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Background/Objectives: Myxofibrosarcomas (MFSs) are aggressive soft-tissue sarcomas (STSs) that often arise in the upper and lower limbs. MFSs are a highly infiltrative sarcoma subtype with a high positive margin rate and poor clinical outcomes. Their management involves multidisciplinary team (MDT) input, with the mainstay of treatment being a wide surgical resection to remove the whole tumour, but this can be challenging due to the infiltrative nature of MFSs through fascial planes. Appropriate pre-operative imaging is therefore essential for surgical planning. Currently, MRI imaging is the modality of choice to assess the soft-tissue extent of MFSs; however, it does not always reliably predict tumour extent, especially when an MRI shows high-signal curvilinear projections, known as “tails”, which often represent tumour extension and increase the risk of positive margins and local recurrence. Methods: This feasibility study therefore aimed to investigate whether the addition of an FDG PET-MRI and DWI MRI is superior for the local staging of MFSs compared to a standard MRI, and to assess its practicality for clinical use. Results: Of the eight patients recruited, six completed the required scans, proceeded to surgery, and were included in the data analyses. Five of the six patients had close (<2 mm) or positive margins requiring re-excision. Conclusions: Our results show that combining an FDG-PET and DWI MRI may offer a more accurate local staging of MFSs than a conventional MRI; however, a larger prospective trial is needed to further investigate this pilot data. Nevertheless, this novel feasibly study demonstrates the potential use of PET-MRI and DWI for improving pre-operative planning prior to the surgical resection of MFSs.

Ewing sarcoma and osteosarcoma represent the two most common primary bone tumours in childhood and adolescence, with bone metastases being the most adverse prognostic factor. In prostate cancer, osseous metastasis poses a major clinical challenge. We developed a preclinical orthotopic model of Ewing sarcoma, reflecting the biology of the tumour-bone interactions in human disease and allowing in vivo monitoring of disease progression, and compared this with models of osteosarcoma and prostate carcinoma. Human tumour cell lines were transplanted into non-obese diabetic/severe combined immunodeficient (NSG) and Rag2(-/-/)γc(-/-) mice by intrafemoral injection. For Ewing sarcoma, minimal cell numbers (1000-5000) injected in small volumes were able to induce orthotopic tumour growth. Tumour progression was studied using positron emission tomography, computed tomography, magnetic resonance imaging and bioluminescent imaging. Tumours and their interactions with bones were examined by histology. Each tumour induced bone destruction and outgrowth of extramedullary tumour masses, together with characteristic changes in bone that were well visualised by computed tomography, which correlated with post-mortem histology. Ewing sarcoma and, to a lesser extent, osteosarcoma cells induced prominent reactive new bone formation. Osteosarcoma cells produced osteoid and mineralised \"malignant\" bone within the tumour mass itself. Injection of prostate carcinoma cells led to osteoclast-driven osteolytic lesions. Bioluminescent imaging of Ewing sarcoma xenografts allowed easy and rapid monitoring of tumour growth and detection of tumour dissemination to lungs, liver and bone. Magnetic resonance imaging proved useful for monitoring soft tissue tumour growth and volume. Positron emission tomography proved to be of limited use in this model. Overall, we have developed an orthotopic in vivo model for Ewing sarcoma and other primary and secondary human bone malignancies, which resemble the human disease. We have shown the utility of small animal bioimaging for tracking disease progression, making this model a useful assay for preclinical drug testing.

IntroductionThe differentials of diffuse omental mass include metastasis and mesothelioma. We describe the challenges in recognising our man from Istanbul (known man in an unexpected location) masquerading as mesothelioma, misconstrued as PEComa and the truncated immune-phenotype was finally salvaged by NGS.Case Description77 year old man presented with diarrhoea and weight loss, underwent a CT colonogram that revealed a diffuse omental mass. In the absence of any other primary, this was radiologically suggestive of mesothelioma although there was no history of exposure.A laparoscopic biopsy was performed as CT biopsy was scanty for exhaustive IHC. The tumour was nested, epithelioid with prominent vessels. The cells had pericellular space with spidery cytoplasmic processes. Although distinctive, this was not diagnostic. Extended panel for lineage excluded mesothelial/epithelial nature (Calretinin, D2-40, CAM5.2, CK5/6, 34BE12, WT1, P40, Pan CK, GATA3, GLUT1, PAX8), neuroendocrine carcinoma (Chromogranin, INSM1, Synaptophysin), melanoma (S100, HMB45, Melan A, BRAFV600E, PRAME), sarcoma (CD34, Desmin, Myogenin, Myo D1, ERG) and lymphoma (CD45/LCA, MUM1). The SWI-SNF complex proteins INI1 and SMARC A4 were retained and so was FH with no accumulation of the metabolite 2 SC. NGS and MDM2 FISH was not available but Cathepsin K showed diffuse strong expression although the focal weak expression of SMA and TFE3 were interpreted as negative but TFE 3 was interpreted as focally positive by the external referral pathologist. A probable diagnosis of malignant PEComa (Perivascular Epithelioid Cell tumour) pending MDM2 FISH, molecular confirmation and GPNMB IHC (never received) was made. Referral to the external soft tissue pathologist confirmed lack of expression of other lymphoma/germ cell (CD30, ALK), macrophage (CD68) and more specific soft tissue sarcoma (EMA,MUC4) markers. MDM2 FISH was negative for liposarcoma which was another significant differential. Archer FusionPlex sarcoma panel V2 was negative. Crucially this does not screen for TSC1/2 abnormalities in PEComa but only TFE3 which was negative. Hence the probability of malignant PEComa was retained along with differentials of pseudo-endocrine sarcoma and GLI 1 altered sarcoma. At the regional sarcoma MDT, CD117 and DOG1 were performed - this was diffusely positive for DOG1 (Discovered on GIST1- K9 clone) but not CD117. The possibility of an unusual GIST with PDGFRA mutation was considered and then confirmed on NGS (V546E and Y849C activating mutations in exon 11 and 18 of PDGFRA gene).Our patient reports symptomatic relief from ascites (that developed later) following treatment with 2 cycles of imatinib and awaits interval scans.Abstract P318 Figure 1ConclusionThis case illustrates that GIST must be considered in all difficult to classify intra-abdominal tumours irrespective of cross-sectional imaging and morphology.

The causes of pain after an anterior cruciate ligament (ACL) reconstruction are numerous and may have complex origins. We present an unusual case in which pain after an ACL reconstruction developed secondary to a giant cell tumour of the bone occurring around a fixation screw in the distal femur, with an associated fracture through the femoral tunnel of a previously well-functioning reconstruction. We discuss the aetiology and the treatment of a complex clinical scenario.

Background: Sarcomas are rare, aggressive cancers which can occur in any region of the body. Surgery is usually the cornerstone of curative treatment, with negative surgical margins associated with decreased local recurrence and improved overall survival. Indocyanine green (ICG) is a fluorescent dye which accumulates in sarcoma tissue and can be imaged intraoperatively using handheld near-infrared (NIR) cameras, theoretically helping guide the surgeon’s resection margins. Methods: Patients operated on between 20 February 2019 and 20 October 2021 for intermediate to high grade sarcomas at our centres received either conventional surgery, or were administered ICG pre-operatively followed by intra-operative NIR fluorescence guidance during the procedure. Differences between the unexpected positive margin rates were compared. Results: 115 suitable patients were identified, of which 39 received ICG + NIR fluorescence guided surgery, and 76 received conventional surgery. Of the patients given ICG, 37/39 tumours fluoresced, and surgeons felt the procedure was guided by the intra-operative images in 11 cases. Patients receiving ICG had a lower unexpected positive margin rate (5.1% vs. 25.0%, p = 0.01). Conclusions: The use of NIR fluorescence cameras in combination with ICG may reduce the unexpected positive margin rate for high grade sarcomas. A prospective, multi-centre randomised control trial is now needed to validate these results.

Septic subacromial bursitis is an uncommon disorder with only a few reported cases in the literature. The most common causative organism is Staphylococcus aureus . We report the case of a 61-year-old female with a septic subacromial bursitis where the causative organism was found to be Mycobacterium avium-intracellulare (MAI). The diagnosis was only made following a biopsy, and we use this case to highlight the importance of recognising the need to consider a biopsy and aspiration in atypical situations.

The causes of pain after an anterior cruciate ligament (ACL) reconstruction are numerous and may have complex origins. We present an unusual case in which pain after an ACL reconstruction developed secondary to a giant cell tumour of the bone occurring around a fixation screw in the distal femur, with an associated fracture through the femoral tunnel of a previously well-functioning reconstruction. We discuss the aetiology and the treatment of a complex clinical scenario.