Anatomy- vs. fluence-based planning for prostate cancer treatments using VMAT

Plan quality comparison for cervical carcinoma treated with Halcyon and Trilogy intensity-modulated radiotherapy

Purpose: Varian Halcyon is a novel machine with dual-layer leaves, single flattening filter free (FFF) energy and an enclosed bore. The purpose of this study was to compare the differences in dosimetry and plan parameters of intensity-modulated radiation therapy (IMRT) plans between the Halcyon and Trilogy accelerators. Methods and Materials: A total of 30 IMRT plans from cervical carcinoma patients were retrospectively analyzed on the Trilogy and Eclipse v13.5 treatment planning systems (TPSs). For each patient, a new plan based on Halcyon was created with the same planning parameters and optimization constraints using the Eclipse Version 15.1 TPS. To compare plan qualities, dosimetry parameters regarding planning target volume (PTV), organs at risk (OARs), monitor unit (MU) efficiency, segment size and treatment time were evaluated. Evaluation of the helical diode array system was performed with gamma-index analysis. Results: The dose distribution of the target volume of the Halcyon and Trilogy plans showed no significant difference (p > 0.05). The mean doses of rectum and both femoral heads for Halcyon plans were significantly reduced compared to those for Trilogy plans (p < 0.05). Compared to Trilogy, Halcyon increased the number of MUs from 1542.9±248.3 MU to 2514.9±328.2 MU (p = 0.00) and decreased the delivery time from 11.28±1.36 min to 3.26±0.26 min (p = 0.00). The average segment areas of Halcyon plans for proximal and distal multileaf collimators (MLCs) were 42.1 ± 31.2 cm2 and 28.4 ± 23.7 cm2, respectively, and that of Trilogy plans was 27.3 ± 16.9 cm2. The mean gamma index (3 mm/3%) results for the Halcyon and Trilogy plans were 99.41±0.26 and 99.76±0.32 (p > 0.05), respectively. Conclusions: All Halcyon treatment plans were recognized as clinically acceptable and had statistically better OAR sparing with higher delivery efficiency. The Halcyon system exhibited fast treatment delivery of IMRT with good dosimetric agreement using ArcCHECK.

An automated Monte Carlo QC system for volumetric modulated arc therapy: Possibilities and challenges

PURPOSE

To develop and implement an automated Monte Carlo (MC) system for patient specific VMAT quality control in a patient geometry that generates treatment planning system (TPS) compliant DICOM objects and includes a module for 3D analysis of dose deviations. Also, the aims were to recommend diagnose specific tolerance criteria and an evaluation procedure.

METHODS

The EGSnrc code package formed the basis for development of the MC system. The workflow consists of a number of modules connected to a TPS by means of manual DICOM exports and imports which were executed sequentially without user interaction. DVH comparison was performed in the TPS. In addition, MC- and TPS dose distributions were analysed by applying the normalized dose difference (NDD) formalism. NDD failure maps and a pass rate for a certain threshold were obtained. 170 clinical plans (prostate, thorax, head-and-neck and gynecological) were selected for analysis.

RESULTS

Agreement within 1.5% was found between clinical- and MC data for the mean dose to the target volumes and within 3% for parameters more sensitive to the shape of the DVH e.g. D_98% PTV. Regarding the NDD analysis, tolerance criteria 2%/3 mm were established for prostate plans and 3%/3 mm for the rest of the cases.

CONCLUSIONS

An automated MC system was developed and implemented. Evaluation procedure is recommended with NDD-analysis as a first step. For pass rate < 95%, the evaluation continues with comparison of DVH parameters. For deviations larger than 2%, a visual inspection of the clinical- and MC dose distributions is performed.

Optimal beam margins in linac-based VMAT stereotactic ablative body radiotherapy: a Pareto front analysis for liver metastases

We explored the Pareto fronts mathematical strategy to determine the optimal block margin and prescription isodose for stereotactic body radiotherapy (SBRT) treatments of liver metastases using the volumetric-modulated arc therapy (VMAT) technique. Three targets (planning target volumes [PTVs] = 20, 55, and 101 cc) were selected. A single fraction dose of 26 Gy was prescribed (prescription dose [PD]). VMAT plans were generated for 3 different beam energies. Pareto fronts based on (1) different multileaf collimator (MLC) block margin around PTV and (2) different prescription isodose lines (IDL) were produced. For each block margin, the greatest IDL fulfilling the criteria (95% of PTV reached 100%) was considered as providing the optimal clinical plan for PTV coverage. Liver D_mean, V7Gy, and V12Gy were used against the PTV coverage to generate the fronts. Gradient indexes (GI and mGI), homogeneity index (HI), and healthy liver irradiation in terms of D_mean, V7Gy, and V12Gy were calculated to compare different plans. In addition, each target was also optimized with a full-inverse planning engine to obtain a direct comparison with anatomy-based treatment planning system (TPS) results. About 900 plans were calculated to generate the fronts. GI and mGI show a U-shaped behavior as a function of beam margin with minimal values obtained with a +1 mm MLC margin. For these plans, the IDL ranges from 74% to 86%. GI and mGI show also a V-shaped behavior with respect to HI index, with minimum values at 1 mm for all metrics, independent of tumor dimensions and beam energy. Full-inversed optimized plans reported worse results with respect to Pareto plans. In conclusion, Pareto fronts provide a rigorous strategy to choose clinical optimal plans in SBRT treatments. We show that a 1-mm MLC block margin provides the best results with regard to healthy liver tissue irradiation and steepness of dose fallout.