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 In an unpublished sub-analysis, we divided the results into two settings for comparison (Box).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.

To the Editor: Youl and colleagues are to be commended for their research into the performance of special-interest skin cancer clinicians.1 However, the conclusion that the performance of general practitioners and skin cancer doctors in the diagnosis of skin cancer is similar is highly questionable.

To truly compare the two groups and their diagnostic accuracy, it must be established that the participants were representative of the groups they are supposed to represent. The fact that the participating GPs were largely self-selected, perhaps on the basis of their personal interest in the subject, was a potential flaw that the authors acknowledge.

Further, an examination of diagnostic accuracy should also take into account whether the lesions found were of similar type, size and stage. There was no determination of any qualitative differences (eg, in size or thickness) between the tumours seen and diagnosed by the two groups. When a patient presents, specifically, with a large, tender, hyperkeratotic squamous cell carcinoma (SCC), there is no real test of diagnostic skills. On the other hand, detecting a small early posterior-thigh melanoma or a superficial BCC on a whole body examination is a challenge. Overall, one would expect at least two to three BCCs to be diagnosed for each SCC found2 — however, in the study by Youl et al, GPs found a similar proportion of each type of lesion, suggesting that perhaps a large number of BCCs were not being detected at all in the GP group.

The lower incidence of whole body examinations in the GP group suggests that a higher proportion of asymptomatic lesions may have been missed by the GP group and thus not included in their sensitivity/specificity data. This possibility could have been examined by noting the site of lesions found: identification and diagnosis of lesions in areas covered by clothing or footwear may be more likely on whole body examination.

Although Youl et al reported that the diagnostic sensitivity for melanoma among skin cancer clinic doctors was twice that of GPs, a re-examination of the data with all of the above in mind may well reveal that skin cancer clinic practitioners are performing even better than suggested.

That being said, the number of melanomas found per week by the skin cancer group (0.25 melanomas/doctor/week) in the study by Youl et al is much lower than in our own dedicated primary care skin cancer clinic (1.47 melanomas/doctor/week, based on audit data gathered between February and September 2007).

Good medical care relies on accurate diagnosis and appropriate treatment, and the earlier the diagnosis is made, the less costly and less invasive the treatment and the greater likelihood of a cure. Surely we should all be working together — skin cancer clinic practitioners and GPs, alike — towards achieving optimum outcomes for our patients. To that end, a wider range of higher education in skin cancer medicine is being encouraged by the University of Queensland’s Master of Medicine program3 and the Skin Cancer College of Australia’s fellowship program,4 and bodies such as the Skin Cancer Society of Australia and the Royal Australian College of General Practitioners have been working together to introduce an accreditation process for those practising primary care skin cancer medicine.5,6

In reply: We thank Heal and Raasch, and Keir for their comments and suggestions. First, as acknowledged in our article, it is possible that general practitioners with an interest in skin cancer may have been over-represented in our sample. It is also the case that the comparison between GPs and skin cancer clinic doctors may have been affected by characteristics of patients attending each type of practice. Patients attending skin cancer clinics are self-selected (often worried about a specific skin lesion), while for those attending GPs in mainstream practice, skin lesions are more likely mentioned during a consultation for something else.1 Case selection, which helps increase diagnostic ability by Bayesian principles (improving pretest probability of malignant lesions), may thus be more likely for skin cancer clinic doctors than GPs.2

Our study represented a broad cross-section of skin cancer clinics, and the number of melanomas excised per doctor per week ranged from 0 to 1.7. The ratio of basal cell carcinomas to squamous cell carcinomas excised ranged from 0.1 to 6.0 for mainstream GPs and 0.8 to 8.0 for skin cancer clinic doctors. Whether clinical and histological features of skin lesions or the type of skin examination undertaken influence diagnostic accuracy was beyond the scope of our initial study. This question will be the subject of future analyses.

It has been suggested that a limitation of our study was that it did not assess or compare the number of skin cancers each group missed. However, as specifically stated in our article, the aim of our study was to examine diagnostic accuracy of excised or biopsied lesions. To examine the sensitivity and specificity of all lesions (excised and non-excised) and of screening examinations would require a different study design.

We disagree that meaningful comparisons between the two groups cannot be made from the data collected in our study. Our prospective study included over 11 000 skin excisions or biopsies from a large group of mainstream GPs and from doctors working in a variety of skin cancer clinics. One of the most important outcome measures of our study was the degree of accuracy of skin cancer diagnoses within the primary care setting.

We have demonstrated that primary care practitioners, whether mainstream GPs or skin cancer clinic doctors, diagnose skin cancer with similar, high levels of accuracy. This is a reassuring result, particularly in a country with the world’s highest incidence of skin cancer.