Truebeam 2

Measurements were made on a Truebeam^® 2.0 linear accelerator (Varian Medical System, Palo Alto, CA, USA) between January 2017 and May 2017.
Three measurement processes were assessed:

ArcCheck^® detection phantom associated with SNC patient™ software v6.7.1 (AC)

EPID As 1200^® associated with Portal Dose Image Prediction PDIP^® v13.0 (PDIP)

Combination of AC and PDIP which consisted of accumulating the verification information of the two systems (Combined).

The experiment was carried out in TrueBeam 2.0 (Varian Medical System Inc., Palo Alto, CA, USA) linear accelerator capable of delivering flattened and UF x ray beams of 6 MV (6FF), 10 MV (10FF), 15 MV (15FF) and 6 MV‐FFF (6UF), 10 MV‐FFF (10UF). Even though this study expresses the impact of high‐Z material on flattened and unflattened beams of respective energies, we have included additional flattened 15 MV to examine the impact of high‐Z material. In this study, we used stainless steel (SS316) and titanium alloy (Grade 5) high‐Z materials (Table 1) which are austenitic grades (nonmagnetic). These two high‐Z materials were studied to imitate the biocompatible generally used in implants. The effective atomic number (Z_eff) of stainless steel (SS316) and titanium alloy (Grade 5) are 29.23 and 22.15 and average mass number (A) of 56.32u, 46.7u, respectively. The composition of stainless steel (SS316) and titanium (Grade 5) material are as follows.19 Breadth, width, and thickness of stainless steel and titanium alloy dimensions are 3 × 3×2.5 cm³. A special RW3 slab of about 2.5 cm thickness was prepared to accommodate these high‐Z material inserts.

All patients were initially positioned via the conventional laser set-up procedure, in which patients were manually aligned so that the set-up fiducial lines marked on the immobilization head mask matched the laser crosslines prealigned to the isocenter of our LINAC system (TrueBeam 2.0, Varian Medical Systems, Palo Alto, CA). Patients were subsequently positioned by the CBCT image guidance set-up procedure, in which patients underwent CBCT scanning in the laser set-up position and were repositioned, if necessary, by 6D CBCT guidance. The laser set-up procedure was performed daily from the beginning to the end of the treatment, whereas the CBCT set-up procedure was performed prior to the first treatment and generally once per week thereafter. However, if the set-up difference was >2 mm in any axis, the CBCT set-up was performed on at least three consecutive days; if the difference persisted, the set-up fiducial lines were refined to the CBCT corrected position.
The positions of the laser and CBCT set-ups for each patient were logged in real time into the ARIA record and verification system (Varian Medical Systems). These records were extracted and analyzed in the study.