Grading and characterization of soft tissue tumors on magnetic resonance imaging: the value of an expert second opinion report

Since its introduction to clinical imaging more than 2 decades ago, MRI has radically modified the practice of preoperative assessment of STT. The accuracy of this technique in grading and characterization was studied in many previous papers [5‐9]. The reported results were however based on analysis of MRI examinations by groups of dedicated musculoskeletal (MSK) radiologists in single institutions with a great deal of expertise in the subject. As MRI has become a widespread technique, performed in many general hospitals, the current study aimed to compare the accuracy of MRI reports made by radiologists in community hospitals and second opinion reports made by experts in MSK radiology.

In our series, the overall agreement in MR grading of STT (including both histological and nonhistological confirmed cases) between EC and RC is good. This may reflect adequate core knowledge of the referring radiologists regarding MR diagnosis of STT. On the one hand, this may be due to the high level of general radiological education in our country. On the other hand, continuing education and evaluation of feedback by reading the second opinion report may have contributed to this good overall agreement. Furthermore, evolving experience in the imaging diagnosis of STT may also have resulted in an improved diagnosis in the EC. Unfortunately, because of the relatively low number of referred cases/year, quantification and statistical analysis of the learning curves of both institutions are not possible.

However, despite the good overall agreement, there were some significant differences in grading between the RC and EC.

In the EC, there were no false negatives (misdiagnosis of a malignant tumor as a benign lesion), compared to a minority of malignant tumors misdiagnosed on MRI as benign tumors in the RC.

More systematic and meticulous analysis of a combination of multiple parameters, experience with a larger series of STT and knowledge of sometimes subtle imaging signs may have contributed to this better prediction of malignancy in the EC. A typical example consisted of a soft tissue lymphoma, incorrectly diagnosed by the referring center as a schwannoma, mainly because of its fusiform morphology. Although the MR appearance of soft tissue lymphoma may be nonspecific in most cases [12], certain useful signs have been reported previously. Most ST lymphomas are of hyperintense signal on T2-weighted images, but a relatively low to intermediate signal on T2-weighted images may be seen as well [13, 14]. Other potential useful signs in the MR diagnosis of ST lymphoma are the homogeneity of the lesion on all pulse sequences and the absence of central necrosis in relatively large tumors [14], the growth pattern along the fascia [15], surrounding lymphangitis and the “wrapped-around” sign of lymphoma surrounding bony structures [16, 17]. False negatives may have a major impact on patient management and the ultimate prognosis. This underscores the value of a second opinion MR report. In our series, false-positive diagnoses (misdiagnosis of a benign tumor as a malignant lesion) were less frequent in the EC compared to the RC, which may reduce the number of unnecessary biopsies. The impact of a false-positive imaging diagnosis is less pronounced as this will not result in radical modification of the treatment strategy. However, since biopsy is recommended in all cases where imaging remains “indeterminate” [18], a number of unnecessary biopsies are unavoidable. Typical lesions that may be misinterpreted as a malignant tumor are desmoid tumors because of their aggressive growth pattern on imaging [19‐21]. A number of lipomatous tumors may be misinterpreted as potential malignant lesions because they contain nonlipomatous components. Gaskin et al. already stated that when an extremity or body wall lesion is considered suggestive of well-differentiated liposarcoma, 64% of these lesions will turn out to represent benign lipoma variants containing nonlipomatous elements [22]. In our series, four histopathologically proven lipomas and one hibernoma were initially interpreted as well-differentiated liposarcoma, because of lesion inhomogeneity and intralesional nonlipomatous components. Recently, Toirkens et al. confirmed that subcutaneous lipoma variants and liposarcoma variants may show overlapping MR characteristics [23]. Even in retrospect, these false-positive diagnoses may be unavoidable, but when a lipoma has to be differentiated from a well-differentiated liposarcoma, there is no change in the treatment regime, as both tumors are treated by simple a shell-out procedure. In the group of histopathologically proven tumors (group 1 of our series), the EC performed better than the RC in suggesting a correct TS diagnosis. This is also attributed to the increased knowledge and experience with rare phenotypes of STT in the EC, which were previously studied in case reports, review articles and small series [19, 24‐27].

Our study also confirms previous published data showing that prediction of the TS diagnosis on MRI is more accurate for benign than malignant STTs. In 2004, Gielen et al. reported that a correct tissue-specific diagnosis of benign STT could be predicted on MRI in 50% of cases [5]. A slightly higher percentage (58%) was reported by Berquist et al. [7]. In our series, the TS diagnosis was correct in 50% of malignant lesions in the EC versus 38.5% in the RC. The TS diagnosis was correct in 71.8% in the EC versus 51.6% in the RC.

Among the 65 soft tissue lesions without histopathological proof (Table 6), a concordant tissue-specific MR diagnosis was made by both centers in the majority of cases (78.5%). These cases presented with a typical MRI appearance such as hemangioma, lipoma, etc. Concordance in the MR diagnosis was extremely valuable for the referring physician, as it allowed a more confident diagnosis based on the combination of clinical findings and imaging. Aggressive diagnostic procedures and treatment regimes could be avoided in these patients, and a reliable wait-and see policy could be implemented.

We acknowledge the limitations of our study.

First, there was a relatively high number of malignant STTs in our series (n = 26, 16.8%) compared to the estimated prevalence between 5.1 and 15.5% in the literature [28]. This may be due to a selection bias caused by the referral policy by the clinician for an MR examination. A second limitation is the potential pathology bias. The pathologist was aware of the imaging diagnosis of both centers when making his final histopathological diagnosis, which might bias the pathologist in the diagnosis of STT [29]. A third limitation is the high number of non-histologically proven STTs (N = 65). However, because of the evidence of benignity based on a combination of imaging, clinical findings and follow-up, it would have not been considered good medical practice if these patients had undergone invasive diagnostic procedures. The last limitation is due to the fact that the MR protocol was not completely uniform as intravenous administration of gadolinium chelates was not performed in all patients.

In conclusion, a second opinion imaging report by an expert center is useful to enhance the overall accuracy in diagnosis.