Abstract Book

ESTRO 37

S574

Conclusion The designed HDR-BT phantom and inserts have been evaluated in accordance with the ESTRO booklet 8 guidelines. Interests of this phantom are multiples; on the one hand machines quality controls can be done quickly and simply. Indeed, source dwell position and dose delivered can be made in the same acquisition. On the other hand, the 6-channels insert allows quantitative verification of patient treatment plans before irradiation (position and dose). PO-1023 Verifying brachytherapy applicator models using an imaging panel M. Van den Bosch 1 , G. Fonseca 1 , R. Voncken 1 , M. Bellezzo 1 , F. Verhaegen 1 1 MAASTRO Clinic, Department of Radiation Oncology, Maastricht, The Netherlands Purpose or Objective A common applicator to treat cervical cancer patients with High-Dose-Rate (HDR) brachytherapy is a ring applicator. The highly curved source channel may lead to snaking of the source cable. In case of snaking, the cable position deviates from the center of the channel resulting in a source positioning error. It is common practise to exclude the source positions close to the tip of the channel for planning. However, the snaking effect is not limited to the end of the channel. As a result, the true source position differs from the planned dwell position in the Treatment Planning System (TPS) when assuming the source trajectory is perfectly in the center of the source channel. In this study, we quantify the positioning error using a default TPS model (the source trajectory is at the center of the channel) and a modified source trajectory derived with imaging panel (IP) measurements. Material and Methods A ring applicator (Varian) was attached to a vertically placed IP to mimic the orientation of the applicator during a treatment. Above the applicator a needle was placed. In this needle the HDR source was temporarily positioned to generate a projection image of the applicator. Next, the source was sent to dwell positions 129.4cm to 121.3cm with detrimental steps of 3mm. The center of the source was estimated using a 2D Gaussian fit to the dose profile measured by the IP and projected on top of the applicator geometry to determine the true source trajectory. The final result is a 2D picture that shows both the applicator geometry as the true source trajectory. This picture is imported into the TPS. The source trajectory of the default TPS applicator model was adjusted according to the derived true source trajectory. The mean and maximum errors were determined for the default and modified TPS models. Results As shown in Fig 1 the true source trajectory is not at the center of the channel but touches the inner and outer channel edge. The mean and maximum error were 2.9 and 3.9mm, and 0.0 and 1.2mm for the default and the modified model, respectively. The remaining error of the modified model could partly be explained by snaking in the third dimension (out-of-plane). However, the default model improved when the dwell positions were shifted 3mm along the source trajectory towards the channel tip. The mean and maximum error dropped to 0.0 and 1.2mm, respectively. Fig 1c shows the error per dwell position.

Conclusion This study shows that the source trajectory can be determined accurately using an IP. The default TPS model could be improved by shifting the dwell positions by 3mm towards the channel tip. The mean and maximum error then equal our modified model. In the future, we want to repeat these experiments to verify the results are consistent in time. Furthermore, we want to derive the 3D source trajectory using the IP signal intensity. The simplification that the source is always at the center of the source channel might be a robust solution when the snaking pattern is not consistent in time. PO-1024 Transperineal TRUS-mpMRI fusion prostate biopsy using brachytherapy treatment planning systems ONLY. M. Dabkowski 1 , E. Gruszczynska 2 , A. Lewicki 3 , J. Palucki 4 , M. Durzynska 5 , A. Kulik 1 , M. Szymanski 6 , W. Rogowski 7 , A. Kasprowicz 1 1 The Maria Sklodowska-Curie Memorial Cancer Center, Department of Brachytherapy, Warsaw, Poland 2 The Maria Sklodowska-Curie Memorial Cancer Center, Department of Physics, Warsaw, Poland 3 Centre of Postgraduate Medical Education, Urology Clinic, Warsaw, Poland 4 The Maria Sklodowska-Curie Memorial Cancer Center, Department of Radiology, Warsaw, Poland 5 The Maria Sklodowska-Curie Memorial Cancer Center, Department of Pathology, Warsaw, Poland 6 The Maria Sklodowska-Curie Memorial Cancer Center, Urology Clinic, Warsaw, Poland 7 Central Clinical Hospital of the MSWiA, Urology Clinic, Warsaw, Poland Purpose or Objective Focal prostate brachytherapy (FPB) is an emerging treatment modality for selected PCa pts. The Poster: Brachytherapy: Prostate