Imaging
Summary of Key Points
• Noninvasive medical imaging often is essential to cancer management at multiple times in the course of the illness.
• Imaging currently is used for screening to detect cancer, characterize lesions, perform locoregional and systemic staging, provide prognostic information, assess response during and after therapy, restage after treatment, perform follow-up of patients for recurrence, and precisely guide biopsies and therapies such as external beam or systemic radiation, brachytherapy, or thermal and other ablations.
• More invasive interventional radiologic procedures also can guide and monitor vascular or intraluminal delivery of treatments such as radioactive microspheres, embolic materials, radiofrequency or cryoablation, and therapeutic drugs.
• Imaging methods range from the traditional anatomic methods—radiograph, computed tomography (CT), and ultrasound—to the more functional methods of magnetic resonance imaging (MRI) and nuclear medicine methods, including positron emission tomography (PET), single photon emission computed tomography (SPECT), and planar nuclear imaging. Hybrid methods combining PET and CT, SPECT and CT, and PET and MRI are growing in importance. Optical imaging is promising but is limited by penetration of light through tissues to superficial structures in most cases.
• Plain films and mammography remain useful techniques, with mammography (including digital mammography) being the main imaging method that has been clearly proven capable of reducing cancer deaths when applied in the screening setting.
• CT remains the cornerstone technology for most oncologic imaging, and CT technology that allows for rapid-sequence angiography is finding new applications, as is three-dimensional reconstruction of CT data sets. Screening data with CT-colonography continues to improve, and in some studies it has been found to be comparable with traditional colonoscopy for colon cancer screening. CT scanning for lung cancer screening appears to be capable of reducing lung cancer death rates when applied to high-risk populations. The radiation dose from CT is a concern, and major efforts to reduce this dose from CT scanning have been implemented in newer CT systems.
• MRI is the imaging tool of choice for central nervous system, spinal, and musculoskeletal neoplasms, as well as for assessing vascular and some hepatobiliary and pelvic lesions. MRI also can be used to detect breast cancers, especially in women with dense breasts. Concerns regarding gadolinium-associated nephrogenic systemic fibrosis have led to cautions in the use of MRI contrast medium in patients with impaired renal function. Newer MRI techniques such as diffusion imaging and complement diffusion contrast MRI appear promising in assessing response to tumor treatment.
• Bone scans using single-photon methods (e.g., technetium-99m methylene diphosphonate) remain the dominant procedure for detecting suspected bone metastases; however, the PET agent fluorine-18 sodium fluoride is increasingly being applied. These techniques may be less sensitive for marrow involvement than MRI and other PET techniques for detecting bone metastases of many tumors.
• PET and PET/CT technology using 18F-fluorodeoxyglucose (FDG) continues to grow in a wide variety of applications, and its use is becoming increasingly routine in the management of patients with cancer at varying states of the disease process. PET is used with increasing frequency in the staging and follow-up of lung, colorectal, and head and neck cancers, as well as lymphomas and other types of tumors, and it is now a routine tool in lymphoma management at several points in the disease. PET with non-FDG tracers is a promising research area with growing clinical applications. In particular, progress has occurred in imaging of prostate cancer with several imaging agents, including U.S. Food and Drug Administration (FDA)–approved carbon-11 choline.
• The fusion of anatomic and functional images to create hybrid “anatomolecular images” with software or dedicated instruments such as PET/CT, SPECT/CT, or the newer PET/MRI devices also is seeing rapid growth in applications in cancer imaging. Fully diagnostic CT scans coupled with PET imaging in the form of PET/CT often provide valuable composite imaging for cancer management. PET/MRI is an evolving technology, and several technical approaches are in clinical use at select medical centers.
• Imaging management for staging lung cancer and characterizing solitary pulmonary nodules often includes FDG-PET in addition to CT when the technology is available because PET-CT has high accuracy in lung cancer assessments compared with CT.
• Imaging management of suspected recurrences of colorectal cancer, head and neck cancer, lymphoma, and many other cancers often now includes the use of PET in addition to CT. Response criteria for FDG-avid lymphomas are now mainly PET-based, and PET assessments of treatment response are increasingly applied. Use of PET at earlier stages in the workup is becoming increasingly common, as is the use of PET in early assessments of the efficacy of cancer therapies. Adapting treatments based on the response seen on PET/CT is also increasingly applied.
• In prostate cancer, available imaging methods remain suboptimal for the detection of primary tumor and early determination of local or systemic tumor spread. MRI nodal contrast agents are promising but not yet routinely available, and MR spectroscopy has had only limited success in the prostate. A variety of MRI sequences, including T2 images, diffusion images, and diffusion contrast enhanced MRI may improve upon purely anatomic MRI approaches for lesion detection and detection of extracapsular involvement. A variety of innovative radiotracers for PET show promise for detecting disease recurrence, and 11C choline is now approved by the FDA in the United States for use in persons with prostate cancer.
• Visceral angiography for diagnostic purposes is being supplanted by CT and MRI methods; however, it remains important as a tool for intravascular delivery of therapies such as chemotherapy, coils, or radioactive microspheres.
• CT, ultrasound, fluoroscopy, and innovative MRI systems can guide interventional procedures such as thermal and cryotherapeutic lesion ablations.
• Highly specific probe-reporter systems are being developed to allow for optical and radionuclide imaging of transfected gene biodistribution and function. These approaches face major regulatory challenges when being translated to humans.
• Combined anatomic and functional information is being applied to allow for more precise planning of external beam radiation therapy, including intensity-modulated radiation therapy and conformal therapy, which are methods that potentially allow for increasing dose escalation and minimization of toxicity to normal tissues.
• Emerging imaging methods are proving increasingly useful in providing information on the physiology and molecular characteristics of lesions, which means that a multiparametric biological imaging phenotype for tumors can be obtained, making it possible to display heterogeneities in tumors. This phenotype can more precisely guide individualized tumor treatment to yield a higher probability of success without excessive toxicity for treatment of the selected neoplastic process.
1. In evaluating computed tomography (CT) scanning for lung cancer as reported in the National Lung Cancer Screening Trial, which of the following statement(s) are true?
A CT screening reduced mortality from lung cancer by 20%.
B 95% of the CT findings were false positives.
C CT screening and chest radiograph screening were equally ineffective in detecting lung cancer.
D CT screening was associated with a decreased all-cause mortality in this trial.
2. Which of the following statements are true with regard to radiation dose in imaging?
A Mammography has a much lower dose than does breast tomosynthesis.
B CT and nuclear medicine procedures represent more than 75% of the imaging-related radiation dose in the United States.
C Age at the time of irradiation is not a major factor in potential radiation-induced carcinogenesis.
D Magnetic resonance imaging (MRI) scans with contrast carry no risk of adverse effects.
3. Which of the following statements are true about imaging methods in cancer?
A Ultrasound is the most cost-effective method for abdominal imaging.
B MRI is the most sensitive method for detecting low numbers of contrast molecules versus nuclear or optical methods.
C X-rays have higher resolution than ultrasound or nuclear imaging methods.
D Fluorodeoxyglucose positron emission tomography (PET) studies cannot be quantified.
1. Answer: A, B, and D. Note that to achieve a reduction in lung cancer and all-cancer mortality in this study, a high prevalence of false-positive findings was required.
2. Answer: B. Mammography and tomosynthesis with modern systems have comparable radiation doses (thus A is false). Younger patients are more at risk of carcinogenesis than are older patients (thus C is incorrect). MRI scans carry no risk of irradiation but carry with them risks of adverse reactions to intravenous contrast, especially in patients with impaired renal function (nephrogenic systemic fibrosis).
3. Answer: C. With regard to A, ultrasound may be the least expensive study to perform, but it may not be the most accurate in the abdomen. Thus it is not the most cost-effective. With regard to B, MRI has low sensitivity for detecting contrast molecules versus optical and nuclear methods. Thus large amounts of MRI contrast must be given to have a suitable signal, in vivo. With regard to D, PET can be quantified for standardized uptake value if performed correctly per National Cancer Institute guidelines.
