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Steroid is commonly initiated for newly recognized brain tumors. For primary CNS lymphoma (PCNSL), however, its lympholytic effect can partially treat the tumor and obfuscate the surgical-pathological diagnosis needed to initiate chemotherapy. Evidence for this remains controversial. In this study we investigated the incidence of the vanishing tumor phenomenon and its association with steroid use. METHODS Patients &amp;gt; age 18 with PCNSL were identified from January 1, 2000 to April 30, 2020 within the University of Michigan Health System. Clinic and operative notes, pathology, imaging, and lumbar puncture results were reviewed. Inclusion required histopathologic diagnosis from brain tissue, cerebrospinal fluid, or autopsy, including diagnosis on repeat biopsy. Patients with prior/concurrent systemic lymphoma, HIV, organ transplant, or diagnosis established from primary ocular disease were excluded. Date(s) of biopsy, steroid initiation, and recurrence (by imaging) were abstracted. We defined vanishing tumor as a lesion whose radiographic improvement or disappearance resulted in deferred or nondiagnostic biopsy, with later confirmation of PCNSL. Steroid use and vanishing tumor were modeled using univariable logistic regression. RESULTS We identified 162 cases of PCNSL, of which 103 patients (63.6%) received steroids prior to biopsy. Twenty-three had vanishing tumor: 11 with deferred biopsies, 12 with non-diagnostic biopsies. Twenty-two of the 23 received steroid. Mean cumulative dose, in dexamethasone equivalents, was 555 + 704 mg prior to nondiagnostic/deferred biopsy (N=17) compared to 186 + 376 mg prior to successful biopsy (N=76) (Wilcoxon p&amp;lt; .001). Median time to recurrence after deferred or non-diagnostic biopsy (N=21) was 53 days (range: 8–648); two cases never recurred. Steroid use increased the risk of vanishing tumor by 14% per 100 mg dexamethasone equivalent (95% CI of odds ratio [1.03–1.29], p=0.02). CONCLUSION Vanishing tumor phenomenon is common in PCNSL, delays diagnosis, and is driven by steroid use.

EPID-27. THE VANISHING TUMOR PHENOMENON IN THE DIAGNOSIS OF PRIMARY CNS LYMPHOMA