Scientific Papers

Identification of dual STRN-NTRK2 rearrangements in a high grade sarcoma, with good clinical response to first-line larotrectinib therapy | Diagnostic Pathology

A 35-year-old woman presented to our oncology clinic for a second opinion for a right gluteal sarcoma initially diagnosed outside the continental United States. She complained of nausea, fever, and worsening pain and numbness form the affected area. The mass had been increasing in size. MRI showed a large, heterogeneous, T2 hyperintense, vascular, and partially necrotic and enhancing mass, centered in the right gluteus medius and maximus muscles, with extension into the right paraspinal musculature, and invasion of the right posterior iliac bone. The initial MRI (50 days ago) showed the mass measuring 10.2 × 9.7 × 7.0 cm (Fig. 1B), and measured 16.5 × 12.9 × 10.4 cm (AP x TV x CC) (Fig. 1C) at first encounter at our institution. PET scan showed no metastasis.

Fig. 1
figure 1

Clinical timeline (A) and imaging (B-E). Before treatment (B: – Day 50 & C: Day 0), a heterogenous mass (yellow arrows in B-E) centers in the right gluteus medius and maximus muscles with extension into the right paraspinal musculature, and invasion of the right posterior iliac bone. Imaging on post-treatment Day 60 (D) and Day 170 (E) show significant decrease in tumor size (MRI#4 of Day 80 is not shown here). 130 days after surgery, a 6.9 cm solid mass in the lower lobe of left lung is biopsied and proven to be a metastatic lesion (F, Day 320, blue arrow with circle), with near complete resolution after the resumption of Larotretinib treatment (G: Day 455, blue arrow with circle & H: Day 560)

A core biopsy was performed and showed a highly cellular mesenchymal neoplasm consisting of round/ovoid cells, with eosinophilic cytoplasm, round to ovoid nuclei with mostly inconspicuous nucleoli, and no significant nuclear pleomorphism (Fig. 2A and B). The tumor was rich in vasculature, with focal staghorn-type vessels (Fig. 2A). Brisk mitotic activity (21 per 10 high-power fields) and tumor necrosis were identified (Fig. 2C). Histologically, the tumor was reminiscent of a malignant solitary fibrous tumor (SFT). It was diffusely positive for CD34 (Fig. 2D), but negative for STAT6 (data not shown). The tumor was also positive for FLI1 (patchy), TLE1 (patchy), and CD99 (focal and weak), and was negative for AE1/AE3, S100, SOX10, ASMA, desmin, ERG, and CD31. LCA stain highlighted the intermixed inflammatory cells (data not shown).

Fig. 2
figure 2

Pathology of the core biopsy. H & E sections show a highly cellular mesenchymal neoplasm of round/ovoid cells with eosinophilic cytoplasm, round to ovoid nuclei and mostly inconspicuous nucleoli without significant nuclear pleomorphism. The tumor is rich in the vasculature with staghorn-type vessels (A & B), and brisk mitotic activity (C, arrows). Tumor is positive for CD34 (D). Magnifications: A, 100X; B-D, 400X

In-house RNA-based NGS gene fusion panel analysis (Archer FusionPlex™ Comprehensive Thyroid and Lung Panel, 18 gene fusion panel) was performed, and two concurrent STRN-NTRK2 fusions were identified. The NGS data showed that both fusions were in-frame and located at the exon-exon boundary. Both fusion RNAs had the same 5’ partner sequence (exon 1–3) of the STRN gene (chr2, NM_003162.3, breakpoint: 37,143,221, Fig. 3A and B). One of the 3’ fusion sequence started from the exon 15 of NTRK2 (chr9, NM_006180.4, breakpoint: 87,475,955, Fig. 3A), and the other 3’ fusion sequence began from exon 16 of NTRK2 (chr9, NM_006180.4, breakpoint: 87,482,158, Fig. 3B). Of note, the fusion product containing exon 1–3 of STRN and exon 16–21 of NTRK2 was the same as the one which was previously identified as an undifferentiated sarcoma in a pediatric patient [14]. The second fusion pattern starting from exon 15 of the NTRK2 had been identified in a ganglioglioma harboring TLE4-NTRK2 fusion.

Fig. 3
figure 3

Molecular characterization of tumor sample by NGS. Two in-frame STRN-NTRK2 fusions with same 5’ partner sequence (exons 1–3) of STRN gene identified by NGS.

Due to the location and extent of the tumor, it was deemed unresectable or would require a morbid upfront surgery, i.e., hemipelvectomy. Given the NGS findings, multidisciplinary tumor board was held, and the patient was started on the selective NTRK inhibitor, Larotrectinib, at 100 mg, BID, as the first-line therapy. The patient noticed a quick amelioration of tumor-related pain and was able to discontinue all pain medicines (previously on 20 mg oxycontin plus 10 mg oxycodone immediate release every 6 h as needed). The patient also reported a significant shrinkage of the tumor after the initial 7-day treatment, and it continued to improve in the following 45 days. On post-treatment day 50, MRI was repeated and showed that the tumor had significantly decreased in size to 6.6 × 7.7 × 7.4 cm (Fig. 1D), approximately 83% reduction compared with the tumor size before treatment. The previously noted focus of enhancement in the right paraspinal musculature was not present in this imaging.

The patient was followed up closely, and MRI was repeated on post-treatment day 90 and day 170. The size of tumor remained stable during this interval (Fig. 1E, day 90: 6.3 × 7.4 × 5.8 cm; day 170: 6.0 × 7.1 × 5.1 cm). During the later period of treatment, the patient started to feel a few nodules around the original tumor expanding with new significant pain. Metastatic workup showed no metastases, and on post-treatment day 190, the patient underwent radical resection of right gluteal soft tissue sarcoma and right ilium, which was uneventful.

The resection specimen revealed a 6.1 × 7.5 × 3.6 cm ill-defined, heterogeneous, fleshy, solid and necrotic mass with a tan-pink cut surface. Histologically, the tumor showed high-grade undifferentiated sarcoma with treatment effects (granulation tissue, myxoid change, fibrosis, foamy macrophages, and necrosis (50% of tumor volume)). It focally invaded into the ilium. Resection margins are free of tumor.

After wound healing was completed, the patient was started on adjuvant radiation therapy (a total of 5000 cgy) given this being a high grade tumor > 10 cm. Larotrectinib was held during adjuvant radiation due to a lack of data on concurrent radiation therapy. Within six weeks after completion of radiation, while still off larotrectenib, patient presented with chest pain and shortness of breath, and chest tightness. Imaging studies showed multiple new large lung nodules. A biopsy of one of the nodules showed metastatic sarcoma. The in-house NGS gene fusion panel (Archer FusionPlex™ Pan Solid Tumor v2 panel, ArcherDX/Invitae, 99 gene panel) showed persistent NTRK2 fusions. The patient was then restarted on palliative larotrectenib, and the symptoms improved within a few days. CAT scan repeated 60 days after initiation of palliative larotrectenib showed complete resolution of her lung metastases. The patient continued larotrectinib with response for 22 months before developing symptomatic right parietal lobe and leptomeningeal disease, and biopsy showed metastatic sarcoma with fusion panel again positive for NTRK2-STRN fusions. Imaging studies to evaluate status of disease showed persistent extracranial response to larotrectenib. Patient was started on compassionate use of second generation NTRK inhibitor PBI-200 with known blood brain barrier penetration. The brain metastasis biopsy was sent for outside laboratory NGS to look for mutations. However, none of the known on-target resistant mutations of TRKB, including solvent front mutations (e.g. TRKBG639R), gate keeper residue mutation (TRKBF633L), xDFG motif mutation (TRKBG667C) [15], and none of the known off-target mutations (e.g., MET amplification, BRAFV600E mutation, and KARS mutations) were identified in this case [15]. Patient had disease control for 3 months on the second generation NTRK inhibitor with eventual worsening of intracranial disease and succumbed to her disease.

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