Serous Borderline Ovarian Tumour

Serous BOT is the most common subtype, accounting for up to 70% of all cases. The median age of presentation is 50 years, with one third of patients being younger than 40 years. They can be of two histological subtypes: classic (accounting for 90% of cases) and cribiform / micropapillary. It is thought that most low-grade serous ovarian carcinomas arise from the cribiform / micropapillary subtype, which is thus considered a pre-malignant lesion. Also, it has been shown that classic serous BOT can progress to the more aggressive cribiform / micropapillary subtype. Serous BOT can harbour KRAS or BRAF mutations. These tumours are often bilateral and measure more than 5 cm in size.

Radiologically, they can appear as cystic, solid or mixed solid and cystic lesions. They are characterized by the presence of papillary projections that have a hierarchical branching pattern (giving them a sea anemone-like appearance) and can have an endophytic, exophytic or combined growth.

On T2-weighted MRI (T2WI), the papillary architecture has a hyperintense signal and the internal branching is hypointense (Figure 2). The hyperintensity is attributed to oedematous papillae and the hypointensity to the fibrous core. On T1-weighted MRI (T1WI), the papillae are inconspicuous but become evident after contrast administration (Figure 3). On DWI, the papilla can show restriction, however the ADC values are higher compared to malignant tumours (Figure 4).

Fig 2: T2WI coronal sequence showing a multicystic tumor with papillary projections. Note that only the hypointense fibrous core is seen (the hyperintense oedematous papillae are the same intensity as the fluid content of the cyst)

Fig 3: T1WI with fat saturation coronal sequence post-gadolinum injection enhancement of the papillary projections

Fig 4: ADC map depicting slight restriction diffusion

Serous BOT can have a more aggressive behaviour compared to other BOT, presenting with peritoneal implants, a feature that can be confused with the peritoneal metastases seen in their malignant counterparts.

Mucinous Borderline Ovarian Tumour

Mucinous BOT are the second most common subtype and can be associated with mature teratomas and Brenner tumours. They have also shown an association with endometriosis, with many mucinous BOT arising in pre-existing endometriotic cysts. The mean age of presentation is 45 years and they often harbour KRAS mutations, while BRAF mutations are rare. These tumours are usually unilateral and are large, often with more than 20 cm.

Radiologically they present as multiloculated cystic tumours, displaying small loculi with a honeycomb pattern (numerous densely aggregated loculi with 5 – 10 mm), that have different signal intensities on both T1WI and T2WI (owing to the presence of viscous content, blood or debris), giving them a stained-glass appearance, which is characteristic of mucinous neoplasms (Figure 5 and Figure 6).

Fig 5: T1WI axial sequence showing a multicystic tumor with loculi displaying diferent signal intensities, including one with high T1 signal intensity

Fig 6: T2WI axial sequence showing a multicystic tumor with loculi displaying diferent signal intensities. The locule presenting with high signal intensity in the T1WI sequence shows low signal intensity in the T2WI, known as T2 shading

Microcysts that are hypointense on T2WI and display a reticular pattern of enhancement are frequent in mucinous BOT and are not usually associated with malignant mucinous ovarian carcinoma and thus this characteristic helps differentiating between these two entities (Figure 7). It must be noted that, however, sometimes the microcysts are so small that they can simulate a solid tumour, so it is of great importance to carefully look for the reticular enhancement.

Fig 7: T1WI with fat saturation axial sequence after gadolinium injection shows a reticular pattern of enhancement, a feature that helps differentiate borderline from malignant mucinous tumours.

If a mucinous BOT coexists with pseudomyxoma peritonei, one must consider the hypothesis of an ovarian metastasis from a primary tumour originating from the gastrointestinal tract, namely in the appendix.

Seromucinous Borderline Ovarian Tumour

Seromucinous BOT are the third most common type of BOT, accounting for approximately 8% of cases and manifest predominantly in women with 35 – 40 years old. They have been recognized as a distinct entity in the revised 2014 WHO Classification of Tumours of Female Reproductive Organs, being previously considered a subtype of mucinous BOT (endocervical-like mucinous BOT). As is the case for mucinous BOT, seromucinous BOT show a strong association with endometriosis, with many arising within endometriotic cysts.

Genetically they are associated with KRAS mutations and loss of ARID1A expression.

Radiologically, these tumours show characteristics similar to serous BOT, being bilateral in up to 30% of cases and having between 5 – 10 cm. They present as a unilocular or paucilocular cyst associated with a papillary architecture with internal branching (Figure 8), with contrast-enhancement, usually with an endophytic growth pattern. However, the intracystic content can display hyperintensity in T1WI and hypointensity in T2WI, therefore also presenting features of mucinous BOT (Figure 9 and Figure 10).

Fig 8: T2WI oblique coronal sequence depicting a biloculated predominantl cystic tumour with slightly hypointense fluid. Note the hyperintense papillar projection with a hypointense core, similar to the ones seen in serous borderline ovarian tumours.

Fig 9: T1WI axial sequence depicting the same tumour as in Figure 8. Note that the fluid is hyperintense and that the papillary projections appear hypointense.

Fig 10: T1WI oblique coronal sequence before (A) and after (B) contrast administration, showing the enhancement of the papillary projections. Note that due to the hyperintense cystic content, the enhancement can be difficult to perceive and the use of subtraction sequences can help in this setting.

Uncommon Borderline Ovarian Tumours

Uncommon BOT comprise the endometrioid, clear cell and Brenner subtype. They affect slightly older patients, and account for approximately 5% of all BOT. Their radiological features are not as well documented due to their lower frequency.

Endometrioid BOT has a strong association with endometriosis, with many arising within pre-existing endometriotic cysts and can harbour ARID1A mutations. They also have an association with endometrial lesions, namely hyperplasia and endometrial carcinoma.

A case series examining the radiological features of endometrioid BOT has postulated that they are mixed solid and cystic tumours, with the solid component corresponding to either large solid areas or papillary projections in tumours with the adenofibromatous or intracystic pattern, respectively. Due to their association with endometriosis, the cystic component may be hyperintense on T1WI and hypointense on T2WI (Figure 11, Figure 12 and Figure 13).

Fig 11: T1WI axial sequence showing a tumor in the right ovary with hyperintense locules and a isointense locule (compared to muscle)

Fig 12: T2WI axial sequence showing a tumor in the right ovary with hypointense locules (corresponding to the hyperintense content in the T1WI, suggesting blood products) with a slightly hyperintense area

Fig 13: T1WI with fat saturation after contrast administration axial sequence showing enhancement of a solid componente corresponding to the slightly hyperintense area in the T2WI

Clear cell BOT are usually found in postmenopausal women and they can harbour ARID1A and PIK3CA mutations. They can also show an association with endometriosis, however it is less strong compared to other BOT subtypes. Unlike the more common subtypes, invasion is not seen with clear cell BOT, so even microinvasion must raise the suspicion of a clear cell carcinoma. They can manifest with an adenofibromatous morphology and their radiological features can resemble either benign cystadenofibroma or endometrioid BOT.

Brenner BOT occur more frequently in women older than 50 years and they can harbour PIK3CA and KRAS mutations. They have a mixed appearance, with both solid and cystic components, as opposed to benign Brenner tumour, which are predominantly solid. Borderline and benign Brenner tumours can coexist within the same specimen. Not many reports exist depicting the radiological features of Brenner BOT. However, they are usually described as cystic lesions with papillary projections that show an intermediate signal on T2WI, coupled with solid hypointense components, corresponding to the coexisting areas of benign Brenner tumours. Calcifications can exist in the benign component and are best depicted with CT.

After treatment, either with radical surgery or with a more conservative approach, these patients need to be followed-up for a long period (usually more than five years) because they can have late recurrences.