ABSTRACT

Objective

To evaluate the morphological and metabolic characteristics of incidentally detected elastofibroma dorsi (EFD) on F-18 florodeoksiglukoz (FDG) positron emission tomography/computed tomography (PET/CT) and their longitudinal changes in oncologic patients.

Material and Methods

We retrospectively reviewed 42 197 PET/CT scans performed at our institution between January 2019 and September 2023. EFD was incidentally identified in 20 patients (0.05%). Patient demographics, primary malignancy, lesion localization, dimensions, and maximum standardized uptake values (SUV_max) were recorded. Measurements were obtained before treatment and at the next 3‑month follow‑up. Statistical analyses included Mann‑Whitney U, Shapiro-Wilk and Spearman correlation tests; significance was set at p<0.05.

Results

The cohort comprised 17 females (85%) and 3 males (15%) with a median age of 67 years (range, 47-83). Primary diagnoses were breast cancer (n=8, 40%) and various other malignancies (n=12, 60%). Lesions were bilateral in 75% of cases. Pre‑treatment lesion size ranged from 10 to 55 mm; median SUV_max was 2.4 (right) and 2.5 (left). No significant differences in baseline size or SUV_max were observed between breast and other cancers. A moderate correlation existed between right and left SUV_max (r=0.641; p=0.010). After 3 months, only the left longest diameter showed a statistically significant decrease (median, 45.0 mm vs. 43.0 mm; p=0.034), which may reflect measurement variability or positional factors rather than true biological change. SUV_max values remained stable.

Conclusion

Incidentally detected EFD on PET/CT exhibits low to moderate and stable FDG uptake and predominantly bilateral localization. Recognition of its characteristic features can prevent unnecessary interventions.

INTRODUCTION

The integration of positron emission tomography/computed tomography (PET/CT) with F-18 fluorodeoxyglucose (F-18 FDG) is a valuable functional imaging technique in oncology. Initially a research tool, the fusion of PET with CT to create PET/CT facilitated its widespread clinical adoption, offering substantial diagnostic capabilities (1). This modality is crucial in oncological management, encompassing initial diagnosis, staging, restaging, treatment planning, and patient monitoring. Indeed, F-18 FDG PET/CT is routinely employed for these purposes in numerous malignancies (2). However, it is important to recognize that F-18 FDG uptake is not specific to malignant processes. As F-18 FDG is not tumor-specific, its accumulation can also occur in infectious and inflammatory conditions. Nevertheless, malignant lesions, unlike most benign lesions, often exhibit sustained tracer retention during delayed imaging phases (3). This non-specificity can lead to diagnostic challenges, as various benign conditions that demonstrate F-18 FDG uptake can mimic malignancy. For instance, tuberculosis and other granulomatous diseases such as sarcoidosis can exhibit F-18 FDG avidity comparable to that of malignant tissues, posing diagnostic difficulties (4). Elevated F-18 FDG uptake can also be seen in specific physiological states and in various benign lesions, with such benign uptake being reported in over 25% of patients undergoing PET/CT examinations.

Elastofibroma dorsi (EFD) is a benign soft-tissue neoplasm typically located in the inferior subscapular region. Although typically found there, EFD has also been documented in other locations, including the axilla, ischial tuberosity, greater trochanter, elbows, and rarely, the stomach, rectum, and omentum. EFD was initially described in 1961 as a slow-growing pseudotumor characterized by the proliferation of fibroblasts and the accumulation of abnormal elastic fibers (5). The reported incidence of EFD varies considerably depending on the age of the population and detection methods. Autopsy studies indicate an EFD prevalence of 7-24% in women and 11-17% in men (6, 7). Furthermore, pre-elastofibroma-like morphological changes, including degenerated elastic fibers, have been noted in up to 81% of autopsy cases (7). In a study of 258 patients undergoing chest CT for unrelated reasons, EFD was identified in 2% of patients (8).

The patients with EFD are predominantly asymptomatic. Symptomatic individuals may present with shoulder pain or a palpable, enlarging soft-tissue mass (9, 10). Diagnosis is often incidental and is made via CT or magnetic resonance imaging (MRI), based on characteristic imaging features (11). On CT, EFD typically appears as a lenticular, non-encapsulated mass with attenuation similar to adjacent muscle, interspersed with hypodense striations corresponding to entrapped adipose tissue (12).

F-18 FDG PET/CT is an established tool for staging, restaging, monitoring therapy response, and prognostic stratification of diverse malignancies. Its applications range from common adult cancers to pediatric malignancies, where it aids in evaluating treatment efficacy and guiding therapeutic decisions (13). Early identification of treatment-resistant diseases using non-invasive methods, such as PET/CT, is of considerable clinical importance, facilitating timely adjustments to treatment strategies (14). Nevertheless, the literature on F-18 FDG PET/CT characteristics of EFD is primarily confined to sporadic case reports (15, 16). Notably, beyond static descriptions, a significant knowledge gap exists regarding potential temporal changes in EFD appearance on sequential PET/CT scans. Elucidating this temporal evolution is essential for accurately differentiating benign EFD from tumor progression, particularly in oncological patients. Therefore, this study aimed to address this gap by evaluating the longitudinal F-18 FDG PET/CT features of incidentally detected EFD in oncologic patients.

MATERIAL and METHODS

Patients

This retrospective, single-center investigation reviewed all 42,197 oncologic F-18 FDG PET/CT scans performed at our institution between January 2019 and September 2024, among which EFD was incidentally identified in 20 patients (incidence rate: 0.05%). Demographic data, primary malignancy classifications, anatomical localizations of the EFD, lesion dimensions, and maximum standardized uptake values (SUV_max) values were documented. This investigation exclusively evaluated patients with incidental detection of EFD on F-18 FDG PET/CT.

We obtained approval for this study from the Samsun University Ethics Committee (ethics committee approval no: GOKAEK 2025/2/18, date: 24.01.2025).

F-18 FDG PET/CT Imaging

PET/CT scans were obtained 60 minutes after the injection using an integrated scanner (Philips Medical Systems, USA). All patients fasted for a minimum of 6 hours prior to the intravenous administration of 5 MBq/kg F-18 FDG. The pre-injection blood glucose levels were measured to ensure that they were below 200 mg/dL. During the distribution phase, patients remained in a supine position in a quiet room. Initially, an unenhanced CT scan with 5 mm slice thickness from the base of the skull to the inferior border of the pelvis was performed using a standardized protocol (140 kV and 80 mA). Subsequently, the PET scan was acquired from the base of the skull to the inferior border of the pelvis (8-10 bed positions, 1.5 minutes per bed position) without repositioning the patient on the table. The patient was permitted to breathe normally during PET and CT acquisitions. F-18 FDG PET images were reconstructed with CT-based attenuation correction.

Statistical Analysis

Semiquantitative evaluation of the PET component images was conducted by measuring SUV_max to assess the metabolic activity of the EFD. EFD size was determined using the longest and shortest diameters of the transaxial CT component images.

All data were analyzed using STATA/MP v.16 software (StataCorp LLC, Texas, USA). The Shapiro-Wilk test was employed to assess normal distribution, and numerical variables were provided as median (25^th-75^th percentile) values. Accordingly, Mann-Whitney U test was used to compare the two groups. The association between numerical variables was evaluated using Spearman’s correlation analysis, and post-treatment changes were analyzed using the Wilcoxon test. Significance was accepted at p<0.05 (*) for all statistical analyses.

RESULTS

A total of 20 EFD patients were analyzed. The cohort included 17 females (85%) and 3 males (15%), with an age range of 47 to 83 years. The most common primary diagnosis among patients was breast cancer (8 patients, 40%), followed by endometrial cancer (3 patients, 15%) and ovarian cancer (3 patients, 15%). The EFD lesions were bilateral in most cases (75%). Initial lesion size ranged from 10 mm to 55 mm. The SUV_max values were low (range: 2.0 to 6.8), typically falling between 2.0 and 3.0 (Table 1). F-18 FDG PET/CT findings revealed bone metastases were present in 3 patients (15%), while cervical and axillary lymph node metastases were observed in 3 patients (15%). One patient with malignant melanoma exhibited a subcutaneous hypermetabolic density in the left medial region (5%). One patient with ovarian carcinoma had multiple peritoneal metastatic nodes (5%). Notably, 12 patients (60%) had no metastatic involvement based on the F-18 FDG PET/CT findings (Table 1).

Prior to treatment, the mean lesion size was not significantly different between breast cancer and other cancers (Figure 1). Similarly, no association was observed between pre-treatment SUV_max values and demographic characteristics. Furthermore, there was no statistically significant correlation between lesion size and SUV_max values (Table 2).

Analysis of lesion size and metabolic activity before and after treatment revealed slight reductions in lesion dimensions (Table 1), although statistical significance was observed only in specific cases (Table 3). Notably, left-sided lesions showed a significant decrease in longest size after treatment (45.0 vs. 43.0 mm; p=0.034), while SUV_max values remained unchanged (Table 3).

DISCUSSION

EFD is a slow-growing benign connective tissue tumor that is most commonly located in the subscapular region of elderly individuals, particularly in females. With the increasing use of F-18 FDG PET/CT in oncologic imaging, incidental detection of EFD has become more frequent (5). In our study, 20 patients with incidentally detected EFD on PET/CT were analyzed. The majority were female (n=17, 85%), with an age range of 47 to 83 years and bilateral involvement was observed in 15 patients (75%). The predominance of female patients in this cohort is consistent with previous studies reporting a higher prevalence and frequent bilateral occurrence of EFD in elderly women, potentially attributable to hormonal or biomechanical influences (7, 17). However, the FDG uptake observed in EFD lesions may pose a diagnostic dilemma, as it can mimic metabolically active malignant lesions. This study evaluated the characteristics of incidentally detected EFD lesions on oncologic F-18 FDG PET/CT, analyzed their temporal treatment-related metabolic behavior, and discussed their clinical implications.

Metabolic Activity and Other Imaging Characteristics

The SUV_max values of EFD lesions in our cohort ranged from 2.0 to 6.8, with a median SUV_max of 2.4 for the right site and 2.5 for the left sites (Figure 2). Literature findings support the observation that EFD generally exhibits low to moderate FDG uptake, but with a stable metabolic profile over time (9).

EFD, while benign, can mimic other soft tissue masses on imaging, necessitating careful evaluation to avoid misdiagnosis and unnecessary intervention (18). The potential for misclassification highlights the necessity of integrating clinical findings with imaging characteristics and, when uncertainty persists, pursuing tissue diagnosis to definitively exclude malignancy. A comprehensive diagnostic approach, moving beyond SUV_max alone, is essential to overcome the diagnostic challenge posed by variable F-18 FDG uptake in EFD. This involves the meticulous integration of key clinical indicators (such as older age, female predominance, and often asymptomatic presentation) with specific imaging patterns. Crucially, the high incidence of bilaterality, observed in 75% of our cohort, has emerged as a powerful diagnostic clue that strongly favors EFD over typically unilateral malignant lesions. When these clinical factors and the robust finding of bilaterality are combined with the characteristic morphological features on CT, the diagnostic confidence for EFD can be significantly increased, often obviating the need for invasive procedures in cases with typical presentations. Ultimately, an accurate diagnosis relies on a combination of imaging features, clinical context, and pathological correlation when necessary, which requires awareness of entities such as low-grade fibromyxoid sarcoma, which can present diagnostic challenges (19). It is essential to note that MRI excels in delineating soft-tissue lesions, relying solely on imaging studies for a definitive histological diagnosis is accurate in only a minority of cases (20). Thus, familiarity with the imaging characteristics of EFD is crucial for radiologists and nuclear medicine physicians to ensure the accurate interpretation of F-18 FDG PET/CT scans in oncologic patients. On CT imaging, EFD typically appears as a lenticular, non-encapsulated lesion with hypodense striations interspersed within the muscular tissue, corresponding to alternating fibrous and fatty components (12). On PET/CT imaging, EFD demonstrates low-to-moderate FDG uptake commonly within the 2.0-3.0 SUV_max range, which can help distinguish it from malignant soft tissue involvement (15, 16). These imaging features are crucial in differentiating EFD from malignancies, particularly in oncological patients where soft tissue metastases or sarcomas are a major concern (11).

In our study, lesion sizes ranged from 10 mm to 55 mm, with a median diameter of 45 mm observed bilaterally. However, no statistically significant correlation was found between lesion size and SUV_max values, suggesting that the metabolic activity of EFD is not solely volume-dependent but may be influenced by underlying histological characteristics such as stromal composition or vascularity. Importantly, 60% of the patients in our cohort exhibited no evidence of metastatic disease, emphasizing the need for caution in interpreting incidental soft tissue findings in oncologic imaging.

Collectively, these observations support the interpretation of EFD as a benign, degenerative fibroblastic process rather than a hypermetabolic neoplastic entity (7). Nevertheless, in the differential diagnosis of soft tissue lesions such as EFD, a structured approach is essential, particularly to distinguish between neoplastic and non-neoplastic entities. In this context, the World Health Organization’s 2013 classification of soft tissue tumors serves as a valuable framework for guiding clinical decision-making, including the appropriate use of biopsy in diagnostically uncertain cases (21).

Pre- and Post-treatment Changes

Few studies have evaluated longitudinal metabolic changes in EFD using F-18 FDG PET/CT (22). In our study, serial 18 F-FDG PET/CT scans revealed no significant changes in SUV_max values before and after oncologic treatment (p>0.05). In our study, a statistically significant reduction was observed in the longest diameter of left-sided EFD lesions following oncologic treatment, although the absolute magnitude of change was minimal (median: 45.0 mm to 43.0 mm; p=0.034). While this dimensional decrease may be incidental, it raises the possibility of subtle structural remodeling in response to systemic therapy or positional factors. Interestingly, existing literature, including the case series by Erhamamci et al. (23), does not report spontaneous or treatment-associated reduction in lesion size, instead focusing on static lesion characteristics and surgical outcomes. This discrepancy may reflect differences in patient selection (symptomatic vs. oncologic surveillance populations), imaging follow-up intervals, or underlying biological behavior. Although elastofibromas are generally considered metabolically and structurally stable, our findings suggest that minor variations in lesion size can occur over time, warranting cautious interpretation. Further prospective imaging studies are needed to determine whether such dimensional changes are reproducible and clinically relevant, or simply represent measurement variability, mechanical influences, or postural shifts during imaging acquisition.

Study Limitations

This study acknowledges several limitations. Firstly, its retrospective design intrinsically poses risks of selection bias and incomplete clinical documentation. Secondly, as a single-center study, the generalizability of our findings may be constrained by institutional imaging protocols, equipment variations, and patient population characteristics. Thirdly, the relatively small sample size (n=20) limits the statistical power of subgroup analyses, particularly concerning treatment-related changes.

Additionally, the diagnosis of EFD was based solely on imaging characteristics without histopathologic confirmation, which, while ethically justified in typical cases, limits definitive validation. In clinical practice, biopsy is not routinely performed for incidentally detected EFD with classic imaging findings; however, histological confirmation remains essential in atypical or diagnostically equivocal presentations.

Lastly, variations in treatment regimens and follow-up intervals among patients precluded standardized assessment of temporal changes in lesion behavior, thereby limiting conclusions regarding the potential impact of oncologic therapy on EFD morphology or metabolism. Future studies with a prospective design, larger cohorts, and histologic correlation are warranted to address these limitations and to better define the natural course of EFD in oncologic populations.

CONCLUSION

EFD is characterized by its bilateral subscapular localization, low-to-moderate FDG avidity, and stable metabolic behavior as observed on PET/CT, which supports its benign nature. The integration of morphologic and metabolic findings enhances diagnostic specificity and reduces unnecessary interventions. Future research should prioritize the development of standardized imaging criteria and the study of longitudinal behavior to refine oncologic interpretation.