Question- Growing left upper arm mass

22-year-old Asian male presenting with concerns of a sporadic non-painful mass on the left upper arm. There is no history of trauma, and the patient is otherwise in good health with no past medical history, current medications, or known allergies. Physical exam revealed a non-mobile mass palpable along the lateral mid-humerus with no superficial skin changes. Laboratory tests noted eosinophilia. What is your diagnosis? (Figs. 1, 2, 3, and 4).

Fig. 1
figure 1

Anteroposterior radiograph of left upper arm

Full size image
Fig. 2
figure 2

Ultrasound of left upper arm. a Longitudinal grayscale ultrasound, b Longitudinal color Doppler ultrasound, c Transverse color Doppler ultrasound, d Ultrasound guided biopsy of left lateral upper arm

Full size image
Fig. 3
figure 3

MRI of left upper arm. a Coronal T1-weighted MRI, b Axial apparent diffusion coefficient (ADC) MRI, c Axial Proton Density (PD) fat suppressed MRI, d Axial diffusion weighted MRI, e Axial T1W gadolinium enhanced MRI

Full size image
Fig. 4
figure 4

Low power histopathology images of the biopsy. a H&E stain, 20X shows fibroadipose tissue involved by a lymphoid infiltrate with associated fibrosis producing a vaguely nodular architecture. b H&E stain, 200X shows a reactive follicle with numerous surrounding eosinophils. c H&E stain, 400X shows increased vascularization of the follicles with scant proteinaceous debris d H&E stain, 400X further reveals numerous associated eosinophils with focal loose eosinophilic abscess formation

Full size image