Gynaecological Imaging in Oncology

Ultrasound

Ultrasound (US) (transabdominal and/or transvaginal) is the primary imaging technique for examining the female pelvis. Currently, the main role of US in gynaecological oncology includes the investigation of postmenopausal bleeding, identification of a suspected pelvic mass and characterisation of ovarian or other pelvic masses. In addition, US has become invaluable in guiding a wide selection of invasive procedures such as transabdominal and transvaginal US (TVUS) guidance of fluid aspiration, tissue sampling or drain placement. US guidance for placement of brachytherapy devices for cervical and endometrial malignancies reduces malpositioning of the applicator.

A full bladder is mandatory for transabdominal US, as it displaces gas-filled bowel loops and provides a sonic window to image the pelvic organs. Transabdominal US utilises 3.5–5.0 MHz transducers. TVUS is optimally performed with an empty bladder, as this reduces the distance between the probe and the pelvic organs. TVUS can therefore employ higher-frequency transducers (5.0–7.5 MHz) to provide greater anatomical detail and delineation of pathology. Colour, power and spectral Doppler also provide additional information regarding associated vascularity of uterine and adnexal pathology.

Ultrasound has many advantages: it is relatively inexpensive, provides multiplanar views, is widely available and lacks ionising radiation. Its portability allows use in virtually any setting, including the US suite, operating room, patient bedside or radiotherapy suite; however, US also has a number of limitations: it is operator dependent and image quality varies with patient body habitus. Although transvaginal, sonohysterography and endorectal US provide improved spatial resolution, they are not as useful as CT or MRI in the staging of pelvic malignancies, including the evaluation of regional extent or metastatic spread.

Computed Tomography

CT is the most commonly used imaging technique for evaluating the extent of gynaecological malignancies and for detecting persistent and recurrent disease, although there is increasing use of MRI. CT-guided biopsy of omental cake or pelvic mass can be used to obtain histological confirmation of ovarian cancer and to confirm tumour recurrence.

CT images of the abdomen and pelvis are acquired in the portal venous phase, 70 seconds following an injection of intravenous low-osmolar iodinated contrast medium; this enhances blood vessels and viscera, allowing easier identification of enlarged lymph nodes and intraparenchymal lesions. Oral contrast medium is utilised to opacify the small and large bowel, which allows detection of bowel serosal deposits and the differentiation of bowel loops from pelvic and nodal disease.

Advantages of CT include wide availability, fast data acquisition and high spatial resolution. Disadvantages of CT include the use of ionising radiation, degradation of image quality by body habitus or metallic hip prosthesis and the risk of morbidity and mortality associated with iodinated contrast agents. Although CT is useful in the advanced stages of pelvic malignancy, it often has limited utility in characterising early-stage disease.

Magnetic Resonance Imaging

The role of MRI in the evaluation of gynaecological malignancies has evolved during the past two decades. MRI has been shown to be superior to CT in the local staging of endometrial and cervical cancer and can be a useful problem-solving tool in the evaluation of ovarian cancer, due to its superior soft-tissue and spatial resolution. In addition, there is evidence that MRI may aid the differentiation of post-treatment changes such as radiation fibrosis from recurrent tumour. The accuracy of MRI assessment of lymph node invasion is similar to that of CT, as both rely primarily on size criteria to detect lymph node metastases; however, MRI can detect differences in signal intensity in nodes replaced by tumour.

The basic imaging protocol for gynaecological MRI includes T ₁ weighted images (T ₁ weighted image) of the pelvis in the axial plane and T ₂ weighted images (T ₂ weighted image) in the axial and sagittal planes. High signal intensity within an adnexal mass on T ₁ weighted image may represent fat or haemorrhage. Therefore, fat-saturation T ₁ weighted images (T ₁ weighted FS) are mandatory to differentiate between the two, as fat will become low signal intensity following fat suppression. Staging of gynaecological malignancies requires large field-of-view axial T ₁ weighted image and/or T ₂ weighted image of the pelvis and abdomen to identify enlarged lymph nodes, hydronephrosis and bone marrow changes. High-resolution, small field-of-view, axial oblique (short-axis) T ₂ weighted fast spin-echo (T ₂ weighted FSE) images taken perpendicular to the endometrial cavity are essential in accurately evaluating the depth of myometrial invasion in endometrial carcinoma. In the assessment of patients with cervical cancer, high-resolution, small field-of-view, axial oblique (short-axis) T ₂ weighted FSE images perpendicular to the endocervical canal are crucial for identification of parametrial invasion.

Dynamic multiphase contrast-enhanced MRI using a three-dimensional (3D) gradient echo T ₁ weighted image after intravenous gadolinium is often utilised in staging patients with endometrial cancer; it is also useful in the characterisation of complex adnexal lesions and can aid in the detection of small peritoneal and serosal implants, when ovarian cancer is suspected. Routine use of contrast-enhanced T ₁ weighted imaging is not recommended for staging of patients with cervical carcinoma. However, it may be very useful for accurate delineation of small cervical tumours in patients being considered for fertility-sparing surgery (i.e. trachelectomy) and differentiation of tumour recurrence from radiation fibrosis in patients with previously treated cervical cancer.

Increasingly, diffusion-weighted imaging (DWI) is routinely incorporated into MRI staging protocols for uterine malignancies. DWI is useful for accurately determining the depth of myometrial invasion in patients with endometrial cancer, which can be particularly helpful in cases of tumours that are either iso- or hyperintense relative to the myometrium on T ₂ weighted image. DWI may replace dynamic contrast-enhanced imaging in the future, particularly when the use of intravenous contrast medium is contraindicated. DWI may also improve detection of drop metastases in the cervix or metastatic foci outside the uterus, such as in the adnexa or peritoneum.

In order to optimise the images, patient preparation is important, including fasting for at least 4 hours before MRI and the use of an anti-peristaltic agent to reduce the bowel motility and thus motion artefact. Emptying the bladder can reduce ghosting artefacts as well as improving patient comfort.

Although MRI is still relatively expensive, it has been shown to minimise costs in some clinical settings by limiting or eliminating the need for further expensive and/or more invasive diagnostic or surgical procedures. Advantages of MRI include superb spatial and tissue contrast resolution, no ionising radiation, multiplanar capability and fast (i.e. breath-hold and breathing-independent) techniques. MRI is the technique of choice for patients with previous reactions to iodinated IV contrast media or impaired renal function. However, MRI is contraindicated in patients with implants such as pacemakers, neural stimulators or cochlear implants, certain vascular clips and metallic objects. Some patients may experience claustrophobia, causing difficulty in completing the examination or requiring sedative premedication in subsequent MR examinations.

Positron-Emission Tomography/Computed Tomography

The main applications of 2-[ ¹⁸ F]-fluoro-2-deoxy- d -glucose (FDG) PET/CT in gynaecological oncology are the staging of cervical cancer and detection of recurrent ovarian, cervical and endometrial cancer. PET/CT takes advantage of the biochemical changes associated with malignancy that are more specific than, and often precede, the structural changes visualised by conventional means. Specifically, FDG-PET/CT exploits the accelerated rate of glycolysis that is common to neoplastic cells to image tumours. FDG-PET/CT does not have the same spatial resolution or soft-tissue contrast as US or MRI, but image fusion techniques that overlay FDG-PET images onto MR images are available. There are several disadvantages of FDG-PET/CT. False negatives can occur with lesions smaller than 0.5 cm and with certain tumour types that demonstrate low metabolic activity. False-positive results can be seen with inflammatory or infective aetiology, postoperative changes or reactive lymphadenopathy, including chronic inflammatory disorders such as granulomatous disease.

Detection, Diagnosis and Staging

Endometrial carcinoma is typically diagnosed at endometrial pipelle sampling or hysteroscopic biopsy, with imaging being reserved to evaluate the extent of disease. Endometrial cancer staging is based on the International Federation of Gynecology and Obstetrics (FIGO) surgical-pathological staging system, revised in 2009 ( Table 34.2 ). Full FIGO staging comprises a total abdominal hysterectomy (TAH), bilateral salpingo-oophorectomy (BSO), peritoneal washings and retroperitoneal lymph node dissection. Although imaging is not part of the FIGO staging system for endometrial cancer, both MRI and CT play an increasingly crucial role in risk stratification and surgical treatment planning. Imaging provides essential information regarding prognostic factors such as tumour stage, depth of myometrial invasion, extrauterine extent and lymph node involvement.

Ultrasound

Transvaginal (TVUS) is superior to transabdominal US for imaging endometrial abnormalities. The most common US appearance of endometrial cancer is non-specific thickening of the endometrium. Endometrial thickening in endometrial carcinoma is indistinguishable from that found with hyperplasia or a polyp; however, the diagnosis of endometrial cancer should be considered when the endometrial/myometrial junction is disrupted or the endometrial surface is irregular ( Fig. 34.1 ). In postmenopausal women, the endometrium should measure less than 4 mm; above this threshold, the patient should be referred for further investigation with hysteroscopy. In sonohysterography, fluid is instilled into the endometrial cavity, and endometrial cancer may appear as an intracavitary polyp or as asymmetric thickening of the endometrial lining. Doppler, colour and 3D US have been advocated to improve endometrial carcinoma detection.

The use of US is limited to the evaluation of stage I disease with emphasis on the evaluation of the depth of myometrial invasion. For the evaluation of myometrial invasion, the presence and continuity of the hypoechoic halo that surrounds the outer layer of the endometrium is assessed (i.e. intact, focally disrupted or totally disrupted). The extent of myometrial invasion is then estimated by measuring the distance from the central lumen of the uterus to the distal junction between tumour and normal myometrium. TVUS has been reported to have a sensitivity of 77%–100%, a specificity of 65%–93% and an overall accuracy of 60%–76% in assessing the degree of myometrial invasion.

Computed Tomography

On contrast-enhanced CT, endometrial cancer is seen as a hypodense mass relative to normal enhancing myometrium, but delineation of the tumour is difficult as there is relatively little contrast difference between tumour and the myometrium.

CT is most commonly used in the assessment of advanced disease and performs as well as MRI in identifying extrauterine spread and enlarged lymph nodes. Its greatest clinical impact is in confirming side-wall extension in stage III tumours, detecting pelvic lymphadenopathy and distant metastases. Limitations of CT include a tendency to understage endometrial carcinoma because of limited soft-tissue resolution, which leads to inaccurate estimation of depth of myometrial invasion. In addition, CT has a limited accuracy in evaluation of cervical stromal invasion and bladder or rectal mucosal invasion.

Magnetic Resonance Imaging

MRI is considered the most accurate imaging technique for preoperative assessment of endometrial carcinoma due to its excellent soft-tissue contrast resolution. The European Society of Urogenital Imaging (ESUR) and the Royal College of Radiologists cancer imaging guidelines also recommend MRI for staging of high-grade endometrioid adenocarcinomas (type I histology) as well as serous papillary and clear-cell adenocarcinomas (type II histology).

Although endometrial cancer may demonstrate high signal intensity on T ₂ weighted sequences, it is more typically heterogeneous and may even be of low signal intensity. Following IV contrast medium administration, there is early avid enhancement of normal myometrium. Endometrial cancer enhances more slowly than the adjacent myometrium, allowing identification of small tumours, even those contained by the endometrium. In the later phases of enhancement, the tumour appears hypointense relative to the myometrium ( Fig. 34.2 ). Endometrial cancer demonstrates restricted diffusion on DWI (high signal intensity on DWI and low signal intensity on corresponding apparent diffusion coefficient (ADC) maps), when compared with the normal endometrium and myometrium. This enables accurate depiction of the depth of myometrial invasion and is particularly useful when the myometrium is distorted by the presence of fibroids or adenomyosis. DWI can also aid the detection of extrauterine metastatic disease, within the peritoneum and adnexa, as well as drop metastases in the cervix/vagina.

MRI is significantly superior to US and CT in the evaluation of both depth of myometrial invasion and tumour invasion into the cervical stroma. The overall staging accuracy of MRI has been reported to be between 85% and 93%.

Stage I

Stage I endometrial cancers represent tumours confined to the uterine corpus. Stage Ia tumours invade less than 50% into the myometrium and may show disruption or irregularity of the junctional zone (JZ). If the low signal intensity of the JZ is intact, a stage Ia tumour is highly probable. The presence of intermediate signal intensity tumour within the outer 50% of the myometrium indicates deep myometrial invasion—stage Ib disease ( Fig. 34.3 ).