Yongbok Kim

Retrospective assessment of HDR brachytherapy dose calculation methods in locally advanced cervical cancer patients: AcurosBV vs. AAPM TG43 formalism

AbstractPurposeThis retrospective analysis was completed to investigate the use of a model‐based dose calculation algorithm (MBDCA) AcurosBV, for use in HDR BT treatments for locally advanced cervical cancer treated with tandem and ovoid applicators with interstitial needles.MethodsA cohort of 32 patients receiving post‐EBRT HDR brachytherapy boost with a prescription dose of 5.5 Gy × 5 fractions to the high‐risk clinical target volume (CTVHR) were selected for this study. For standard TG43 dose calculation, applicators were manually digitized on the planning images, while for AcurosBV calculations, solid renderings of Titanium Fletcher Suite Delclos (FSD) applicators included in BrachyVision were matched to those used clinically and Ti needles were manually digitized. The dose was recalculated using Varian's AcurosBV 13.5 and dose‐to‐medium‐in‐medium (Dm,m) was reported. EQD2 values for targets and organs at risk were compared between dose calculation formalisms. D90% and D98% values were reported for the high and intermediate‐risk CTVs, and values were reported for OARs including bladder, rectum, sigmoid, bowel, and vagina. Due to variability within the patient cohort, the dosimetric impact of AcurosBV was investigated corresponding to planning image modality (CT vs. CBCT), presence of Ti needles, and contrast within vaginal balloons used to stabilize implants. AcurosBV showed lower dosimetric values for all plans compared to TG43.ResultsThe average ± standard deviation of dosimetric reduction in D90% was 4.33 ± 0.09% for CTVHR and 4.12 ± 0.09% for CTVIR. The reduction to OARs was: 4.99 ± 0.15% for bladder, 7.87 ± 0.16% for rectum, 5.79 ± 0.17% for sigmoid, 6.91 ± 0.14% for bowel, and 4.55 ± 0.14% for vagina.ConclusionsAcurosBV should be utilized for HDR BT GYN cases, treated with tandem and ovoid applicators, with high degrees of heterogeneity and calculated in tandem with TG43.

Commissioning of the Varian universal interstitial cylinder system for HDR brachytherapy of gynecological cancer

AbstractPurposeThis paper outlines the commissioning of the Varian (VMS, Varian Medical Systems, Palo Alto, CA) Universal Interstitial Cylinder (UIC) applicator set for Ir‐192 HDR brachytherapy. The UIC was commissioned for use with CT and MRI and a custom phantom was designed to avoid the introduction of water‐like materials into the needle guide tracks. Various marker strands were investigated to determine which allowed the most accurate reconstruction of source positions.MethodsPlanar kV and MV imaging, along with physical measurements and autoradiographs, were used to commission the physical dimensions of all components of the UIC applicator set. CT and MR imaging was used to further commission one configuration of the UIC with UCP and eight interstitial needles in a simulated clinical setup using a GYN phantom. Three different methods of channel identification were compared – no radio opaque markers, VMS numbered markers, or nylon coated stainless steel leader wires – to see which best aided in channel identification and image registration. An HDR MRI Lumen marker (C4 Imaging, LLC) was used to verify any applicator rotation on MR scans during image registration. Three types of GYN phantoms were investigated – wet towel, gelatin, and ground beef. Dimensions of all components were compared with vendor provided information, including the solid applicator models, which are based on the computer‐aided design model files of the specific applicators.ResultsThe dimensions of the applicators could be validated using physical measurements, kV and MV planar imaging, and CT scans. The ground beef based GYN phantom best eliminated the introduction of water into the needle guide tracks that was found when using a water or gel‐based phantom. CT scans using no radio opaque markers did not allow the plastic needles to be visualized well enough to digitize source positions. CT scans with VMS markers showed significant artifact. CT scans with the nylon coated stainless steel wires provided the best visibility of the needle locations to aid in digitizing source positions. The use of an MR marker allowed the channel to be identified on the MR scan and confirm rotation for image registration.ConclusionsThe UIC set and applicator configuration was commissioned for CT and MR based treatment planning. The plastic components of the UIC applicator set pose challenges to the commissioning process but the use of radio opaque markers seen on CT combined with MR image registration allow the source positions within the needles, as well as the location of the end of the needles, to be digitized appropriately. A ground beef phantom minimized the fluid introduced into the needle guide track, minimizing any unintended MR and CT signal in the needle guide tracks.