The growth pattern on a basal cell carcinoma report: nodular, superficial, infiltrative, basosquamous

WHO names the subtypes and Britain sorts them. The fourth edition of the WHO skin classification (2018) listed ten: nodular, superficial, micronodular, infiltrating, sclerosing or morphoeic, basosquamous carcinoma, pigmented, with sarcomatoid differentiation, with adnexal differentiation, and fibroepithelial (of Pinkus). The fifth edition (2025) lists the same nine minus pigmented basal cell carcinoma, which no longer has a heading of its own and survives as a related term under the general ICD-O code. The UK reporting dataset uses the WHO classification and divides growth pattern into low and high biological risk (RCPath G123).

The low-risk patterns. **Superficial** basal cell carcinoma is made of multiple small collections of follicular germ cells in contact with the epidermis or hair follicles, most widely defined as not extending beyond the papillary dermis, and all published thickness definitions are under 1 mm; it is often called multicentric or multifocal, which the dataset calls a misnomer since the apparently separate islands are usually continuous. It is often accompanied by a stromal reaction in the upper dermis that may be the only abnormality visible in a small biopsy. The dataset notes that there is no consensus on whether superficial basal cell carcinoma is in situ or invasive, which is why its title avoids the phrase invasive basal cell carcinoma altogether. **Nodular** basal cell carcinoma grows as nodules of varying size, sometimes cystic or with keratin cysts, larger than the micronodules below; it is the commonest pattern. **Fibroepithelial** basal cell carcinoma (of Pinkus) is contested, with a long argument over whether it is a basal cell carcinoma or a benign trichoblastoma; WHO and therefore the UK dataset treat it as low risk.

The high-risk patterns. **Infiltrating** basal cell carcinoma grows as irregular islands and strands with a jagged or spiky outline. **Sclerosing or morphoeic** is that same infiltrating pattern with dense stromal fibrosis around it, which is what makes a morphoeic tumour feel like a scar and makes its edge impossible to see. **Micronodular** grows as small round nodules of follicular bulb size, defined as under 0.15 mm across, roughly fewer than 25 cells wide; the dataset asks that the term be kept for tumours that also infiltrate at the edge. All three are reported together in the UK as **infiltrative basal cell carcinoma**, because, in the dataset's words, there is no clinical value in distinguishing them for management or treatment, and they often co-exist in one tumour with overlapping forms. Naming them individually is optional.

**Basosquamous** carcinoma is not a growth pattern but a differentiation: a basal cell carcinoma containing a moderately or severely atypical or frankly malignant squamous component. The term has been used loosely for collision tumours and for the poorly defined metatypical basal cell carcinoma, but there is reasonable evidence that basal cell carcinoma with squamous atypia recurs and metastasises more often, so the UK dataset asks for it to be identified and treats it as high risk. The many other differentiations a basal cell carcinoma can show, pigmented, adenoid, keratotic, clear cell, granular, with eccrine, apocrine, sebaceous or follicular components, need not be recorded at all: they do not change what is done.

In practice most tumours are composite, containing low-risk and high-risk patterns at once. There is no evidence about what percentage or position of a high-risk pattern matters, so the UK rule is pragmatic: the overall risk status is read from the highest-risk pattern present, irrespective of percentage or location, and once a high-risk component is recorded the low-risk ones need not be.