ESTRO 2024 - Abstract Book

S4786

Physics - Quality assurance and auditing

ESTRO 2024

guidelines outlined in the AAPM TG-218 report, which stipulates the appropriate use of gamma-index for the PSQA procedure.

Material/Methods:

The Uganda Cancer Institute (UCI) commenced IMRT and VMAT treatments in April-2022 and May-2023 respectively. The analysis was divided into three treatment regions: Head&Neck (nasopharyngeal, oral cancers, etc.) thoracic (breast, esophageal, etc), and pelvis (prostate, cervical and rectal cancers). The IMRT data analysis included the first 95 patients, totaling to 147 plans, while the VMAT analysis included the first 67 patients, totaling to 67 plans. At IMRT commencement, most plans were planned using sequential fractionation compared to VMAT plans where SIB techniques were used. The planning and treatments were done with TrueBeam accelerator, Eclipse TPS 16.1, utilizing the Anisotropic Analytical Algorithm, 2.5 mm grid and 6/10 MV. For each plan a verification plan was created using the TPS, scheduled/irradiation accomplished and subsequently obtaining the measured dose data. The MatriXX Software, is an IMRT/VMAT PSQA system manufactured by Iba dosimetry, 2D-array of 1521 ionization chambers and can cover 25.3 x 25.3 cm2 (6.5 mm resolution) field-size at the isocenter. The Portal Dosimetry system uses aSi-1200 EPID, is also a 2D IMRT/VMAT PSQA system manufactured by Varian medical system, with a 0.34 mm resolution and can cover an active field-size of 40.0 x 40.0 cm2 at the isocenter. The portal dosimetry license was not available at IMRT commencement. The process involves transferring the generated treatment plans and measured dose files to the two software for each patient verification, calculating the gamma-pass--rate. A TL is defined as the boundary within which a process is considered to be operating normally, while the AL is the amount the quality-measure is allowed to deviate without risking harm to the patient also defining limit value for when clinical action may be required. The determination of the TL and AL involves the calculation of the means and variance from the IMRT/VMAT PSQA measurements over a time-frame. The gamma-pass-rates were computed at 3% dose-difference and 3mm dose-to-agreement. The TL and AL were determined for each group according to TG-218 guidelines using the equations below:

Where X̅ is the group mean gamma-pass-rate, σ2 is the group variance, T is the target value, β is a constant value = 6.0 and n is the number of plans in any group. The control limits mark the tolerance limits. Analysis was done to compare differences among the different groups.

Results:

The distribution of the analyzed Head&Neck, Thoracic and pelvis plans were 34 (23.1%), 30 (20.4%), and 83 (56.5%) respectively for IMRT while they were 21 (31.3%), 12 (17.9%), and 34 (50.8%) respectively for VMAT. The computed IMRT TL were 83.7%, 85.7% and 86.6% for Head&Neck, Thoracic and pelvis plans respectively while for VMAT they were 96.6%, 98.8% and 99.2% respectively. The MatriXX measurements lower gamma passing rate is due to its spatial resolution vs portal dosimetry but not IMRT vs VMAT. The mean gamma-pass-rate, TL and AL were all lowest for Head&Neck plans, followed by Thoracic and highest with pelvis plans (Figure-1).