MSCT assists ovarian mass identification

November 1, 2006
Athina Tsili, MD
Athina Tsili, MD

,
Evangelos Paraskevaidis, MD
Evangelos Paraskevaidis, MD

,
Stavros Efremidis, MD
Stavros Efremidis, MD

Ovarian cancer accounts for nearly 3% of all cancers among women. It is the second most common gynecological malignancy in the U.S., following carcinoma of the uterine corpus.

Ovarian cancer accounts for nearly 3% of all cancers among women. It is the second most common gynecological malignancy in the U.S., following carcinoma of the uterine corpus. Estimates suggest that in the U.S., more than 20,000 new cases of ovarian carcinoma will be detected in 2006 and the disease will cause more than 15,000 deaths.1 Ovarian cancer causes more deaths than any other cancer of the female reproductive system owing to its silent nature. The disease will have reached an advanced stage in 70% of patients at initial presentation.

Characterization of an ovarian mass is an important part of preoperative evaluation. Referral of patients with suspected adnexal masses to a gynecologic oncologist for staging laparotomy results in better survival rates. Laparoscopic surgery has been used to manage benign adnexal masses with minimal surgical morbidity. Transvaginal ultrasound is the most practical modality for the assessment of adnexal masses, though its specificity in distinguishing benign masses from malignant lesions varies from 60% to 98%.2,3 While MRI can detect and characterize adnexal masses accurately, this modality is reserved for problem-solving cases owing to its high cost.4 Now advances in multidetector technology are improving the performance of CT in diagnosing ovarian masses.

DIAGNOSTIC TOOLS

Ovarian masses present a special diagnostic challenge. Benign lesions are common and greatly outnumber ovarian malignancies. Pretreatment characterization of adnexal masses can provide a basis for optimal preoperative planning if the mass turns out to be cancerous. It can also reduce the number of unnecessary laparotomies in patients with benign lesions.

Bimanual pelvic examinations and evaluations of serum CA-125 levels have failed to detect ovarian cancer consistently. Both techniques often have sensitivities below 50%.5 Ultrasound remains the study of choice in the initial evaluation of suspected ovarian masses. It is inexpensive, noninvasive, and widely available, and it has a high negative predictive value for the characterization of adnexal masses.

Transabdominal and transvaginal ultrasound should be performed together to assess morphological and Doppler features of ovarian lesions. Reported sensitivity for the characterization of adnexal masses with gray-scale criteria ranges from 85% to 97%. Documented specificity varies from 56% to 95%.2 Doppler ultrasound helps identify vascularized tissue.

Doppler ultrasound can also be used in conjunction with pulsed Doppler ultrasound to identify vessels for waveform analysis. Waveforms can be analyzed using the resistive index and the pulsatility index, which both increase with increasing distal vascular resistance. Resistive indices less than 0.4 to 0.8, and pulsatility indices less than 1.0, are generally considered suspicious for malignancy.3,6

Doppler ultrasound has yielded variable results when differentiating between benign and malignant adnexal masses. Reported sensitivities range from 50% to 100%, and specificities from 46% to 100%.3 Results from a meta-analysis comparing ultrasound techniques for characterizing ovarian masses showed that combinations of gray-scale morphologic assessment and tumor vascularity imaging were significantly better than Doppler arterial resistance measurements, color Doppler flow imaging, or gray-scale morphologic assessment alone.6

MRI has a potential role in the characterization of ovarian masses. Several studies indicate that gadolinium-enhanced MRI offers the best assessment of complex adnexal masses.4,7-9 The modality demonstrates a high detection rate and an accuracy of 91% in lesion characterization.8 Its high cost, however, means that it is best reserved for cases in which ultrasound findings are inconclusive or indeterminate.

EXPANDING ROLE FOR CT

The main role of CT in ovarian cancer has traditionally been to provide staging information for preoperative planning and determination of surgical resectability, to show tumor response to therapy, and to allow the detection of persistent or recurrent disease.10 Single-slice CT and MRI are equally accurate, and either modality can be used to stage advanced ovarian cancer.11,12

The introduction of multislice CT, and especially 16-slice scanners, has improved the diagnostic performance of the modality. Scanning is faster, and thin-slice acquisition is feasible.13,14 Routine use of 0.8-mm section collimation makes it possible to evaluate the adnexae in detail and to detect small lesions. Near-isotropic voxels mean that high-resolution multiplanar and 3D images can be reconstructed. This has greatly improved the ability of CT to distinguish between ovarian and extra-ovarian intraperitoneal pelvic masses and to assist in the differentiation of benign ovarian masses from malignancies.

MSCT scanners may additionally improve the sensitivity of CT in the evaluation of peritoneal metastases. Multiplanar reformatting is allowing the detection of subcentimeter implants and visualization of curved structures, such as the diaphragm, paracolic gutters, and pelvis (Figure 1). Three-D displays may aid tumor resection by depicting the disease as seen at surgery (Figure 2) and its relationship to viscera and blood vessels.

We have devised our own protocol for evaluating ovarian masses with a 16-slice scanner (see table). Patients receive 1000 mL of water orally 30 minutes before the examination. Abdominal scanning generally begins after administration of 120 mL of nonionic iodinated contrast material, at a rate of 3 mL/sec. Imaging is initiated 70 seconds after contrast injection during the portal phase. Both dose modulation and automatic current setting (DoseRight) are used. Considering that the tube current is modulated across the patient's body, based on a planning scan projection radiograph and body asymmetry (dose modulation function), the mean mAs per rotation at each phase for each scan is calculated at 110 mAs.

This protocol generates a large number (700 to 800) of noisy axial images, which can be difficult to study. Image interpretation is instead performed on transverse, coronal, and sagittal reformatted images. These multiplanar reformatted images are usually 4-mm thick and acquired at 3-mm intervals. MPR images are useful for a detailed evaluation of certain anatomic regions, such as the diaphragm (coronal and sagittal MPR), paracolic gutters (coronal MPR), and the pelvis (coronal and sagittal images).15 We have found the evaluation of some anatomic areas to be more reliable with certain anatomic planes, e.g., the coronal plane for the evaluation of the paracolic gutters. Three-D volume-rendered images are also acquired. It takes approximately one minute to generate MPR and 3D images on a 16-slice CT scanner.

DIFFERENTIAL DIAGNOSIS

The following CT features are suggestive of a benign ovarian mass:10,16

  • lesion diameter less than 4 cm;
  • presence of entirely cystic components;
  • lack of internal structure;
  • wall thickness less than 3 mm; and
  • absence of ascites or findings indicative of invasive abdominal disease (e.g., peritoneal metastases, lymphadenopathy).

Diagnosis of serous cystadenoma can be made if CT shows a homogeneous unilocular or multilocular tumor filled with serous fluid. The tumor should not enhance after contrast administration, though some enhancement may be seen in the tumoral wall or septa, which should be less than 3-mm thick. Observation of a thin-walled multilocular cystic mass containing liquids with differing CT attenuation (owing to mucin) is considered to represent a mucinous cystadenoma.

Fat attenuation within a cyst, with or without the presence of calcification, should lead to a diagnosis of mature cystic teratoma.17 Visualization of a hyperdense lesion (50 to 70 HU) that does not enhance after contrast administration is likely to be a benign hemorrhagic adnexal mass (probably a hemorrhagic cyst or an endometrioma). A wholly solid mass that demonstrates homogeneous enhancement is also more likely to be benign.

CT features used to diagnose an ovarian malignancy are similar to criteria used with other imaging techniques: 4,16,18

  • lesion diameter larger than 4 cm;
  • presence of bilateral adnexal masses;
  • part-cystic, part-solid mass;
  • enhancement of solid mass elements after contrast administration; and
  • presence of necrosis in solid mass.

Imaging characteristics of the wall or septa are very important in the characterization of cystic and solid-cystic ovarian lesions. Identification of a wall or septum greater than 3-mm thick, with irregularity or papillary projections, and enhancement on contrast administration (Figure 3) are used as signs to confirm malignancy. The presence of ancillary findings, such as pelvic organ or sidewall invasion, ascites, lymphadenopathy, and peritoneal, omental, or mesenteric metastases will confirm diagnosis of a malignancy.

MSCT does have a number of limitations, however. These include difficulties revealing microscopic involvement of the adnexae and occasional difficulties defining whether a large adnexal mass is unilateral or bilateral. Imaging evaluation of borderline tumors also remains problematic, due to the similarity of their morphologic features with those of benign ovarian masses. This is true, however, whichever imaging modality is used.

Another limitation of MSCT relates to the misinterpretation of fibrous tissue within adnexal masses as solid elements. These lesions may appear to have solid, enhancing parts on CT, leading to a diagnosis of malignancy (Figure 4). MRI, on the other hand, provides a true diagnostic assessment of such lesions. The accuracy of MRI in characterizing adnexal masses containing fibrous tissue is well known and derives from its superb contrast resolution.19

MSCT is an excellent modality for detecting peritoneal involvement, but care must be taken to avoid confusion with the wide spectrum of tumorlike conditions that may mimic malignancy.20 Coexistence of ascites and peritoneal disease in patients with an ovarian mass is generally considered to characterize malignancy, although this may not always be confirmed histologically. Ascites, infiltration of the peritoneal ligaments and mesenteries, and the pathways of spread are common to neoplastic and inflammatory diseases.

In conclusion, 16-slice CT enables rapid, accurate abdominal examinations. MPR views and 3D reconstructions are generally devoid of artifacts and offer excellent anatomic detail.

DR. TSILI is a radiologist in the clinical radiology department, PROF. PARASKEVAIDIS is head of the gynecology and obstetrics department, and PROF. EFREMIDIS is head of the clinical radiology department, all at the Hospital of Ioannina, Panepistimioupolis, Ioannina, Greece. Assisting in the preparation of this manuscript were Constantine Tsampoulas, M.D., and Nikolaos Dalkalitsis, M.D., both at the same institution.

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