Secondary Primary Carcinoma Arising on the Flap Skin in the Oral Cavity—Case Series

<b>Background:</b> Oral squamous cell carcinoma (OSCC) causes considerable morbidity and mortality rates, posing a major global health burden. The management of the OSCC is multidisciplinary, but still the gold standard is surgical resection and reconstruction of the postablative defect. The appearance of secondary primary OSCC is not uncommon; however, it is quite rare that it appears on the skin of the flap that was used for reconstruction during the previous surgical therapy. <b>Methods:</b> We present three cases in which a secondary primary OSCC appeared on the skin of two radial forearm free flaps and two on regional pectoralis major flaps. <b>Results:</b> Our case series show that, although relatively rare, there is a chance of a secondary primary tumor on the flap used for intraoral reconstruction after the first oncological reconstruction. According to the latest and available literature, there is still no explanation of the underlying mechanism that leads to this occurrence. <b>Conclusions:</b> The learning point of this case series should be that, aside from the neck metastasis or recurrence of the primary oral cancer, the clinicians should also bear in mind that the flap itself should be physically examined in detail.