MRI, CT, and PET/CT Reveal How Imaging Tracks a Teen’s Rare Shoulder Sarcoma

A previously healthy 16-year-old girl walked into a clinic in Trento, Italy, with months of worsening right shoulder pain and a growing inability to move her arm. There was no trauma, no fever, no weight loss, and no history of systemic disease. What clinicians ultimately found was a giant synovial sarcoma, an aggressive soft tissue malignancy that predominantly strikes adolescents and young adults, arising in one of the least common locations for this tumor type. The case, published as a case report in Frontiers in Nuclear Medicine, illustrates how a coordinated battery of imaging techniques—ultrasound, magnetic resonance imaging, computed tomography, and whole-body 18F-FDG PET/CT—can detect, stage, and monitor a rare pediatric cancer with remarkable precision, and how metabolic imaging can reveal a treatment response long before the tumor itself begins to shrink.

Synovial sarcoma is among the most common non-rhabdomyosarcoma soft tissue sarcomas in adolescents and young adults, yet it remains a rare and formidable diagnosis. Tumors arising in the shoulder region account for approximately 6 percent of all synovial sarcomas, and the disease typically does not originate from inside the joint but from the surrounding soft tissues. The tumor often progresses slowly, taking months to years before patients present for evaluation, which can result in a significant delay in diagnosis. In this case, the girl’s symptoms had been present for several months and had gradually worsened, eventually prompting clinical evaluation when the pain began interfering with daily activities and sports participation. Physical examination revealed localized bulging of the right shoulder, reduced range of motion, and tenderness on palpation of the periarticular region.

The diagnostic journey began with plain radiography and ultrasound of the shoulder. The radiograph revealed a soft tissue mass of increased density adjacent to the joint, without significant calcification or osseous involvement. Subsequent ultrasound demonstrated a large heterogeneous soft tissue mass involving the periarticular region and extending into adjacent muscular compartments. The lesion appeared poorly defined, with marked internal heterogeneity—findings that immediately raised suspicion for an underlying malignant soft tissue tumor. These initial assessments, while not diagnostic on their own, served as the critical first step in a multimodal imaging strategy that would ultimately characterize the tumor from both anatomical and functional perspectives.

Magnetic resonance imaging, performed on a 1.5-T scanner with a standard musculoskeletal protocol including T1-weighted, T2-weighted, and STIR sequences without intravenous contrast medium, demonstrated a large soft tissue mass measuring approximately 18 centimeters in maximum diameter. The lesion showed a multilobulated configuration with marked internal heterogeneity and areas of variable signal intensity on T2-weighted sequences, characterized by both cystic and solid components, with no enhancement in the areas containing cysts. Fluid-fluid levels were noted together with the characteristic triple sign, consisting of the coexistence of low, intermediate, and high signal intensity regions. These features suggested the diagnosis of synovial sarcoma. The tumor involved several muscles of the shoulder girdle, including the deltoid, infraspinatus, and subscapularis muscles, and extended into the surrounding soft tissues. MRI is considered the reference modality for local staging due to its excellent soft tissue contrast and its ability to assess tumor extent and relationship with surrounding structures, information that is essential for surgical planning and for defining target volumes in radiotherapy.

However, MRI findings in synovial sarcoma are not pathognomonic, and additional imaging techniques are often required to fully characterize the disease. In this case, the evaluation of possible bone involvement was initially challenging due to periosteal reaction and inflammatory changes. For this reason, contrast-enhanced computed tomography was performed and confirmed the absence of direct invasion of the humeral head. Moreover, contrast-enhanced CT provided important information on the enhancement pattern of the lesion, contributing to the characterization of tumor heterogeneity. Most synovial sarcomas demonstrate heterogeneous enhancement with reduced or absent enhancement in areas of cystic necrosis or internal septations, and this case was no exception. The CT findings, combined with the MRI data, allowed clinicians to confidently exclude bone invasion while gaining a deeper understanding of the tumor’s complex internal architecture.

To complete staging, a whole-body 18F-FDG PET/CT was performed three weeks after the MRI, acquired 60 minutes after tracer injection according to the institutional pediatric oncology protocol. The examination revealed a highly heterogeneous pattern of metabolic activity within the primary tumor, with a maximum standardized uptake value (SUVmax) of 12.3, a mean activity (SUVmean) of 1.5, and a peak activity (SUVpeak) of 7.7. Areas of intense uptake corresponded to viable tumor tissue, while regions of absent uptake corresponded to necrosis or fluid components. A key diagnostic finding was the identification of metabolically active lymph nodes in the ipsilateral axilla—information that had direct implications for disease staging and therapeutic planning. PET/CT proved particularly valuable in identifying regional lymph node involvement that was not clearly detected on morphological imaging, confirming its well-recognized higher sensitivity in detecting nodal and distant disease in pediatric sarcomas.

Histopathological examination of the excisional biopsy confirmed the diagnosis of monophasic poorly differentiated synovial sarcoma. Molecular testing demonstrated the presence of the characteristic chromosomal translocation t(X;18)(p11;q11) involving the SS18 gene, the genetic hallmark of this tumor type. Following multidisciplinary discussion, the patient started multimodal therapy consisting of combination chemotherapy with ifosfamide and doxorubicin, according to the EpSSG NRSTS 2005 protocol. After completion of three cycles of chemotherapy, which were well tolerated with no relevant treatment-related complications, a follow-up imaging evaluation was performed to assess treatment response.

The follow-up imaging told a fascinating story about the biology of treatment response. Contrast-enhanced CT demonstrated persistent tumor volume without clear evidence of disease progression, but with changes in the enhancement pattern of the solid tumor component. Meanwhile, the whole-body 18F-FDG PET/CT demonstrated a clear reduction in metabolic activity of the primary tumor compared with baseline, consistent with an early metabolic response to therapy. SUVmax decreased from 12.3 at baseline to 6.7 at follow-up, and SUVpeak decreased from 7.7 to 2.6. These structural changes corresponded to the reduction in metabolic activity observed on follow-up PET/CT, suggesting an early biological response to therapy preceding measurable tumor shrinkage. This finding highlights the ability of functional imaging to detect early treatment response before morphological tumor shrinkage becomes evident, a phenomenon that has been reported in high-grade soft tissue sarcomas and supports the role of PET/CT in early response assessment and treatment monitoring. While tumor size remains a widely used parameter for response evaluation, changes in metabolic activity may represent earlier and more sensitive indicators of therapeutic effectiveness—information that would not have been captured by size-based criteria alone.

Semiquantitative metabolic parameters, including SUVmax, SUVmean, and SUVpeak, are commonly used to characterize tumor biology and monitor response to therapy. Among these, SUVpeak, which reflects metabolic activity within a small, standardized tumor volume, may provide a more robust and reproducible measure than single-voxel metrics in heterogeneous lesions. Although considerable variability exists in reported thresholds due to differences in scanners and acquisition protocols, in synovial sarcoma a baseline SUVmax above approximately 5 has been associated with higher histological grade and worse clinical outcomes, supporting its role as a clinically relevant imaging biomarker for risk stratification. In addition, metabolic imaging with 18F-FDG PET/CT can help identify metabolically active tumor regions within heterogeneous masses, which are more likely to correspond to areas of higher cellularity and biological aggressiveness. These regions may represent optimal targets for biopsy and may support biologically guided radiotherapy planning, including potential dose escalation to high-risk tumor subregions while sparing surrounding normal tissues, thereby improving local tumor control and reducing treatment-related toxicity.

Interestingly, the follow-up PET/CT also detected multiple areas of symmetrical intense FDG uptake bilaterally in the neck area, the posterior mediastinum, bilateral costovertebral margins, and bilateral suprarenal fossa, extending to perinephric fat, which anatomically correlated to fat at CT—consistent with functioning brown fat. Although typically considered a physiological finding, recent evidence suggests that brown fat activation on FDG PET/CT may reflect systemic metabolic status and has been associated with improved survival outcomes in oncologic patients. In the present case, this finding was interpreted as a physiological phenomenon; however, its potential prognostic implications deserve further investigation, particularly in pediatric and adolescent populations. The case underscores the importance of an integrated imaging strategy in the management of pediatric soft tissue sarcomas and highlights the clinical value of combining complementary imaging techniques in rare and complex presentations. The main limitation of this report is the relatively short duration of follow-up, which does not allow assessment of long-term outcomes. Nevertheless, individual case reports remain valuable in the context of rare pediatric malignancies, where clinical experience is limited and each well-documented case contributes to improving diagnostic awareness and management strategies. At the time of the most recent follow-up, the patient reported good tolerance of therapy and maintained a positive outlook regarding treatment.

Subject of Research: Multimodal imaging with MRI, CT, and 18F-FDG PET/CT for staging and response assessment of pediatric synovial sarcoma

Article Title: Case Report: Role of MRI, CT, and 18F-FDG PET/CT in staging and response assessment of pediatric synovial sarcoma

Article References: Picori, L., Maccio, L., Meneghello, L., Caldarella, C., Donner, D., & Feraco, P. (2026). Case Report: Role of MRI, CT, and 18F-FDG PET/CT in staging and response assessment of pediatric synovial sarcoma. Frontiers in Nuclear Medicine, 6, Article 1870009. https://doi.org/10.3389/fnume.2026.1870009

Image Credits: AI Generated

DOI: 10.3389/fnume.2026.1870009

Keywords: synovial sarcoma, pediatric oncology, MRI, CT, 18F-FDG PET/CT, SUVmax, tumor staging, treatment response, soft tissue sarcoma, ifosfamide, doxorubicin, triple sign