Feasibility study for marker‐based VMAT plan optimization toward tumor tracking

Abstract

This work investigates the incorporation of fiducial marker-based visibility parameters into the optimization of volumetric modulated arc therapy (VMAT) plans. We propose that via this incorporation, one may produce treatment plans that aid realtime tumor tracking approaches employing exit imaging of the therapeutic beam (e.g., via EPID), in addition to satisfying purely dosimetric requirements. We investigated the feasibility of this approach for a thorax and prostate site using optimization software (MonArc). For a thorax phantom and a lung patient, three fiducial markers were inserted around the tumor and VMAT plans were created with two partial arcs and prescription dose of 48 Gy (4 fractions). For a prostate patient with three markers in the prostate organ, a VMAT plan was created with two partial arcs and prescription dose 72.8 Gy (28 fractions). We modified MonArc to include marker-based visibility constraints ("hard"and "soft"). A hard constraint (HC) imposes full visibility for all markers, while a soft constraint (SC) penalizes visibility for specific markers in the beams-eye-view. Dose distributions from constrained plans (HC and SC) were compared to the reference nonconstrained (NC) plan using metrics including conformity index (CI), homogeneity index (HI), gradient measure (GM), and dose to 95% of planning target volume (PTV) and organs at risk (OARs). The NC plan produced the best target conformity and the least doses to the OARs for the entire dataset, followed by the SC and HC plans. Using SC plans provided acceptable dosimetric tolerances for both the target and OARs. However, OAR doses may be increased or decreased based on the constrained marker location and number of trackable markers. In conclusion, we demonstrate that visibility constraints can be incorporated into the optimization together with dosimetric objectives to produce treatment plans satisfying both objectives. This approach should ensure greater clinical success when applying real-time tracking algorithms, using VMAT delivery.

Constrained optimization towards marker-based tumor tracking in VMAT

“…Initially, to establish the quality of the treatment plans produced by MonArc, we compared clinical VMAT plans from Eclipse TPS to optimized non-constrained plans developed in MonArc on the same dataset, in a previous study [15]. We observed that MonArc-based plans were fairly inferior in terms of dose conformity to the targets and OARs.…”

“…MonArc uses a cost function that determines the optimal shapes and weights for all the MLC field apertures in a VMAT delivery, due to the user-prescribed target dose requirements and OAR constraints. As described in our prior work [15], we modified the script to incorporate fiducial marker visibility in the BEV of each MLC aperture that accompanies individual control points in the plan, by adding visibility constraint parameters to the conventional dosimetric term evaluation in the cost function [15]. All MonArc optimized plans were imported into Eclipse ® (Progressive Resolution Optimizer v13.6.23: Varian Medical Systems, USA) and doses recalculated with the Acuros XB (Acuros External Beam, v13.6.23) algorithm, using the dose visualization and comparison tools readily available in the commercial TPS.…”

“…conventional plans developed using only dosimetric constraints), to analyze the discrepancies. Preliminary feasibility investigations of VMAT plans developed with the added visibility constraints were completed as described in our previous work [15] and are briefly reviewed in section 2.…”

“…In a feasibility study [15], we established the viability of this technique on a dynamic thorax phantom and one prostate and lung patient dataset, which all had three fiducial markers inserted/implanted. For a more detailed clinical analysis in this work we now examine on five liver, ten prostate and five lung patient datasets respectively, with different tumor sizes and geometric characteristics, to validate our approach.…”

“…In our previous work [15], we customized the radiotherapy optimization development software (MonArc) to establish the feasibility of combining dosimetric and marker visibility constraints for optimized VMAT treatment planning. We demonstrated the basic practicality of this approach and produced treatment plans that were clinically acceptable in terms of the required dosimetric tolerances but also ensured minimum levels of fiducial marker visibility.…”