High FDG activity in focal fat necrosis: a pitfall in interpretation of posttreatment PET/CT in patients with non-Hodgkin lymphoma

Repeatability of metabolic tumor burden and lesion glycolysis between clinical readers

The Metabolic Tumor Volume (MTV) and Tumor Lesion Glycolysis (TLG) has been shown to be independent prognostic predictors for clinical outcome in Diffuse Large B-cell Lymphoma (DLBCL). However, definitions of these measurements have not been standardized, leading to many sources of variation, operator evaluation continues to be one major source. In this study, we propose a reader reproducibility study to evaluate computation of TMV (& TLG) metrics based on differences in lesion delineation. In the first approach, reader manually corrected regional boundaries after automated detection performed across the lesions in a body scan (Reader M using a manual process, or manual). The other reader used a semi-automated method of lesion identification, without any boundary modification (Reader A using a semi- automated process, or auto). Parameters for active lesion were kept the same, derived from standard uptake values (SUVs) over a 41% threshold. We systematically contrasted MTV & TLG differences between expert readers (Reader M & A). We find that MTVs computed by Readers M and A were both concordant between them (concordant correlation coefficient of 0.96) and independently prognostic with a P-value of 0.0001 and 0.0002 respectively for overall survival after treatment. Additionally, we find TLG for these reader approaches showed concordance (CCC of 0.96) and was prognostic for over -all survival (p ≤ 0.0001 for both). In conclusion, the semi-automated approach (Reader A) provides acceptable quantification & prognosis of tumor burden (MTV) and TLG in comparison to expert reader assisted measurement (Reader M) on PET/CT scans.

Pitfalls in Oncologic Imaging of the Pericardium on CT and PET/CT

In the oncologic setting, misinterpretation of fluid in pericardial recesses as mediastinal adenopathy or benign pericardial findings as malignant can lead to inaccurate staging and inappropriate management. Knowledge of normal pericardial anatomy, imaging features to differentiate fluid in pericardial sinuses and recesses from mediastinal adenopathy and potential pitfalls in imaging of the pericardium on CT and PET/CT is important to avoid misinterpretation.

Pericardial Recesses Mimicking Mediastinal Adenopathy on CT

Thin-section computed tomography (CT) has improved the detection of pericardial recesses and sinuses. Physiologic fluid in the pericardial recesses and sinuses can mimic mediastinal adenopathy. The misinterpretation of pericardial recesses and other benign pericardial entities in the oncologic setting can lead to inappropriate staging and management. Knowledge of the anatomy of the pericardium with emphasis on the imaging of different pericardial recesses on CT is important to avoid misdiagnosis, unnecessary further investigations, and/or biopsy.

Pitfalls of a Mixed Metabolic Response at PET/CT

Although the term mixed metabolic response is commonly used in PET/CT reports, it should be a red flag to reconsider the assumptions made by the PET scan reader. Fluorine 18 fluorodeoxyglucose (FDG) PET/CT is recognized as an accurate imaging method for detecting response to cancer therapies. Critical clinical decisions regarding therapy are dependent on accurate interpretation of findings. The use of standardized terminology for response assessment, such as that in the Positron Emission Tomography Response Criteria in Solid Tumors (PERCIST), is highly recommended. With PERCIST, treatment response is categorized as complete metabolic response, partial metabolic response, stable metabolic disease, or progressive metabolic disease. Mixed metabolic response is not included in PERCIST. Rather, it is used colloquially to describe a scenario in which scanning performed after systemic cancer therapy reveals divergent findings, with some tumor foci responding and others not responding or even seen progressing. In PERCIST, mixed metabolic response should be described as stable metabolic disease or progressive metabolic disease. However, the PET/CT reader may also wish to suggest that individual tumors have heterogeneous genetic and/or other characteristics and consequently a mixed response to therapy. The concept of tumor heterogeneity is gaining momentum in cancer research and thus possibly leading to options for therapy targeted to oligometastases that are not responding. However, the authors suggest exercising extreme caution when PET/CT findings appear at first to reflect what some might call a mixed response. In addition, they have found that FDG PET/CT findings are often confounding owing to the simultaneous presence of two or more unrelated disease processes. Common examples include synchronous neoplasms, inflammatory processes, and treatment-related effects. Thus, an apparent mixed response is a red flag to reconsider whether all of the FDG-avid findings are actually metastases of the same cancer. Common mimics of a mixed metabolic response that do not represent true tumor heterogeneity are highlighted to improve the FDG PET/CT reader's recognition of these lesions.^©RSNA, 2019.

Fat necrosis after abdominal surgery: A pitfall in interpretation of FDG-PET/CT

OBJECTIVE

We describe FDG-PET/CT findings of postoperative fat necrosis in patients following abdominal surgery, and evaluate their changes in size and FDG uptake over time.

METHODS

FDG-PET/CT scans from January 2007-January 2016 containing the term 'fat necrosis' were reviewed. Lesions meeting radiological criteria of fat necrosis in patients with prior abdominal surgery were included.

RESULTS

Forty-four patients, 30 males, mean age 68.4 ± 11.0 years. Surgeries: laparotomy (n=37; 84.1 %), laparoscopy (n=3; 6.8 %), unknown (n=4; 9.1 %). CTs of all lesions included hyperdense well-defined rims surrounding a heterogeneous fatty core. Sites: peritoneum (n=34; 77 %), omental fat (n=19; 43 %), subcutaneous fat (n=8; 18 %), retroperitoneum (n=2; 5 %). Mean lesion long axis: 33.6±24.9 mm (range: 13.0-140.0). Mean SUVmax: 2.6±1.1 (range: 0.6-5.1). On serial CTs (n=34), lesions decreased in size (p=0.022). Serial FDG-PET/CT (n=24) showed no significant change in FDG-avidity (p=0.110). Mean SUVmax did not correlate with time from surgery (p=0.558) or lesion size (p=0.259).

CONCLUSION

Postsurgical fat necrosis demonstrated characteristic CT features and may demonstrate increased FDG uptake. However, follow-up of subsequent imaging scans showed no increases in size or FDG-avidity. Awareness of this entity is important to avoid misinterpretation of findings as recurrent cancer.

KEY POINTS

• Postsurgical fat necrosis may mimic cancer in FDG-PET/CT. • Follow-up of fat necrosis showed no increase in FDG intensity. • CT follow-up showed a decrease in lesion size. • FDG uptake did not correlate with time lapsed from surgery.

Pictorial review of 18F-FDG PET/CT findings in musculoskeletal lesions

We herein reviewed ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) positron emission tomography (PET)/computed tomography (CT) findings in a number of musculoskeletal lesions including malignant tumors, benign tumors, and tumor-like lesions with correlations to other radiographic imaging modalities, and described the diversity of the ¹⁸F-FDG PET/CT findings of this entity. Malignant primary musculoskeletal tumors are typically ¹⁸F-FDG avid, whereas low-grade malignant tumors show mild uptake. Benign musculoskeletal tumors generally show a faint uptake of ¹⁸F-FDG, and tumor-like conditions also display various uptake patterns of ¹⁸F-FDG. Although musculoskeletal tumors show various uptakes of ¹⁸F-FDG on PET/CT, its addition to morphological imaging modalities such as CT and MRI is useful for the characterization and differentiation of musculoskeletal lesions.

Fat-Containing Hypermetabolic Masses on FDG PET/CT: A Spectrum of Benign and Malignant Conditions

OBJECTIVE

This article focuses on identifying the imaging appearances of hypermetabolic fatty masses and masslike lesions on PET/CT and understanding the diagnostic challenges radiologists may face while interpreting findings of these lesions on PET/CT. This article provides an approach to aid in the diagnosis of these lesions and the appropriate management of patients.

CONCLUSION

Both malignant and benign fat-containing masses and masslike lesions can show hypermetabolic activity on PET/CT. Although the differential diagnosis is broad, clinical history, anatomic location, and knowledge of anatomic variants and imaging features can help radiologists avoid misinterpretation of benign fatty lesions as malignancy.

How We Read Oncologic FDG PET/CT

¹⁸F-fluorodeoxyglucose (FDG) PET/CT is a pivotal imaging modality for cancer imaging, assisting diagnosis, staging of patients with newly diagnosed malignancy, restaging following therapy and surveillance. Interpretation requires integration of the metabolic and anatomic findings provided by the PET and CT components which transcend the knowledge base isolated in the worlds of nuclear medicine and radiology, respectively. In the manuscript we detail our approach to reviewing and reporting a PET/CT study using the most commonly used radiotracer, FDG. This encompasses how we display, threshold intensity of images and sequence our review, which are essential for accurate interpretation. For interpretation, it is important to be aware of benign variants that demonstrate high glycolytic activity, and pathologic lesions which may not be FDG-avid, and understand the physiologic and biochemical basis of these findings. Whilst FDG PET/CT performs well in the conventional imaging paradigm of identifying, counting and measuring tumour extent, a key paradigm change is its ability to non-invasively measure glycolytic metabolism. Integrating this "metabolic signature" into interpretation enables improved accuracy and characterisation of disease providing important prognostic information that may confer a high management impact and enable better personalised patient care.

The acetabular fossa hot spot on 18F-FDG PET/CT: epidemiology, natural history, and proposed etiology

OBJECTIVE

To describe a benign focus of increased activity in the acetabular fossa (the acetabular fossa hot spot, AFHS) on (18) F-FDG PET/CT that can mimic a neoplasm.

MATERIALS AND METHODS

(18)F-FDG PET/CT images from four patient populations were examined. Group 1 (n = 13) was collected from a search of radiology reports and used to define the AFHS and for hypothesis generation. Group 2 (n = 1,150) was used for prevalence of AFHS. Group 3 (n = 1,213) had PET/CT and MRI pelvis within a week of each other and was used to correlate metabolic and anatomic findings. Group 4 (n = 100) was used to generate the control group. Data were collected on demographics, common comorbidities, underlying cancer diagnosis and status, and hip symptoms.

RESULTS

Prevalence of AFHS was 0.36 % (95 % CI 0.10-0.91 %). None progressed to malignancy or was associated with cancer status. The majority (71 %) were on the left, and 6 % were bilateral. Mean SUVmax of the AFHS was 4.8 (range, 2.7-7.8). Male patients were more likely to have the AFHS (OR = 8.69, 95 % CI 1.88-40.13). There was no difference with respect to other collected data, including hip symptoms. Average minimum duration of AFHS was 346 days (range, 50-1,010 days). Readers did not detect corresponding hip abnormalities on MRIs.

CONCLUSIONS

AFHS is a benign finding that may be caused by subclinical ligamentum teres injury, focal synovitis, or degeneration of acetabular fossa fat. Despite uncertainty regarding its etiology, recognition of AFHS as a benign finding can prevent morbidity associated with unnecessary biopsy or initiation of therapy.

Explorative analyses on the value of interim PET for prediction of response in pediatric and adolescent non-Hodgkin lymphoma patients

BACKGROUND

This study is to evaluate the predictive value of FDG-PET (PET) in pediatric and adolescent patients suffering from non-Hodgkin lymphoma (pNHL) in comparison to information provided by conventional imaging methods (CIM).

METHODS

Imaging was performed at baseline and at interim (after 2 cycles of chemotherapy). The response assessment in PET was carried out visually and semi-quantitatively, the latter one by use of percentage decrease in SUVmax from baseline to interim (ΔSUVmax). The PET-based results were compared to the findings by CIM. Progression-free survival (PFS) was analyzed using Kaplan-Meier curves (KM) and log-rank test.

RESULTS

The final study included 16 patients (mean follow-up time, 60.2 months (range, 4.0 to 85.7 months)). Relapse occurred in four patients. Visual PET compared to CIM revealed higher sensitivity (3/4 vs 1/4) and NPV (6/7 vs 10/13), and equal PPV (3/9 vs 1/3), but lower specificity (6/12 vs 10/12) and accuracy (9/16 vs 11/16). False-positive findings in PET at interim were predominantly observed in patients presenting bulky disease (5/6), whereas CIM was true-negative in all of these cases. KM analyses revealed no significant differences in 5-year PFS neither for CIM (76.9% vs 66.7%; p = 0.67) nor for visual PET (85.7% vs 66.7%; p = 0.34) nor for ΔSUVmax (88.9% vs 57.1%; p = 0.12).

CONCLUSIONS

The predictive value of iPET in pediatric patients suffering from NHL was limited due to considerably high amount of false-positive findings, especially in patients suffering from bulky disease. However, due to our limited sample size, final conclusions cannot be drawn and, thus, call for further evaluation of PET in pNHL in larger and more homogenous patient series.