Low-Energy Brachytherapy Sources for Pelvic Sidewall Treatment

Abstract

Purpose: The CivaSheet (CivaTech Oncology, Inc., Research Triangle Park, NC) is a Pd-103 brachytherapy device with radiation directionality for the treatment of cancer. The device has individual elements (i.e., CivaDots) spaced in a rectangular array (0.8 cm center-to-center spacing) within a flexible bioabsorbable substrate to mitigate the hotand coldspots of radiation dose typically prevalent with Pd-103 implants. The current study compares dose distributions and critical structure doses for the CivaSheet (CivaTech Oncology, Inc., Research Triangle Park, NC) to those of conventional, low dose-rate (LDR), low-energy photon-emitting brachytherapy seeds. Materials and Methods: The VariSeed (Varian Medical Systems, Palo Alto, CA) LDR brachytherapy treatment planning software is the most popular system. Version 9 enables directional orientation of the sources away from the imaging scanning axis. Post-implant CT scans (120 kVp) of the first patient treated with the CivaSheet for cancer at the pelvic sidewall were compared for three low-energy seeds (model 200 Pd-103, model 6711 I-125, and model CS-131 Rev2 Cs-131). Based on starting with a 6x12 array, a total of 70 CivaDots (1.34 U each, 93.8 U in total) were implanted for a prescription dose of 60 Gy to 0.5 cm with customization of device sizing within the operating room to conform to the surgical tumor bed. Dose distributions were assessed near the tumor bed and adjacent to critical structures (i.e., bowel and bladder). Source strength for each element of the CivaSheet was 1.340 U. For a direct comparison at the prescription depth, theoretical meshes containing Pd103, I-125, and Cs-131 seeds were set at source strengths of 1.285 U, 0.189 U, and 0.957 U each, respectively, and scaled inversely with the dose-rate constant and half-life. The contoured bowel and bladder volumes were 433 cm and 289 cm, respectively. Clinically-relevant dose-volume histogram (DVH) metrics of 2 cm and 0.1 cm were determined. For the CivaSheet, differences between dose to water-inwater and dose to tissue-in-tissue were evaluated. These treatment planning data were obtained using Monte Carlo methods according to the methodology of the TG-43U1 and AAPM/ESTRO TG-186 reports. Commissioning results for these treatment planning data are included in another presentation. Results: The CivaSheet imaged clearly. Even with a metal prosthetic hip, all 70 CivaDots were discernable. However, spatial localization of individual CivaDots, and determination of their orientation and resultant radiation directionality, would benefit from improvements in the acquired 0.3 cm slice thickness and large field-of-view (50 cm). Dose distributions are depicted in the Figure. The directional source minimized radiation dose away from the implant in comparison to meshes loaded with conventional brachytherapy seeds. For the bowel, doses to 2 cm volumes for the CivaSheet, Pd-103 seeds, I-125 seeds, and Cs-131 seeds were 3.0, 25.8, 31.8, and 34.7 Gy, respectively. Doses to 0.1 cm volumes were 6.9, 46.5, 51.0, and 52.5 Gy, respectively. For the bladder, doses to 2 cm volumes for the CivaSheet, Pd-103 seeds, I-125 seeds, and Cs-131 seeds were 12.3, 41.7, 44.7, and 45.3 12.3 Gy, respectively. Doses to 0.1 cm volumes were 39.1, 102.6, 89.6, and 85.7 Gy. Dose generally increased with increasing photon energy of the source. The CivaSheet reduced the bowel and bladder dose metrics by factors of 7 and 2 over the seed-based results. For the same CivaSheet source strength, DVH metrics for dose to tissue-in-tissue for these organs were similar to those of dose to water-in-water, largely due to the 2% difference in dose-rate constants (0.566 and 0.579, respectively) and the tissues being within a few centimeters of the CivaSheet implant. Conclusions: Gold shielding in the CivaDots attenuates radiation for directional bracytherapy. The CivaSheet provides a therapeutic target dose while substantially minimizing critical structure doses.