ESTRO 2021 Abstract Book

S1445

ESTRO 2021

Measurements performed with the ICProfiler TM detector revealed a discrepancy of 1.9 mm in the beam centre position between both opposite collimator angles for the 6FFF MV beam. Along successive iterations adjusting 2T I Ctrl and 2T Bal parameters, such discrepancy was reduced without compromising the beam symmetry. BB measurements confirmed a reduction of the focal spot misalignment: average lateral CAX displacement decreased from Δ=1.68 mm to Δ=0.31 mm (Fig.1). Importantly, we noticed that this adjustment had a beneficial impact in one of our clinical SBRT treatmentsas we observed an improvement in 2D dose profiles during treatment verification (Fig.2).

Figure 1. Field CAX (a) before and (b) after beam focal spot misalignment correction.

Figure 2. 2D dose profile (a) before and (b) after beam focal spot misalignment correction.

Conclusion The procedure we followed in our clinic to detect and correct misalignments of the beam focal spot is straightforward, fast, and can be easily implemented. According to our experience, verification of the focal spot should be included in the routine QA tests on a regular basis. Misalignments of the focal spot may imply a detriment in the quality of clinical treatments, being of particular importance for SBRT treatments where high dose gradients are required. PO-1720 SunCHECK Machine, an automated QA software solution: A centres 5 year experience evaluation G. Martin 1 , K. Fogarty 2,1 , D. Egleston 1 , L. Howard 1 , M. Gilmore 1 1 The Clatterbridge Cancer Centre, Medical Physics, Liverpool, United Kingdom; 2 St. Lukes Radiation Oncology Network, Medical Physics, Dublin, Ireland Purpose or Objective Evaluate the key stages of the SunCHECK Machine implementation, including; initial viability, sensitivity testing, commissioning, long term use with 9 linacs and time saving quantification. Materials and Methods Initial viability: Qualitatively compare the results of automated and manual analysis for MLC based QA only. Sensitivity testing: For a broader range of tests, performance was manually adjusted using independent methods to be outside of expected level/s. Tests included; picket fence (0.5mm leaf offset), VMAT QA – dose rate and gantry speed (DR+GS) first section 9.01% lower (all sections ideally equal) and leaf speed (LS) (first section 2.01% lower), Winston-Lutz (WL) (1mm X, Y and Z offset), flatness (1cm sheets of solid water introduced at G180), symmetry (beam symmetry steered 1%, 2% and 3% using Starcheck MAXI ) and field size adjusted in 1mm increments from 26.7cm to 27.3cm. Commissioning: QA patients were produced for each QA frequency, with a QA plan for each test (details of QA included in table attached), baselines acquired, SunCHECK analysis templates produced, baselines uploaded and automation tested. Long term use: The software has been used on 9 linacs for 1 year and now evaluated. Time saved: The time taken to complete QA measurements and analysis using legacy and SunCHECK Machine was compared (details in table).