Multiple ‘skip’ lesions in oropharynx and contralateral synchronous primary in hypopharynx detected on FDG PET/CT in case of oral cavity cancer

Sir,

We present a rare case of a 61-year-old man, a chronic betel nut chewer, who presented with an oral ulcer and difficulty in opening the mouth. On examination, an ulcerated lesion, about 2 cm × 2 cm, was seen in the right retromolar trigone (RMT). Biopsy confirmed it to be of squamous origin. Also, present was a palpable contralateral cervical node in lower neck (Level III) region. This prompted a whole body 18-F Fluorodeoxyglucose Positron Emission Tomography/computed tomography (FDG PET/CT) study for pre-treatment staging. Maximum intensity projection [(MIP – Figure 1a–c] image showed a large area of tracer uptake in right upper neck region (arrow), with the multiple scattered foci in rest of the neck (arrow-heads). Axial fused PET/CT showed enhancing soft-tissue involving the right RMT (b-arrow), which was the primary site, with contiguous extension into the soft palate. Also seen was mandibular erosion on axial CT (c-arrow-head). However, the other small foci corresponded to enhancing nodular FDG avid soft-tissues involving the base of tongue on the right side [Figure 2a and b – arrow] and post-cricoid [Figure 2c and d – arrow] region, in right para-midline location. These were suggestive of ‘skip’ lesions. Incidentally was seen was an enhancing FDG avid ulcerated soft-tissue thickening in left pyriform sinus [Figure 3a and b – arrow] with an enlarged left level III cervical node [Figure 3a and b – arrow head]. Biopsy was suggestive of a second site of primary of squamous cell origin. Patient was deemed inoperable and is on palliative chemotherapy.

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Figure 1 MIP (a) image showing large area of tracer uptake in right neck region (arrow) with multiple small foci of FDG uptake in neck region (arrow-heads). Axial PET/CT (b) and CT (c) images show FDG avid thickening involving right retromolar trigone with contiguous extension into soft palate (arrows)

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Figure 2 Axial CT (a and c) and PET/CT (b and d) images show FDG avid enhancing focus in the right base of tongue (arrow) and post-cricoids region suggestive of ‘skip’ lesions.

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Figure 3 Axial CT (a) and PET/CT (b) images show enhancing thickening involving the left pyriform sinus with enlarged draining right Level III cervical node (arrows) suggestive of a synchronous primary

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Concept of ‘skip’ metastases from head and neck cancers is defined in relation to cervical nodes, other than nodes draining the primary site.[1] However, as in our case, spread of cancer from RMT to the floor of mouth and soft palate occurs through pterygomandibular raphe,[2] a thick band of fascia that extends from the pterygoid hamulus to mylohyoid ridge. This explains the easy route of transit from RMT to vallecula. Furthermore, once the cancer spreads to the oropharyngeal mucosa; there is a high risk of mucosal spread to other pharyngeal sites, seen as skip lesions.[3] FDG PET serves as the modality of choice as the focality of uptake helps to pick up these subtle ‘non-nodal skip’ lesions.[4] Palpable cervical node, irrespective of its level and location with respect to primary site, is often assumed to be metastatic from the clinical point of view, from the documented primary site in neck, as seen in our case. However, since Level III cervical nodes drain the hypopharynx, lesion picked up in ipsilateral pyriform fossa (PFS) on PET/CT confirms the node to be metastatic from PFS and not from oral cavity. In a large population-based analysis of 75,087 patients by Morris et al., most common site of second primary in primary head and neck squamous cell carcinoma was lung, followed by head and neck, followed by esophagus.[5] The recent literature also highlights the fact that human papilloma virus positivity poses more risk for second primary than tobacco addiction.[6] Though most of these synchronous sites in head and neck are detected by pan-endoscopy, there are certain inaccessible locations like posterior pharyngeal wall or PFS, as in our case.[7] FDG PET/CT is superior in this respect with a positive predictive value of 93%, negative predictive value of 98% thus also avoids unnecessary endoscopies.[8] However, in our case there was coexistence of skip lesions as well as synchronous primary. Hence, in such a scenario, clinical information of drainage pattern of cervical nodes along with enhancement pattern and multi-focality of tracer concentration on FDG PET/CT helps in picking up skip lesions as well as synchronous primary.