Oral melanotic hyperpigmentation following external beam radiotherapy for mucoepidermoid carcinoma of the parotid gland

Mucoepidermoid carcinomas are tumors that arise from mucous-secreting and squamous cells within salivary glands, most commonly the parotid gland [1] Demonstrating a slight female preponderance and most commonly presenting in middle age, most tumors are low- to intermediate-grade, with a small proportion demonstrating high-grade features. Wide local excision or parotidectomy, with or without neck dissection, is recommended as primary intervention, with adjuvant radiotherapy considered in the context of involved surgical margin(s), perineural invasion, intraoperative tumor spill, high-grade histology, and/or lymph node involvement [2].

During a routine check-up, a healthy 46-year-old Pacific Islander woman was found to have a new, painless mass on her right parotid gland, which she had not noticed earlier. She did not feel any fullness, swelling, or facial weakness. Subsequent computed tomography (CT) imaging characterized the lesion as a 2.6 cm complex solid/cystic mass with mild internal and rim enhancement and suggestion of subdermal extension. Biopsy demonstrated well- to moderately differentiated mucoepidermoid carcinoma, with minimal atypia and predominance of mucous cells. Following head/neck surgeon consultation, total parotidectomy plus lymph node dissection was recommended, as is routine for this diagnosis [1]. Pathology demonstrated a 3.3 cm primary tumor with microscopic extraparenchymal invasion, involvement of superficial and deep surgical margins, without involvement of 29 dissected nodes (pT2N0 R1). Due to surgical margin involvement, an established risk factor for local recurrence [1],[2], which can be associated with additional morbidity from re-resection. As such, the patient was referred to radiation oncology for consideration of adjuvant radiotherapy, which has demonstrated efficacy in reducing the risk of local recurrence [3]. Standard fractionation was elected, prescribed as 66 Gy in 33 once-daily fractions to the unilateral resection bed plus margin, as described elsewhere [4]. A 6 MV photon volumetric modulated arc technique was employed in order to optimize target volume coverage while minimizing unnecessary dose to adjacent organs at-risk (and associated acute and late toxicity risks), as is standard for head/neck radiotherapy [5]. Specifically, the mean oral cavity dose was 11.3 Gy and the maximum oral cavity dose was 22.8 Gy, with mandible, spinal canal, pharyngeal constrictors, and contralateral parotid gland meeting treatment plan objectives. Isodose map is demonstrated in Figure 1.

Just over two weeks into radiotherapy, the patient inquired about new well-demarcated patches of painless oral darkening, noted primarily on the tongue, palate, and right buccal mucosa (Figure 2A). These remained unchanged during the remainder of her treatments, painless and without oral discomfort or mucositis, and focal oral melanocytic hyperpigmentation was suspected (Figure 2B). Excepting anticipated moderate skin xeroderma with dry desquamation, moderate xerostomia, and dysgeusia, the patient completed her course of treatment and remained asymptomatic from these macules, which gradually faded over time, without intervention or complication (Figure 2C and Figure 2D). Skin changes, oral dryness, and taste sensation gradually resolved over 3–6 months as well, and she remains without evidence of recurrence presently.

Melanotic hyperpigmentation of the tongue and oral mucosa secondary to radiotherapy appears to be an uncommon/underreported phenomenon, as there are few reports of similar cases within the medical literature [6],[7]. These cases demonstrated similar features, with otherwise asymptomatic macular hyperpigmentation, not associated with mucosal denudement, discomfort, or long-term pigmentation changes.

Oral hyperpigmentation is likely an underreported phenomenon, and appears to be largely cosmetic, with stigmata resolving gradually over time.