CASE REPORT
A 58-year-old Caucasian male, with a negative family history of melanoma, was referred to the Dermatology Clinic in March 2021 for a routine nevi check-up. Under dermoscopy, attention was drawn to an asymmetric lesion of unknown duration located in the lumbar region. The lesion consisted of two components — the left part showing under dermoscopy typical brown pigment network, and the right part composed of an unspecific pink structureless area (Fig. 1A). The patient was invited for a follow-up visit in July 2021 (Fig. 1B). Digital dermoscopy showed growth of the pink structureless area and development of punctate erosions (Fig. 1C). In addition, short fine serpentine vessels became apparent (Fig. 1D). The collision lesion was not taken into consideration, and malignant melanoma was the main suspicion. The whole lesion was surgically excised with a 3-mm margin. Histopathology was consistent with the diagnosis of a collision tumour composed of dysplastic nevus with cytologic low-grade atypia (left part) and basal cell carcinoma (right part) — Figure 2.
DISCUSSION
Collision skin tumours develop quite rarely, and largely because of that, pose a diagnostic challenge. Thus, only a small number of descriptions of them can be found in the English-language literature [1–15]. They are characterized by the presence of at least two different skin neoplasms in the same lesion [1]. They were found to be histopathologically diagnosed and not suspected based on clinical examination in 51.2% of cases [1]. A collision tumour may be composed of benign associations, malignant associations, or benign-malignant associations. The confusing terminology that has been present in the literature so far was finally systematized by Satter et al. [2], who distinguish four types of lesions: collision tumours (composed of originally separate neoplasms with a tendency for merging), combined tumours (composed of intimately admixing cell populations), colonized tumours (composed of one cell population has a tendency for colonizing an underlying second cell population), and biphenotypic tumours (composed of cell populations arising from a common precursor but undergoing divergent differentiation) [3]. If a malignant component is present, correct diagnosis is crucial for proper management [1].
In the study of 41 collision tumours by Fikrle et al. [1], over half of the lesions were misdiagnosed clinically and dermoscopically. Twenty-eight out of 41 collision lesions consisted of at least one malignant component. However, only 3 cases were a collision of two malignant tumours. Melanoma was found to collide most frequently with seborrheic keratosis [1]. Therefore, older patients with multiple seborrheic keratosis undoubtedly require thorough dermoscopic examination in order not to miss melanoma.
Basal cell carcinoma (BCC) was also found to frequently collide with seborrheic keratosis [1]. In the study by Zaballos et al. [4], a collision of BCC and seborrheic keratosis constituted 37.9% of histopathologically proven malignant collision tumours. In the same study, collision tumours composed of BCC and melanocytic nevus accounted for 19.9% of cases [4]. In the study by Fikrle et al. [1], collision tumours composed of BCC were predominantly located on the head and neck area (58.3%).
Three larger histopathological studies on collision tumours have been published to date, all showing a very low rate of collision tumours among biopsied lesions [3, 13, 14]. In the study by Boyd and Rapini, the collision of BCC and melanocytic nevus was found to be predominant [3].
It should be taken into consideration that collision lesions, consisting of benign components in particular, are encountered in daily clinical practice much more frequently than estimated. Most of these lesions, such as a collision of seborrheic keratosis and cherry angioma, are easy to diagnose with the naked eye and they are never biopsied. Therefore, a thorough examination, including dermoscopy, is so important in order not to miss the malignant component, which may be overshadowed by the dominant benign part of the collision lesion.
Dermoscopy facilitates the detection of collision tumours [4, 6, 11]. In the study by Fikrle et al. [1], dermoscopic structures corresponding to the histopathological diagnosis were present in all 41 cases of collision tumours [1]. Therefore, routine use of dermoscopy for evaluation of all skin tumours and close examination of all quadrants of the lesion in order not to miss a minor colliding tumour is recommended [1]. Reflectance confocal microscopy (RCM) has also proved to be of help in the early recognition of collision tumours [15]. Cooperation with histopathologists is crucial. In the above-mentioned study by Fikrle et al. [1], six collisions were initially missed in histopathology. Enough clinical and dermoscopic information should be provided to the pathologist to avoid even a minor element of the collision tumour being unrecognized.
CONCLUSIONS
In conclusion, thorough physical examination, detailed dermoscopic analysis of all four quadrants of the lesion, RCM in case of equivocal lesions, and good cooperation with the dermatopathologist are necessary to increase early clinical diagnosis of collision tumours.
Article information and declarations
Acknowledgements
We would like to thank the patient for her involvement.
Author contributions
All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship for this article, take responsibility for the integrality of the work as a whole, and have given their approval for this version to be published.
Conflict of interest
The authors of this publication declare no conflicts of interest.
Ethics statement
Informed consent was obtained from the patient for participation in the study and publication of the article, including publication of clinical photographs.
Funding
No funding was received for the publication of this article.
Supplementary material
None.
