ESTRO 38 Abstract book

S36 ESTRO 38

reconstruction uncertainties lead to clinically relevant deviations in dose. Material and Methods IVD was performed during 7 fractions of HDR prostate BT. Patients were treated with two HDR fractions of 8.5Gy delivered after 46Gy EBRT. The needles were implanted under trans-rectal ultrasound guidance. The needle reconstruction and organ delineation were done on MRIs. A dosimeter, based on a small luminescence crystal, was placed inside the prostate in a dedicated needle. Dose rates were recorded during the dose delivery at a read-out frequency of 20Hz and analysed post-treatment. The position of each needle relative to the dosimeter were determined with an optimisation process transforming dose rate patterns into positional shifts of the needles. The tracked needle positions were registered to the patient anatomy (relative to the dosimeter) on the planning MRI and used to reconstruct the delivered DVH parameters. Delivered and planned DVH parameters were compared to investigate the effect of positional uncertainties/changes. Results The ST analysis of all 117 needles (fig 1) showed longitudinal needle shifts (caudal-cranial direction) with a spread (1SD) of 1.5 mm and radial needle shifts (towards – away from the dosimeter) with a spread (1SD) of 0.5 mm. Any mean longitudinal shift on a fractional level was interpreted as an offset in the dosimeter position and corrected for before calculating the dose. The resulting changes in the dose distributions lead to a mean±1SD fractional change of -0.2±0.1Gy in prostate D 90 , - 0.1±0.2Gy in urethra D 2cm3 , 0.4±2.1% point in rectum D 2cm3 and 0.1 ± 4.5Gy in bladder D 2cm3 (table 1). One needle was shifted 4 mm in the cranial direction into the bladder wall resulting in a bladder D 0.1cm3 of 22.8 Gy.

panel detector (FPD) embedded in the treatment couch for source tracking and a ceiling mounted x-ray system for imaging and registration 1 . Here, we present the real time capabilities of the system, showing its ability to track the source, measure source position, and compare the measurement to the plan during treatment . Material and Methods An AP radiograph is captured of the phantom with implanted fiducal markers. The treatment plan, consisting of 81 dwell positions across 9 catheters, was registered to FPD coordinate space via the fiducial markers, allowing the tracked source positions to be directly compared to the plan. As the treatment was delivered, the FPD was run in cine mode at 15 fps, continuously capturing auto- radiographs of the source radiation. Each frame was analysed for 2D source position and compared to the planned dwell position. The process is completed in seconds: the analysis of each catheter is performed before the start of the next catheter treatment delivery. Any deviations, if present, are assessed against ‘error signatures’, shown in previous work 2 to correlate with common errors, enabling rapid identification of errors. Plans were delivered both with and without deliberately introduced errors to validate the system in a phantom and to verify that treatment errors can be identified. Results To account for uncertainties and to minimise false positives, an error detection threshold of 5mm was set. When the plan was delivered without introduced errors, 0/81 dwells differed by more than the threshold, thereby, as all dwells were within uncertainty of expected positions, treatment was verified. For one test of the system, the transfer tubes to two catheters were swapped. During delivery, the system correctly identified the error after the first incorrect catheter, allowing user notification before any additional incorrect catheters were delivered. The analysis was performed quickly: feedback was provided in time for the treatment to be interrupted before the start of the next catheter. Once adapted for clinical use, this will allow treatment to be paused and recover the prescribed plan. The success of the real time treatment verification system for other errors will be discussed. Conclusion Our real time HDR brachytherapy treatment verification system can identify gross errors during delivery , and confirm correct delivery with an error detection threshold of 5 mm. The process is performed during treatment , allowing for errors to be identified and corrected before completion of treatment. As more experience is gained with the system, it will be used routinely in real time as a treatment verification system with minimal impact on workflow and patient discomfort. References 1.Smith, R et al (2018) Brachytherapy , 17(1) 111-121 1.Franich, R et al (2018) APESM 41(1) 307 OC-0077 Reconstruction of delivered dose based on in vivo dosimetry in prostate brachytherapy E. Jørgensen 1 , S. Rylander 1 , S. Buus 1 , L. Bentzen 1 , S.B. Hokland 1 , A.K.M. With 2 , G. Kertzscher 1 , K. Tanderup 1 , J.G. Johansen 1 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 2 Örebro University Hospital, Department of Medical Physics, Örebro, Sweden Purpose or Objective Novel in vivo dosimetry (IVD) systems for BT, have enabled source tracking (ST) with a sub-millimetre precision. We have used ST to reconstruct the delivered dose and dose volume histogram (DVH) parameters in HDR prostate treatments. Reconstructed DVH parameters are compared to the treatment plans to investigate, if current applicator