PMC10436465_01
Pathological findings (A) On Hematoxylin & Eosin (HE) staining, tumor cells with well-defined nucleoli and a pale eosinophilic, partially vacuolated cytoplasm resemble a microcystic form, presenting a histology similar to the microcystic form of acinic cell carcinoma. Immunostaining showing that Pan-Trk and S-100, and MUC4 are widely expressed. (B) Fluorescence in situ hybridization (FISH) using the ETV6 break apart probe examined for the presence of yellow (red/green fusion) or green/red fluorescent signals in the tumor cells. Yellow signals are negative, and separate red and green signals are positive. ETV6 split signal of 32% is seen (cutoff, 10%).
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S-100, MUC4, and Pan-Trk showed widely positive results ( Figure 1A ), while immunostaining for mammaglobin revealed partially positive result. Subsequently, fluorescence in situ hybridization (FISH) with an ETV6 (12p13) break-apart probe (Vysis ETV6 Break Apart FISH Probe Kit, Abbott) using a previously reported method revealed split signals in the nuclei ( Figure 1B ).
Gross and computed tomography (CT) images. The upper panels show the gross findings of the local recurrence site before and after treatment, and the lower panels show the CT images of the same site before and after treatment.
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After treatment, the local recurrence in the left parotid region shrank rapidly and completely disappeared 6 months later ( Figure 2 , upper row). Computed tomography (CT) findings also showed that the local recurrent lesion had shrunk deeply after 2 months of treatment and completely disappeared after 6 months of treatment ( Figure 2 , lower row).
Adverse events with entrectinib and patients' clinical course.
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Adverse events experienced during entrectinib treatment and the patients' clinical course are illustrated in  Figure 3 .