Canine multicentric lymphoma

Lymphoma is one of the most common neoplastic diseases in pet dog population, with an annual incidence estimated to be 13-114 per 100, 000 dogs [2]. It comprises 7 to 24% of all canine neoplasia and 83% of all canine hematopoietic tumor [3], making it a significant disease in this species. Lymphoma consists of diverse groups of neoplasia, possessing heterogeneity regarding etiology, clinical and histopathological presentation, treatment response and prognosis. Among all the dogs diagnosed with lymphoproliferative diseases, 84% of the population develop multicentric lymphoma [3].

1.1.1 Signalment

For canine multicentric lymphoma, some species are well-known for increased risks for developing, including Bullmastiff, Rottweiler, Scottish terrier, Boxer, Cocker spaniel, German shepherd, Golden retriever (GR), Labrador retriever (LR) [2-5]. Even though any age dogs can be diagnosed with lymphoma, middle-aged or older dogs with the median age of 6 to 9 years are more likely to be affected [6]. Based on a large-scaled investigation for canine lymphoma in France where 608 cases were studied, the age at occurrence ranged from 1-16 years, with a mean of 8.3 years and a median of 8 years [5]. There is no such evidence of sex predisposition, although intact females associated with lower risk was once reported [6].

1.1.2 Etiology

The etiology of multicentric lymphoma remains unclear and is thought to be multifactorial. Investigated factors include exposure to chemicals, waste incinerators, polluted sites and radioactive waste [3, 5]. DNA repair deficiency is also suspected, based on the finding of significantly increased chromosomal damage following ionizing radiation exposure and bleomycin exposure in lymphocytes of Golden retrievers diagnosed with lymphoma compared to which of normal Golden retrievers and mixed breed dogs [7].

1.1.3 Clinical presentation

Generalized, non-painful lymphadenopathy is the most common symptom in affected dogs. Clinically, most dogs show no signs. However, the swollen lymph nodes, especially those of mandibular or prescapular region, sometimes can still result in precaval syndrome, characterized by edema of head, neck and forelimbs, when the sizes become large enough to compress the circulation of local lymphovascular system (e.g.

cranial vena cava). Dogs with precaval syndrome may suffer from dyspnea, noisy breathing sound, choking or difficulty of swallowing. Other nonspecific signs, such as anorexia, vomiting, depression, fever, or polydipsia and polyuria, may be seen as well and usually depend on the location and severity of organs involved.

Laboratory abnormalities associated with multicentric lymphoma included anemia, thrombocytopenia, neutrophilia, or circulating atypical lymphocytes which may indicate bone marrow involvement. Hypercalcemia may be noted as a paraneoplastic syndrome in approximately 10-40% of the clinical cases [8] and tends to associated with the

[8, 9].

Internal organ involvement can also result in abnormal imaging findings. In thoracic radiographs, abnormal pulmonary parenchymal patterns and enlargement intrathoracic (sternal, tracheobronchial and cranial mediastinal) lymph nodes are noted in 37% and 26 to 40% of dogs with multicentric lymphoma, respectively [10]. Pleural effusion may be presented as well. There are still other differentials accounting for the similar image changes, such as bronchopneumonia or chronic bronchial changes.

However, so far, the information of the correlation or consistency between the abnormal imaging changes mentioned above and the cytological or histopathological findings from other diagnostic methods (i.e., bronchoalveolar lavage or lung biopsy) regarding determining pulmonary infiltration by multicentric lymphoma is scarce. Only one study in 1993 showed cytology finding of bronchoalveolar lavage (BAL) was more sensitive compared with radiographic evaluation of lungs, with 66% and 34% positive result, respectively [11]. In abdominal radiographs or ultrasonography, sublumbar lymphadenopathy, hepatomegaly and splenomegaly with echogenicity changes are three most common findings, ranging from 46 to 75% in dogs affected [10].

1.1.4 Staging

Evaluation of clinical staging is essential once the diagnosis of multicentric lymphoma confirmed on a dog. To provide the disease extent and tailor the treatment plan. The most widely adopted staging system is World Health Organization’s clinical staging system for lymphoma in domestic animals proposed in 2011 by Valli et al. [3]

(Table 1).

To define a clinical stage, information, such as hematology data, image details for liver and spleen, or bone marrow evaluation, is required. Among numerous diagnostic

tools, application of more sensitive staging tests can detect previously unnoted lesions, especially those that are asymptomatic, and sometimes leads to stage migration.

Substitution of abdominal radiography with ultrasonography and quantitative blood count (QBC) with complete blood count (CBC) both result in significant stage migration from less advanced to advanced stages (i.e. stage IV & V) [12]. Addition of splenic and hepatic cytology may help to distinguish between stage IV and III [13];

however, the addition of bone marrow cytology will not have a significant impact on staging [12]. Interestingly, even though the variable degree of stage migration is commonly noted among different studies, these changes of diagnostic methods exert no significant influence on either remission rate, remission duration, or survival in these studies [12, 13]. A possible explanation that disease extent may not be an essential determinant was proposed [13]. Therefore, application of additional diagnostic methods, especially more invasive ones, may be limited by the prognostic value in practice.

1.1.5 Immunophenotype

Immunophenotyping provides information of whether clonality exists and what type of cells comprising the disease. The ways of immunophenotyping include flow cytometry (FC), immunohistochemistry (IHC), immunocytochemistry (ICC) and polymerase chain reaction for antigen receptor rearrangement (PARR), according to the sample submitted [14-19]. For canine lymphoma, the distribution of B-cell, T-cell and unclassified subtype was estimated about 51.2 to 74%, 26 to 36% and 19.5%, respectively [18, 20, 21]. In the ability of correctly predicting immunophenotype (with IHC serving as a gold standard), FC was showed to be superior to PARR, with the

70%, respectively [18]. However, if a fresh sample is not available or the result of flow cytometry is confounding, PARR is still highly suggested for immunophenotyping due to its high specificity of 98% [18].