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Spiradenomas (SA) and cylindromas (CS) are benign skin appendage tumours, commonly arising in the head and neck area, previously known as tumours with eccrine differentiation; it is assumed today that they are neoplasms of the folliculosebaceous apocrine unit [1]. These tumours are very closely related and can be seen either as separate entities or combined as spiradenocylindromas [2]. The clinical and dermoscopic aspects of both tumours, however, are not pathognomonic; histopathological evaluation remains the gold standard of diagnosis. We herein report two clinical cases, where high-frequency ultrasound (HFUS) allowed a thorough characterization and oriented the diagnosis towards benign adnexal tumours. A 74-year-old man was referred to the dermatologic Department with a completely resected malignant melanoma of the upper back, with a Breslow thickness of 6.8 mm (pT4a cN0 cM0). Upon clinical assessment prior to performing re-excision and sentinel node biopsy according to European Guidelines, a 1.5 cm large, solitary, pinkish-purple, solid nodule in the parietal area was identified (Figure 1a). The lesion developed in the last year, having slow growth and being slightly painful upon manipulation. The differential diagnosis included basal cell carcinoma (BCC), Merkel cell carcinoma (MCC), glomus or adnexal tumour and skin metastasis. Dermoscopy revealed a structureless pattern, pinkish areas and arborizing vessels. HFUS (AplioA i800 from Canon Medical Systems, 17LH7 probe) and superb microvascular imaging (SMI) of the lesion displayed a well-defined dermo-hypodermal lesion composed of aggregated nodules in a ‘rosette-like’ pattern, no epidermal involvement, heterogeneous echogenicity, posterior acoustic enhancement and decent peripheral and central vascularization (Figure 1b−d). No hyperechoic spots (characteristic for BCCs) or chaotic vascularization pattern (often seen in metastases or MCC) were identified. Furthermore, the slow growth of the tumour was also inconsistent with a malignant process, these knowingly being characterized by faster growth. A glomus tumour was also excluded, as these lesions, even though painful, usually only involve the superficial dermis. Upon suspicion of an adnexal tumour, a local excision was performed. The histopathologic examination showed a well-circumscribed, non-encapsulated, dermal tumour without connection to the surface epithelium. Several aggregated nodules of different sizes, composed of monomorphic basaloid cells, intermingled with larger pale cells, little droplets of basement membrane material and lymphocytes could be identified. Furthermore, small intratumoral haemorrhages and a few ductal structures were also reported (Figure 1e). A 65-year-old patient presented with an isolated, exophytic growing, nodular, firm tumoral lesion at temporal level, with an irregular and alopecic surface, pink-brown discoloration and telangiectasia (Figure 2a). The lesion had appeared 2 years ago as an erythematous macule that developed slowly to a well-circumscribed, painless plaque of increased consistency, adherent to the deep planes. Dermoscopy showed a coloured background with pink/white/yellow areas, arborizing telangiectasia, dilated blood vessels, blue globules and fine white lines surrounding the blue globules. Differential diagnosis included BCC, trichoepithelioma, cyst and adnex tumour. HFUS revealed a well-defined, solid, bilobed lesion extending into the subcutaneous tissue, with different shades of echogenicity within, arranged in a ‘jigsaw-like’ pattern. Hyperechoic lines within the lesion, corresponding to fibrous septa, were seen separating the hypoechoic areas of cell nests. Singular anechoic areas within the lesion were identified as corresponding to fluid deposits. On colour Doppler, ample central and peripheral vascularization was identified (Figure 2b,c). Histology showed a sharply circumscribed lesion of the dermis extending into the subcutis with two cell populations: basaloid cells, displaying dark nuclei, localized at the periphery, in close apposition (palisade-like) and larger pale cells located in the middle of the cellular lobules, displaying pale nuclei. At the periphery, a dense eosinophilic and PAS -positive material surrounding each lobule like a thick basement membrane was seen. The nests of basaloid cells, fitting together in a jigsaw-like appearance lead to the diagnosis of CS (Figure 2d). SA and CS are benign adnexal tumours, representing two morphological variants of the same neoplasm. Malignant transformation is rare, and it has been mostly described in cases of multiple lesions of the scalp [3]. SA, CS, trichoepitheliomas and sometimes parotid gland tumours can arise together in Brooke−Spiegler syndrome, an autosomal dominant inherited disease caused by mutations in CYLD gene located on chromosome 16q12-q13 [4]. CS appearing as multiple papules or nodules of the scalp, coalescing to each other, like a turban-like structure, is named a turban tumour, whereas for solitary lesions, no family history association was reported [5]. From a histological point of view, CS presents epithelial aggregates of basaloid cells in a ‘jigsaw-puzzle’ appearance, while SA consists of similar, fewer but larger epithelial aggregates scattered by lymphocytes, myoepithelial and epithelial cells in a cribriform pattern. Immunohistochemical stains are less important for diagnosis. Both tumours are positive for low molecular cytokeratin, ductal structures can be highlighted by CEA, myoepithelial cells by SMA and S100. Clinically, CS are usually firm, not painful, singular or multiple, pinkish tumours of various sizes, with localized loss of hair, most seen in the head and neck area, with the scalp being extensively involved. SA, on the other hand, usually appears as solitary, painful lesions of the head and neck, trunk or extremities. They are mainly located in the dermis and subcutaneous tissue, appearing as bluish, firm nodules [6]. The pain often accompanying SA is supposed to originate from the nervous plexus surrounding the connective tissue capsule. Dermoscopy of CS usually identifies arborizing vessels on a whitish-yellowish background, structureless light pink areas surrounded by white linear structures, while SA usually have a bluish-pink background, with branched vessels and occasional blue-ovoid clods [7]. However, the clinical and dermoscopic appearance alone are often not enough to correctly diagnose these entities. The use of HFUS in the pre-therapeutic setting can increase diagnostic accuracy and orient the diagnosis. For SA, differential diagnosis usually includes other painful tumours, as well as schwannomas, hemangiomas, leiomyomas, glomus tumours and skin endometriosis. Schwannomas are usually oval, well-defined hypoechoic lesions, with little Doppler flow and a characteristic rat tail sign on HFUS, referring to a thin, hyperechoic line extending from the tumour, representing the nerve along which schwannomas usually develop [8]. Hemangiomas are hypo- or hyperechoic lesions with increased vessel density. Leiomyomas are well-defined, oval, hypoechoic structures most seen at calves or ankles while glomus tumours are oval, hypoechoic and intense vascularized lesions. Lastly, skin endometriosis usually appears on US as ill-defined, irregular, heterogeneous lesions which change size according to the menstrual cycle [9, 10]. Sonographic findings of SA have been previously reported and included, just like in our case: well-defined lobulated masses of the dermis and subcutis, without connection to the overlying epidermis, heterogeneous echogenicity, dorsal acoustic enhancement and scattered vascular flow both in the centre and periphery of the lesion [11]. HFUS with the typical sonographic findings of lobulated structures with central mixed echogenicity, arranged in a ‘rosette-like pattern’, very accurately correlates to the histological appearance of SA. For singular CS, clinical diagnosis can also be challenging as it can mimic epidermal cysts, BCCs, trichoepitheliomas and so forth. Epidermal cysts are well-circumscribed, round-oval, avascular structures of the subcutaneous tissue with a dorsal acoustic enhancement, lateral shadowing and a porus connecting the lesion to the surface as typical sonographic findings. Also, BCCs and trichoepitheliomas usually display on HFUS characteristic hyperechoic spots which are not seen in CS [12]. The sonographic aspect of CS tumours, displaying different shades of echogenicity within the lesion appears to correspond to the ‘jigsaw-puzzle’ aspect previously described histologically for these tumour entities. The presence of singular anechoic areas within the tumour, as shown in our case, most probably corresponds to ductal lumens filled with secretion on the histological assessment. HFUS in dermatology can improve the diagnostic accuracy of CS and SA. While both represent benign sweat gland tumours, their differentiation from other entities by means of clinical aspect, dermoscopy, growth rate, presence or absence of pain is challenging without an imaging tool to help narrow down the list of differential diagnoses. SA can be differentiated by means of HFUS from other painful tumours by its typical appearance consisting of lobulated masses of the dermis and subcutis, like ‘rosettes’, heterogenous echogenicity, posterior acoustic enhancement and scattered vascular flow. For CS, the ‘jigsaw-puzzle’ aspect within the lesions, referring to different areas of echogenicity blending like pieces of a puzzle, histologically corresponding to the ‘jigsaw’ appearance of the tumour stroma might help clinicians better orient the diagnosis. Nonetheless, while HFUS is a fast, noninvasive method enabling an accurate evaluation of skin tumours in terms of size, shape, vascularization and histomorphological aspects, it is an operator-dependent examination, requiring adequate training to avoid misdiagnoses and increase diagnostic accuracy. Diana Crisan: conceptualization, methodology, clinical management, data collection, writing – original draft, writing – review and editing, supervision. Tina Weiss: histopathological analysis, data analysis, validation, writing – original draft and review. Karin Scharffetter-Kochanek: histopathological analysis, validation, review. Yusef Sallum: data collection, clinical management, writing review. Ioana-Ancuta Ungureanu: histopathological analysis, validation, writing review. Maria Crisan: clinical management, data analysis, writing, review, and supervision. The authors would like to acknowledge Heiko Grandel for his photographic and formatting expertise. Open Access funding enabled and organized by Projekt DEAL. The authors received no specific funding for this work. The authors have nothing to report. All patients in this manuscript have given written informed consent for participation in the study and the use of their de-identified, anonymized, aggregated data and their case details (including photographs) for publication. Ethical Approval: not applicable. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.