In a patient with a progressively enlarging triceps region mass over three months and MRI findings strongly suggestive of a malignant soft‑tissue tumor, what is the most appropriate next investigation to obtain a definitive diagnosis?

Core Needle Biopsy for Suspected Soft Tissue Sarcoma

Core needle biopsy is the most appropriate next investigation to confirm the diagnosis in this patient with a progressively enlarging triceps mass and MRI findings strongly suggestive of malignancy. 1

Rationale for Core Needle Biopsy

Percutaneous core needle biopsy is the standard approach to establish histopathological diagnosis for suspected soft tissue sarcomas. 1 This technique provides sufficient tissue for accurate histologic diagnosis while minimizing the risk of tumor contamination compared to open biopsy procedures. 1

Key Technical Considerations

• Multiple cores should be taken to maximize diagnostic yield, typically performed under image guidance (ultrasound or CT) by a radiologist. 1
• The biopsy must be planned by a specialist sarcoma multidisciplinary team so that the biopsy tract can be safely removed during definitive surgery. 1, 2
• Meticulous hemostasis is essential during the procedure to prevent hematoma formation and tumor seeding along tissue planes. 2

Why Other Options Are Inappropriate

Fine Needle Aspiration (Option A)

• Fine needle aspiration is not recommended as a primary diagnostic modality for suspected soft tissue sarcomas because it yields insufficient tissue for definitive histologic diagnosis, grading, and molecular characterization. 1

Excisional Biopsy (Option B)

• Excisional biopsy without prior staging imaging and multidisciplinary planning is a critical error that can lead to inadequate margins, tumor spillage, and the need for more extensive re-resection, adversely affecting functional outcome and survival. 1
• Excisional biopsy is appropriate only for superficial lesions smaller than 5 cm; it is not indicated for deep or large tumors. 1
• Performing excisional biopsy before appropriate imaging and multidisciplinary planning can seed tumor cells along the tract, compromise surgical margins, and worsen functional and survival outcomes. 1

Incisional Biopsy (Option C)

• Incisional (open) biopsy is generally reserved for situations where core needle biopsy has failed to yield diagnostic tissue or when the lesion is not amenable to percutaneous sampling. 3
• Open biopsy carries higher risks of contamination of unaffected tissue planes, leading to recurrence and potentially unnecessary amputations if not performed with meticulous technique. 3

Critical Management Algorithm

1. Ensure MRI with contrast has already been completed to characterize the lesion and guide biopsy planning. 1, 4

2. Refer immediately to a specialist sarcoma multidisciplinary team before performing the biopsy, as this patient meets high-risk criteria (deep location, progressive growth, MRI findings suggestive of malignancy). 1

3. Coordinate with the MDT to plan the biopsy tract location so it can be excised en-bloc during definitive resection. 1, 2

4. Perform image-guided core needle biopsy with multiple passes to obtain adequate tissue. 1

5. Obtain frozen-section analysis during the procedure to ensure diagnostic material has been obtained. 2

Essential Pitfalls to Avoid

• Never perform any biopsy before obtaining MRI and specialist consultation, as this can compromise surgical margins and worsen oncologic outcomes. 1
• Do not proceed with excisional biopsy in the belief that "complete removal" is therapeutic—unplanned excisions of sarcomas result in positive margins in the majority of cases and require complex re-resection. 1
• Avoid inadequate hemostasis during biopsy, as hematoma can disseminate tumor cells and contaminate additional tissue compartments. 2

Histopathological Requirements

• Histological diagnosis should be made according to the 2020 WHO Classification of Soft Tissue and Bone to determine the grade and stage of the tumor. 1
• A soft tissue specialist pathology review is recommended due to high discrepancy rates (8-11% for major discordance, 16-35% for minor discordance) when biopsies are performed outside sarcoma centers. 1
• Tumor grade should be provided using a recognized system such as the FNCLCC grading system, as this directly impacts treatment planning and prognosis. 1