Room: Exhibit Hall | Forum 4
Purpose: A 50 kV X-Ray Source (XRS) with spherical applicators is used to deliver IntraOperative Radiotherapy (IORT). The commissioning of this system requires an ion chamber with a small sensitive volume, and pre-2000 these measurements were taken with the PTW TN23342 which under-estimated the depth dose gradient and informally became known as the â€œTARGIT calibrationâ€?. Advancement in chamber design, allow for more accurate depth dose measurements, and the new calibration is known as â€œV4.0â€?. The goal of this study is to validate V4.0 calibration, characterize the dosimetric and radiobiological differences.
Methods: Ionization chamber measurements were performed with a PTW TN34013 and compared with the TARGIT and V4.0 calibration. For spherical applicators (1.5 to 5 cm), we determined target dose, treatment time and calculated Equivalent Dose 2Gy (EQD2).
Results: At all depths investigated, V4.0 calibration demonstrated better agreement with measurement data. Therefore, when prescribing 20 Gy with the TARGIT calibration we're delivering 23.2 to 28.6 Gy at the surface of the applicator and 4.5 to 8 Gy at 1cm depth; the dose difference are largest for the smallest applicator. Further, for clinical endpoints of local-regional relapse we observed a significant increase in delivered EQD2 at the prescription point and at 1 cm depth. At the prescription point a 1.5 cm applicator delivered 28.6 Gy and has an EQD2 that is 95% greater; larger differences such as these are noteworthy when considering tumor control probabilities (TCP).
Conclusion: Our presented framework allows the calibrations to be compared on a dosimetric and radiobiological perspective.