3102 - Monte Carlo Simulation and IMRT Plan Optimization for Dynamically Configurable X-Ray Compensator

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 30

POSTER

Presenter(s)

Billy Loo, MD, PhD, FASTRO - Stanford University, Stanford, CA

C. Qiu¹, A. M. Rauf², D. Chen¹, V. Maradia¹, S. Charyyev¹, S. W. Follmer², and B. W. Loo Jr¹; ¹Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, ²Department of Mechanical Engineering, Stanford University, Stanford, CA

Purpose/Objective(s):

To deliver IMRT rapidly for reducing motion-induced dosimetric uncertainty, we proposed as a replacement for multi-leaf collimators (MLCs) a dynamically configurable X-ray compensator, comprising a multi-layer pin array rather than conventional MLC leaves, perpendicular to the beam axis. This architecture modulates the full-field beam all at once rather than by sequential binary apertures, compatible with much higher dose rate linacs. The objective of this study was to model the dosimetric performance of this configurable compensator in inverse planned IMRT.

Materials/Methods:

A 6-MV flattened photon phase-space source was used to simulate the X-ray beam. A 20-cm cubic water phantom was modeled, containing a 3-cm-diameter water-equivalent spherical planning target volume (PTV) generated by adding a 3-mm margin to the clinical target volume (CTV). The source-to-axis distance (SAD) was set to 100 cm, and the center of the multi-layer pin array was positioned 50 cm from the source along the beam axis. The pin-array architecture consisted of 10 tungsten-alloy layers. Each pin measured 12 cm x 2 mm with a thickness of 6.9 mm, separated by 1-mm inter-layer gaps and 0.5-mm inter-pin spacing. A staggered offset configuration was introduced in alternating pin layers to suppress inter-pin leakage, similar to tongue-and-groove mitigation strategies in MLCs.

TOPAS Monte Carlo simulations were performed to generate dose maps for 60 candidate beam directions evenly distributed along a single coplanar arc with 6-degree angular spacing. For each direction, 10 beams corresponding to the pin layers were modeled, resulting in 600 candidate beam configurations.

An objective function was formulated to penalize the quadratic difference between the delivered and prescribed (2 Gy) doses within the PTV while minimizing the dose to a virtual ring structure located outside the PTV, separated by a 4-mm gap and having a 6-mm radial thickness. The fast iterative shrinkage thresholding algorithm was used to solve the function and determine the optimal directions, layers, and intensity weights.

Results:

After normalization to PTV D95 = prescription dose, the optimized plan achieved PTV V95% of 98.9%, with a Paddick conformity index of 0.94. The gradient index was 2.77, indicating controlled dose falloff.

Conclusion:

This study demonstrates the dosimetric performance of a dynamically configurable X-ray compensator based on a pin-array architecture for IMRT of a simple exemplary case. The multi-layer attenuation-based modulation approach achieved high target coverage and conformality, demonstrating potential for clinical translation.