Adenoid cystic carcinoma is a rare malignancy with a mean crude incidence of 1.0–1.3/100000/year, represent 10% of all epithelial salivary neoplasms and 20% of all malignant salivary gland tumours [1]. The US National Cancer Institute’s Surveillance, Epidemiology and End Results (SEER) programme indicates a decline of ACC in the USA with 5-year, 10-year and 15-year survival rate among all stages of ACC in head and neck patients being 90.34%, 79.88% and 69.22%, respectively [5]. It can be observed in both young and old age however appears most frequently in the fifth to sixth decade with no gender predominance [4, 5].
The tumor predominantly involves the head and neck but it can also involve upper respiratory tract, breast, female reproductive tract, skin, thymus, prostate and esophagus [1]. The lesion can have a variable presentation. It can present as a well-defined mass or an ill-defined mass with infiltration into surrounding structures, where the latter was the presentation in our case. The lesion presented with involvement of middle ear cavity, mastoid, Prussak’s space, orbital apex, cavernous sinus, perineural and adjacent skull base invasion suggesting a late presentation with T4a staging of the tumour. The invasive nature of the tumour is very peculiar. The invasion can occur by direct extension, haematogenous spread or perineural invasion wherein the last method of invasion being the most common. Intracranial invasion of the salivary gland ACC’s occurs predominantly from three routes: the mandibular and maxillary nerves, the internal carotid artery and the eustacian tube [7].
In our case, T2 and FLAIR hyperintensities were seen involving left middle ear cavity, petrous apex and mastoid cavity extending into cavernous sinus and left orbital apex. There was also involvement of left trigeminal and abducens nerves at the entry points into the Meckel’s cave and Dorello’s canal, respectively, which are thickened and edematous with mild signal changes and homogenous post contrast enhancement. There is similar pachymeningeal enhancement in the Suprasellar, pre-pontine cistern, along the internal acoustic meatus and temporal regions on the left side. In retrospect the above findings can be attributed to the typical presentation of ACC however without prior clinical suspicion the above findings can be mistaken for other etiologies as seen in our case where only later the diagnosis of ACC has been made. Similar findings can be seen in other cases as shown in Figs. 4 and 5 which was a case of right malignant otitis externa with oto-mastoiditis, skull base osteomyelitis (involving right petrous apex and clivus) and cellulitis of right prepharyngeal and parapharyngeal space. The above case described in Figs. 4, 5 and 6 is a 66-year-old male patient with history of right ear pain and discharge since 3 months. The ear pain was pricking type and aggravated at night. The patient also had pain radiating to the post aural region, right side of head, right temporal region, retro-orbital pain, right sided facial pain and right eye diplopia. Examination revealed granulomatous discharge in the right external auditory canal right sided, non-visualisation of right tympanic membrane, fistulous discharge in the right preauricular area and right mastoid tenderness. Furthermore, Rinne’s test revealed bone conduction to be greater than air conduction in the right ear and Weber’s test lateralised to the right ear suggesting conductive hearing loss of the right ear. Blood examination revealed neutrophilic leucocytosis, gram stain revealed gram positive cocci and gram negative bacilli. The patient was treated with intravenous antibiotics, antifungals, anticoagulants, analgesics with supportive management after which the patient showed clinical improvement.
Diffusion weighted imaging (DWI) which demonstrates Brownian motion of water molecules within a lesion has been proposed to provide differences between benign and malignant lesions. Studies suggested that most benign lesions (excluding benign Warthin’s tumours) show higher apparent diffusion coefficient (ADC) values compared to malignant lesions [8,9,10]. However there is a wide consensus with various studying showing contradicting findings. Some studies suggest that a significant difference exists between benign and malignant lesions whereas other studies suggest the contrary that the ADC values overlap between benign and malignant lesions. In the study by Chen et al. [6], it was suggested that overlap of ADC values occurs due to tumour heterogeneity and suggests that non-Gaussian diffusion model such as Fractional order calculus (FROC) helps to characterise heterogenous tumours more profoundly than simple Gaussian models which depends on presence of tumour homogeneity. As observed in our case, there is no evidence of diffusion restriction.
Singh et al. [11] shows that there are various sites of perineural spread of ACC depending on the primary location of the tumour. For palate, the potential sites includes the greater and lesser palatine nerves (branches of maxillary nerve, V2) and on to pterygopalatine fossa (PPF) and foramen rotundum. For nasal cavity and maxillary sinus, infraorbital nerve (branch of maxillary nerve, V2) and on to PPF and foramen rotundum. For retromolar trigone and tongue, inferior alveolar nerve and lingual nerves (branches of mandibular nerve, V3) and on to foramen ovale. For parotid gland, facial nerve (stylomastoid foramen, petrous portion internal auditory canal) and auriculotemporal branch of V3. Hence the Pterygopalatine fossa (PPF) is an important landmark for tumour spread as it links inferior orbital fissure, cavernous sinus, orbital apex with foramen rotundum, infratemporal fossa, vidian canal with the pterygomaxillary fissure, sphenopalatine foramen and greater and lesser palatine canals. Singh et al. [11] has also described the important imaging features for perineural spread such as enlargement/erosion of foramen, nerve enlargement/enhancement, obliteration of fat plane around the nerves including PPF, enlargement and convexity of lateral cavernous sinus wall, soft tissue replacement of CSF filled Meckel’s cave and muscular denervation (firstly edema and enhancement followed by atrophy). Lastly, Singh et al. [11] has also suggested how several conditions including infection, inflammation, trauma, vascular lesions and haematoma can mimic ACC. Hence, importance of clinical suspicion must be stressed upon.
Adenoid cystic carcinoma is a rare entity responsible for less than 1% of tumours arising from the head and neck, with only few cases reported in literature. In our case there was no clinical suspicion for the presence of ACC on initial presentation. The study by Badger et al. [2] suggests that the clinical symptoms of PNS such as pain, paresthesia and diplopia without the presence of a known primary head and neck tumor is a rare presentation to suggest tumor metastasis and such individuals are typically misdiagnosed to for trigeminal neuralgia. Imaging findings have been supportive of a skull base infiltration. By considering the age, clinical evaluation and imaging findings, a plausible diagnosis of an infective aetiology was considered and patient was initially diagnosed as CSOM with gradenigo syndrome.
As we have observed, ACC is difficult to distinguish based on imaging findings alone. The above myriad of findings which suggests suspicion for a diagnosis is vague unless there is a high clinical suspicion to suggest potential perineural spread of the ACC presenting as a skull base infiltrative lesion. Secondly, the presentation of ACC is typically more common in an older age group. Various studies have suggested the clinical dilemma that skull base osteomyelitis cannot be differentiated from malignancy as both entities shows skull base erosion, destruction and PNS where skull base malignancy is ruled out only on the basis of multiple negative biopsies for the latter [12,13,14,15,16].
