Pre-treatment evaluation of basal cell carcinoma for photodynamic therapy: comparative measurement of tumour thickness in punch biopsy and excision specimens.

Tumour thickness affects the outcome of photodynamic therapy in basal cell carcinoma (BCC). The aim of this study was to evaluate whether punch biopsy provides reliable information on BCC tumour thickness, by comparing corresponding measurements in biopsy and excision specimens for 48 lesions in 43 patients. BCC tumours were between 0.2 and 6.1 mm thick. The mean depth of the excisions were 0.14 mm greater than that of the biopsies. Bland-Altman 95% limits of agreement were (-1.3, 1.6) mm, but the difference between measurements increased with tumour thickness. A punch biopsy tumour thickness of 1.0 mm yielded an upper 95% predicted limit for excision depth within 2.0 mm. In conclusion, there was reasonable overall agreement between corresponding measurements. A biopsy thickness of 1.0 mm suggests that the tumour will most likely be within the current accepted limits for photodynamic therapy. With increasing tumour thickness, however, individual tumour measurements may differ considerably.