Abstract Book

S72

ESTRO 37

be large. Furthermore, real-time monitoring of 6 degree of freedom (6DoF) target motion has demonstrated substantial intrafraction prostate rotation. Commercial treatment planning systems lack the ability to calculate the dosimetric impact of these rotations. This study extends a fast motion-including dose reconstruction algorithm to 6DoF motion and applies it for evaluation of inter- and intra-fraction prostate rotations. Material and Methods An in-house developed software program for real-time motion-including dose reconstruction (DoseTracker) was extended to handle 6DoF motion of an internal structure within the body contour of a patient. DoseTracker calculates the time resolved dose to a set of arbitrary calculation points in the structure by a pencil beam algorithm, currently assuming water density. In this study, a volume with 50,000 calculation point encompassing the PTV of a prostate cancer patient was used. Deliveries of a dual arc VMAT plan were simulated with machine parameters (gantry angle, MLC positions, MU increment, etc.) and prostate rotation angles being streamed to DoseTracker at a frequency of 21 Hz. Simulations were performed with static prostate rotations about the left-right (LR) axis and with a patient- measured dynamic prostate rotation. The dosimetric impact of the rotations was quantified as the CTV volume covered by 95% of planned mean CTV dose (V95%). Results Dose reconstruction was performed at 1.7Hz demonstrating that real time application is feasible. The interfraction rotations rotated the high dose volume relative to the prostate, but otherwise kept its shape almost unchanged (Fig 1). For the investigated VMAT plan, CTV V95% remained high for rotations up to ~10° and then dropped steeply at larger rotations (Fig 1). The patient-measured dynamic prostate rotation was in mean 4.4° (LR axis), 0.5° (SI axis), and -1.6° (AP axis), but it was highly dynamic with a maximum of 19° (Fig 2A). Interplay effects between the dynamic rotation and VMAT delivery markedly distorted the dose distribution and reduced CTV V95% from 100% to 94.9% (Fig 2B-C). This was in stark contrast to the small dosimetric impact of static rotations.

Results Figure 1c shows an in-house developed software that processes the data in real time showing the current source position and previous dwell positions. Results are instantly available at the end of irradiation without requiring any additional processing time. The method has 0.2 mm spatial resolution and is capable of verifying even a 1 mm interdwell distance (smallest interdwell distance available). It is possible to import catheter contours and reference markers direct from the treatment planning system allowing a direct comparison against measurement results and the treatment plan. Differences between measured and expected dwell positions are within the manufactures specification of 2 mm, except for up to 4 mm intermittent variations observed for one specific dwell position. Conclusion The proposed method overcomes film limitations for brachytherapy applicator verification. A projection of the applicator can be obtained using the 192 Ir gamma photons without requiring any additional source of radiation. The measurement time is reduced considerably since additional processing is not required and several measurements can be performed in sequence. Individual applicator commissioning is already recommended in the literature and we encourage the clinical staff to perform several measurements for the same applicator, which is easy to do with the system proposed here, due to an intermittent problem observed for one applicator used in this work. PV-0143 Dynamic six-degree of freedom dose reconstruction during radiotherapy C.G. Muurholm 1 , T. Ravkilde 2 , S. Skouboe 1 , T. Eade 3 , D. Nguyen 4 , J. Booth 3 , P.J. Keall 4 , P.R. Poulsen 1 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 2 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 3 Royal North Shore Hospital, Northern Sydney Cancer Centre, St Leonards NSW, Australia 4 The University of Sydney, Radiation Physics Laboratory- Sydney Medical School, Sydney, Australia Purpose or Objective Image guided radiotherapy is commonly used for daily tumor alignment, e.g. by x-ray imaging of implanted prostate markers. However, the alignment will typically only include translations although prostate rotations may