ESTRO 36 Abstract Book

S145 ESTRO 36 _______________________________________________________________________________________________

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.

needle movements between MR-scan and treatment.

There was no relation between deviations in measured dose and shifts of needles. E.g. patient 6 and patient 7 have similar shifts but very different accumulated dose deviations. This illustrates how a small shift in a nearby needle can lead to significant changes in the measured dose, making it hard to use the accumulated dose for treatment verification. Conclusion Accumulated dose and dose rate have been measured in real-time for 22 treatments. We have used real-time in- vivo dosimetry to determine the rela tive geometry between needles and dosimeter with high precision. This could potentially lead to real-time treatment verification in BT. OC-0280 Benefit of repeat CT in high-dose rate brachytherapy as radical treatment for rectal cancer R.P.J. Van den Ende 1 , E.C . Rijkmans 1 , E.M. Kerkhof 1 , R.A. Nout 1 , M. Ketelaars 1 , M.S. Laman 1 , C.A.M . Marijnen 1 , U.A. Van der Heide 1 1 Leiden Univers ity Medical Center, Department of Radiation Oncology, Leiden, The Netherlands Purpose or Objective High-dose rate endorectal brachytherapy (HDR-BT) for rectal cancer can be used to increase the dose to the tumor while sparing surrounding organs due to a smaller treated volume and the steep dose gradient. Conventionally, one treatment plan is derived from a planning CT with applicator in situ prior to the start of treatment, which is then used for all further applications (non-adaptive approach). An adaptive approach would be to acquire a repeat CT scan at each application for treatment planning. The purpose of this study was to evaluate the difference in dose conformity and clinical target volume (CTV) coverage between the non-adaptive and the adaptive approach. Material and Methods Eleven patients included in a dose-escalation study were included in this study. Patients received a radical treatment consisting of 13x3 Gy external beam radiotherapy (EBRT) followed by three weekly applications HDR-BT of 5-8 Gy. A planning CT with applicator in situ was acquired at application one and repeat CT scans with applicator in situ were acquired at application two and three. The CTV was defined as residual macroscopic tumor or scarring after EBRT. The CTV, rectal wall without CTV, mesorectum and anus were delineated by an expert

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