Abstract
Planning and delivery in HN‐IMRT has been challenging for the Elekta linac because of numerous machine limitations. Direct aperture optimization (DAO) algorithms have had success in simplifying the planning process and improving plan quality. Commercial adaptations of DAO allow for widespread use in many clinics; however clinical validation of these methods is still needed. In this work we evaluated Pinnacle3 commercial software for HN‐IMRT on the Elekta linac. The purpose was to find a set of planning parameters that are applicable to most patients and optimal in terms of plan quality, delivery efficiency, and dosimetric accuracy. Four types of plans were created for each of 12 patients: ideal fluence optimization (FO), conventional two‐step optimization (TS), segment weight optimization (SW), and direct machine parameter optimization (DMPO). Maximum number of segments (NS) and minimum segment area (MSA) were varied in DMPO. Results showed DMPO plans have the best optimization scores and dosimetric indices, and the most consistent IMRT output among patients. At larger NS (≥80), plan quality decreases with increasing MSA as expected, except for MSA<8 cm2, suggesting presence of local minima in DMPO. Segment area and MUs can vary significantly between optimization methods and parameter settings; however, the quantity ‘integral MU’ remains constant. Irradiation time is linearly proportional to total plan segments, weakly dependent on MUs and independent of MSA. Dosimetric accuracy is independent of DMPO parameters. The superior quality of DMPO makes it the choice for HN‐IMRT on Elekta linacs and its consistency allows development of ‘class solutions’. However, planners should be aware of the local minima issue when pushing parameters to the limit such as NS<80 and MSA<8 cm2. The optimal set of parameters should be chosen to balance plan quality and delivery efficiency based on a systematic evaluation of the planning technique and system constraints.PACS number: PACS: 87.55.D, 87.55.de
Highlights
44 Worthy et al.: HN-IMRT on Elekta Linac close proximity of critical structures to large and irregular target volumes.[4, 5] in simultaneous integrated boost (SIB) HN-IMRT plans, the primary tumor and subclinical disease are concurrently treated with one treatment plan
Results show that the improved target coverage and decreased plan scores seen for direct machine parameter optimization (DMPO) plans in comparison to segment weight optimization (SW) plans is statistically significant with all p-values
With respect to plan monitor units, beam-on time is only weakly dependent on the plan MUs with a Pearson correlation coefficient of 0.343. These results demonstrate that the estimation of beam-on time for HN-IMRT plans on an Elekta linac should be largely based on NSactual instead of the plan MUs, typically the dominant factor used for predicting treatment times for standard conformal radiation delivery
Summary
44 Worthy et al.: HN-IMRT on Elekta Linac close proximity of critical structures to large and irregular target volumes.[4, 5] in simultaneous integrated boost (SIB) HN-IMRT plans, the primary tumor and subclinical disease are concurrently treated with one treatment plan. The X1-diaphragm stops at the central axis G, such that the unwanted radiation is largely blocked by the MLC and backup Y-diaphragms In this approach, the MLC leaves are advanced past the open segment area (ABCD), so that the backup Y-diaphragm can shield the minimum gap (area AFGH) between the opposing MLC leaves.[15] The Y-diaphragms are much thinner than the X-diaphragms, having a thickness of only 3 cm.[16] the transmission through the minimum gap (AFGH) is ~10%, significantly larger than the 2% transmission through the MLC leaves alone (area ABEF) and the 0.5% transmission through the X-diaphragms.[19] Second, constraint (b) prevents MLC leaf inter-digitization, and the 1 cm minimum gap requirement makes leaf sequencing and optimization of segment shapes very difficult. Constraint (c) restricts segment shape possibilities by limiting leaf over travel distance, but to a much lesser extent in comparison to constraint (b) All of these adverse effects caused by the Elekta machine limitations lead to very complex HN-IMRT plans with large differences observed between optimized and deliverable dose distributions
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
