Intranodal palisaded myofibroblastoma (IPM), also known as “intranodal hemorrhagic spindle cell tumor with amianthoid fibres”, is a rare and benign primary mesenchymal neoplasm of the lymph node with myofibroblastic/smooth muscle cell differentiation. Cytological findings of IPM are rarely described in literature. We herein describe the characteristic cytological findings of IPM in the inguinal lymph node of a 55 years old male with a high suspicion of a malignant tumour, highlighting cytomorphological clues for its accurate identification along with brief review of literature.

• Palisaded myofibroblastoma

• Amianthoid fibers

• Lymph node

• Cytology

Hemlata J, Swati M, Hena K, Seema K (2022) Cytomorphological Clues for Correct Diagnosis of Intranodal Palisaded Myofibroblastoma (Intranodal Hemorrhagic Spindle Cell Tumor with Amianthoid Fibers): Report of an Entity Rarely Recognized on Cytology with Brief Review of Literature. Ann Clin Cytol Pathol 8(1): 1142.

FNA: Fine Needle Aspiration; GIST: Gastrointestinal Stromal Tumour; H&E: Hematoxylin and Eosin; IPM: Intranodal Palisaded Myofibroblastoma; IHC: Immunohistochemistry; MGG: May-Grunwald-Giemsa; PAP: Papanicolaou; RCC: Renal Cell Carcinoma; SMA: Smooth Muscle Actin

Intranodal palisaded myofibroblastoma(IPM) is a rare, benign primary mesenchymal neoplasms involving the lymph node [1], described first in 1989, IPM is characterized by proliferation of myofibroblastic/smooth muscle cells admixed with eosinophilic amianthoid fibers that blend indiscernibly with lesion cells [2,3]. Data on aspiration cytology features of this rare neoplasm is limited to a few case reports [4-9]. We report such case with characteristic cytology findings, subtle diagnostic features are highlighted which will help as diagnostic clues for correct identification of this entity on aspiration cytology along with brief review of literature.

A 55 years old male was presented with a painful gradually increasing left groin mass. CECT showed a circumferential mucosal thickening in the anal canal along with a well-defined hypoechoic mass (3×2.5×2.5cm) in the groin. Colonoscopy guided biopsy from rectum confirmed the diagnosis of adenocarcinoma. With the clinical suspicion of metastasis, FNAC of inguinal swelling was performed and obtained highly cellular smears with large cohesive clusters of bland spindled cells (Figure 1A).

Figure 1: A-B) A cellular smear showing large cluster of cohesive spindle cells (A, MGG x100) along with loosely arranged, dis-cohesive singly scattered cells(B,PAP x 100)). C) Clusters with central traversing blood vessel (PAP x100). D-E) Tendency to form palisades around green-tinged fibrillary material (D,PAP x 200) and central, orange-colored, acellular matrix core (E,PAP x 100). F) “Amanthiod like structures” with central dense pink stroma surrounded by spindle to ovoid cells (MGG x400). G) On higher power, spindle cells exhibit either nuclei blunt-ended or tapered (MGG x 200). H) Calcification at the periphery of cell clusters (PAP x100). I) Occasional round-shaped, intranuclear inclusion bodies are seen (MGG x 400). J) Histologic examination reveals spindle cells arranged in sheets and short intersecting fascicles with palisading along with scattered islands of collagen with peripheral stellate like extensions resembling amianthoid fibers are noted (H&E x 200). K&L).The spindle cells of IPM are immunoreactive for smooth muscle actin and cyclin D1 (H&E x 200).

Short fascicles and singly lying cells were also seen intermingled with large clusters (Figure 1B). Many large clusters showed frequent traversing blood vessels within the cores, reminiscent of renal cell carcinoma (RCC) like vascular pattern (Figure 1C). The cells had either blunt-ended or tapered nuclei, granular chromatin with ill-defined cytoplasm (Figure 1G), and showed palisading and clustering around fibrillary material (Figure 1D), which was green to orange coloured on PAP (Figure 1E), and dense bright pink to metachromatic on MGG (Figure 1F). Frank haemorrhage was not seen though occasional hemosiderin macrophages were observed in the background. Focal dystrophic calcification (Figure 1H), and occasional intranuclear inclusions were also eminent (Figure 1I). No mitosis, distinct atypia or necrosis was identified. The tumor cells were negative for pancytokeratin on immunocytochemistry. Cytological diagnosis of low-grade spindle cell lesion was given and lesion was excised for further characterization. Afterwards an excisional biopsy of intact lymph node was evaluated and a thin peripheral rim of lymphoid tissue was identified underneath the fibrous capsule along with a tumor mass composed of interlacing fascicles with uniform spindle cells showing abundant eosinophilic cytoplasm and insignificant atypia, mitosis and necrosis. The cells were arranged in palisading pattern with ‘amianthoid’ like fibers characterized by an eosinophilic center surrounded by a paler stellate-shaped periphery (Figure 1J). Extravasation of RBCs and minimally scattered hemosiderin deposits were seen throughout the lesion. The cells were immunopositive for smooth muscle actin (SMA) (Figure 1K), vimentin and showed nuclear expression of Cyclin-D1 (Figure 1L) while immunonegative for pancytokeratin, S100 protein, CD34, desmin, CD117/C-kit and DOG-1 favoring of “intranodal palisaded myofibroblastoma”.

IPM is a rare benign mesenchymal neoplasm most commonly seen in inguinal lymph nodes as painless slow growing lump, though sporadic case reports from submandibular, cervical and axillary nodes are available which are characterized by intranodal proliferation of spindle cells with smooth muscle differentiation, often with the presence of amianthoid like fibers [6,10].

On histopathology, characteristic findings include an outer rim of compressed lymphoid tissue separated from the tumor by a thick pseudo-capsule and tumor composed of fascicles of spindle cells characterized by elongated nuclei with dispersed chromatin, fibrillary eosinophilic cytoplasm and indistinct cell margins showing palisading [1,2]. A striking feature is the presence of stellate areas of collagen deposits described as “amianthoid fibers” in the literature [5]. Various studies have established the nature of amianthoid fibers as collagen fibers with center composed of type I collagen and periphery of type III collagen [4]. Scattered foci of hemosiderin pigment and extravasated erythrocytes may be seen. Mitosis or necrosis is rare.

Cytological descriptions are limited to a few reports and stress on variable cellularity and presence of amianthoid like fibers [4- 9]. Most of the studies reveal typically high to moderate cellularity as also observed in our case (Table 1).

Bland looking spindle cells with blunt to tapered ends showing palisading as described by Dei et al. [4], and Sood et al. [7], were also noted and help to distinguish IPM from schwannoma which characteristically shows buckled nuclei in fibrillary background. As highlighted by previous studies dense metachromatic stroma and amianthoid like fibers can be visualized on MGG and PAP stains. Likewise a scant aspirate showing small fragments and singly lying cells with minimal matrix formation can be a diagnostic challenge and recognition of blunt ended nuclei will be helpful in such cases. We also noticed that in absence of amianthoid fibers, presence of traversing blood vessels (RCC like vascular pattern) within the clusters could be a helpful diagnostic feature as reported by Sood et al. [7]. Similarly, intranuclear inclusions could be a helpful feature. Another interesting feature was very scant to minimal hemosiderin granules in the background inspite of abundant hemorrhage in the sections. We consider this could be due to sampling error in our case; needle may have traversed more cellular part of the node rather than hemorrhagic part.

Immunohistochemically, the spindle cells of IPM show positivity for SMA and vimentin while are negative for S100, CD34 and desmin, they show a low proliferating index of Ki67. Recently IPM has been shown to have a strong expression for cyclin D1. It is essential to differentiate IPM from other soft tissue tumours, which include Schwannoma, Kaposi’s sarcoma, intranodal leiomyoma/leiomyosarcoma and metastatic GIST. Intranodal schwannomas in particular are most commonly confused with IPM; however they are composed of spindle cells with frequent nuclear palisading and buckled nuclei in a fibrillary background and lacks amianthoid fibers. Kaposi sarcoma in lymph node aspirates shows loosely cohesive aggregates of bland spindle-shaped cells with prominent streaking and nuclear crush artifact on variably hemorrhagic background on cytology. Leiomyomas/Leiomyosarcomas are characterized by spindle cells with cigar shaped nuclei without amianthoid fibers and infrequent palisading. Metastatic GIST shows minimal atypia, occasional mitosis and lacks amianthoid fibers.

Intranodal-palisaded myofibroblastoma has distinctive and fairly reproducible cytomorphological features. Apart from bland spindle cells and amianthioid fibers as essential diagnostic clues, intranuclear inclusions and dystrophic calcification can be useful features for diagnosis. Hemorrhagic background or hemosiderin laden macrophages may be scarce in cellular aspirates. In difficult cases immunopositivity for SMA and Cyclin D1 is helpful. Recognition of these features will help in correct identification of this benign entity on cytology.

Published research is exempted from ethics committee approval as the aspirate and excision biopsy were performed for diagnostic purposes after obtaining informed consent.

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