Dear Editors, Most basal cell carcinomas (BCC) can be effectively controlled with local treatment. However, in certain histological subtypes, including infiltrative BCC, tumor nests infiltrate in cord-like or micronodular patterns, which often obscure tumor boundaries both clinically and histopathologically [1]. It has also been reported that recurrent lesions may exhibit atypical patterns of tumor spread [2].

An elderly man in his 80s presented with a recurrent lesion on the posterior neck. Ten years earlier, he had undergone complete surgical excision of a basal cell carcinoma (BCC) at another hospital with a 5-mm margin to the level of the subcutaneous tissue, and the surgical margins were histopathologically negative. Approximately 1 year before presentation, an erythematous plaque appeared at the same site and gradually enlarged.

At presentation, an ill-defined erythematous plaque measuring approximately 7 cm in diameter was observed extending from the occipital region to the posterior neck, accompanied by a palpable subcutaneous nodule (Figure 1A). Computed tomography showed a corresponding high-density lesion without lymph node enlargement or distant metastasis. Skin biopsy confirmed recurrent BCC.

The tumor was excised with a 1-cm margin and the defect was reconstructed using a full-thickness skin graft. Histopathological examination revealed infiltrative BCC with deep invasion and superficial lateral microextension beyond the clinically apparent lesion (Figures 1B–D). The lateral margin was positive, and the minimum distance between the tumor and the deep surgical margin was 0.5 mm. Three weeks after the initial excision, mapping biopsies were performed at 13 sites to assess lateral tumor spread. The biopsies extended approximately 5 mm in depth to include the superficial subcutaneous tissue. Approximately 6 weeks later, additional excision was performed, with the resection boundary determined by connecting tumor-negative mapping biopsy sites (Figures 1E,F). Histopathological examination confirmed complete tumor clearance. No recurrence was observed at 6 months after the final excision.

Histopathological examination of the primary excision specimen revealed that tumor nests were distributed predominantly within the subcutaneous tissue, with only limited continuity with the epidermis. Such a distribution may be difficult to recognize clinically as a superficial lesion [3] and may lead to underestimation of tumor extent at the time of the initial excision. Because the surgical margins were judged to be negative, the lesion was followed without further treatment and eventually recurred after a long interval.

In the recurrent lesion, the tumor formed a nodular mass within the deeper subcutaneous tissue, suggesting a more aggressive growth pattern [4]. At the same time, the tumor retained subtle lateral microextension within the superficial subcutaneous layer beyond the deep nodular component, making accurate margin determination difficult. This growth pattern may explain why the tumor extent was difficult to appreciate clinically and why conventional margin determination based on surface findings was insufficient in this case.

Changes in skin coloration and a palpable subcutaneous nodule were used as clinical landmarks for estimating the tumor boundary; however, margin determination based solely on palpation was insufficient to capture the lateral tumor spread within the superficial subcutaneous layer.

Although Mohs micrographic surgery is recommended for high-risk BCC [5], its availability varies among institutions. In retrospect, given the difficulty in clinically determining the tumor extent in this case, temporary coverage with artificial dermis at the initial excision, followed by definitive reconstruction after histopathological confirmation of clear margins, might have been a reasonable alternative approach. In situations where Mohs surgery is not feasible, mapping biopsy may be useful for assessing tumor extent and guiding appropriate surgical margins.