ESTRO 2021 Abstract Book

S1334

ESTRO 2021

Shenzhen Hosipital, Radiotherapy Department, Shenzhen, China; 3 PeKing University Shenzhen Hosipital, Radiotheray Department, Shenzhen, China

Purpose or Objective The MU delivered per field size, i.e., MU density, may affect the stability of patient-specific QA measurement, especially when the MU density is very small. The aim of this study is to investigate the stability of gamma passing rate of patient-specific QA measurement using EPID and Delta4 in a wide range of MU density. Materials and Methods Virtual phantom: Square fields with the field size of 4×4cm² to14×14cm² were created using MLC in virtual water phantom in the Eclipse treamtent planning sytem (Varian medical systems, USA). MU ranging from 12MU to 400 MU were delivered in the fields above. Therefore, MU density was ranged from 0.8MU/cm to 100MU/cm .Subsequently, dose verification were conducted on a Trilogy linac (Varian medical systems, USA) using EPID (Type aS1000) and Delta (ScandiDos AB, Sweden) respectively. Gamma passing rate (3%/3mm,local gamma) were calculated using the portal dosimetry or Delta4 software analysis. Patient plans: 39 patients treated with volumetric modulated arc therapy plans were retrospectively selected and patient- specific QA meausrements were conducted using EPID and Delta4. Gamma passing rate (3%/3mm,local gamma) was calculated for both EPID and Delta4. The results were compared and analyzed. Repeat the procedure of the above using only half of the MUs delivered in the patient-specific QA, which meant MU density was only half of the original plans. The gamma passing rate was compared between two different MU densities in EPID and Delta4, respectively. SPSS 26.0 software (IBM, Armonk, NY) was used for data analysis. Results As shown in Figure 1, the gamma passing rates of virtual phantom tests were stabilized for larger MU density. However, for EPID measurements, when MU density was less than 12MU/cm, the gamma passing rate dropped dramatically. For Delta4, the drop did not occur until MU density decreased to 2MU/cm. For patient QA measurements using EPID, similar results were observed, and the gamma passing rate of the half MU density group is lower than the the original MU density group (96.48±1.78% (Mean±SD) vs 96.81±1.65%, p<0.001 ) .For example, as shown in Fig 2, the gamma passing rate with the original MU density was 97.5% (Figure 2(a)), but it dropped to 95.2% when only the half MU density was applied(Figure 2(b)). In addition, similar failure pattern was observed for those two MU density, and lower MU density shows lower gamma passing rate. For patient QA measurements using Delta4, there is no significant difference between the two MU density groups (99.50±0.54% vs 99.52±0.50%, p=0.266).