The radiologic (CT/MRI)-pathological correlations of the salivary duct carcinoma (SDC) with hyaline degeneration and peripheral nerve invasion

Abstract

Objectives:

Salivary duct carcinoma is an aggressive tumour commonly showing local invasion and/or nerve palsy. However, their CT/MRI findings, especially, regarding T2WI, and the diffusion-weighted-image (DWI), were not well known. In this study, we correlated the CT/MRI appearance and the pathological findings containing the nerve invasion cases such as a facial nerve.

Methods:

We reviewed 14 cases of SDC (parotid = 11, submandibular = 2, minor salivary gland = 1) pathologically proven peripheral nerve involvement. Their CT findings of all patient including dynamic contrast-enhancement study､MRI (n = 9) and DWI (n = 6) were also analyzed with histopathological correlation.

Results:

On contrast-enhanced CT, the solid component was moderately enhanced. On MRI, T2WI central low signal core (n = 6) with peripheral high intensity rim (n = 5) was frequently observed except heterogeneous low and high (n = 1), diffuse low (n = 1), and high (n = 1) signal cases. The hyaline degenerative area located in the tumour core was poorly enhanced. Eleven tumours had an ill-defined margin, reflecting invasive tumour growth. On DWI, they showed high signal [the central low and peripherally high (n = 4), and diffuse (n = 1), heterogeneously high signal (n = 1)]. The mean ADC value was 1.148 ~ 0.961 x 10–3 mm²/s. With pathological correlation, the central low signal area on T2WI reflected hyaline degeneration. The sites of gross nerve involvement were revealed as tubular or branching structures on CE-CT (n = 3), and MRI (n = 1).

Conclusions:

(1) We frequently observed a central low signal area on T2WI/DWI in SDC. With histopathological correlation, it corresponded to the central hyaline degeneration with the peripheral viable tumour. 2) The gross nerve involvement might be detected as a strongly enhancement structure.

Introduction

Salivary duct carcinoma (SDC) is an uncommon tumour, known as a highly aggressive nature. Pathologically, this tumour showed great similarities to the ductal carcinoma of the breast,¹ which is the reason why this tumour was described as “salivary duct carcinoma (SDC)”. The parotid gland (related to Stensen‘s duct) is the most commonly involved site among the salivary gland. SDC accounts for 0.9–6% of all parotid grand’s tumours.^1–3 In up to of 27–57% cases, SDC develops from pre-existing pleomorphic adenoma,⁴ but it can also occur sporadically. Patients are usually elderly males (mean age: 55 to 61 years old). It presented as a rapidly growing mass with possibilities of early distant metastasis, local recurrence and high mortality. Facial paralysis was frequently observed in up to 40–60% cases.^5,6 They tended to extend along intraparotid (extracranial portion) facial nerves. Intracranial extension through foramen ovale along mandibular nerve or via facial nerve in the parotid gland was also reported as an one form of peri-neural extension. However, CT and/or MRI findings of the nerve invasion of SDC were not precisely described mainly due to the relatively low incidence and poor resolution to evaluate the nerve fibers.

Methods and materials

Fourteen patients of SDC (parotid = 11, sub-mandibular = 2, oral minor salivary gland = 1) cases with pathological nerve invasion (Gross = 3, Microscopic = 8) in Nagoya City University hospital who underwent CT and MR examinations before resection (n = 11) or biopsy (n = 3) were reviewed from April 2006 to March 2018. All subjects except two cases were male with an age range of 56–78 (mean 65.6). The tumour size was 11 × 9−88 × 80 mm in diameter. And the pathological stages were as follows; T2 (n = 2), T3 (n = 5), T4a (n = 6), T4b (n = 2) (N0 = 4, N1 = 7, N2b = 4) according to the Union for International Cancer Control (UICC) TNM staging system 7). The distant site of asynchronous metastasis was lung (n = 5), brain (n = 3) and bone (n = 3) in eight patients. CT scans were performed by the 64–320 row multi-slice volume scanner: Somatom Definition (Siemens, Erlangen, Germany) or Aquillion One (Canon, Tokyo, Japan). The plain CT (n = 15) and dynamic contrast CT(n = 11) study (30, 120 sec) after the i.v. administration of 80–100 ml iodinated contrast material at a rate of 2.5 ml s⁻¹ were scanned with 2.5–5 mm section thickness (120 kVp, 200 mA,). The CT value (Hansfield Unit), for each lesion, in all phases were compared. A plot representing the time-attenuation curve was constructed for each lesion.

MR images were obtained using a 1.5 T unit (Avanto, Siemens, Erlangen, Germany) in six patients and a 3 T unit machine (Ingenia, Phillips, The Netherland) in four patients. In these eight patients, axial T1W spin-echo (SE) images (repetition time/echo time (TR/TE) 466–680/11–20 ms) and T2W fast SE images (TR/TE 2500–5200/77–90) were obtained. In five cases, axial fat-suppressed T2W (TR/TE 3300–5200/90–100), coronal T1W or fat-suppressed T2W image (WI), and DWI (b factor = 0 & 1000 s/mm2; n = 7) with ADC map was obtained. The DWI data were processed on a voxel-by-voxel basis by use of the known relationship with the b factor, and the apparent diffusion coefficient (ADC) map was derived using linear regression. The ADC value of the solid component was also calculated. In nine cases, gradient contrast dynamic study [TR/TE 4.51/2.39: 0, 30, 60, 90, 120, 150 and 180 s after the i.v. injection of 0.2 nmol/kg of Gadolinium-DTPA (gadopentetate dimeglumine)], post-contrast coronal and/or sagittal T1WI with fat suppression technique were scanned. Other parameters of MRI were as follows; slice thickness: 3–5 mm (intersection gap: 1–1.25 mm), an acquisition matrix: 256 × 192–256 (128 × 128 on DW images) and a field of view (FOV): 24 × 22–24 cm. We retrospectively investigated the dynamic CT and MRI findings regarding the presence of central low intense area on T2WI, tumour’s margin, the presence of extra gland extension, and the possibility of lymph node metastasis, time-density(CT) or time-intensity(MRI) curve obtained from dynamic contrast-enhancement study. Time to peak enhancement (TTP) and washout ratio (WR) obtained from time-intensity curves (TICs) at DCE-MRI. Boundary definition was classified as well- or ill-defined. The lymph node was defined as positive when it had a minimal axial diameter of more than 10 mm or central low density suggesting necrosis. Section thickness was 3–5 mm and the pixel size was 0.46 – 0.88 mm. The consensus was always obtained in cases of discrepancy between the readers. Japan national board-certified radiologists (NK YS) and pathologist (YY) reviewed separately correlated the MR images and pathological findings. Written informed consent was obtained from all patients regarding the risk and benefits of CT and/or MRI examinations with the administration of the i.v. contrast-enhanced materials. The informed consent of all patients regarding the purpose of this retrospective study and the risk of the CE-CT and/or MRI examinations were obtained. The institutional IRB approval was obtained for reviewing patient clinical data retrospectively.

Results

The patient characteristics with CT and MRI findings are summarized in Table 1. The tumour size was 12 × 10～88 × 80 mm (mean 35.8 × 29.8 mm) in the greatest dimension. Plain CT showed an iso-slightly high dense tumour (Figure 1) with punctuating calcifications (n = 3). On post-contrast CT, they were moderately enhanced without cystic degeneration nor necrotic portions (Figure 1b). On T1WI, they showed iso-slightly high signal intensity. On T2WI, the central low signal core (n = 6/9) with peripheral high signal intensity (viable cell area) rim (n = 5) was commonly observed (Figure 2a) except almost low signal (n = 1), heterogeneously high (n = 1) and diffuse high signal cases (n = 1). The high signal intensity of DWI reflected high cellular hyperintense rim on fat suppressive T2WI (Figure 2b and c). However, infiltrative growth of tumour without capsulation was commonly observed (All cases showed totally or partially ill-defined margin). On DCE-CT and/or MRI, the peripheral dominant solid area showed early enhancement with late washout enhancement pattern (n = 2) with central poorly or less-enhanced core (Figure 2d–g). The early enhancement with persistent pattern (n = 2), gradual enhancement pattern (n = 6) were also observed. They showed peripherally dominant (n = 7), and heterogeneous (n = 5), relative homogeneous (n = 2) enhancement without cystic component, necrosis, hemorrhage, nor calcified foci. On DWI (n = 6), they showed high signal intensity [the central low and peripherally high (n = 4) (Figure 2b), and diffuse high (n = 1) and heterogeneously high signal (n = 1)]. The mean ADC value was 1148–961 × 10⁻³ mm^2/s (Figure 2c).

Table 1.

The summary of CT/MRI findings of 14 SDC cases

Patient	Age (yr)/Sex/Site	TN classification	Nerve	Meta	T2WI	Mean ADC Value (10 3 mm2/s)	TIC (Time Intensity Curve)
1	63 M Pa	T3N1			Int(Core+)	0.961	C
2	56 M Pa	T4aN0	Gro	+	Low		B
3	72M Sub	T4aN1	MIc	+	Sl h(Core+)		C
4	64 M Pa	T4bN2b	Gro	+	Sl h	1.123	A
5	62 M Pa	T4a N1	Mic		Sl h(Core+)	1.148	C
6	69 M Min	T2 N2b	Mic
7	67M Sub	T3N2b	MIc	+
8	62 M Pa	T4a N1	Mic		Inter low(Core+)	0.977	C
9	71 M Pa	T4aN0	Gro	+	Hetero sl h(Core+)		C
10	78 M Pa	T3N0	Mic		Slh(Core+)	1.121
11	73 F Pa	T3N1	MIc				B
12	51 M Pa	T2N2b	Mic	+	Slh(Core+)	0.994	C
13	84 M Pa	T3N1		+			A
14	79 F Pa	T2 N2a		+

Gro, Gross; Int, Intermediate; Mic, Microscopic; Sl h, Slightly high.

Figure 1.

A 63-year-old male with a “Fried-Egg” sign. Non-contrast CT (a) showed a slightly high-density tumour measuring 30 × 26 mm in the superficial lobe of the left parotid gland post-contrast study (b) shows less enhancement in the central area (green arrow; 56→70 H.U.) than the peripheral area (red arrow; 53→92 H.U.).

Figure 2.

MRI Axial T2W imaging (a) demonstrates low signal intensity with irregular thin intermediate to high signal-rim on T2WI. The peripheral rim shows high signal intensity on DWI (ADC = 0.962 × 10-3 mm²/s (low signal on ADC map) (a, c). The dynamic post-contrast T1W MRI (d-g) demonstrated gradual peripherally dominant enhancement with a little late washout (d) plain T1WI. (g) fat-suppressive dynamic Gd-enhanced T1WI (e) 30 s, (f). 90 s, (g 180 s). The microscopic specimens showed ductal lesions comprising pleomorphic tumour cells with a peripheral portion of the cribriform growth pattern (h, i). In the centre, the eosinophilic hyalinized degeneration area was also noted (h). The pathological stage was pT3N1M0. Microscopically faint facial nerve involvement was observed. There were no recurrences or metastases within 3 years of follow-up.

With pathological correlation, the central area showed fibrosis (hyaline degeneration) (Figure 2h) and the peripheral area showed viable infiltrative tumour growth (Figure 2i). Generally, the low signal area on T2WI correlates high tumour cellularity or fibrosis. Moreover, our study could clearly reflect the central hyaline degeneration (Figure 2h) at the site of pre-existing pleomorphic adenoma containing some chondroid components for the first time literally.

It is assumed to be mainly caused by a pre-existing pleomorphic adenoma (n = 6) by the presence of adenomatous cells and chondroid matrix. The degree of enhancement of viable adenocarcinoma (SDC) was moderate. However, the central hyalinized area was poorly or less-enhanced with non-restricted diffusion (Figure 2b and c).

The peripheral nerve involvement including facial nerve of SDC was confirmed in 78.6% (11[gross:3 micro:8]/14) cases pathologically, of which three cases demonstrated tubular or branching excessive enhanced structures on CE-CT (n = 3) (Figure 3a and b) and/or MRI(n = 1). Lymph node metastasis was observed in seven cases (N1 = 5, N2a = 1, N2b = 4). The asynchronous distant metastases (lung5, brain3, bone3) were found on PET-CT and/or contrast-enhanced brain MRI.

Figure 3.

A 56-year-old patient with progressive facial paralysis in 1 year‘s duration. Non-contrast CT (a) showed a slightly high-density nodule with indistinct margin measuring in 12 × 10 mm in the deep lobe of it. parotid gland. On the post-contrast (CE), 3-mm slice section CT, the tubular avid enhancement structure (arrows) continuous to the tumour was observed (b). The MRI examination showed an ill-defined low-intensity area due to the central hyaline degeneration on T2WI (not shown). Pathology shows the tubular avid enhancement structure corresponds to the facial nerve involvement both on the macrospecimen (c) and HE stain (d). The photograph of the gross specimen showed the abnormal enlarged tubular structure (arrows) underneath the tumour, which exhibited a lobulated solid tumour with infiltrative growth of the tumour cells without encapsulation. HE stain shows component of a tumour with the cribriform nest and intraluminal comedo-necrosis resembling ductal carcinoma of the breast was revealed (arrows. HES×400). The final pathological staging was T4aN0M0.

Discussion

SDC of the salivary gland usually arose from primitive (potent reserve) cells of the excretory ducts.^2–4 SDC is a rare malignant epithelial tumour and is characterized histopathologically by multiple neoplastic epithelial nests similar to the ductal carcinoma of the breast. Although the first case was described by Kleinsassar et al² in 1968, SDC was not recognized in the WHO classification of salivary gland tumours until 1991.¹ SDC represents 9–10% of the salivary gland tumour of malignancy. The M/F ratio is 4/1 and most patients present after the age of 50. The parotid is most commonly involved (about 90%), but submandibular, sublingual, minor salivary gland, maxillary and laryngeal originated tumours have also been reported. It may arise from pleomorphic adenoma and chronic obstructive sialadenitis.⁴ In cases arising as carcinoma ex pleomorphic adenoma, rapid growth on a longstanding pre-existing mass is commonly reported. SDC is known to be an aggressive tumour with a worse prognosis. This is due to its local invasiveness and metastatic potential. The facial nerve involvement correlates with the clinically aggressive behaviour of the tumours^1,3. Nearly 50% of the patients of nerve involvement die within 4 to 5 years. Therefore, the detection of a faint nerve invasion of SDC is useful in the pre-operatvie image examination in order to avoid the unnecessary sacrifice of peripheral nerve. Cervical metastatic adenopathy and peri-lymph nodal invasion were not uncommonly identified.Radical surgery is the primary treatment followed by neck lymph node excision. However, the rate of locoregional recurrence is high and the prognosis for survival is poor in surgically treated case of insufficient margin. Lymphatic invasion and peri-neural, extraparotid invasion are also further indicators of poor prognosis. Post-operative radiation therapy is mandatory in advanced cases, whereas chemo-radiotherapy is generally conducted for metastatic conditions. The prognosis may be improved in tumours measuring <2 cm⁶; however, the five-year recurrence-free survival rate remains at ~30%.^2–8 Cytological differential diagnoses of SDC include mucoepidermoid carcinoma, adenocarcinoma, metastatic adenocarcinoma and oncocytic carcinoma. The most relevant morphological feature is the presence of an intraductal component. This diagnostic ductal lesion comprises pleomorphic, epithelioid tumour cells with solid, papillary, cystic and cribriform growth patterns, so-called ‘Roman bridge’ formation and intraductal comedo-necrosis (Figure 2i). Regional recurrence and distant metastases to the lung, bone and liver were frequently observed literally.^4–6 Under the current definition of SDC, it is a tumour that consists of solid invasive cancer nests with polygonal atypical cells surrounding an area of comedo-like necrosis. Mitotic figures are usually abundant. Sarcomatoid spindle cell growth pattern, mucin rich and invasive micro-papillary variants were also reported.⁸ In the review of patients with sarcomatoid SDC, 25% of patients were dying within 2 years⁸.

Immunohistochemically, cytokeratin, CEA, c-erd-B2 EMA, LeuM1 and androgen receptors (AR) are usually positive, and apocrine receptor, and GCDFP-5 may be positive focally^1,9 . The Mib-1 index is usually high with an average value of 43 (25–80)%. human epidermal growth factor 2 (HER2)/neu protein was also positive in most SDC. De Palma et al¹⁰ suggested that patients with HER2-positive SDC treated with trastuzumab (an anti-HER2 monoclonal antibody) demonstrated promising results. Therefore, patients with HER2 subtype SDC may benefit from targeted therapies using anti-HER2 monoclonal antibodies, or HER2 tyrosine kinase inhibitors, such as lapatinib. The utility of both anti-androgen therapy with/without chemotherapy for a patient with advanced SDC was also attempted.¹¹

Boundary and invasion to adjacent structures

In 2005, SDC was included as an independent entity by the WHO, labeling it as an aggressive adenocarcinoma, which resembled high-grade breast ductal carcinoma.

The invasive carcinoma consists of irregular glands and cords of cells that frequently elicit a prominent desmoplastic reaction. Gross findings consist of a tumour of variable size, usually firm with a variable cystic component. Infiltration of the adjacent parenchyma is usually obvious.

An ill-defined margin reflects the invasive growth of tumour cells and the frequent lymph nodal metastasis reflects a tendency of lymphatic involvement. Although, the mucoepidermoid tumours usually had peri-tumoural inflammatory changes rather than invasive tumour growth. Deep structural invasion (parapharyngeal space, muscle or bone), indistinct margin, and subcutaneous fat infiltration were poor indicators like other malignant salivary gland tumours.^12,13

Salivary gland lesions showing indistinct margin were summarized in Table 2. Almost all lesions were either inflammatory or high-grade malignant tumours including metastatic lesions. Therefore, if the inflammatory entities could be ruled out by the clinical course or laboratory data, the possibility of malignancy should be suspected.

Table 2.

The Peri-neural Extension (Nerve Involvement) in head and neck tumour

Adenoid cystic carcinoma

Squamous cell carcinoma(pharyngeal or cutaneous)

Basal cell carcinoma

Mucoepidermoid carcinoma

Polymorphous low-grade adenocarcinoma of minor salivary gland tumour

Melanoma(desmoplastic)

Lymphoma(neurolymphomatosis)

Rhabdomyosarcoma

T2WI Low-Intensity central core

SDC was described to occur as de novo cancer or as the malignant component of carcinoma ex pleomorphic adenoma (secondary SDC).

Fibrotic (central desmoplasia) or hyaline degeneration of the pre-existing PA are usually observed in secondary SDC. Punctuate calcification was observed in 50% of cases in de-novo SDC,¹⁴ whereas gross calcification was founded only in one case on a conventional CT scan. Our radiologic pathological correlation revealed that the low signal intensities of T2WI (Figure 2a) reflecting hyaline degeneration/admixed stromal fibrosis. The hyaline degeneration of the central core was assumed to be correlated with pre-existing pleomorphic adenoma (n = 4) by the presence of adenomatous cells with a vague chondroid matrix (Figure 2i). In contrast-enhancement study, the tumour showed peripheral dominant (n = 4), or central oriented (n = 3), relatively homogeneous (n = 1) enhancement without cystic, haemorrhagic, necrotic component nor calcification. On dynamic contrast-enhanced CT, all solid components showed a gradual upward persistent enhancement pattern (central: 47-58-89 and peripheral: 42-50-60 H.U.) like dynamic MRI (Figure 2d–g). Motoori et al. reported that the gradual upward enhancement on dynamic MR images and hypo-intensity on STIR and T2W images may be a clue for making a diagnosis of SDC.¹⁴ The necrosis was common findings in SDC,¹⁵ so they sometimes showed necrotic entirely cystic mass (20%), or heterogeneous tumour containing the area of the fluid-fluid level. In our study, 6 of 9 (66.6%) tumours showed a central low signal core on T2WI. On CT all cases were slightly high density except for cystic (necrotic) area. With histological correlation, the central low signal area on T2WI corresponded to the central hyaline degeneration and/or high tumour cellularity.¹⁶ This central low with peripheral high signal intensity pattern should be called “the Fried-Egg” sign.

Peri-neural extension (Facial nerve involvement)

The nerve involvement of SDC was confirmed in 78.6% (11[gross:3 micro:8]/14) cases pathologically, of which three cases showed tubular or branching enhanced structures (Figure 3b) on CE-CT (three cases) with 3-mm slice thickness on the delayed phased fat suppressive Gd-enhanced T1WI MRI (5-mm slice thickness)(one cases). Peri-neural spread (PNS) refers to the extent of tumour cells along the nerve sheath or nerve itself (Figure 3c and d). The enlargement and avid contrast-enhancement were observed. This phenomenon is more frequently associated with carcinoma of minor or major salivary glands (e.g. adenoid cystic carcinoma), pharyngeal squamous cell carcinoma, basal cell carcinoma, melanoma and lymphoma¹⁷ Table 2). To preserve the facial nerve at surgical resection, it is important to make a correct diagnosis regarding the presence of PNS or neural involvement.

This neural invasion was also an indicator of a poor prognosis like a lymphatic invasion or extra parotid invasions such as fat, skeletal muscle and dermis^1.3. However, due to the poor spacial resolution, almost microscopic nerve invasion was not observed except thin slice CE-CT or high pitch matrix MRI in this study. The Bell’s palsy (60%) is the main aetiology of peripheral facial nerve paralysis followed by a traumatic origin (17%), inflammatory or infectious causes (10%) and tumoural causes (6%). The symptoms were also dependent on the lesion sites, such as intracranial, internal ear, mastoid, stylomastoid foramen or parotid gland.¹⁸ Immediately after passing the stylomastoid foramen, the facial nerve leaves branches for the cervical muscles (auricular, digastric, stylohyoid) and then enters the parotid to give rise to muscular temporofacial and cervicofacial branches in contact with the submandibular area and the external jugular vein.¹⁹

The tubular/linear enhancements in the parotid gland should be considered not only the active inflammation but also the tumour involvement if they were continuous to the known tumour. Retrograde peri-neural spread in progressive facial palsy without known tumoural origin was also reported.²⁰ They were diagnosed with the abnormal enhancement of the facial nerve with abnormal enlargement. These unsuspected tumours were described in not only the parotid but also the submandibular gland. The PNS could be detected not only in CT (Figure 3b) but also in MRI²¹ and FDG-PET²² However, it should be noted that the Gd-enhancement of tympanic and intramastoid portions is normal due to peri-neural arteriovenous plexus²³ but, on the other hand, the enhancement of other portions including intra/peri-salivary gland is abnormal.

Perineural tumour spread disrupts the blood-nerve barrier and results in increased permeability of the endoneurial capillarie.²⁴ This allows leakage and accumulation of iodinated or paramagnetic contrast agents that leads to nerve enhancement which is more readily detected on MR imaging than CT.²⁵

Diffusion-weighted image(DWI) and Carcinoma ex- adenoma

In 5/6 cases, the tumour showed high signal intensity [the central low & peripherally high (n = 3), and diffuse high signal (n = 2)] on DWI. The mean ADC value was 1.148–0.961 × 10⁻³ mm² s⁻¹. With histopathologic correlations, the peripheral component correlates hyper-cellular portions, and the central component is accorded with the hypo-cellular degenerative area. This phenomenon might show an evidence of pre-existing pleomorphic adenoma (PA) portion leading to the hyaline degeneration, although PA often accompanies the chondromatous and/or myxoid degeneration. The mean ADC value of malignant tumours (1.08  ±  0.1, 1.04  ±  0.1 × 10⁻³ mm² s⁻¹) was significantly lower [p  =  0.001] than that of benign lesions (1.69  ±  0.2, 1.72  ±  0.3 × 10⁻³ mm² s⁻¹). A threshold of ADC of 1.33  ×  10⁻³ mm² s⁻¹ was used for differentiating malignant parotid tumours from benign lesions.^26,27 According to the WHO histological classification published in 2017, malignant derivatives of PA of salivary gland origin were called “malignant mixed tumours”. The carcinoma in pre-existing pleomorphic adenoma (CXPA), such as SDC and myoepithelial carcinoma usually arises in 10–15 years (Table 3), but a sudden rapid growth (average 3–6 months) was reported. Pain and facial nerve paralysis are often present. CXPA exhibiting a widely invasive phenotype usually exhibits more poorly circumscribed mass than benign PA or early minimally invasive (breach of the PA capsule) and characterized as irregular tumour margin, heterogeneous signal-intensity, tumour infiltration into surrounding tissue, and low signal intensity on T2WI. On the other hand, PA often shows heterogeneous low-to-intermediate signal intensity on T1WI and intermediate-to-high signal intensity on T2WI. The central low signal MR finding in our series seems to reflect the histological nature of SDC suggesting pre-existing PA. Histologically, SDC is characterized as having both intraductal and infiltrating components, the intraductal component is usually accompanied by comedo necrosis and, in some cases, areas of hemorrhage, necrosis, and cystic degeneration are observed although its appearance diverts from wholly solid to predominantly cystic.²⁸ Usually, the intraductal and the infiltrating components show high cellularity. The tumour component is occasionally accompanied by prominent fibrosis or myxoid stroma which are leading to the hyaline degeneration.^29,30 These histological features were also seen in our patients and seemed to be reflected in the low to intermediate signal intensities on T2WI. The tumour often shows a non-capsulated and invasive growth pattern into the vessel or nerve. This seemed to be reflected in the ill-defined margins and extra parotid infiltration. A previous article reported that the ADC values of PA are generally higher than those of malignant tumours.^31,32 Malignancy can be suggested based on the findings of the DWI and ADC map except for cases in which malignant components may be too small to detect even in the pathological specimen or necrosis in carcinoma exhibits high ADC values due to hypocellularity, comparative evaluations using contrast-enhanced CT/MRI may be necessary.³³ Recently, diffusion tensor imaging with the fractional anisotropy (FA) and the mean diffusivity (MD) value has been introduced for the differentiation between benign and malignant salivary gland tumours.³⁴

Table 3.

The carcinoma arising from pre-existing pleomorphic adenoma (CXPA)

SDC

Myoepithelial carcinoma

Adenocarcinoma not otherwise specified

Squamous cell carcinoma

NET(neuroendocrine tumotr):small cell carcinoma

Oncocytic carcinoma

Carcinosarcoma

Spindle cell carcinoma

Synovial sarcoma

Dynamic CT/MRI study relating the differential diagnosis of salivary gland tumour

Both dynamic contrast-enhanced MRI and multiphasic CT have comparable high accuracy in characterizing the different histological types of parotid gland tumours especially for discrimination between Warthin and other tumours.

Time to peak enhancement (TTP) and washout ratio (WR) were obtained from time-intensity curves (TICs). TIC was usually classified into four types³⁵; type A, time to peak more than 120 s (this is considered a gradual enhancement); type B, time to peak 120 s or less, with a high washout ratio (30%) (this is considered early enhancement and high washout); type C, time to peak 120 s or less, with low washout ratio (<30%) (this is considered early enhancement and low washout); and type D, flat (seen in markedly cystic tumours). For example, pleomorphic adenoma commonly showed type-A (gradual enhancement curve), Warthin tumour showed type-B (rapid enhancement with highly washout curve) and most histologically another salivary gland tumour often showed type-C (rapid - moderate increase and relative moderate washout pattern³⁶ like our study of SDC. Other valuable findings of the DCE investigation in the differential diagnosis of SDC were that CT value in delayed-phase was significantly higher in SDC than in adenoid cystic carcinoma.³⁶ However, discrimination between malignant and benign salivary gland tumours was not possible solely based on dynamic curves. Fine needle aspiration cytology or surgery is required for a definite histopathological diagnosis. Samir. S et al. stated that a significant difference as regards washout rate between recurrent tumours and post-treatment changes.³⁷ It has been also investigated for the differentiation of tumour recurrence and post-treatment fibrosis in regions³⁸ Abdel Razek et al³⁹ introduced the Dynamic Susceptibility Contrast Perfusion-Weighted Magnetic Resonance Imaging for pre-operative evaluation. Multi-parametric MRI using arterial-spin labeling with the calculation of tumour blood flow, DWI (ADC value) and proton MR spectroscopy would be used soon in differentiating subtypes of parotid tumours.⁴⁰

Several limitations of this study were as follows: 1) Because of the rare occurrence, the number of subjects including gross facial nerve involvement was small, 2) the comparison of other histological types of carcinoma in pre-existing pleomorphic adenoma (CXPA) was mandatory regarding the frequency of hyaline degeneration and nerve invasion. 3) Regarding the evaluation for the nerve invasion, more thin slice scan less than 2 mm would be expected both on CT and MRI.

In conclusion, 6 of 9 (66.6%) SDC showed heterogeneously low-to-intermediate signal intensity on T2WI with a central low signal core. On CT (n = 14), they were slightly high density except for the cystic area. With histological correlation, the central low signal area on T2WI and DWI corresponded to the central hyaline degeneration (n = 6). This central low signal with peripheral high signal pattern should be called “Fried-Egg” sign 2). The peripheral nerve involvement of SDC was confirmed in 11 cases pathologically, of which three cases demonstrated finger-like branching enhanced structure on CE-CT/MRI.

Therefore, when we observe the central low signal on T2WI and finger-like tubular enhancement in evaluating the indistinct salivary gland tumour, we can speculate the high-grade malignant tumour, such as SDC (SDC) with facial nerve invasion at high confidence.