Purpose: To quantify a novel method intended for quick adaptation of VMAT plans. Based on the changes of filtered radon transformations of dose distributions – differences in the pseudofluence (DPF) - related to patient’s changes of geometry a procedure will be developed. Therefore, the quantification of the correlation between the angular dependent dose projections, DPF and resulting field shape changes needs to be quantified.
Methods: In a planning study, primary geometries of a planning target volume (pPTV) and organs at risk (pOARs) in a phantom were systematically translated or deformed towards a secondary sPTV and sOARs. The PTV is concave, partially surrounding one of the OARs. A primary optimized plan was further optimized to adapt the dose distribution to sPTV and sOARs. The adapted plan was controlled towards a newly optimized plan to secure its sufficient quality. DPF was generated from primary and secondary dose distribution. Its peaks were related to the shifts of the multi-leaf-collimator (MLC) position from primary to the secondary plans. All beams from the same direction and from opposed directions were involved. DPF and monitor unit -weighted differences of leaf positions were compared.
Results: A dependence of leaf position changes and DPF is derived and is presented. Negative DPF correlate with the increase of open beam areas, negative DPF with MLC shifts decreasing open beam areas. Integrals of DPF-peaks and MLC-shifts are both linearly growing with structure translations. The resulting calibration function can predict leaf motions in deformation cases.
Conclusion: A correlation between leaf position changes of the MLC and the pseudo fluence is quantified and first predictions for beam adaption for simple OAR/ target combinations are shown. In future, derived dependence will be used to quickly adapt VMAT plans without the need to reoptimize plans.
Funding Support, Disclosures, and Conflict of Interest: Work was supported and partially funded by Elekta AB (Stockholm, Sweden)
Radiation Therapy, Optimization, Deformation