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Imaging in Oncology
Imaging in Oncology
Imaging plays a key role in the diagnosis and assessment of treatment response of primary malignancies and metastatic disease throughout the body. Computed tomography (CT) and magnetic resonance imaging (MRI) are the primary imaging modalities used for this purpose, with ultrasound (US), radiography, and conventional angiography used in more specific capacities. Nuclear medicine, including positron emission tomography (PET)/CT and PET/MRI, will be addressed in a separate chapter.
Computed Tomography
CT is the primary imaging modality used in many different types of malignancy. Modern CT scanners are capable of producing highly detailed, thin-slice volumetric datasets reformatted in multiple planes or as three-dimensional renderings. Image acquisition is sufficiently rapid to acquire the dataset in less than 1 minute, making CT particularly suited for patients who have difficulties with remaining still, following breath-holding instructions, or claustrophobia. The intravenous administration of iodinated contrast media provides optimal assessment of the vasculature, mediastinum, solid organs, and bowel and is preferred for most oncological CT examinations. Approximately 75 to 125 mL of contrast is typically administered during an examination. Risks from contrast administration include allergic reactions along the spectrum from urticaria to anaphylaxis, contrast-induced nephropathy, and radiation exposure. Premedication with corticosteroids and antihistamines or intravenous hydration may be necessary if allergic reactions or contrast-induced nephropathy, respectively, is of concern.
Magnetic Resonance Imaging
MRI has long been used for evaluation of the central nervous system, spine, and musculoskeletal systems. More recently, MRI has gained more widespread use in imaging head and neck and abdominopelvic malignancies due to its superior soft-tissue contrast and anatomic depiction. The lack of ionizing radiation makes MRI a particularly attractive alternative in imaging of appropriate pediatric malignancies and screening for malignancy in at-risk populations. Nearly all oncological MR examinations will be performed using the intravenous administration of gadolinium contrast, typically in doses ranging up to 20 to 30 mL. Adverse reactions related to gadolinium injection are less common than for iodinated CT contrast, but unique risks such as nephrogenic systemic fibrosis and deposition within the central nervous system must be considered, and gadolinium is unsafe to administer during pregnancy. In addition, MRI examinations typically take at least 15 to 30 minutes and may extend up to an hour depending on complexity and anatomic regions being scanned, during which time the patient must be able to lie still and supine within the relatively small bore of the magnet. The powerful magnetic field also creates potential complications with metallic and electronic implants such as cardiac pacemakers, spinal stimulator devices, aneurysm clips, or joint prostheses. Thus, patients must be carefully screened for compatibility prior to performing an MRI.
Ultrasound
US has a more targeted role in oncological imaging compared with MRI or CT. Although it has superior spatial resolution and few contraindications, US can be limited owing to patient body habitus, overlying structures, and depth of penetration. In addition, US is more operator dependent than CT or MRI. Common uses in oncological imaging for which US is a primary imaging modality are evaluation of small body parts such the thyroid gland, testes, gallbladder, ovaries, and lymph nodes. US is also a preferred tool for image-guided biopsies for diagnosis and staging of malignancy, along with CT.
Imaging Strategies
The imaging appearance of commonly encountered primary and metastatic malignancies in the chest, abdomen, pelvis, central nervous system, head and neck, and spine will be presented using examples of CT, MRI, and US. PET/CT and PET/MR examples will be briefly mentioned, with more extensive discussion in a separate chapter. Imaging protocols can vary from institution to institution; consultation with a local radiologist can be very helpful in optimizing oncological imaging for diagnosis and staging.
Chest Imaging
Primary Tumors of the Lung
Lung cancer is the leading cause of cancer-related deaths in the United States. Early diagnosis and complete surgical resection are essential for improved survival; however, this proves difficult, as many patients are asymptomatic at the time of diagnosis, and greater than two-thirds of lung cancers are detected at an advanced stage.
Imaging Strategies
The chest radiograph is most commonly used imaging modality for numerous clinical scenarios when assessing pathology in the chest. As a result, a majority of lung cancers are initially discovered on chest radiograph ( eFig. 11.1 ).
CT and positron emission tomography/CT (PET/CT) are the most widely used modalities to stage and assess treatment response in lung cancer. MRI may provide additional detail in challenging cases, typically to assess involvement of the chest wall, mediastinum, major vascular involvement, or invasion into the brachial plexus.
Lung Cancer Screening
As a result of the National Lung Screening Trial, the US Preventive Services Task Force has given a grade B recommendation for lung cancer screening (LCS) using low-dose CT (LDCT) for scanning high-risk current and former smokers. To direct appropriate management, the American College of Radiology (ACR) has established the Lung-RADS (Reporting and Data System) classification scheme to standardize the screening lexicon, interpretation, and recommendation of detected nodules ( eFig. 11.2 ). As reimbursement for LCS in appropriate patient populations improves, the number of detected lung cancers is expected to rise nationally.
Tumors of the Lung
Primary lung malignancies can be divided into two major histological categories: non–small cell carcinoma and small cell lung carcinoma. Non–small cell lung cancer (NSCLC) accounts for the vast majority of lung cancers. Non–small cell cancer subtypes include adenocarcinoma, squamous cell, and large cell.
Adenocarcinomas include both mucinous and nonmucinous tumors along a spectrum consisting of adenocarcinoma in situ, minimally invasive adenocarcinoma, lepidic predominant adenocarcinoma, and invasive adenocarcinoma. Mucinous adenocarcinomas typically present on CT as solid nodules or foci of airspace opacification with air bronchograms ( eFig. 11.3 ). Nonmucinous tumors manifest along a spectrum as solid or subsolid lesions, either nonsolid (ground glass) or part solid (ground glass and solid components). Increasing overall size, increasing size of a solid component, increasing attenuation of the ground-glass component, air bronchograms, distortion of adjacent vasculature, and indentation are markers of more severe disease.
Squamous cell carcinomas are often centrally located tumors that can present a single solitary nodule or mass with irregular borders. In some instances, these tumors may result in intraluminal obstruction of a bronchus, resulting in segmental or lobar collapse. Cavitation can frequently be seen in squamous cell lung cancers and can serve as a distinguishing feature on imaging ( Fig. 11.1 ).
Large cell carcinoma, also known as giant cell carcinoma, is a poorly differentiated NSCLC. Large cell carcinomas are characterized by their rapid growth pattern, large size, and predilection for early metastases to the mediastinum and brain ( Fig. 11.2 ).
Small cell lung cancers account for 13% to 15% of all lung cancers, are the most common neuroendocrine-type tumor, and have a strong association with cigarette smoking. Small cell lung cancers typically arise from lobar or main bronchi, commonly manifesting as a large centrally located mass within the lung parenchyma and often involving the hilum and mediastinum. CT imaging may demonstrate confluent soft tissue encasing mediastinal structures ( Fig. 11.3 ). Occasionally, small cell lung cancer presents as a single peripheral solitary tumor without significant lymphadenopathy.
Staging of Lung Cancer
Staging of lung malignancies provides a consistent nomenclature classifying the anatomic extent of disease that allows for physicians to apply appropriate treatment strategies to patients with lung cancer. The Union Internationale Contre le Cancer (UICC) and American Joint Committee on Cancer (AJCC) are the official bodies that define, review, and refine stage classification systems; the most current system is the eighth edition.
Anatomic extent of tumor consists of three components: extent of primary (T), involvement of lymph nodes (N), and distant metastases (M). Each component is divided into several categories, with combinations of T, N, and M corresponding to specific stage groups.
The tumor descriptor of lung cancer is based on size of the primary tumor, extent of invasion of local structures, and presence and location of additional ipsilateral tumor nodules. Tumor size is determined by measuring the greatest long axis measurement of the primary tumor, typically performed with contrast-enhanced CT to allow for demarcation between tumor and adjacent atelectatic lung, as well as multiplanar reconstructions. Furthermore, cross-sectional imaging can be used to evaluate extent of local invasion of the parietal pleura, visceral pleura chest wall, superior sulcus, phrenic nerve, parietal pericardium, ribs, diaphragm, mediastinum, heart and great vessels, trachea, esophagus, vertebral bodies, and the recurrent laryngeal nerve. As a result of its superior contrast resolution, MRI is typically superior to CT for evaluation of chest wall and pleural invasion. Furthermore, it is important to note that chest wall invasion can be present pathologically in the absence of imaging findings. Lung nodules distinct from the primary tumor are typically well evaluated with CT. Additional lung nodules can typically be attributed to one of the following: a solid primary lung cancer with one or more solid tumors of the same histological types (intrapulmonary metastasis), separate primary lung cancers, multiple lung cancer nodules, or pneumonic-type lung cancer manifesting as diffuse or multinodular areas of consolidation.
The nodal descriptor is based on the absence or location of cancer spread to regional lymph nodes. Nonregional lymph node spread is considered a distant metastasis and classified using the M descriptor. Lymph node descriptions are based on the anatomic location of lymph nodes rather than number or size of actual nodes. A multimodality approach is used to characterize the N descriptor consisting of CT, PET/CT, esophageal US, endobronchial ultrasonography, and mediastinoscopy ( Fig. 11.4A ). Lymph nodes with a size larger than 1 cm in the short axis are considered suspicious on imaging. Unfortunately, size criteria are not very sensitive or specific as a multitude of pathological processes separate from lung malignancy can result in enlarged lymph nodes. When using the size criterion greater than 1 cm, CT has a sensitivity of 55% and specificity of 81%. FDG-PET/CT has proven superior to CT in detecting mediastinal lymph node metastasis with a sensitivity of 77% and specificity of 86%. Fluorodeoxyglucose–PET/CT (FDG-PET/CT) for lymph nodes less than 1 cm is suboptimal, with a sensitivity of only 32.4%. Regional lymph node maps with labels assigning numerical levels for each anatomic location are used to describe the location of lymph nodes. The International Association for the Study of Lung Cancer (IASLC) map is used in the current edition of the TNM staging manual. Ipsilateral peribronchial and/or hilar and intrapulmonary nodes are classified as N1. Ipsilateral mediastinal and/or subcarinal involved nodes are N2. Finally, contralateral mediastinal or hilar, ipsilateral/contralateral scalene, or supraclavicular involved nodes are N3.
The M descriptor is based on the presence of metastases and is used to describe their location and multiplicity. The M1 descriptor is given when any distant metastasis is present. In the current staging system, a distinction is made between regional metastatic disease (M1a), solitary (M1b), or multiple (M1c) distant metastatic disease. Regional metastatic disease (M1a) is defined by involvement of the pleura, pericardium, or contralateral pulmonary nodules ( eFig. 11.3B ). M1b refers to solitary extrathoracic metastasis, whereas M1c refers to multiple extrathoracic metastases, either in a single organ or multiple organs. Common sites of extrathoracic disease include the adrenal glands, liver, brain, and skeleton ( Fig. 11.4B ). PET/CT imaging in lung cancer can be used to detect occult metastatic disease, which is essential for treatment planning as it may alter management. PET/CT is limited in detection of brain metastases; for this reason, brain MRI is the mainstay for evaluation of intracranial metastatic disease.
Follow-Up Imaging for Lung Cancer
CT and PET/CT play crucial roles in evaluating response to therapy and assessing for possible treatment complications. Patients with lung cancer can be followed with CT, PET/CT, or a combination of the two imaging modalities to evaluate treatment response. PET/CT may prove useful to evaluate for changes in metabolic activity. The role of MRI is limited but can supplement CT in the setting of superior sulcus tumors or in evaluating tumors invading the chest wall, mediastinum, or spinal cord.
Lymphoma
Lymphomas are malignancies of hematopoietic cells of specific lineages, including T and B lymphocytes and histiocytes, composed of two primary classifications: Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL). NHL compromises 90% of lymphomas; HL comprises the remaining 10%. Classically, HL and low-grade NHL present with involvement of the lymph nodes within the chest. CT imaging may demonstrate nodal disease manifesting as enlarged lymph nodes (> 1 cm), increased in number, with homogeneous density. Typically, the paratracheal and anterior mediastinal lymph nodes are involved ( Fig. 11.5 ). Lymphoma can also present as a soft-tissue mass within the anterior mediastinum that conforms to surrounding structures. Low-density or cystic areas can be seen within the mass ( Fig. 11.6A ). Imaging may show associated pleural or pericardial effusions and chest wall invasion ( Fig. 11.6B ). Pulmonary involvement of lymphoma is rare; however, it can manifest as pulmonary nodules or masses with or without cavitation, ground-glass opacities, or endobronchial masses.
NHL and HL can also be seen in patients with posttransplant lymphoproliferative disorder (PTLD). PTLD can be divided into subgroups based on the presence of the Epstein-Barr virus. Imaging manifestations of PTLD include solid pulmonary nodules or masses, consolidation, ground glass, or interstitial disease.
CT and PET/CT play a key role in imaging of lymphoma. CT can be employed for initial diagnosis and subsequent follow-up imaging to assess location and size of enlarged lymph nodes or soft-tissue masses. PET/CT provides the added benefit of evaluating metabolic response to treatment based on changes in FDG uptake within the nodal soft tissue. Furthermore, PET/CT is beneficial when evaluating extranodal tissue, such as occult bone marrow lesions. PET/CT has been fully incorporated into staging and response assessment of FDG-avid lymphoma. In instances in which FDG uptake is low, CT remains the mainstay for follow-up evaluation of lymphoma.
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