Truebeam accelerator

When all QA-steps were completed, the MRI-only treatment plan was approved and passed on to daily treatment on a TrueBeam accelerator (Varian Medical systems, Palo Alto, California, USA). Patients were positioned with corresponding fixation as during MRI and aligned using the patient tattoos. Set-up verification was performed with daily kilo voltage (kV)-image registration. The synthetic GFMs, represented in the sCT-DRR, were manually registered towards the physical GFMs seen in the orthogonal kV-image pairs. Operators were instructed to match the center of the GFM in the orthogonal kV-images to the center of the corresponding synthetic GFM in the sCT-DRR. From the eleventh patient and forward, the cylindrical GFM shape was added to the sCT as a RT structure around the synthetic GFMs. This was an attempt to facilitate easier detection of prostate and GFM rotations.

The same set of beam data (including percentage depth dose curve, profiles and output factors) used by AAA and measured in a three-dimensional water scanning system (PTW, Germany) for field sizes from 3 × 3 to 40 × 40 cm² were imported in Eclipse treatment planning system (Version 10.0, Varian Medical Systems, Palo Alto, CA) for the configuration of AXB. All data presented in this study were collected from a commissioned Varian Truebeam™ accelerator equipped with a Millennium 120 multileaf collimator (MLC, with spatial resolution of 5 and 10 mm for the central and outer 20 cm, respectively.

The commercial TPS evaluated in this study is Varian Eclipse Version 11.0 (Varian Medical Systems, Palo Alto, CA, USA) with the model‐based AAA dose calculation algorithm. The 6, 10, and 15 MV photon beams from a Varian TrueBeam accelerator are used in the study. The heterogeneity correction is employed in all AAA dose calculations; and, unless otherwise specified, the calculation grid size is 2.5 mm, which is a typical size in clinical practice. Although the grid size may affect the skin dose calculation accuracy, switching from 2.5 mm grid size to 1 mm grid size, which is smallest for AAA in Eclipse, only slightly improves the accuracy.20 In addition, the calculation time is much longer with 1 mm grid size, making it clinically unattractive. Furthermore, the grid size for MC dose calculations in this study (see below) is also set at 2.5 mm, which makes the comparison meaningful and justified. To avoid confusion, the skin dose is defined in this study as the mean dose to the skin structure of 5 mm thickness for the CT based dose calculations. To quantify the skin dose, the skin was contoured to be an area of 2 × 2 cm², corresponding to a volume of about 2 cm³, which is of clinical interest. The skin dose predicted by Eclipse is compared with that of MC calculations which are benchmarked by measurements in phantoms. The term “entrance dose” is used for the phantom measurements.

Dose measurements were performed using gafchromic MD-V3 film, which had been tested in a wide dose range, namely 1–100 Gy [53 (link),54 (link)]. Using the Truebeam accelerator (Varian Medical Systems, Inc., of Palo Alto, CA, USA), we constructed a calibration curve by irradiating films with known doses of 0, 1, 3, 5, 10, 15, 20, 30, 40, and 50 Gy. The linearity of dose and exposure with Monitor Units (MU) were also checked.
CT scans of every setup configuration were acquired with a LightSpeed™ Pro16 (General Electric Medical Systems, Waukesha, WI, USA) and fed to a TPS (Eclipse, Varian Medical Systems, Inc., of Palo Alto, CA, USA) to calculate the MU required for the irradiation of each sample. Gafchromic films were placed under the holder samples (flask or uvette) to check the accuracy of the dose delivery.

On the Eclipse treatment planning system V13.5, the aforementioned RapidPlan model was applied to estimate the best achievable DVHs under five photon beam qualities from a Varian TrueBeam accelerator equipped with Millennium 120 multi-leaf collimator (MLC), including 6-MV flattened (6X), 6-MV flattening-filter-free mode (6F), 8X, 10X and 10F respectively, for 20 historical patients that were not included in the model library. Higher energies are not used at our center for the consideration of secondary neutron contamination [19 ], hence were not tested in this study. Without any human intervention, VMAT plans were optimized using the RapidPlan-generated patient-specific objectives [20 ], keeping the original beam geometries of the clinical plans unchanged. The prescription dose was 41.8Gy for PTV and 50.6Gy for PTV_boost. The volume dose was calculated using analytical anisotropic algorithm (AAA). All plans were normalized to cover 95% target volume with 100% prescription dose before comparison.

For pre‐clinical validation, 17 anonymized intensity‐modulated treatment plans were selected for analysis. All plans were generated in Eclipse. Dose was calculated with the AcurosXB algorithm (v11). The treatment machine for all plans was a Varian TrueBeam accelerator with high‐definition (0.25 and 0.5 cm) MLCs. The accelerator is equipped with 6 and 10 MV flattened photon beams. Plans were chosen to encompass a clinically relevant variety. Nine plans used volumetric modulated arcs (VMAT) for delivery. Of the VMAT plans, six used a beam energy of 6 MV, and three used a beam energy of 10 MV. The eight remaining plans utilized sliding window intensity‐modulation (IMRT) at static gantry angles for delivery. Of the IMRT plans, two used 6 MV, three used 10 MV, and three were mixed‐beam (6 and 10 MV). Both IMRT and VMAT plans incorporated tracking of the primary jaws to minimize MLC leakage. Treatment plan characteristics for these 17 cases can be found in Table 1.
For the post‐implementation clinical plan comparison, the first 36 treatment plans evaluated using Mobius were chosen for inclusion in this study. The only requirement for inclusion was intensity modulation; this included IMRT as well as VMAT. Of these 36 clinical cases, 33 used a beam energy of 6 MV, one used a beam energy of 10 MV, and two used mixed energy (6 and 10 MV).