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their differentiation from invasive cancers. In DCE delayed enhancement and MRI type 2 timeintensity curves may be seen in BT (Fig. 2) (Thomassin-Naggara et al. 2013).

Differential Diagnosis

Borderline tumors are indistinguishable from invasive cancers due to overlap in imaging findings such as irregular thickened walls, enhancing vegetations, and solid components. Low ADC values of solid aspects in DWI favor cancer (Zhao et al. 2014a). Peritoneal implants or lymph node involvement is not a useful criterion for differentiation as these may be also found in approximately 30% of serous BT (Leake et al. 1992). Hemorrhagic and mucinous contents may be seen both in mucinous BT and mucinous cystadenomas. Features favoring BT are honeycomb appearance, thick septa (5 mm), and papillary projections of >5 mm (Zhao et al. 2014b).

Recurrent Ovarian Cancer

Although the initial response to treatment is good, persistence or recurrence of ovarian cancer remains a major problem. This is reflected by the overall likelihood of relapse for all stages of ovarian cancer of 62% and of up to 85% for women presenting with advanced ovarian cancer (Birrer 2016).

Survival correlates with the disease-free interval before tumor recurrence and the residual disease following primary cytorective surgery (Jayson et al. 2014; Ozols et al. 2001; Birrer 2016). Pelvic relapse develops after an average of 1.8 years and hematogenous metastases (liver, spleen, pleura, lungs, and brain) after an average of 2.5 years (Burghardt 1993). The pelvis, particularly the vaginal vault and the cul-de-sac, is the most common site of tumor recurrence, and it is followed by abdominal peritoneal implants. Typical abdominal locations include the surface of the diaphragm and liver, paracolic gutters, the largeand small-bowel surface, and mesentery (Kwek and Iyer 2006). Lymph node metastases are typically located in the para-aortic region and found in 18–33% (Burghardt 1993). Smalland large-bowel obstruction is a common complication in patients with recurrent ovarian cancer and remains the leading cause of mortality (Birrer 2016).

Unlike in primary ovarian cancer, recurrent ovarian cancer is not strongly associated with ascites. In one study, ascites was only found in 38% of patients with relapse of ovarian cancer, and in the vast majority, the amount of fluid detected was small (Forstner et al. 1995). Furthermore, small amounts of ascites were also demonstrated in patients without evidence of tumor recurrence. Large amounts of ascites are more likely to occur in platinum-resistant disease (Birrer 2016). Serum tumor markers (CA-125) are the cornerstone in the surveillance of patients with ovarian cancer. A rising CA-125 level in a patient in a clinically complete remission is highly predictive of recurrence. However, this may precede the median time to physical or radiographic evidence of recurrent disease by 4–6 months (Birrer 2016).

Imaging Findings

Recurrent ovarian cancer most frequently presents as solid or as mixed solid and cystic lesions located within the pelvis (Fig. 21). Entirely cystic lesions are rarely found (Forstner et al. 1995). In CT, recurrent disease usually displays moderate contrast enhancement. In MRI, the imaging findings depend on the morphology of the lesions. Usually smaller lesions display low to intermediate SI on T1-weighted images and intermediate to high SI on T2-weighted images. Contrast-­ enhanced images and DWI improve the detection of peritoneal surface lesions. Diffuse or focal peritoneal thickening presents peritoneal carcinomatosis. The pattern of peritoneal involvement is similar to primary ovarian cancer, with diffuse thin lining of the peritoneal surfaces to plaquelike lesions or nodules emerging from the peritoneal surfaces. Diffuse ascites is usually a sign of diffuse peritoneal recurrent disease. Omental caking is encountered only in patients treated with primary chemotherapy. Small-bowel obstruction is a typical complication as ovarian cancer advances and occurs in 5–42% (Low et al. 2003). Signs of malignant bowel obstruction include bowel dilatation, an obstructing mass, focal mural thickening, and peritoneal carcinomatosis (Low et al. 2003). A pseudo-small-bowel obstruction pattern can mimic small-bowel obstruction. It is typically­

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a	c

b

Fig. 21  Central pelvic recurrence (arrow) with rectal (R) invasion is demonstrated on Gd T1WI (a) and confirmed by its restricted diffusion on the ADC map (b). The PET/

CT follow-up (c) after 6 months of chemotherapy demonstrates reduction in size but still vital residual cancer

encountered late in the course of the disease and is caused by tumor infiltration of the myenteric plexus of the small bowel (Ozols et al. 2001). Resection of recurrent disease, which is usually performed in pelvic recurrence, is only considered successful when complete resection without residual tumor is possible. Preoperatively, it is crucial to assess pelvic sidewall invasion rather than tumor size (Sala et al. 2013).

Differential Diagnosis

Postoperative hematomas, adhesions between bowel loops, or localized trapped fluid may mimic recurrent disease. Benign forms of diffuse peritoneal thickening such as a result of postoperative inflammatory complications or bacterial peritonitis cannot be differentiated from peritoneal relapse. Furthermore, chemical peritonitis follow-

ing intraperitoneal chemotherapy also results in diffuse peritoneal thickening (Low et al. 2015).

Value of Imaging

Although large randomized trials showed no survival benefit of routine postoperative follow-up with CA-125, this tumor marker plays a pivotal role in monitoring patients with ovarian cancer in clinical practice (Birrer 2016; Spencer and Perren 2010). Imaging in conjunction with CA-125 is used to assess disease progression and response to therapy. Baseline CT examinations after surgery or before chemotherapy have been ­advocated to allow an objective follow-up (Sala et al. 2013). However, in many institutions imaging is only performed when tumor markers persist or increase or when the patients present with clinical symptoms. Demonstration of recurrence is pivotal in