Keratoacanthoma

Case 332   Sept 2011 (P. McKee)  M 79 nodule cheek 

One high power view.  Textbook glassy cytoplasm. 

Case 542  6.7.12  (R. Carr)  F 70 nodular skin lesion on arm. 

Regressing phase.  Classic crateriform architecture; lichenoid reaction; ground glass cytoplasm and intra-epithelial elastic fibres. 

Early proliferative phase: often quite striking cellular pleomorphism with highly proliferative infiltrative (not pushing circumscript borders like well differentiated SCC) borders with prominent elastic incorporation.  Central (often abrupt) transition to highly differentiated and distinctive pilar keratinisation. Incorporation of elastic can be a feature of poorly differentiated SCC but not of well differentiated SCC (that usually have much more pushing mitotically inactive borders ).  In RC’s view elastic incorporation is rarely a true diagnostic consideration in the histological differential diagnosis of an early KA. 

Case 593   17.9.12 (P. McKee)  M 40 lesion on arm 

Regressing phase with incidental tattoo pigment. 

Case 694 12.2.13 (P. McKee) F 64 “squamous” lesion 

Textbook regressing KA 

Case 1172  19.12.14 (R. Carr) F 75 arm 

RC argues the case for KA but not universally agreed.  Perineural invasion is apparently allowed in KA  

Ref:  Godbolt AM.  KA with perineural invasion. Australas J Dermatol 2001; 42(3):  168-71. 

Case 1309   30.6.15  (U. Sundram)  F 45 keratotic lesion, leg 

Superficial biopsy cut tangentially not showing full architecture.  Coexistent tattoo.  Pigment artefact due to plastic coverslip. 

Case 1551 3.6.16 (R. Carr)  M 55 destructive finger-tip lesion (sub-ungual KA -amputation) 

Typical glassy change.  These lesions tend not to regress.  Can be marker of incontinentia pigmenti.  

Case 1907  19.9.17  (R. Carr)  M 80 8 week history (*teaching) 

Typical features here: symmetry; abrupt maturation in centre of islands; no acantholysis; no follicular mucin 

Atypical features:  small size; pushing borders not reahing sweat coils; no elastic entrapment; lacks significant nuclear pleomorphism. 

Case 2119  20.7.18  (R. Carr)  F 72 neck (*teaching) – textbook case 

Detailed discussion of comparison with follicular SCC. 

KA features: symmetrical, exo-endophytic lesion with a rounded/arciform profile to the base, a well-formed epidermal collarette, an abrupt transition from basaloid peripheral cells to central cells with glassy eosinophilic cytoplasm and tricholemmal keratinisation, engulfed collagen and elastic fibres, a well-formed laminated keratin-filled crater, neutrophil microabscesses, and regressional features including lichenoid reaction, peripheral individual cell necrosis and fibrosis. We allow infiltrative proliferative peripheries with cellular pleomorphism limited to the peripheral cell layers provided that central maturation is present in all areas of the tumour. 

FSCC: distinctive circumscribed lobular profile with frequent presence of follicular mucin; prominent dyskeratosis, ‘dirty’ parakeratosis (hyperchromatic enlarged nuclei and prominent cellular debris), ulceration and zonal necrosis. 

Case 2682  16.10.20 (R. Carr)  F 45 cheek ?SCC 

Regressed KA.  Residual lichenoid reaction.  p16 is mosaic and p53 wild type which fits with KA or a benign / reactive proliferation. The EVG in the perineural focus shows evidence of the prior more infiltrative proliferation (foci of trapped collagen & elastic). KA with perineural invasion are particularly common in the central face and peri-oral location and they may also show venous invasion at this location too. 

Case 2697   06.11.20 (R. Carr) F 75 ?KA 

IHC: wild type p16 (strong mosaic), wild type p53 (moderate to strong in periphery but weakens in mid zone and no moderate or strong in the inner third), peripheral only Ki67 all compatible with KA. 

Case 2792  19.3.21 (R. Carr)  F 87 Nodule calf 

Clinical morphology, age of patient and site favour KA, as does the histology.  R. Carr’s scoring system 9/32 (two assessors), below threshold for fSCC. 

R. Carr discusses potential biology of KA’s. 

Case 4600 25.2.25 (J Costa-Rosa)  M 75 fast growing lesion on wrist 

Regressed phase with cup-shaped cutaneous horn on a bland base.