixed, paraffin-embedded tissue sections. Slides were placed in a pH 6 citrate buffer for heat-mediated antigen-retrieval method using a water bath for 40 min at 90°C. Tissues were incubated overnight at 4°C with the following primary antibodies: anti-CD3 a (cluster of differentiation 3) antibody (ready to use), anti-CD79a a (1:25 dilution), and anti-Ki-67 a (1:25 dilution). In order to assess specificity, a normal lymph node was used as a positive control and primary antibody omission was used for negative controls. Immunostaining was evaluated quantitatively. The percentage of positive cells was counted in 10 high power fields of view. The neoplastic cells were strongly positive for CD3 (70% reactivity; Fig. 4) and negative for CD79a. Ninety percent of the neoplastic lymphocytes were positive for Ki-67. In the lymph nodes, sinusoidal neoplastic cells had the same immunohistochemical staining characteristics. The results of immunohistochemistry were consistent with a T-cell lymphoma of high-grade malignancy.
Although organs such as the liver, spleen, or lungs were not evaluated histologically, their macroscopic appearance was not suggestive of neoplastic involvement. Because of the muscular tropism of the lymphoma, the presence of very few neoplastic cells in the lymph nodes, and the immunohistochemical staining results, a diagnosis of presumptive primary muscular anaplastic large T-cell lymphoma was made. Only 1 case of canine primary muscular lymphoma has been reported in veterinary medicine literature. 6 In that single case, the neoplastic tissue involved the skeletal muscles ofthe hind limb and myocardium as well as cutaneous tissue. The neoplastic cells were CD3 negative.
In humans, muscular lymphoma rarely has been described and occurs mostly at the extremities of the thighs in middle-aged to aged patients. 7 The reason for a greater incidence at this location is not well understood. A relationship to a higher incidence of trauma in these areas can be speculated. The space-occupying effect of the tumor that increases the interstitial pressure within muscle can lead to a compartment syndrome and clinical signs of pain. 1 Most cases of muscular lymphoma are intermediate- to high-grade tumors, usually of B-cell origin. 7 The etiology of extranodal lymphomas in human patients is usually unclear. Chronic inflammation, due to an infectious agent or an autoimmune disease, seems to be a strong predisposing factor. 7
The dog in the present case had an infiltrative primary muscular anaplastic large T-cell lymphoma involving ventral cervical, diaphragmatic, intercostal, and thigh muscles. A history of masticatory myositis was reported 4 years ago. As suggested in humans, perhaps this previous chronic inflammatory condition predisposed the patient to the development of lymphoma.
Histologically, the neoplastic cells showed marked tropism to myofibers. These neoplastic cells were of relatively large size, with well-defined cytoplasm, a round, centrally located nucleus, and prominent nucleoli. In dogs, the classification of lymphomas is not completely established. Usually, the modified human Kiel classification is used despite some differences between lymphoma of humans and dogs. 3,4 A large number of entities seen in humans have not been observed in dogs, and some of them are likely to be species-specific. In association with the modified human Kiel classification, 4 immunohistochemistry is one of the most useful tools to establish a definitive diagnosis. Unfortunately, diagnosis of canine lymphoma by immunohistochemistry is limited by the lack of species-specific antibodies. Use of anti-human antibodies may be associated with poor specificity and a high background staining, making the interpretation of immunohistochemical staining difficult.
At clinical examination, the differential diagnosis of cervical enlargement includes traumatic and inflammatory processes (e.g., hematoma and abscess) and neoplasms (e.g., thyroid carcinoma, lipoma, and hemangiosarcoma). The microscopic differential diagnosis for round cell tumors includes lymphoma, plasmacytoma, mast cell tumor, and some metastasizing neoplasms such as anaplastic carcinoma or melanoma. 6
In summary, the present report describes a putative primary muscular anaplastic T-cell lymphoma in a dog that was strongly infiltrative and associated with a short survival time (6 weeks). Further reports of muscular lymphoma will be needed to understand the behavior of the tumor and predisposing factors. However, because of its rarity, significant information on this particular form of lymphoma will probably be infrequently reported.
Footnotes
a.
Dako Denmark A/S, Golstrup, Denmark.