SU‐E‐T‐454: Comparative Dosimetry of the Xoft Cervical Applicator and HDR Ir‐192 Henschke Applicator

Purpose: To evaluate HDR dosimetry for the Xoft Axxent ™cervical tandem and ovoid (T&O) applicator using the 50‐kV electronic brachytherapy (eBx) source. The titanium wall of this Henschke‐style applicator filters lower energies which flattens the radial dose function. Dose distributions around the T&O for 50‐kV‐Ti were investigated with comparison to Ir‐192. Methods: TG‐43u parameters were derived from film measurements of the 50‐kV‐Ti. To account for varying filtration effects on anisotropy from tip to barrel in the tandem, a mixed source model using the anisotropy at two dwell positions was implemented. Stylized geometry of cervical PTV, rectum and bladder were created on a CT scan of a gelatin phantom with the 15‐degree T&O. Plans for each source were optimized on BrachyVision™ using identical constraints and normalization. The GMP‐192 source was used for the Ir‐192 plans. Results: Both plans were normalized at 6‐Gy (D95) and have classic pear‐shaped doses. Doses at Points A‐H are within 5% for each plan, but are 20% lower for 50‐kV‐Ti at Point B. Along the tandem length peak Ir‐192 doses are 50% higher. The Ir‐192 dose dip at the tandem tip is absent for 50‐kV‐Ti. PTV DVHs are similar except for the effect of the peak tandem surface doses. PTV coverage (D95) for both is 6 Gy (+/− 1%) and the median dose for both was 950 cGy (+/− 2%). The peak dose region (D5) is 14% lower for 50‐kV‐Ti (18.1 Gy vs 20.7 Gy). D5 of rectum and bladder are 13% and 20% respectively for 50‐kV‐Ti and Ir‐192. Median doses to critical structures are 18–20% lower for 50‐kV‐Ti. Conclusion: PTV doses for Ir‐192 HDR and 50‐kV‐Ti eBx have similar target coverage, except at the tandem surface where doses were lower for eBx. Critical organs dose outside the PTV will be lower for eBx. Funding for this study was providing by Xoft, Inc — an iCad subsidiary