ESTRO 2021 Abstract Book

S1463

ESTRO 2021

We collected the data generated during daily QA checks on 6 MV, 10 MV and 15 MV photon beams generated by two linacs, an Elekta Precise from 2005 to 2019 and an Elekta VersaHD during the 2020. The dosimetric systems for daily QA associated to these linacs were the PTW QC6 Plus and PTW QUICKCHECKwebline, respectively. Five machine QA parameters, output constancy (F), beam flatness (H), beam symmetry along gun target direction SGT) and left-right direction (SLR) and beam quality factor (BQF) were retrospectively analyzed. Shewhart charts and Exponentially Weighted Moving Average (EWMA) charts were obtained for each parameter. The central line (CL), the upper and lower control limits (UCL and LCL) and the process capability indexes, Cp and Cpk, were calculated. A Gage R&R study was also performed on our two dosimetric systems. Finally, an Ishikawa diagram was built to trace back the root causes for the failures in machine QA. Results SPC results reported that F, H, SLR, SGT and BQF metrics are within the specification limits for both linacs. For all parameters, EWMA charts are not found to be beyond control for any of the variables. Following the TG 142 QA protocol, all analyzed metrics were within the clinical tolerance range, indicating that the both linacs had never compromised the treatment quality. However, Shewhart charts and EWMA charts detected major differences in control for the two linacs, with charts related to the VersaHD linac reporting a considerably high level of control for all variables. For example, at a 2% specification level for the F linac output values, the process capability indexes Cp,k ranged from 1.11 to 1.27 for the Precise linac and from 3.35 to 3.63 for the VersaHD linac. These results indicated the actual processes and mechanical capabilities rather than the vendor recommendations and they have been used to established the tolerances levels for our QA practice. Figure 1 shows an example of SPC for the 6MV photon beams of both linacs.

Conclusion We investigated the usefulness of SPC for the effective control detection in dosimetric machine QA. SPC technique has proven to be an efficient strategy in order to define local action limits, enabling improvements in the QA process. PO-1741 A feasibility study: collimator angle, gantry spacing and sliding window can improve HNC plans? P. Ferrari 1 , M. Haller 1 , S. Bou Selman 2 , M. Rosa 2 , S. Hofer 1 , J.M. Waskiewicz 2 , M. Maffei 2 1 Azienda Sanitaria dell'Alto Adige, Servizio di Fisica Sanitaria, Bolzano, Italy; 2 Azienda Sanitaria dell'Alto Adige, Servizio di Radioterapia Oncologica del Comprensorio Sanitario di Bolzano, Bolzano, Italy Purpose or Objective To assess the performance of 6 HNC treatment plans determining the effect of collimator angle (CA), gantry spacing (GS) and sliding window sequencing (SW) on PTV coverage and organ at risk dose reduction Materials and Methods 12 patients underwent VMAT treatment planning techniques in head neck cancer (HNC), were retrospectively analyzed. 6 arcs were implemented in order to avoid shoulders (from arc angle 186° to 235°, 270° to 90°, 125° to 174°, clockwise and counterclockwise). Our reference plan is based on 3° GS, no SW, CA of 350° for CW arch's and of 10° for CC arch's (3°GS_350°10°). 5 plans with different combination of CA (improving experience of Kim (1)), GS and use of sliding window sequencing were compared with reference plan: 3°GS_100°10°, 2°GS_350°10°, 2°GS_100°10°, 2°GSSW350°10° and 2°GSSW100°10°. All plans were optimized using the same constrains. Statistical analysis in PTVs coverage and organ risk sparing was performed. Percentage changes in D98, D95, average, D1 and D2 for PTV and in DMAX, D1 and average for organ risks were calculated respect reference plan. Data were expressed as mean ± standard deviation. Unpaired Student’s t- test was used to compare inter-group means. A P-value equal to or less than 0.05 was considered significant.

Results