ESTRO 36 Abstract Book

S114 ESTRO 36 _______________________________________________________________________________________________

OC-0228 DVH criteria for prostate in vivo EPID dosimetry R.F.M. Van Oers 1 , E. Van der Bijl 1 , I. Olaciregui-Ruiz 1 , A. Mans 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective In our department in vivo EPID dosimetry is used for dose verification of all treatment plans. The algorithm uses EPID images acquired behind the patient to reconstruct the in vivo 3D dose distribution. This is then automatically compared to the planned dose distribution and alerts are generated when deviations are detected. These alerts are based on γ-analysis. Gamma values combine dose-difference and distance-to-agreement in a single metric, but this metric contains no information on the clinical relevance of deviations. Furthermore, γ- analysis can be insensitive to systematic under- or overdoses in plans with inhomogeneous dose distributions. Dose-volume histograms (DVHs) are widely used for evaluation of treatment plans, and readily understood by clinicians. Moreover, differences in DVH parameters can be linked more straightforwardly to clinical relevance. This makes DVH-based criteria an attractive alternative to γ-criteria for in vivo EPID dosimetry alerts. In this study we investigated the correlation between γ- and DVH-parameters of the PTV for prostate treatments, and compared the alert rates for different criteria in order to propose a suitable set of DVH-criteria for clinical implementation. Material and Methods In vivo 3D dose distributions were reconstructed for the first three fractions of 95 prostate VMAT treatments, and then averaged for the evaluation of each treatment. The γ-analysis was done with global 3%/3mm settings. DVHs were obtained for the PTV (prostate + seminal vesicles). Calculated γ-parameters were mean γ, the near-maximum γ (γ1%), and the γ-passrate (γ%<1); our current criteria also include the isocenter dose difference (ΔDisoc). Calculated DVH-parameters were the difference in near- maximum dose (ΔD2), median dose (ΔD50), and near- minimum dose (ΔD98). We obtained alert rates for different sets of criteria on these DVH-parameters. These were compared to alert rates for the current γ-based criteria. There are two alert levels, higher-priority “error” and lower-priority “warning”. Results The strongest correlation was found between γ-mean and |ΔD50|, Pearson’s r=0.95. All other γ- and DVH parameters were also strongly correlated, with r values around 0.85.

treatments, juxtaposed with sets of DVH-criteria of similar alert rate.

Table 1: Alert rates (% of treatments) for different sets of γ- and DVH-criteria. The top set of γ-criteria corresponds to “warning level” alerts, the bottom set corresponds to “error level” alerts. Conclusion A strong correlation was found between γ- and DVH- parameters of the PTV; a set of DVH-criteria that performs comparably to the current γ-criteria can easily be chosen. OC-0229 EPID dose response in the MR-Linac with and without presence of a magnetic field I. Torres Xirau 1 , I. Olaciregui-Ruiz 1 , B. J. Mijnheer 1 , U. A. van der Heide 1 , A. Mans 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Image-guided radiotherapy systems are being investigated and clinically implemented aiming for online and real-time adaptation of the treatment plan. The use of Electronic Portal Imaging Devices (EPIDs) for independent in vivo dose verification in the Elekta MR-Linac is being developed. One of the challenges for MR-Linac portal dosimetry is the presence of a small magnetic field at the EPID level. In the presence of a magnetic field, the secondary electrons that actually deposit the dose in the scintillator of the EPID will be affected by the Lorentz force possibly leading to a B-field induced dose redistribution. The aim of this study was to analyze and quantify the effects of the B-field on the EPID images acquired on the Elekta MR-Linac. Material and Methods The Elekta/Philips MR-Linac combines a 1.5T magnetic resonance imaging scanner with a linear accelerator and is equipped with an on-board EPID. A magnetometer (MetroLab THM1176) was used to measure the strength of the magnetic B-field at the surface of the EPID. To assess the reproducibility of the panel readouts, a 10x10 cm 2 field was irradiated 10 times in two consecutive days and the value of the on-axis region (averaged 5x5 pixels) of EPID images was recorded. During the installation of the MR-Linac in our institute, EPID images were acquired before the B-field was ramped up and repeated with B- field one month later. To study the on-axis response of the EPID as function of field size with and without the magnetic B-field, square fields were irradiated with field sizes varying from 2 to 20 cm. Furthermore, EPID images acquired with and without B-field were compared by means of a 2-D γ-analysis (local 2%,1mm, 20% isodose) and X-Y EPID lateral profiles were compared by visual inspection. Results The magnetic field measured on top of the panel did not exceed 2.5 mT, yielding an electron trajectory radius of approximately 1.20 m. The reproducibility of EPID central axis values for ten irradiated 10x10 cm 2 fields was 0.3% (1 SD). The normalized on-axis EPID response as function of field size with and without the presence of the magnetic field is shown in Figure 1 together with their ratio.

Figure 1: Relation between γ-mean and ΔD50 for the analyzed prostate VMAT plans. The indicated alerts are generated by the “error level” sets of γ- and DVH-criteria. Table 1 shows alert rates for different sets of γ- and DVH- criteria. The two highlighted sets of γ-criteria are the ones currently used in our clinic to generate alerts for prostate