Orbital Myeloid Sarcoma Masquerading as Orbital Cellulitis: A Diagnostic Challenge in an Adult Patient.

Myeloid sarcoma (MS), also known as granulocytic sarcoma or chloroma, is a rare extramedullary manifestation of acute myeloid leukaemia (AML). It represents a tumour-like accumulation of myeloid blasts outside the bone marrow. Orbital involvement is uncommon and may mimic inflammatory or infectious conditions, creating significant diagnostic challenges. We report a 24-year-old female who presented with a two-week history of progressive left orbital pain and swelling, initially managed as presumed orbital cellulitis with oral flucloxacillin and topical chloramphenicol, which subsequently progressed to worsening swelling, eye pain, and proptosis despite antibiotic therapy, prompting further investigation. Orbital imaging demonstrated a retro-orbital mass compressing the optic nerve. Peripheral blood film demonstrated circulating blast cells, and subsequent orbital biopsy confirmed MS. Cytogenetic analysis revealed t(8;21)(q22;q22), RUNX1-RUNX1T1 fusion, along with FLT3-ITD and KIT mutations, confirming AML. Following confirmation of the diagnosis, the patient received induction chemotherapy using the DA60 regimen (daunorubicin and cytarabine) combined with gemtuzumab ozogamicin (Mylotarg) as per institutional protocol. Following achievement of remission, consolidation chemotherapy was delivered with two cycles of high-dose cytarabine (HiDAC) in combination with midostaurin, administered via continuous ambulatory drug delivery (CADD) during the second consolidation cycle. Midostaurin was subsequently continued as maintenance monotherapy in view of the confirmed FLT3-ITD mutation. Adjuvant orbital radiotherapy was delivered to a total dose of 25 Gy in 12 fractions. Visual acuity recovered from a baseline of 6/24 to 6/6 at follow-up, with complete resolution of proptosis. The first complete morphological remission was confirmed approximately four weeks after commencement of induction chemotherapy, with serially declining minimal residual disease (MRD) levels on PCR monitoring. This case highlights the importance of maintaining a high index of suspicion for malignant causes in atypical or treatment-resistant orbital lesions. Orbital MS can closely resemble orbital cellulitis or idiopathic orbital inflammation, delaying accurate diagnosis. Early biopsy and haematological workup are essential for timely intervention. Prompt initiation of AML-directed therapy and multidisciplinary management can significantly improve both visual and systemic outcomes.