Brain Tumor Response to Stereotactic Radiosurgery Revealed on FDG-PET/CT

Objectives: Stereotactic radiosurgery (SRT), such as gamma knife therapy, can be a highly effective treatment modality for treating brain tumors less than 2 cm in diameter[1]. However, its’ efficacy can be difficult to monitor, as the response is often confounded by the treatment-induced inflammation and tissue remodeling. The MRI tissue properties of T1 and T2 usually are not definitive in specifying the underlying pathology. Hence there has been a reliance on the presence of contrast enhancement. However, even this has limitations in specificity[2]. We investigated the patterns of metabolism following SRT, with the goal of defining PET’s ability to assess SRT response.

Methods: Twenty patients who had undergone SRT for either primary brain tumors of any grade or metastatic disease were studied with both conventional MRI and FDG-PET/CT. The metabolic profile at the target site of SRT characterized by spatial distribution, morphology and magnitude of metabolically tissue. This study was approved by the UBIRB ID STUDY00000193. Patients were mostly male (14 males, 5 females). The mean patient age was 54 years (range: 24-68). In most (17) cases, the time between gammaknife and FDG-PET was less than one year, although in two cases clinical suspicion of relapse warranted FDG-PET imaging after 3 yrs. Tumor pathologies included: Glioblastoma multiforme (8), Ovary, lung and breast mets (4), Astrocytoma (5), Oligodendrioglioma(1), Meningioma(1) and Gliosarcoma (1).

Results: On the FDG-PET imaging obtained after the radiosurgery, 10 (52%) had recurrence, 6 (31%) remission (4 with the inflammatory remnant) and in three patients(15%) brain metastasis in locations other than the gamma knife site were also found. We identified 3 physiologic metabolic patterns at the SRT target site: 1) An ominous pattern of irregularly contoured and located hypermetabolic tissue; 2) A regularly contoured pattern of hypermetabolism, typically composed of a thin rim of homogenous metabolism around the SRT target site; 3) Absent metabolism or severe hypometabolism. These patterns are related to Karnofsky Performance Status.

Conclusion: Although contrast enhanced MRI is highly sensitive for presence of pathology, it has low specificity for the cause of that pathology. Ring lesion enhancement can be present with both radiation-induced inflammation and tumor recurrence, a distinction needed for rationale management in neuro-oncology. We show that FDG-PET is extremely useful in providing definitive diagnosis necessary for subsequent rational management. The uniqueness of each pattern reflects the underlying tissue response to SRT. Pattern I: Indicates local recurrence of high grade neoplasm. Pattern II: Reflects treatment-induced inflammation and tissue remodeling. Pattern III: Indicates treatment-induced tissue necrosis. FDG-PET/CT can be used to monitor SRT tissue response[3-5]. We are in the process of comparing the relative diagnostic utility of MR diffusion weighted imaging vs. PET/CT and we are gathering long-term outcomes in these patients for a further cohort analysis. Research Support: William E. Mabie, DDS, and Grace S. Mabie Fund