The updated ESSR consensus guidelines aim to provide feasible best practice expert opinion pertaining to soft tissue tumor imaging. In comparison with the previous ESSR recommendations [11], the revised guidelines are updated to the current literature and re-structured. They provide minimal requirements and an optimized strategy in a systematic approach and contain relevant details.

The Delphi process was chosen as the panelists could perform their scoring anonymously and without the necessity to meet personally for rating [5]. However, additional face-to-face-meetings proved useful to clarify open questions regarding the procedure and to discuss concerns and re-phrasing of statements without consensus.

The extended expert panel included specialists from twelve European countries. The ESSR represents the European musculoskeletal radiologists [12]. Recruiting the panelists from the dedicated Musculoskeletal (MSK) tumor subcommittee of the ESSR allowed to form an adequate expert panel of active, representative, and leading specialists [13]. As group consensus (which reflects a considerably high level of agreement) could be reached in the majority of statements, and group agreement even in the remaining ones, this paper may help to provide feasible imaging algorithms taking into account different national infrastructure and approaches.

In this first part, the statements reflect the situation that any radiologist is confronted with in a patient with a newly suspected soft tissue tumor. Part I of our consensus therefore contains the imaging algorithm that we would recommend for primary diagnosis. It also contains detailed description of imaging methods for the tumor itself and the role of tumor reference centers and guidelines.

In the following paragraphs, we present a selection of the most clinically relevant statements with short discussion (the numbers correspond with Tables 1 and 2; the remaining comments are provided online).

Primary diagnosis, local imaging

1.1.1. Regarding the past medical history, a standardized checklist, primarily filled out by the patient, and discussed with the radiologist, is considered advisable. The patient or the referring clinician should also be asked to provide previous imaging if available.

1.1.2. Information about clinical symptoms and clinical examination findings should be available for the radiologist.

The past medical history of the patient is considered important and has to be taken into account not only by the clinician, but also by the radiologist. A standardized checklist, primarily filled out by the patient, and discussed with the radiologist, is considered advisable [14]. The information that should be available for example includes recent trauma [15, 16], anticoagulation [17], and a history of previous surgery or of radiation therapy [18,19,20]. Of note, patients often report a recent trauma that they relate to the tumor, which, however, may be unrelated, and misleading [15, 16]. It is very important that the diagnostic process is not prolonged during the process of obtaining this information.

The patient or the referring clinician should also be asked if, where and when, previous imaging had been performed. The previous imaging studies and their radiological report should be provided to the assessing radiologist (if available) [11].

1.2.1. Ultrasound (US) is considered the appropriate initial triage imaging modality for a suspected soft tissue tumor, if accessible by US and small (< 5 cm). When US diagnosis is not typical for a diagnosis, refer to Magnetic Resonance Imaging (MRI) or even biopsy.

Caveat: MRI should be performed prior to biopsy (if it will add to lesion characterization), not afterwards.

1.2.2. MRI is the imaging technique of choice for characterization and local staging of large (> 5 cm) musculoskeletal soft tissue masses and masses with indeterminate ultrasound features.

Primary MRI should be considered instead of US if, there is a clinical suspicion of malignancy, if the mass is deep, rapidly enlarging, and if there is osseous or joint involvement.

1.2.3. Computed tomography (CT) can be considered instead of MRI for complex thoracic/ abdominal / other deep masses. CT should be performed in case of complex thoracic/ abdominal / other deep masses if MRI is unavailable or is contraindicated.

Ultrasound represents the initial triage imaging modality for accessible and small suspected soft tissue tumors [21,22,23,24]. Ultrasound is highly accurate for diagnosis of specific superficial lesions with typical ultrasound features [23, 25].

MRI is the modality of choice for the characterization and local staging of soft tissue tumors in most cases [26,27,28]. CT and MRI may have complementary roles, with the capability of CT to demonstrate intralesional mineralization patterns and potential bone involvement [29]. A deep soft tissue mass incidentally found at CT usually requires MRI examination. Tissue-specific evaluation and multiplanar capability of high-resolution MRI permit better tumor localization and characterization of pelvic/retroperitoneal masses [27, 30].

Suspicious or likely malignant tumors should undergo biopsy [11].

Role of tumor centers and guidelines

2.1. Criteria for referral to a sarcoma treatment center include: Any patient with a tumor ≥ 5-cm, or with indeterminate or suspicious US/MRI findings, or with clinical suspicion of malignancy; Any patient with indeterminate MRI findings or those suspicious for malignancy.

Teleradiologic second opinion from a tumor center is appropriate in patients with indeterminate or suspicious MRI findings. It should be offered to the local hospitals in all patients in whom soft tissue sarcoma is suspected.

2.2. Patients with suspicion of sarcoma should be referred to the tumor reference center before biopsy or surgery (minimal requirement).

In patients with large, indeterminate, or suspicious tumors, a tumor reference center should be contacted for referral or teleradiologic second opinion, to avoid delay in diagnosis or unplanned surgery (“whoops procedure”) [31,32,33,34], both of which can result in a potentially worse prognosis [35,36,37,38]. A second opinion MRI report from an expert center increases the overall accuracy in the diagnosis of soft tissue tumors, with fewer false-negative and false-positive diagnoses [39,40,41].

Biopsy of suspected appendicular soft tissue sarcoma should be performed by a tumor radiologist-specialist, using image guidance, to minimize adverse outcomes, and with minimal delay [42].

In case of unplanned surgery of sarcoma, the patients should immediately be referred to a sarcoma center for further evaluation and treatment, in order to avoid a potentially worse prognosis [35].

Local radiologists should implement guidelines for early imaging by ultrasound and MRI with a designated pathway. Adherence to those guidelines should on the one hand help prioritize onward referral for suspicious lesions [22], and on the other hand help reduce the volume of benign lesions referred [22, 43].

The imaging strategies that become necessary when the histologic diagnosis is already known will be covered in consecutive parts of our guidelines. This includes recommendations for whole-body staging in the primary diagnosis, for therapy control, and for follow-up imaging, as well as special aspects and pitfalls.

If these guidelines lead to more standardized examinations, the resulting data may be better suited for multicenter studies, with an improved possibility to collect and analyze comparable large data volumes. Thus, these guidelines may help to develop more individualized imaging protocols for the diagnosis of soft tissue tumors in the future.

Limitations

Our consensus has got several limitations: The panelists came from European countries only. However, access to MRI is limited in many other parts of the world. In those areas, US and—if accessible—CT have to replace MRI. MRI contrast agents may be too expensive. Our guidelines take those points into account only to a certain extent. Tumor reference centers may be too distant, and teleradiologic consultation may not be available. In less-developed countries, only some parts of this consensus will be applicable at the moment. It is envisaged that these guidelines will however provide added impetus to health care professionals in these countries to canvass and optimize resources for better patient outcomes. The Delphi method itself has also got some limitations. Firstly, it is dependent on the expertise of the panelists. This was mostly overcome by including only ESSR-approved tumor specialists. Secondly, the possibility for open discussion is limited. On the other hand, this allows for distribution of critical remarks anonymously. Thus, the Delphi method had the advantage not to be biased by dominant participants. Thirdly, the process was time-consuming. This is a disadvantage that has been described for guidelines that contain multiple statements, such as our consensus [5]. As the Delphi process requires commitment to take part in several questionnaire rounds, we aimed to provide sufficient time for the participants to answer. Finally, it should be emphasized that these guidelines reflect the current knowledge and will require further updates in the future.