Impact of Time Interval Between Diagnostic and Planning MRI on Tumor Volume and Anatomic Displacement for Brain Metastases Undergoing Stereotactic Radiosurgery.

PURPOSE/OBJECTIVE(S) Stereotactic radiosurgery (SRS) is increasingly used in brain metastasis patients. However, the impact of the timing on tumor volume change and tumor anatomical displacement between the diagnostic and treatment planning imaging is not well studied. MATERIALS/METHODS Consecutive patients diagnosed with brain metastases and treated with SRS between July, 2017 and June, 2020 were evaluated. Only patients who underwent an institutionally standardized diagnostic MRI (MRI-1) and a treatment planning MRI (MRI-2) with identical imaging parameters were included. The impact of histology, inter-scan interval, lesion location, volume and size were evaluated on final lesion size, volume and anatomic displacement. Correlations were assessed using Kendall's rank correlation. The Kruskal Wallis rank sum test was used for all other comparisons. RESULTS We included 101 eligible patients (531 lesions) with median inter-scan interval of 8 days (range: 1-42); 50% were NSCLC and 25% breast cancer. The three most common locations of lesions were the frontal (181 lesions, 34%), temporal (96, 18%), cerebellar (91, 17%) lobes. Median MRI-1 diameter and volume were 0.50 cm (Range: 0.23-0.89 cm) and 0.06 cm3 (Range: 0.01-4.62 cm3), respectively. Median percentage change in lesion diameter and volume were +9.5% (Range: -8.4% to +61.2%) and +20% (Range: -20% to +200%), respectively. Overall, there was a statistically significant change in percent tumor volume change between MRI-1 to MRI-2 (P = 0.01). Directional displacements exceeded 1 mm in 18 (3%) lesions in right to left (RL), 28 (5%) superior to inferior (SI), and 25 (4.7%) anterior to posterior (AP) directions. Overall, 355 (67%) lesions had any displacement in at least 1 direction between MRI-1 and MRI-2 and 205 (38%) in all 3 directions. Median lesion centroid displacement was 0.48 mm (0.10-3.09 mm) and was larger than 1 mm in 66 (12%) lesions. Differences were also observed across tumor histologies (P = 0.01); melanoma brain metastases had the largest magnitude changes. Tumor displacement also differed across various histologies with largest displacement in renal cell carcinoma metastases (P < 0.001). Magnitude of displacement was also significantly associated with tumor location (P = 0.03) with the largest displacement in parietal lobe lesions. Across all lesions, there was a difference in tumor size change in patients treated on corticosteroids (P = 0.01). CONCLUSION Volumetric and positional changes can occur during the planning process for SRS and are histology and location specific. Additional analyses are key to understanding the impact of each of these variables, corticosteroids, and systemic therapy to better understand and model tumor dynamics. AUTHOR DISCLOSURE T. Kutuk: None. R.P. Tolakanahalli: None. A. Williams: None. M.C. Tom: Research Grant; Blue Earth Diagnostics. J.D. Vadhan: None. H. Appel: None. M.D. Hall: Research Grant; Florida Department of Health; Proton Collaborative Group (PCG) Executive Council. D. Wieczorek: None. S.D. Davis: None. M.W. McDermott: Consultant; Stryker, Diende. V. Siomin: None. M.D. Chuong: Research Grant; ViewRay, AstraZeneca. Honoraria; ViewRay, Sirtex, Accuray. Speaker's Bureau; ViewRay, Sirtex, Accuray. Advisory Board; ViewRay, Advanced Accelerator Applications. Travel Expenses; ViewRay, AstraZeneca, Accuray; Particle Therapy Cooperative Group. Manages the operations and activities within the proton center; Miami Cancer. M.P. Mehta: None. A. Gutierrez: Honoraria; ViewRay, Elekta. R. Kotecha: None.