Considerations in Treatment Planning and Dosimetric Specifications of Permanent 131Cs Brachytherapy Implantation in Treatment of Brain Tumor – An Institutional Experience

Abstract

<h3>Purpose/Objective(s)</h3> Local recurrence after the operative management of several types of brain tumors remains common and is the rationale for the frequent use of adjuvant radiation. This report details the dosimetry and normal tissue exposure to organs at risk (OARs) for patients with brain tumors treated with a novel strategy of maximum safe resection (MSR) and permanent ¹³¹Cs brachytherapy implantation. <h3>Materials/Methods</h3> From 01/2019 to 02/2022, thirty-two patients with 35 brain tumors underwent MSR followed by permanent ¹³¹Cs source implantation using a collagen tile brachytherapy device. The pre-op GTV delineated on MRI within 30 days was used to estimate the number of tiles needed. The post-operative thin-slice (1 mm) CT and contrast-enhanced MRI were obtained within 24 hours of surgery. Dosimetry analysis was performed using a brachytherapy planning tool. A dose of 60 Gy was prescribed to the high-risk clinical target volume (HR-CTV), defined as the brain parenchyma within 5 mm isotropic expansion from the resection cavity, respecting unviolated anatomical borders. Implant quality was evaluated with the D₉₀, V₅₀, V₁₀₀, V₁₅₀, V₂₀₀ metrics for the HR-CTV; D_max and D_mean OAR metrics were recorded. When applicable, the residual gross tumor volume (GTV_r), defined as persistent, nodular contrast enhancement on post-operative T1-weighted MRI, was delineated, and the D₉₀ to GTV_r was recorded. <h3>Results</h3> The median volume was 17.8 cc (2.3-119.2 cc) for the pre-op GTV, 15.8 cc (4.6-78.3 cc) for the resection cavity, 21.2 cc (9.5-58.3 cc) for HR-CTV, and 1.2 cc (0.2-28.9 cc) for GTV_r. 36% (16/35) cavities were larger than pre-op GTV by an average of 9.1 cc (<0.1-31.3 cc). The median D₉₀ was 63.4 Gy to HR-CTV (n=35, 31.7- 148.3 Gy), and 68.7 Gy to GTV_r (n=24, 22.8-113.8 Gy). The median V₅₀, V₁₀₀, V₁₅₀, V₂₀₀ of HRCTV were 100% (91.1-100%), 93.2% (41.9-100%), 11.1 cc (3.7-51.9 cc), and 4.7 cc (1.2-43.7 cc). The dosimetric objective D₉₀ ≥ 54 Gy to the HR-CTV was met in 69% (24/35) of cavities; the objectives V₅₀ ≥ 99% and V₁₀₀ ≥ 90% were met in 83% (29/35) and 54% (19/35) of cavities. OAR metrics were summarized in Table 1. <h3>Conclusion</h3> Permanent ¹³¹Cs brachytherapy implantation can provide adequate dosimetric coverage of the HR-CTV and selective sparing of OARs after MSR.