Grossly, the rectum revealed a grayish black, nodular, friable tumoral growth with ulcerated mucosa measuring 4x3cm located 0.7cm away from the ano-rectal junction and 2cm from the distal resected margin (Figure 1). Tumor did not extend into the peritonealized segment. Microscopically, tumor was composed predominantly of oval to spindle shaped cells arranged in sheets and cords (Figure 2A, {M}). Moderate nuclear atypia, brisk mitosis (>10/10HPF) and abundant intracellular and extracellular brownish black pigment were noted (Figure 2B). Morphological features were of “Malignant melanoma”. Tumor was ulcerating the overlying mucosa (Figure 2C) and infiltrating into the muscularis propria with sparing of serosa. Just adjacent to this tumor, a smaller component of well-differentiated squamous cell carcinoma was observed in the ano-rectal mucosa measuring approximately 6mm in diameter (Figure 2A,{S}). This was composed of malignant squamous cells infiltrating superficially into lamina propria as small nests (Figure 3A) and single atypical cells (Figure 3B). The two tumors were occurring side by side without any significant admixture. Squamous cell carcinoma component was seen in continuity with the anal mucosa. No perineural or lymphovascular invasion was evident. The surgical margins were free of tumor and fourteen lymph nodes isolated did not reveal tumor metastasis.

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Figure 1: Gross photograph showing a nodular, black, friable growth in the rectum.

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Figure 2: Photomicrographs showing collision tumor in the anorectum. A) Predominant component of malignant melanoma (M) and a minor component of squamous cell carcinoma (S) (H&E; x100). B) Large, atypical, cells having pleomorphic nuclei with prominent eosinophilic nucleoli and abundant intracellular and extracellular brownish black melanin pigment. (H&E; x200). C) Malignant melanoma component with partly ulcerated colonic mucosa (H&E; x200).

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Figure 3: Photomicrographs showing squamous cell carcinoma component of the collision tumor. A) Nests of malignant squamous cells invading into lamina propria (H&E; x200). B) Invasive front showing single tumor cell infiltration into lamina propria (arrows) (H&E; x400).

Immunohistochemical staining distinguished the two tumor components. Melanoma cells were immunoreactive for HMB-45 (Figure 4A) and S-100 (Figure 4B) while pancytokeratin (CK) was negative. Squamous cell carcinoma component was immunoreactive for CK (Figure 4C) and negative for HMB-45 and S-100. Thus, a diagnosis of collision tumor of rectum comprising malignant melanoma (T4N0M0) and squamous cell carcinoma (TisN0M0) was rendered.

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Figure 4: Photomicrographs showing immunohistochemical profile of the collision tumor. A) Diffuse positivity of HMB 45 in malignant melanoma (HMB 45; x400). B) Positive immunoexpression of S-100 in malignant melanoma (S-100; x200). C) Cytokeratin positive squamous cell carcinoma component (arrow) with negative reaction in the melanoma cells (Pancytokeratin; x100).

Some authors believe that collision is merely a chance occurrence of two unrelated tumors².Others advocate that a common carcinogenic stimulus induces certain micro environmental changes leading to development of the second tumor⁶. Other theories suggested are common origin from pluripotent cancer stem cell which differentiates into two components with simultaneous proliferation of two different cell lines¹¹. In our case, malignant melanoma was a predominant component and the changes produced by it in the milieu could have stimulated the immediately adjacent ano-rectal mucosa to undergo increased proliferation and neoplastic transformation into squamous cell carcinoma. Collision tumors need to be differentiated from composite tumors. The latter develop as a result of single mutation in the precursor cell with the clone undergoing a divergent differentiation at some point leading to formation of two neoplasms that are intimately mixed³. On the other hand collision tumors result from two separate molecular events and exist without significant tissue admixture. Molecular studies provide evidence regarding the origin of tumor from the same or different clones. If two tumors arise independently, the genetic alterations are expected to be different from each other because of different tumor origins and vice versa^12,13.

Due to rarity of these tumors the biological behavior is unpredictable and it is still debatable whether the most predominant or the more histologically aggressive component of the collision tumor will determine the prognosis. In our case, possibly the more aggressive and predominant melanoma component was the key determining factor in the final outcome which was the death of patient within a year of clinical presentation. Therefore, appropriate treatment strategies may ultimately be dependent on the individual biological aggressiveness of each of the tumor components.

In conclusion, to best of our knowledge collision tumor of malignant melanoma and squamous cell carcinoma, rectum has not been described in English literature till date. This case report highlights the need for accurate identification of both components of the tumor for determining the overall prognosis and planning treatment strategies.

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