Diagnosing skin cancer in primary care: how do main-stream general practitioners compare with primary care skin cancer clinic doctors?

To the Editor: In a recent article, Youl and colleagues provided information about the ability of doctors to accurately diagnose skin lesions that they excise or biopsy.1 We wish to offer some comments about their comparison between general practitioners and skin cancer clinic doctors.

First, in the study by Youl et al the behaviour of GPs and patients in mainstream practice was different from that of doctors and patients in skin cancer clinics, as indicated by the comparative frequency of whole body skin examinations performed (GPs, 30.4%; skin cancer clinic doctors, 73.2%).1 The study did not indicate the circumstances under which each decision to excise took place. Did patients become aware of a new or changing skin lesion and bring it to the attention of the doctor, or did the diagnosis result from a whole body skin check that might have revealed an earlier, previously unnoticed and more subtle lesion? It may be useful to separate basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) into histological subtypes, as early superficial BCC and intraepidermal SCC may be more difficult to diagnose than other subtypes.2

Second, the casemix of non-melanotic skin cancers in the two groups of doctors was quite different, with a BCC : SCC ratio of 1.1 : 1 for GPs and 2.1 : 1 for skin cancer clinic doctors. The difference in casemix was reflected in a study of our own3 in which we described the histology of 1247 lesions excised by doctors, including 76 lesions removed by one doctor in a designated skin cancer clinic. In an unpublished sub-analysis, we divided the results into two settings for comparison (Box). Like Youl et al, we found that the casemix of non-melanotic skin cancers was significantly different for the two groups of doctors (P < 0.001), but in our study the BCC : SCC ratio was much higher for skin cancer clinic doctors (4 : 1) than for GPs (0.6 : 1). We believe this most likely reflects an increased pick-up of BCC in skin cancer clinics, owing to the higher frequency of full body skin examinations and the consequent detection of lesions of which the patient is unaware.

Third, the reported sensitivity and specificity in the study by Youl et al refers only to excised lesions. There is no information given about the lesions that practitioners decided not to excise. The sensitivity and specificity of skin examinations can only be determined if all relevant skin lesions are assessed, thereby giving an accurate representation of the number of true- and false-negative diagnoses. However, this would require multiple excisions, which would be clinically unacceptable. An important limitation of the study is that it does not assess or compare how many skin cancers each group of doctors missed.

In conclusion, although the study by Youl et al provides comprehensive information about diagnostic accuracy, we do not feel — based on the information available — that a meaningful comparison between the two groups of doctors can be made.