Abstract
Background and ObjectivesIntra-parenchymal brain surgical resection cavities usually contract in volume following low dose rate (LDR) brachytherapy implants. In this study, we systematically modeled and assessed dose variability resulting from such changes for I-125 versus Cs-131 radioactive sources.MethodsResection cavity contraction was modeled based on 95 consecutive patient cases, using surveillance magnetic resonance (MR) images. The model was derived for single point source geometry and then fully simulated in 3D where I-125 or Cs-131 seeds were placed on the surface of an ellipsoidal resection cavity. Dose distribution estimated via TG-43 calculations and biological effective dose (BED) calculations were compared for both I-125 and Cs-131, accounting for resection cavity contractions.ResultsResection cavity volumes were found to contract with an effective half-life of approximately 3.4 months (time to reach 50% of maximum volume contraction). As a result, significant differences in dose distributions were noted between I-125 and Cs-131 radioactive sources. For example, when comparing with static volume, assuming no contraction effect, I-125 exhibited a 31.8% and 30.5% increase in D90 and D10 values (i.e., the minimal dose to 90% and 10% of the volume respectively) in the peripheral target areas over the follow-up period of 20.5 months. In contrast, Cs-131 seeds only exhibited a 1.44% and 0.64% increase in D90 and D10 values respectively. Such discrepancy is likewise similar for BED calculations.ConclusionResection cavity contractions affects Cs-131 dose distribution significantly less than that of I-125 for permanent brain implants. Care must be taken to account for cavity contractions when prescribing accumulative doses of a radioactive source in performing the brain implant procedures.
Highlights
IntroductionIntraoperative permanent brain implants have been performed since the 1980s for treating large brain metastases [1]
Brain metastases are the most common intracranial malignancy
Dose distribution estimated via TG-43 calculations and biological effective dose (BED) calculations were compared for both I-125 and Cs-131, accounting for resection cavity contractions
Summary
Intraoperative permanent brain implants have been performed since the 1980s for treating large brain metastases [1]. I-125 has been our radioactive source of choice for performing permanent brain implants [2,3,4]. I-125 possesses a half-life of 60 days and emits photons with a mean energy of approximately 0.3 MeV. Cs-131 has emerged as an alternative for permanent brain implants [56]. Cs-131 possesses a half-life of 9.7 days and emits photons that reach peak energies of 0.29 to 0.34 MeV [7,8,9]. Intra-parenchymal brain surgical resection cavities usually contract in volume following low dose rate (LDR) brachytherapy implants. We systematically modeled and assessed dose variability resulting from such changes for I-125 versus Cs-131 radioactive sources
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