ESTRO 2024 - Abstract Book

S3656

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

rotation angle from control point to control point has to be restricted, as large changes deteriorate treatment plan quality. In addition, the number of beamlet dose calculation substantially increases with the large number of control points used for VMAT. Thus, for VMAT the collimator angle settings per control point were pre-determined. For this purpose, the area between the PTV structure extended by an isotropic margin of 0.5 cm and the conformal MLC opening (whitespace area) is determined as a function of the gantry and collimator angle settings. This leads to a gantry-collimator whitespace area map to which an A* path finding algorithm with a maximum gradient of 4°/5° (collimator/gantry) was applied to determine the optimized gantry-collimator path. This path was then passed to the hDAO for optimization. For the resulting optimized apertures, a final dose calculation followed by a re-optimization of the MU-weights of the apertures was performed using MC dose calculations and the implementation of the ETHOS beamline. The developed optimization framework was first tested for an academic situation. For this purpose, a c-shaped planning target volume (PTV) structure enclosing a spherical organ at risk (OAR) structure were placed in a cylindric water phantom with diameter and height of 30 cm. A second test included a clinically motived head and neck cancer (HNC) as well as a prostate cancer case. 12 field IMRT and 2 arc VMAT treatment plans were generated with both static and dynamic collimator angles. Final dose distributions were compared based on dose volume histogram (DVH) parameters. For the academic phantom, by applying the dynamic collimator approach, the homogeneity index (HI) for VMAT and IMRT was improved by 14% and 22% respectively, while for the conformity index (CI) as defined by Van’t Riet and Paddick [1, 2], the improvement for VMAT and IMRT was 4% and 11%, respectively. Thereby, the mean OAR dose was increased by 1% for VMAT and reduced by 5% for IMRT. For the clinically motivated HNC and prostate cases, the benefit of the dynamic collimator was somewhat mixed. For the HNC case, the improvement of the HI for VMAT and IMRT was 4% and 14% respectively. In the prostate case, the HI decreased for VMAT by 8% but improved for IMRT by 13%. The improvement for the CI in the HNC case for VMAT and IMRT was 1% and 6% respectively. For the prostate case, the CI for VMAT decreased by 4% but improved for IMRT by 5%. For the two clinically motivated cases no differences in OAR dose were found, except for the right parotid for which the mean dose decreased by 4% for VMAT and 3% for IMRT. Results:

Conclusion:

A optimization framework for VMAT and IMRT treatment plans with dynamic collimator rotation for the O-ring ETHOS or Halcyon treatment system was successfully developed. Treatment plan quality improvements are more pronounced for IMRT compared to VMAT for the cases investigated in this study. The framework can now be used to investigate the potential benefit of dynamic collimator radiotherapy for a larger patient cohort. Supported by Varian, a Siemens Healthineers Company.

Keywords: VMAT, dynamic collimator rotation, Monte Carlo

References:

[1] Van't Riet A., Mak A.C., Moerland M.A., Elders L.H. and van der Zee W. (1997), A conformation number to quantify the degree of conformality in brachytherapy and external beam irradiation: Application to the prostate. Int. J. Radiat. Oncol. Biol. Phys., 37:731–736.