Cutaneous lymphocytosis is a nodular disease of skin with a diffuse pattern histologically. 61% were solitary. The most frequently affected site was the thorax(43.5%). Other affected sites included legs (17.4%), pinnae (17.4%), flank (13%), neck (13%), abdomen (8.7%), foot s (8.7%), between the scapulae (4.35%), elbow (4.35%), caudal thigh (4.35%), hip (4.35%), digit (4.35%) and planum nasale (4.35%). The cells are small lymphocytes and are CD3 +. Half had intraepithelial clusters of lymphocytes. It is a slowly progressive disease but some develop internal nodules.
Gilbert S, Affolter VK, Gross TL, Moore PF, Ihrke PJ (2004) Clinical, morphological and immunohistochemical characterization of cutaneous lymphocytosis in 23 cats. Veterinary Dermatology 15 (1), 3–12.
Tarsal Lymphoma
Many of us recognised a unique lymphoma that just involved the tarsus and distal hind limb of cats. Burr et al (2014) published a series of these from multiple institutions.
This begins as a high grade subcutaneous primary nonepitheliotropic lymphoma of the distal hind limb/tarsus of an older cat. In the 23 cases reported by Burr et al (2014) 5 had popliteal lymph node involvement and 13 had lymphoma in other sites at last followup. Median survival was 190 days. 13 cats had immunophenotyping and 8 were B cell, 4 were T cell, one was null cell but was amyloid positive (B cell) and one was potentially TCRBCL.
Burr HD, Keating JH, Clifford CA, Burgess KE. Cutaneous lymphoma of the tarsus in cats: 23 cases (2000-2012). J Am Vet Med Assoc. 2014; 244: 1429-1434.
Subcutaneous panniculitis like T cell lymphoma is a subtype of lymphoma that is localized in the subcutaneous fat and is primarily septal in distribution. Some cases have panniculitis and necrosis of fat too.
Roccabianca P, Dell'Aere S, Avallone G, et al. Subcutaneous panniculitis-like T-cell lymphoma: Morphological, immunophenotypical and clonality assessment in six cats. Vet Dermatol. 2024; 35: 207-218
Plasma cell tumors of the skin of cats are very rare. Traditionally they are divided into multiple and solitary extramedullary plasma cell tumor.
These are very rare, and nothing is published about their prognosis. Many have amyloid.
Majzoub M, Breuer W, Platz SJ, Linke RP, Linke W, Hermanns W. Histopathologic and immunophenotypic characterization of extramedullary plasmacytomas in nine cats. Vet Pathol. 2003; 40: 249-253.
There are 4 main histiocytic tumors in cats - Feline progressive histiocytosis, histiocytic sarcoma, feline pulmonary Langerhans cell histiocytosis and hemophagic histiocytic sarcoma. The first 2 can affect the skin. Histiocytic sarcoma can be in the skin or internal organs. The latter 2 affect the internal organs only.
Clinical presentation
FPH presents with single or multiple skin masses of the head, limbs or trunk. They may increase and decrease in size over time. Some progress to a systemic form with metastasis to lymph nodes and internal organs including liver, spleen, kidneys, lungs and bone marrow.
Immunohistochemistry
The phenotype of FPH is that of a histiocytic dendritic macrophage origin and thus they are CD18 positive. A dendritic cell origin is suggested as they are CD1a and CD1c positive. Those that are E-cadheran positive may be of Langerhans cell type.
Histology
Lesions are highly cellular nonencapsulated and poorly circumscribed masses and they have dermal involvement in almost all cases (25 of 26). They are relatively uniform with minimal anisokaryosis and mitoses number up to eight per high-power field with the presence of atypical mitotic figures being common. Only a few cases have multinucleate giant cells. There usually clusters of small reactive lymphocytes in the lesions. Epitheliotropism occurs in about half of the cats (11 of 26 and intra lymphatic or intravascular aggregates are present in about one quarter of cases (6 of 26). Some cases have marked pleomorphism.
Prognosis
In a study of 26 cats, there was complete follow-up for 19 cats and nine died of the disease either from local reoccurrence or progression including additional skin nodules or confirmed metastasis. Median survival of cats that died was 96 days. 14 of the 19 cats had either local reoccurrence or progression of the disease. Five of 19 cats survived with no recurrence or progression.
In a study with 6 cats, median survival was 470 days (range 20-2890 days).
Coste M, Prata D, Castiglioni V, Minoli L, Etienne-Raffestin C-L, Boulouha L, Moreau S, Lagadic M. Feline progressive histiocytosis: a retrospective investigation of 26 cases and preliminary study of Ki67 as a prognostic marker. J Vet Lab Diagn 2019; 31: 801-808
Hirabayashi M, Chambers JK, Sumi A, Harada K, Haritani M, Omachi T, Kobayashi T, Nakayama H, Uchida K. Immunophenotyping of Nonneoplastic and Neoplastic Histiocytes in Cats and Characterization of a Novel Cell Line Derived From Feline Progressive Histiocytosis. Vet Pathol. 2020; 57: 758-773.
Histiocytic sarcomas have cells with marked nuclear atypia. There are 2 types - those predominantly of a round or polygonal phenotype or those that are predominantly spindle cell in type.
Survival for localized cutaneous histiocytic sarcoma is difficult to assess.
Hirabayashi M, Chambers JK, Sumi A, Harada K, Haritani M, Omachi T, Kobayashi T, Nakayama H, Uchida K. Immunophenotyping of Nonneoplastic and Neoplastic Histiocytes in Cats and Characterization of a Novel Cell Line Derived From Feline Progressive Histiocytosis. Vet Pathol. 2020; 57: 758-773.
Feline mast cell tumours of skin are separated into traditional mast cell morphology and histiocytic mast cells. These later group may be progressive histiocytosis or other histioctyic neoplasia and the use of that term is relegated to history.
The majority of feline cutaneous mast cell tumors are cured by excision. There is a small group that act aggressively by metastasising to the lymph node or going systemic. A mitotic count of 5 p10HPF was predictive.
Prognosis
Mitotic count is the most consistent, but not necessarily the most accurate, determinant of whether a cMCT will metastasise. 5 or higher mitoses per 10HPF is a cut off.
Pleomorphism with uninuclear giant cells (cytomegaly and karyomegaly) and nuclear pleomorphism (including a wavy nuclear membrane) is not in itself a poor prognostic indicator. Johnson et al (2002) reported on 15 cases and only one was aggressive. 14 of the 15 had a mitotic count of 1 per 10HPF or less!
Sabattini and Bettini (2010) examined 25 cMCT and CD117 was positive in 13 of 25 tumors. 5 cats died of MCT related disease. Multiple tumors, pleomorphism, KIT immunoreactivity, mitotic count and Ki67 staining correlated with an unfavorable outcome.
Sabatini et al (2013) found that cytoplasmic KIT localisation was predictive, like mitotic count. KIT mutation status was not predictive.
Sabattini and Bettini (2019) examined 63 cats with cMCT and divided them into cats that survived 1000 days - 48 cats; low grade - and those that developed lymph node metastases, multiple cutaneous nodules (>10) or visceral disease. Cats with high grade tumors had high grade >5 mitotic figures per 10 high-power fields and at least 2 of the following 3 criteria: tumor diameter >1.5 cm, irregular nuclear shape, and nucleolar prominence/chromatin clusters. Cats that lived 1000 days included 24 that were still alive and 14 that died - none had MCT related disease. Of the 15 cats with high grade tumors - aggressive disease, 5 had lymph node metastasis, 8 of the 15 had disseminated cutaneous involvement (>10). 9 of the 15 developed splenic mast cell tumor. Median overall survival was 349 days.
Arz et al (2023) reported on cats with lymph node removal both with and without evidence of lymphadenopathy and low grade mast cell tumors. They found 10 of 17 cats with low grade MCT had nodal metastasis in their population.
Arz R, Chiti LE, Krudewig C, Grieco V, Meier V, Fejös C, Stefanello D, Nolff MC. Lymph node metastasis in feline cutaneous low-grade mast cell tumours. J Feline Med Surg. 2023; 25
Johnson TO, Schulman FY, Lipscomb TP, Yantis LD. Histopathology and Biologic Behavior of Pleomorphic Cutaneous Mast Cell Tumors in Fifteen Cats. Vet Pathol 2002; 39: 452-457
Sabattini S, Bettini G(2010) Prognostic Value of Histologic and Immunohistochemical Features in Feline Cutaneous Mast Cell Tumors. Vet Pathol 2010 47: 643-653
Mitotic index most important, but not accurate!Sabattini S, Guadagni Frizzon M, Gentilini F, Turba ME, Capitani O, Bettini G. Prognostic significance of Kit receptor tyrosine kinase dysregulations in feline cutaneous mast cell tumors. Vet Pathol. 2013; 50: 797-805.
Sabattini S, Bettini G. Grading Cutaneous Mast Cell Tumors in Cats. Vet Pathol. 2019; 56: 43-49.