ESTRO 38 Abstract book

S940 ESTRO 38

volume averaging effects. For this reason, various small- field-suitable dosimeters (e.g. diodes, scintillators) are recommended in the literature in place of the large ionisation chambers typically used for beam configuration data measurements in larger fields. The objective of this study was to evaluate whether relatively large volume ionisation chambers, that are unsuitable for other small field applications, might be used for accurate TMR or PDD measurements. Material and Methods PDD and TMR measurements were performed in an IBA BluePhantom 3D water tank with 100 cm source-to-surface distance and 100 cm source-to-detector distance, respectively; using four dosimeters: PTW 60017 (Diode E), IBA CC04, PTW 31010 (Semiflex) and PTW 30013 (Farmer); with active volumes of 0.00003, 0.04, 0,125 and 0.6 cc. Data was acquired for four field sizes (0.5×0.5, 1×1, 2×2 and 3×3 cm²) produced using an Elekta linear accelerator. Lateral scans were repeated for each field size to verify the positioning of the dosimeter in the centre of the field. Dose was measured to depths of 25 cm. Measurement data was resampled to a resolution of 0.5 mm, where necessary, and smoothed with a mean filter with a 2 mm window. Root-mean-square deviations (RMSD) were calculated between diode measurements (the "gold standard" data) and measurements acquired using the other dosimeters. Results PDD and TPR measurement data varied with detector volume (see figure). The largest disagreement in PDD profiles existed between the PTW 60017 diode and PTW 30013 chamber measurements at the smallest field size, with an RMSD of 11.2%. The corresponding RMSD for TMR data was 2.5%. The impact of a larger active volume is reduced when measuring TMRs, where the field size varies minimally with depth.

Purpose or Objective Advanced radiotherapy dose verification using an EPID- based method such as a commercial software, Dosimetry Check (DC) has been extensively highlighted in recent years and its validity in verifying dose has becoming the major topic of interest in avoiding dosimetric errors [1- 4] . In this work, we emphasised on the implementation of the commercial available DC [5-6] dose reconstruction system to address the significant of nonuniform backscatter effect from the Varian aS1000 EPID arm [4-5] to clinical VMAT pretreatment Head-and-Neck. Material and Methods The feasibility of applying the developed backscatter correction method [7] to a clinical VMAT Head-and-Neck case was investigated. A phantom was used with an optimised VMAT irradiation beams and all plans were calculated in local TPS. The assessment included, i) improvement of the beam profiles along in-line direction for dose reconstructed by DC, ii) pass-rate of the gamma criteria of 3%/5mm and 3%/3mm and iii) pass rate for gamma volume index. Results By using the correction method, an average percentage difference for DC dose relative to TPS dose improved from 4.2% to 1.7% in the in-line profiles (Fig.1). For gamma evaluation of 3%/5mm calculated in DC, about 95% and 97% of points passing gamma at coronal and sagittal planes respectively whereas more than 85% of points passing gamma for 3%/3mm gamma criterion at both planes. Gamma volume index calculated by DC for a ROI outlined within the Head-and-Neck phantom also improved from 73% before correction to 87% after correction and from 89% to 95% for a 3%/3mm and 3%/5mm gamma criterion respectively (Table1).

Conclusion The results indicate that accurate TPR measurements may be obtained using dosimeters otherwise considered unsuitable for small field applications. While the PTW 30013 is not suitable for small field dosimetry, it provides a useful example of how TMR measurements are less sensitive to the active volume of the dosimeter than PDD measurements. If small-field-suitable dosimeters are unavailable, TMR measurements may provide more accurate beam characterisation data than PDD measurements. EP-1744 Enhancing the accuracy in VMAT dose verification by the use of EPID-based commercial software Y. Md Radzi 1 , R.S. Windle 2 , G.D. Lewis 2 , E. Spezi 3 1 Universiti Sains Malaysia, School of Physics, Penang, Malaysia ; 2 Velindre Cancer Centre, Department of Medical Physics, Cardiff, United Kingdom ; 3 Cardiff University, School of Physics, Cardiff, United Kingdom

Fig.1

Table 1 Conclusion

This ‘proof-of-concept’ of the novel correction shown to give benefit to pretreatment VMAT verification techniques especially to critical plan such as Head-and-Neck using DC as a dosimetric verification tool. Ultimately, the software