ESTRO 36 Abstract Book

S143 ESTRO 36 2017 _______________________________________________________________________________________________

HDR prostate plan. The fluctuation could be reduced to <0.5% by mixing the Y 2 O 3 :Eu and YVO 4 :Eu phosphors in a ratio 1-to-10. The stem signal of the ruby, Y 2 O 3 :Eu and YVO 4 :Eu ISDs was up to 3%, 1% and 2%, respectively, of the total signal, and the photoluminescence was <1%, when the BT source moved 8 cm away from the detector and 1 cm from the fiber-optic cable. In contrast, the stem signal of the PSD was up to 70%.

T2W MRI with 2mm slice thickness for treatment planning. Dose rates were measured using a fiber-coupled Al 2 O 3 :C luminescent crystal placed in a dedicated needle in the prostate. The dose measurements were analysed retrospectively. The total accumulated dose was compared to the predicted dose. Secondly, the measured dose rate originating from each dwell position in a needle was compared to the predicted dose rate obtained from the dose planning system. The discrepancies between measured and predicted dose rates were assumed to be caused by geometrical offsets of the needles relative to the dosimeter from the treatment plan. An algorithm shifted each treatment needle virtually in radial and longitudinal directions relative to the dosimeter until optimal agreement between the predicted and measured dose rates was achieved. Results Table 1 shows the relative difference between the measured and predicted accumulated dose and the average radial and longitudinal shifts of 337 needles in 22 treatments. The average shifts are expected to correspond to systematic uncertainties in dosimeter positions, and the standard deviations reflect the shift of needles relative to the dosimeter. Two treatments were not further analysed because of dosimeter drift by >15mm.

Conclusion Red-emitting ISDs based on ruby, Y 2 O 3 :Eu are suitable for HDR BT treatment verification in real time. Their large signal intensities and emission properties facilitate accurate detector systems that are straightforward to manufacture and use which can result in widespread dissemination and improved patient safety during BT. OC-0279 Removing the blindfold - a new take on real- time brachytherapy dosimetry J. Johansen 1 , S. Rylander 1 , S. Buus 1 , L. Bentzen 1 , S.B. Hokland 1 , C.S. Søndergaard 1 , A.K.M. With 2 , G. Kertzscher 3 , C.E. Andersen 4 , K. Tanderup 1 1 Aarhus University Hospital, Department of oncology, Aarhus C, Denmark 2 Örebro University Hospital, Department of Medical Physics, Örebro, Sweden 3 The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston- TX, USA 4 Technical University of Denmark, Center for Nuclear Technologies, Roskilde, Denmark Purpose or Objective Although in-vivo dosimetry has been available for decades it is still not a standardized verification tool in brachytherapy (BT). Major limitations are that in-vivo dosimeters only provide point dose information and that the steep dose gradient leads to strong positional dependency. The aim of this study is to examine whether it is possible to utilise in-vivo dosimetry for evaluation of the implant geometry during irradiation in addition to post :Eu and YVO 4

The longitudinal and radial shifts of each needle are plotted in Fig. 1. The relative needle-dosimeter geometry was determined with sub-millimetre precision for 98% of the treatment needles (error bars in Fig. 1). More than 90% of the needles were shifted less than 4mm longitudinally and 2mm radially, which is consistent with typical uncertainties in needle and dosimeter reconstructions and

hoc dose verification. Material and Methods

This study includes in-vivo dosimetry measurements from 22 HDR BT procedures for prostate cancer. Needles were placed in the prostate guided by TRUS with a subsequent