Clinical Significance of Lung Nodules Reported on Abdominal CT

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1 Cardiopulmonary Imaging Original Research Alpert et al. Significance of Lung Nodules on Abdominal CT Cardiopulmonary Imaging Original Research Jeffrey B. Alpert 1 John P. Fantauzzi Kira Melamud Heather Greenwood David P. Naidich Jane P. Ko Alpert JB, Fantauzzi JP, Melamud K, Greenwood H, Naidich DP, Ko JP Keywords: abdominal CT, lung nodules DOI: /AJR Received February 4, 2011; accepted after revision August 8, All authors: Department of Radiology, Thoracic Imaging Section, NYU Langone Medical Center, 560 First Ave, IRM 236, New York, NY Address correspondence to J. B. Alpert (jbalpert@hotmail.com). AJR 2012; 198: X/12/ American Roentgen Ray Society Clinical Significance of Lung Nodules Reported on Abdominal CT OBJECTIVE. The objective of our study was to identify the significance of lung nodules reported on abdominal CT. MATERIALS AND METHODS. Abdominal CT reports from a 1-year period were reviewed for the terms nodule, nodular, or mass in reference to the lung bases. Patients with prior chest or abdominal CT examinations were excluded; the study population included patients with an initial abdominal CT study and at least one follow-up chest or abdominal CT examination. Two thoracic radiologists characterized nodules in consensus. Radiology and clinical records were reviewed for nodule growth and clinical diagnoses. RESULTS. The term nodule, nodular, or mass in reference to the lung bases was reported in 364 of 12,287 abdominal CT studies (3%). Of 125 patients with no prior CT examination, 42 had undergone follow-up chest CT, abdominal CT, or both. Common imaging indications included abdominal pain (13/42, 31%) and preexisting malignancy (n = 7, 16.7%). Regardless of the indication for imaging, 16 (38.1%) had malignancy that was known (n = 13) or newly diagnosed (n = 3) on the initial abdominal CT. Three of 42 patients (7.1%) had malignant nodules representing metastatic disease: Nodule growth was seen in one patient with preexisting colon cancer, one patient with newly diagnosed metastatic pancreatic cancer, and a third with known bladder cancer. The latter patient had suspected lung metastases that were confirmed on chest CT 1 day later. Three of the 16 patients (18.8%) with preexisting or newly diagnosed cancer had malignant nodules. No malignant nodules were identified without such history. Six patients (14.3%) had an infection. CONCLUSION. Lung nodules incidentally detected on abdominal CT were rarely malignant and were seen only in the setting of an underlying abdominal malignancy. Knowledge of such history is of critical importance to both the clinician and the radiologist. Dedicated chest CT is most useful when assessing pulmonary nodules in patients with localized malignancy. T he clinical significance of a lung nodule incidentally discovered on abdominal CT, which provides an incomplete view of the lung parenchyma, is unclear. The malignant potential of a detected lung nodule is uncertain, and the presence of other nodules or other potentially significant lung abnormalities is unknown. Current guidelines regarding the management of small pulmonary nodules incidentally detected on chest CT have been recommended by the Fleischner Society [1] and are based primarily on nodule size and patient risk factors. Specifically, the likelihood of an incidental lung nodule being clinically significant increases as nodule size increases [2, 3]. The Fleischner Society clearly states that clinical data such as patient age and comorbidities should also influence management recommendations [1]. Despite a greater understanding of the significance of incidental lung nodules detected on chest CT, few studies in the literature describe the incidence or outcome of lung nodules incidentally reported on abdominal CT and it is uncertain whether similar management guidelines should be applied to such nodules. Preliminary data from a retrospective study of noncalcified lung nodules on abdominal CT showed that 11% of patients with follow-up CT had malignant nodules based on nodule growth or tissue sampling (Wu CC et al., presented at the 2009 annual meeting of the Radiological Society of North America [RSNA]). In a study of the report- AJR:198, April

2 Alpert et al. ing rates of unsuspected lung nodules on abdominal CT, Rinaldi et al. [4] noted that 16 of 44 patients (36%) with noncalcified lung nodules had a new or growing nodule when compared with prior or follow-up studies, although these patients were not clearly stratified on the basis of known preexisting disease in particular, malignancy. Given the limited data available, the goal of the current study was to retrospectively determine the clinical significance of incidental lung nodules detected on abdominal CT in a patient population who later underwent follow-up chest CT, abdominal CT, or both. Materials and Methods Patient Selection This study was approved by the institutional review board and was compliant with HIPAA. Reports of all abdominal CT studies over a 1-year period (January 1, 2005, to December 31, 2005) were electronically searched for the word nodule, nodular, or mass, and these reports were reviewed to ensure this term referred to the lung bases. The following study population was then selected: patients with no prior CT study who had undergone an initial abdominal CT examination and then underwent at least one follow-up chest or abdominal CT examination (Fig. 1). Although some patients had undergone initial chest CT and abdominal CT concurrently, the study focused on patients who initially underwent abdominal CT alone. CT Technique At our institution, abdominal CT examinations were performed on MDCT scanners ranging from 16 to 64 detector row configurations (Sensation and Definition, Siemens Healthcare). The abdominal CT protocol used 120 kv, a reference tube current exposure time product of mas, and a gantry rotation time of 0.5 second. Images were obtained at end-expiration beginning just above the domes of the diaphragm. The amount of thorax included in the examination varied because of differences in patients expiratory lung volumes. Contiguous 4-mm axial images were reconstructed using a low-frequency algorithm. The chest CT protocol entailed the use of 120 kvp, a reference tube current time product of mas, and a gantry rotation time of 0.33 second. Images were obtained during a single inspiratory breath-hold, and contiguous 5-mm axial images were created using low- and high-frequency algorithms for soft-tissue and lung interpretation, respectively. CT studies performed with IV contrast material used nonionic contrast material (iopromide [Ultravist 300, Bayer HealthCare]), and dose was based on patient weight. The injection rate and scan delay time varied depending on the clinical indication for the examination. Image Review The visualized lungs included on each initial abdominal CT study were reviewed in consensus by two thoracic imaging trained radiologists with 11 years and 1 year of postfellowship experience on a PACS workstation. All lung nodules were counted, and the size, shape, margins, and attenuation of the two largest nodules were noted. The nodules were measured using electronic calipers, and the maximum dimension was recorded. Shape was characterized as round, ovoid, irregular, rectangular, or triangular. Nodular margins were described as smooth, irregular, poorly defined, or lobulated. Attenuation was assessed as solid, part solid, or ground-glass. The term part solid referred to nodules with both solid attenuation and groundglass attenuation. The location of the nodule was recorded on the basis of lobar anatomy, central versus peripheral (outer two thirds of the lung) and whether the nodule was adjacent to the pleura or a fissure. Nodules were described as cavitary or noncavitary, and any calcifications were noted. Completely calcified nodules were excluded. The presence of other lung abnormalities, such as emphysema, bronchial wall thickening, bronchiectasis, bronchopneumonia, or bronchiolitis, was recorded by the thoracic radiologists utilizing definitions from the Fleischner Society s glossary of terms [5]. Pleural effusion and lymphadenopathy were noted if present. Subsequently, follow-up chest CT, abdominal CT, or both were reviewed. Lung nodules seen on the initial abdominal CT study were reevaluated for Abdominal CT performed between January 1, 2005 and December 31, 2005 (n = 12,287) Reports with mention of nodule, nodular, or mass pertaining to lung bases (n = 364) These 364 CT reports correspond to 338 patients. Prior chest or abdominal CT (n = 213) Follow-up chest or abdominal CT performed (n = 42) No prior chest or abdominal CT (n = 125) Concurrent chest and abdominal CT performed (n = 18) a change in size, morphology, or attenuation. New lung nodules were characterized, as were nodules outside the area covered by the initial abdominal CT. Other thoracic abnormalities were also evaluated similar to the initial abdominal CT study. Clinical Information The electronic medical records of all patients in the study group up to July 2009 were reviewed to gather information about medical history, the indication for imaging, imaging results, and patient outcome. The time intervals between the initial CT study and first and last follow-up CT studies were recorded. Any history of malignancy was recorded; for patients with a history of malignancy, the date of diagnosis, tumor site, and cell type were noted when available. Results A total of 12,287 abdominal CT examinations were performed at our institution over a 1-year period both in an inpatient and acute care setting and in an outpatient setting (Fig. 1). Of these studies, 364 abdominal CT reports (3%) included the word nodule, nodular, or mass pertaining to the lung bases; these 364 studies were examinations of 338 patients. Of the 338 patients, 125 (37%) had no prior chest or abdominal CT, whereas 213 (63%) did have a previous CT examination and were excluded from the study. Of the remaining 125 patients, 42 (33.6%) who underwent follow-up CT in the form of chest CT (n = 15), abdominal CT (n = 13), or both (n = 14) comprised the study population. Eighteen of the 125 patients (14.4%) had concurrent chest CT performed Calcified nodules (n = 3) No follow-up CT and no concurrent CT (n = 62) Fig. 1 Flowchart illustrates selection of patient cohorts. 794 AJR:198, April 2012

3 Significance of Lung Nodules on Abdominal CT at the time of the initial abdominal CT. Sixtytwo of the 125 patients (49.6%) had no followup chest CT or abdominal CT and no concurrent chest CT. Three of the 125 patients (2.4%) were excluded because the nodule identified on abdominal CT was completely calcified. Study Population Forty-two patients (25 men, 17 women; average age, 67.5 years; range, years) underwent an initial abdominal CT examination (33 inpatients and nine outpatients) followed by at least one thoracic or abdominal CT examination. The most common clinical indications for the initial abdominal CT study were abdominal pain (n = 13); other gastrointestinal symptoms (n = 8) such as nausea, vomiting, diarrhea, obstruction, or jaundice; genitourinary indications (n = 10) such as hematuria, hydronephrosis, or renal calculus; vascular disease including abdominal aortic aneurysm (n = 4); and a preexisting malignancy (n = 7) related to endometrial carcinoma (n = 2), bladder carcinoma (n = 2), hepatocellular carcinoma (n = 1), colon carcinoma (n = 1), or lymphoma (n = 1). Although seven patients were imaged for a clinical indication of a preexisting malignancy, 13 of 42 patients (31.0%) had a known diagnosis of malignancy at the time of the initial abdominal CT study. The additional six patients were imaged for nonspecific clinical indications, such as abdominal pain. Preexisting cancers were predominantly abdominal or pelvic malignancies (n = 10), with others representing lymphoma (n = 1), esophageal carcinoma (n = 1), or CNS malignancy (n = 1). For three patients without a known malignancy at the time of imaging, a new diagnosis of cancer was the result of the initial abdominal CT: pancreatic cancer (n = 2) or gynecologic malignancy (n = 1). Therefore, a total of 16 of 42 patients (38.1%) had a preexisting or newly diagnosed malignancy after the initial abdominal CT. On the initial abdominal CT examination, a total of 104 isolated lung nodules were identified in 40 of 42 patients (mean, 2.6 nodules per patient; range, 1 11 nodules). The two remaining patients had innumerable nodules in a diffuse centrilobular pattern consistent with infectious bronchiolitis. Because up to two dominant nodules were individually characterized for each of the 42 patients, 61 isolated lung nodules (mean, 6.9 mm; range, 2 23 mm) were assessed (Table 1). Nodules were predominantly round (33/61, 54.1%) or ovoid (n = 20, 32.8%), had smooth borders (n = 45, 73.8%), had solid attenuation (n = 58, 95.1%), and were located in the lung periphery (n = 55, 90.2%). Peripheral nodules abutted a pleural surface in 40 of 55 instances (72.7%), whereas the remaining 15 nodules (27.3%) were located adjacent to a fissure. One of the 61 nodules (1.6%) was partially calcified. None was cavitary. Other thoracic abnormalities were frequently identified. Bronchial wall thickening (n = 23), bronchiectasis (n = 4), and emphysema (n = 7) were documented. Four patients had either focal or diffuse clustered centrilobular nodules consistent with bronchiolitis, and two patients had findings consistent with bronchopneumonia, resulting in six of 42 patients (14.3%) with CT evidence of infection. Pleural effusion was identified in eight patients. Nodule outcome The average time to the first follow-up CT was 122 days (range, TABLE 1: Lung Nodules Characterized on Initial Abdominal CT Characteristic Nodule 1 Nodule 2 Total No. (%) Nodule Shape Round (54.1) Ovoid (32.8) Irregular (13.1) Rectangular Triangular Margins Smooth (73.8) Irregular (11.5) Poorly defined (8.2) Lobulated (6.6) Attenuation Solid (95.1) Partially solid Ground-glass (4.9) Location LUL Lingula (1.6) LLL (39.3) RUL Middle lobe (16.4) RLL (42.6) Location Central (inner one third) (9.8) Peripheral (outer two thirds) (90.2) Touching pleura (72.7) Or fissure (27.3) Total 55 Partially calcified (1.6) Cavitary Note LUL = left upper lobe, LLL = left lower lobe, RUL = right upper lobe, RLL = right lower lobe. AJR:198, April

4 Alpert et al. Fig year-old man with colon cancer. A, Axial abdominal CT scan viewed in lung window setting reveals ovoid 3-mm nodule (arrow) in right lower lobe. B, Follow-up CT scan obtained 308 days after A shows nodule (arrow) has increased in size to 6 mm. A days), or nearly 4 months. The average time to the last follow-up CT was 362 days (range, days), or nearly 1 year. Three of 42 patients (7.1%) had nodules on the initial abdominal CT study that proved malignant. Two (4.8%) had nodules that had increased in size on follow-up CT (Table 2). One had preexisting bladder cancer with suspected metastases that were confirmed on the initial abdominal CT, which showed retroperitoneal adenopathy and nodules at the lung bases; nodules up to 10 mm were scattered throughout the lungs on follow-up chest CT performed 1 day later. One of the two patients with nodule growth had colon cancer diagnosed 2 months before the initial abdominal CT examination, which was performed because of abdominal pain. This patient had an ovoid, smooth, solid nodule in the right lower lobe that was shown to have grown from 3 to 6 mm when it was reevaluated 308 days (? 10 months) later (Fig. 2). This patient also had several new lung nodules, adenopathy, and low-density liver lesions consistent with metastatic disease on follow-up CT. The second patient was diagnosed with metastatic pancreatic cancer as a consequence of the initial abdominal CT, which was performed because of severe abdominal pain. Numerous lung nodules were seen in addition to a pancreatic mass and abdominal adenopathy. On follow-up chest CT performed 50 days later, an ovoid, smoothly marginated, solid nodule that contacted the pleura of the right lower lobe had increased from 0.6 to 1 cm (Fig. 3) and other lung nodules had increased in both size and number. Of the remaining 39 patients, follow-up CT showed no change in the size of the nodules (mean, 274 days, or approximately 9 months; range, days) in 31 patients (73.8%); a decrease in nodule size (time to follow-up, days) in two patients (4.8%); and nodule resolution, occurring over a mean period of 443 days (range, , or nearly 15 months), in six patients (14.3%). Stable nodules (47 nodules in 31 patients) were frequently round (n = 25) or ovoid (n = 17), smoothly marginated (n = 38), and solid (n = 45). All the nodules that had decreased in size (three nodules in two patients) were round and solid but were inconsistently marginated. Nodules that had resolved (seven nodules in six patients) were commonly solid (n = 6) but were inconsistent in shape and margin. Risk stratified according to malignancy history Of the subset of 16 patients with a preexisting or newly diagnosed malignancy, 18.8% (n = 3) had malignant nodules. No TABLE 2: Patients With Nodule Growth Documented on Follow-Up CT Indication for Initial Abdominal CT Abdominal pain; recently diagnosed colon cancer Size of Nodule on Initial CT (mm) Characteristics of Nodule 3 Ovoid, smooth, solid; RLL, peripheral Abdominal pain 6 Ovoid, smooth, solid; RLL, peripheral Note RLL = right lower lobe. Time to Size Increase (d) Size of Nodule on Follow-Up CT (mm) B patients without malignancy had incidental nodules that were proven malignant. No primary lung malignancy was discovered among the 42 study subjects. Additional nodules on follow-up chest CT On follow-up CT, 22 of 42 patients (52.4%) had additional nodules (n = 59) in the portions of the lungs that had not been imaged on the initial abdominal CT. Four of these 42 patients had a mass or nodule in the superior chest that was larger than the initially detected nodule at the lung bases (Table 3), although only three (7.1%) had a significant additional nodule or mass that was larger than 8 mm. One of these four patients was the patient with metastatic bladder cancer and malignant lung nodules: The largest nodule on the initial abdominal CT was 7 mm, although other nodules measuring up to 10 mm were seen on follow-up chest CT performed 1 day later. Continued follow-up CT of another patient showed resolution of the larger nodule, indicating an infectious or inflammatory cause. The two remaining patients had no further follow-up imaging. Patients With Concurrent Chest and Abdominal CT In addition to the 42 patients who underwent abdominal CT alone, 18 of 125 patients % Increase in Nodule Size Between Initial CT and Follow-Up CT Patient Outcome Metastatic colon cancer diagnosed on follow-up CT Metastatic pancreatic cancer diagnosed on initial abdominal CT; Disease progression on follow-up CT 796 AJR:198, April 2012

5 Significance of Lung Nodules on Abdominal CT Fig year-old woman with advanced pancreatic cancer. A, Axial abdominal CT scan viewed in lung window setting shows dominant 6-mm ovoid nodule (arrow) abutting pleura. B, Follow-up CT scan obtained 50 days after A shows nodule (arrow) has grown to 10 mm and that there are new metastatic nodules. (14.4%) underwent chest and abdominal CT concurrently. Of these 18 patients (10 men, eight women; average age, 65.3 years; range, years; 11 inpatients, seven outpatients), the clinical indication for imaging was malignancy in 13 (72.2%). Regardless of the clinical indication for imaging, 11 of 18 patients had a preexisting malignancy at the time of CT and three others had a newly diagnosed malignancy at the time of CT, producing a total of 14 of 18 patients (77.8%) with a preexisting or newly diagnosed malignancy. Ten of 18 patients (55.6%) with concurrent CT studies had malignant nodules determined by tissue diagnosis or by clinical or imaging follow-up, representing metastatic disease in eight patients and primary lung malignancy in two. The likelihood of having a malignant nodule was 71.4%, 10 of 14 patients, in the presence of a preexisting or newly diagnosed malignancy. None of the patients without a preexisting or newly diagnosed malignancy had malignant nodules. Patients Without Follow-Up or Concurrent CT Sixty-two patients underwent neither followup CT nor concurrent CT. Of these 62 patients, 34 had no documented follow-up whatsoever. Two patients died during the hospital course when the initial CT was performed: One had widespread metastatic colon cancer and acute respiratory failure, and the other had bowel ischemia related to atherosclerotic disease. Twenty-three patients later underwent imaging studies unrelated to the incidentally discovered nodules. Three patients had clinical follow-up but no further imaging, and malignancy (melanoma) was later diagnosed in one. Discussion In our study, only a small percentage (3%) of abdominal CT examinations reported lung A nodules. This percentage is lower than that described by Rinaldi et al. [4]; they found a discordance between the high number of incidental noncalcified lung nodules on abdominal CT (95/243 patients; 39% of patients studied) and the low number of patients nodules reported by the interpreting radiologist (8/95, 8.4%) [4]. Although some nodules may not be detected by interpreting radiologists, it is likely that small nodules may not be reported when detected because of the presence of dense calcification or the low probability of clinical significance given small size. In patients with no prior chest or abdominal CT who underwent abdominal CT alone, the clinical suspicion of thoracic disease at the time of the initial abdominal imaging was presumably low. We have shown that only a small percentage (7.1%) of nodules reported on abdominal CT proved clinically significant and that all were seen in patients with a known or newly diagnosed malignancy. In two of these cases, nodule growth over time represented metastatic disease in patients with malignancy diagnosed either 2 months earlier or as a consequence of the initial abdominal CT. Over a mean time of nearly 1 year (362 days) of CT follow-up, the majority of lung nodules characterized on the initial abdominal CT were stable (73.8%), had decreased in size (4.8%), or had resolved (14.3%). The clinical significance of small lung nodules incidentally detected on chest CT even in high-risk smokers has been shown to be small [6 8]. As the Mayo Clinic CT Screening Trial investigators and other groups have reported, fewer than 1% of small 4- or 5-mm lung nodules in patients without a history of cancer were malignant even among high-risk smokers [2, 9, 10]. The likelihood of malignancy among nodules smaller than 3 mm has been reported as 0.2%, with malignancy rates of 0.9%, 18%, and 50% for nodules measuring 4 7 mm, 8 20 mm, and larger than 20 mm, respectively [2]. Therefore, the recommendations established by the Fleischner Society use nodule size and patient risk factors for malignancy as criteria for determining the appropriate follow-up of small nodules. The Fleischner Society suggests more aggressive evaluation of nodules measuring 8 mm or greater [1]. TABLE 3: Patients With Larger Lung Nodules Seen Outside the Area Covered by the Initial Abdominal CT Examination Indication for Initial Abdominal CT Right lower quadrant abdominal pain Bladder cancer, acute renal failure Size of Largest Nodule (mm) Initial CT Outside FOV of Initial CT Time to Follow-Up CT (d) Patient Outcome No known malignancy; no further follow-up available Metastatic malignancy suspected before imaging; widespread metastases confirmed Fever, diarrhea No known malignancy; no further follow-up available Bladder cancer Decrease in 22-mm nodule on further follow-up B AJR:198, April

6 Alpert et al. The Fleischner Society guidelines do not apply to patients with a known or suspected malignancy because nodule follow-up in these patients is based on clinical factors, including cell type, tumor site, and stage of disease. Our investigation found that all patients with malignant nodules had a clinical diagnosis or suspicion of malignancy, whereas all nodules of patients without a diagnosis of malignancy were benign. In a retrospective review of incidental lung nodules detected in 334 nonscreening chest CT studies, Benjamin et al. [11] reported that 10% of nodules less than 10 mm in the long axis represented metastatic malignancy. Those investigators also reported a malignancy rate of small nodules in patients without a known malignancy to be as low as 1%, in accordance with other studies [3, 9, 12]. Thus, knowledge of an existing malignancy diagnosis including the time of diagnosis and tumor type, grade, and stage is of critical importance when small nodules are seen on either chest CT or abdominal CT [13, 14]. The incidental nodules characterized on abdominal CT in our investigation were frequently round (54.1%) or ovoid (32.8%), had smooth borders (73.8%), and had solid attenuation (95.1%). Most nodules were peripheral in location (90.2%), close to a pleural (72.7%) or fissural (27.3%) surface. Several screening studies indicate that nodule characteristics such as these are predictive of benign behavior. In a study of small solitary nodules detected on screening CT and followed over time, Takashima et al. [15, 16] found that polygonal nodule shape and a 3D ratio of greater than 1.78 were highly specific for benignity [15, 16]. Predominantly solid and peripheral subpleural nodules were frequently benign [15, 16]. Similarly, at 1-year follow-up of solid indeterminate nodules detected at baseline screening CT, Xu et al. [17] found no malignancy arising from nodules that were round; smooth; or attached to vessels, pleura, or fissures. Furthermore, Ahn et al. [18] found no primary lung cancer arising from perifissural nodules among 146 screening patients followed over 7 years. Additional lung nodules and other thoracic findings were often seen on follow-up chest CT. Importantly, 7.1% of patients had additional lung nodules on follow-up CT that were greater than 8 mm, larger than those identified at the lung bases on the initial abdominal CT. This finding supports the findings of Wu et al. (2009 RSNA meeting), who reported that a small percentage (6%) of patients had nodules on followup CT that were more than 2 mm larger than those detected on the initial abdominal CT. Although additional nodules in these patients may be of uncertain clinical significance, these findings suggest that in most patients dedicated chest CT after the detection of an incidental nodule at the lung bases would provide little additional data. Conversely, these findings also illustrate the need for established imaging recommendations regarding who should undergo dedicated chest CT after the detection of an incidental nodule at the lung bases. No incidental primary lung cancer was identified, and although all malignant nodules were discovered in patients with an underlying malignancy, the impact of nodule detection on abdominal CT on clinical outcomes is unclear given the small number of patients. Two of three patients with malignant nodules already had evidence of distant metastatic disease after the initial abdominal CT (pancreatic and bladder cancers). The third patient had a 2-month diagnosis of colon cancer with no other findings of advanced disease at the time of the initial abdominal CT and was reported to have only a small lung nodule at the lung bases. Growth of this nodule was detected on chest CT 10 months later when additional lung nodules, adenopathy, and liver metastases were also noted; it is unclear if and when follow-up imaging of this nodule would have altered patient outcome. Regardless, our findings suggest that follow-up chest CT may be most useful in patients with a local malignancy and no proven distant metastases. Other clinically important findings were noted at the lung bases in a small number of patients. A total of six patients (14.3%) had evidence of bronchopneumonia or infectious bronchiolitis, findings that would benefit from antibiotic therapy. Our assessment of patients with concurrent thoracic and abdominal CT examinations showed, not surprisingly, that they had a higher rate of malignant nodules (55.6%) than the study population (7.1%). The higher rate of malignant nodules presumably reflects a higher clinical suspicion of thoracic disease; thus, nodules among these patients should not be considered incidental. Malignancy was the most common clinical indication for imaging (77.2%), higher than among the study population (16.7%). More patients who underwent concurrent CT studies also had a preexisting diagnosis of malignancy (61.1%) compared with the study population (31.0%). The limitations of our study relate to its retrospective nature. Our identification of patients relied on the original radiologist who interpreted abdominal CT for clinical purposes. Therefore, nodules may not have been reported despite detection given the associated low clinical suspicion. Conversely, referring physicians may have decided against follow-up imaging. Some patients underwent follow-up abdominal CT rather than chest CT, and there may have been significant findings in the superior chest that remained unidentified. Some patients had short-interval follow-up, limiting the opportunity to detect malignant behavior and potentially underestimating the presence of malignant nodules. Several patients had a known malignancy at the time of abdominal CT, and it is unclear why patients with a known malignancy did not undergo concurrent chest CT, perhaps because of a remote history of malignancy. Patients may have undergone CT elsewhere before the initial CT at our institution or may have undergone followup CT at other institutions, unknown and unavailable to us. Finally, the amount of lung included in the area covered by imaging varied. Although exact measurement of the amount of lung was not performed, the investigation reflects clinical practice at our institution, which follows a standard abdominal CT protocol. Among patients with a low probability of thoracic disease, incidental lung nodules on abdominal CT rarely showed malignant behavior; malignancy was seen in only a small number of patients with a known or newly diagnosed abdominal malignancy. No primary lung malignancy was identified among the study population, and over a mean period of nearly 1 year, most nodules were stable, had decreased in size, or had resolved. Knowledge of an underlying malignancy is of critical importance to both the clinician and the radiologist; our findings suggest that after discovery of an incidental lung nodule on abdominal CT, dedicated chest CT would be of greatest benefit to patients with a localized malignancy. However, continued investigation would be helpful to determine whether preexisting management recommendations for incidental lung nodules on chest CT can be applied to those identified at the lung bases on abdominal CT. References 1. MacMahon H, Austin JH, Gamsu G, et al.; Fleischner Society. Guidelines for management of small pulmonary nodules detected on CT scans: a statement from the Fleischner Society. Radiology 2005; 237: Midthun DE, Swensen SJ, Jett JR, et al. O-127 Evaluation of nodules detected by screening for lung cancer with low dose spiral computed tomography. (abstract) Lung Cancer Abstracts of 798 AJR:198, April 2012

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