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Editorial |
1 Melanoma and Skin Cancer Research Institute, Royal Prince Alfred Hospital, Camperdown, New South Wales 2050, Australia
2 Sydney Melanoma Unit, Royal Prince Alfred Hospital, Missenden Road, Camperdown, New South Wales 2050, Australia
3 Department of Anatomical Pathology, Royal Prince Alfred Hospital, Camperdown, New South Wales 2050, Australia
4 Discipline of Surgery, Faculty of Medicine, University of Sydney, New South Wales 2006, Australia
Correspondence: Address correspondence and reprint requests to: John F. Thompson, MD, FRACS, FACS; Sydney Melanoma Unit, Royal Prince Alfred Hospital, Gloucester House, Camperdown, New South Wales 2050, Australia; E-mail: thompson{at}smu.org.au.
Desmoplastic melanoma (DM) is an unusual variant of cutaneous melanoma that displays many clinical and pathologic differences compared with other types of melanoma. Since the original description of DM in 1971 by Conley et al.,1 the features of more than 600 patients with DM have been documented. These descriptions have shown that DM tends to occur in a slightly older age group than conventional types of melanoma and that it is more frequent in men, is usually amelanotic, and most often occurs in the head and neck region in individuals with chronic sun damage.2 Pathologically, DM is characterized by the presence of atypical spindle cells separated by collagen fibers or fibrous stroma, and there may be no association with an overlying epidermal or in situ melanoma component.2 In view of these features, it often presents a diagnostic challenge, and both clinicians and pathologists who are not familiar with DM or its morphological spectrum may not recognize it as a melanoma. As a result, DM is frequently not diagnosed correctly until it reaches an advanced clinical stage. Neurotropism is a common associated feature, and tumors in which it occurs are termed desmoplastic neurotropic melanomas.3
The prognosis for patients with DM has been the subject of considerable controversy. Of the seven patients described in the original report by Conley et al.,1 five developed local recurrence, three developed nodal metastases, and four died of disease. On the basis of these findings, the authors characterized DMs as "highly malignant, stubbornly recurring and often metastasizing neoplasms." The perception that DM was associated with a worse clinical outcome than other types of melanoma was supported by Egbert et al.4 in 1988, when they reported a series of 23 patients with DM and suggested that DM "is more aggressive than other types of melanoma." However, neither of the aforementioned studies provided information about important pathologic prognostic features of the reported cases, such as their Breslow thickness, ulcerative state, or tumor mitotic rate. This makes it impossible to draw reliable conclusions from these studies about differences in clinical outcome between DM and other types of melanoma. Conversely, several subsequent studies (in which adjustments for tumor thickness were made) indicated that although DM was associated with higher rates of local recurrence, survival times were longer than they were for patients with conventional types of melanoma of similar thickness.5–7 However, a large recent series from the Sydney Melanoma Unit did not detect any difference in survival rates between patients with DMs and those with conventional melanomas8 when allowance was made for other prognostic pathologic variables. Recent reports have also suggested that DMs are less frequently associated with sentinel node (SN) metastases than are non-DMs: reported rates of SN positivity for DMs range from 0% to 12%.9,10
The conflicting results in the literature about the risk of regional node field metastasis and the prognosis for patients with DM are probably attributable to heterogeneity among tumors classified as DMs and to failure to account for differing median tumor thicknesses in various series. The lack of a precise pathologic definition of DM and the often vague descriptions of the inclusion criteria for the various types of tumors in many of the published series of DM are probable additional contributing factors. In this issue of Annals of Surgical Oncology, Hawkins et al.11 report the large experience of cutaneous DM from the Memorial Sloan-Kettering Cancer Center (MSKCC) in New York, and the authors make some progress toward redressing this problem. It is important to note that they recognized a spectrum in the extent of desmoplasia within the tumors, and, unlike all other studies of DM so far reported (apart from two other recently published reports from the same institution9,12), they subclassified DM into "pure" and "mixed" types for their analyses. Melanomas in which the desmoplasia was prominent throughout the vast majority of the tumor were classified as pure DMs, whereas melanomas with only partial desmoplasia in an otherwise nondesmoplastic tumor were classified as mixed DMs. The patient demographics and primary tumor characteristics in this series were similar to those in other reported large series of DM. However, in this study, the authors found that regional nodal metastasis was very uncommon in patients who presented with clinically localized pure DM (1%) compared with those who had mixed DM (10%) or conventional melanomas (6%; P < .05, pure DMs vs. conventional melanomas). They also found that patients with the pure subtype of DM had a favorable outcome compared with those who had mixed DM (5-year melanoma-specific mortality, 11% vs. 31%; P < .01). Despite a more than 2-fold difference in Breslow thickness of the primary tumors, patients with pure DM had a similar melanoma-specific mortality compared with patients who had conventional melanomas. In patients with clinically localized disease at presentation and who were treated definitively by wide local excision with histologically confirmed clear margins, a higher rate of local recurrence was found for DM (pure or mixed; 7%) compared with patients with conventional melanoma (2%; P < .001).
It is important to note that in their study, Hawkins et al. highlight the heterogeneity of DMs. The concept by the MSKCC group of subcategorizing DM into pure and mixed types is a significant contribution to the field. To allow comparison of results from different studies, it is imperative that investigators apply a robust and reproducible pathologic definition for DM and its subcategories. The MSKCC group is to be congratulated in this regard, because they have clearly defined pure DM as "spindle cell melanomas with overwhelming stromal fibrosis"11 and mixed DM as tumors with "only partial desmoplasia associated with an otherwise non-DM."11 Even so, there seem to be some minor differences in what constitutes "overwhelming fibrosis" in separate studies from the MSKCC group. In the current study, desmoplasia was noted to be prominent in 80%–90% or more of the tumor in most melanomas in the pure DM group,11 whereas in another study from the same institution (and presumably involving a similar cohort of patients), prominent stromal fibrosis was required in 
90% of the invasive melanoma for a tumor to be placed in the pure DM subcategory.12 These minor issues aside, other investigators undertaking future studies of DM should be encouraged to adopt the subcategorization used by the MSKCC group and to clearly define how it is applied. The observation that separating DM into pure and mixed subgroups has important prognostic significance highlights the practical implications of this study. If other series confirm the rarity of SN metastases in patients with pure types of DM, there may be no benefit in performing an SN biopsy as a staging procedure in these patients.
Received for publication December 2, 2004. Accepted for publication January 10, 2005.
REFERENCES
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