ESTRO 38 Abstract book

S582 ESTRO 38

Québec, Canada ; 4 The University of Texas, MD Anderson Cancer Center, Houston, USA Purpose or Objective The purpose of this study is to evaluate source tracking in brachytherapy using two state of the art detectors: 1) an inorganic scintillator detector (ISD) and a multipoint plastic scintillation detector (mPSD). The former is significantly energy dependent, while the latter is subject to the stem effect due to lower scintillation intensities. The hypothesis of this study is that source tracking with sub-millimeter precision can be obtained by accounting for these effects. Material and Methods One ISD (CsI) and a mPSD composed by BCF-10, BCF-12, BCF-60 were placed in a 40x40x40cm 3 water phantom. The dosimeters were coupled to two independent data acquisition systems using optical fibers. The dose rates were read out at 40Hz (ISD) and 70kHz (mPSD). The dosimeters were surrounded by nine hollow plastic needles to provide a symmetric configuration for the two dosimeters, fig. 1. A treatment plan was used with ten source dwell positions in each needle and 5mm source steps. The dosimeters were placed in the center depth of the ten dwell positions. Irradiation was performed four times, two times with dwell times of 20s and two times with dwell times of 1s. First, the distances between the individual source positions and the individual dosimeters were determined based on the relation between dose rate and distance (TG43 formalism). The distances were compared to the expected distances between dwell positions and dosimeters based on the phantom geometry. For the ISD measurements, the distance offset was then separated in to a shift along the needle ( z ) and one radially away from the dosimeter ( r ) . This was done by making a joint virtual shift of all the dwell positions in a single needle to best match the measured dose rates to TG43 as described in [1].

Conclusion Source tracking using state of the art scintillation dosimeters can be done with a precision comparable to the free space of the source and dosimeters inside the needles (~0.2mm). The tracking methodology can be transformed into a real-time approach. [1] Johansen et al. Time-resolved in vivo dosimetry for source tracking in brachytherapy, Brachy. 17 (2018) 122- 132 PO-1047 Evaluation of ACE dose calculation model on HDR treatments delivered with a multichannel applicator M. Carrara 1 , P. Caricato 1 , T. Giandini 1 , C. Tenconi 2,3 , A. Cerrotta 4 , B. Papalardi 4 , F. Piccolo 4 , C. Fallai 4 , E. Pignoli 1 1 Fondazione IRCCS Istituto Nazionale dei Tumori, Medical Physics, Milan, Italy ; 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 1, Milan, Italy ; 3 University of Milan, Oncology and Haemato-oncology, Milan, Italy ; 4 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 2, Milan, Italy Purpose or Objective To investigate the performance of the Advanced Calculation Engine (ACE) (Oncentra Brachy 4.5) in comparison to the AAPM-TG43 calculations for 24 HDR vaginal brachytherapy treatments delivered with a multichannel vaginal applicator (MVA), since patient's anatomy and heterogeneities are not considered with the latter algorithm. Material and Methods After the commissioning of ACE according to the indications of the AAPM TG-186 recommendations, this dose calculation model was retrospectively applied on 24 vaginal HDR brachytherapy treatments delivered at our Institution with MVAs of two different diameters (i.e., 25 and 30 mm). The following volumes were all contoured: applicator, free air catheters, sigma, rectum, bladder, cortical bone, adipose tissue and remaining soft tissue, and their corresponding materials and priorities were assigned. Patient plans were then computed using both uniform and HU-based densities and compared with the TG-43 calculations in terms of relative differences between D0.1cc, D1cc and D2cc to rectum, sigmoid and bladder. D90%, D95%, D100%, V90%, V95%, V100% and V150% to the target were all compared, as well as the dose to a point P placed 5 mm above the applicator central tip. Results The average relative differences between the calculated parameters for the 24 patients are shown in figure. All mean differences resulted within ±2%, except the dose to the point P and the V150% to the target. In particular, the dose to P resulted higher if calculated with ACE, most probably due to the reduced attenuation by the air cavity of the central catheter. In some cases (i.e., sigmoid close over the applicator), this increased dose might result in a

Results The deviation between measured and expected distances for each dwell position were within 0.6mm (ISD) and 1mm (mPSD) at source-to-detector distances <40mm and within 1.1mm (ISD) and 1.8mm (mPSD) at larger distances (up to 58mm). Table 1 shows the radial ( r ) and longitudinal ( z ) shifts determined for each needle individually using the ISD. The maximum offset was r =0.49mm and z =-0.79mm. The maximum differences between the four measurements for a single needle was r =0.29mm (Needle 6) and z =0.34mm (Needle 5). The mean±SD offset across all needles and measurements were r =0.054±0.21mm, z =-0.22±0.27mm. This precision corresponds to the precision of the source positioning of the afterloader.