Author: Brett Bocian, Patrick James Jensen, Luis Augusto Perles, Christopher J. Tien 👨🔬
Affiliation: Yale University School of Medicine, University of Miami, The University of Texas MD Anderson Cancer Center, UT MD Anderson Cancer Center 🌍
Purpose: Guidelines for dose prescriptions in endometrial cancer are based upon depth from the cylinder, traditionally 5 mm or 0 mm (i.e., surface). There are no current single-channel vaginal cylinder (SCVC) designs that can produce dose distributions with uniform doses at prescription, particularly along the central axis at or near the SCVC tip. We aim to develop a filter that provides uniformity near the tip, based on results from computational Monte Carlo (MC) simulations.
Methods: Radiation dose distributions were calculated using TOPAS (scorer dimensions of 16x16 cm2, 0.1 mm/bin, 5x108 particle histories per dwell), with specialized components to model the filter (designed in AutoCAD), applicator, 192Ir radiation source (Varian GammaMed Plus), and surrounding water phantom ((30 cm)3). Five dwell positions, spaced 1 cm apart, were simulated separately. The initial filter design was guided by hand-tracing attenuation regions, which were subsequently refined using a pseudo-ray tracing method. Dose uniformity was evaluated based on the standard deviation of the doses received at prescription points. Plan robustness was evaluated by comparing isodose lines observed with +/- 1 mm of uncertainty in dwell positioning.
Results: Point dose along the central axis at the tip + 5 mm was found to be 1.13 Gy lower than the prescription dose (7 Gy). With filtering, the isodose lines demonstrated a more uniform dose distribution. The standard deviation of selected points at the prescribed dose distance with the filter decreased by 52.65% (showing these point doses are more uniform), while the point dose at tip + 5 mm increased from 5.87 Gy without the filter to 6.43 Gy.
Conclusion: Simulations of this filter show promise in increasing uniformity in dose distribution associated with SCVCs, particularly along the central axis. Further design changes, not shown in this study, may improve uniformity.