ESTRO 36 Abstract Book

S144 ESTRO 36 2017 _______________________________________________________________________________________________

needle movements between MR-scan and treatment.

radiation oncologist and a resident radiation oncologist on all repeat CT scans and consensus was reached. The treatment plan of application one was projected on the repeat CT scans to simulate the other applications. Projected treatment plans were categorized as clinically acceptable or unacceptable. Additionally, new treatment plans were derived from the repeat CT scans by an experienced treatment planner. A conformity index, taking into account CTV coverage and dose to organs at risk, was used to quantify conformity of both the projected and the repeated treatment plans. Dose distributions were scaled to a prescription dose of 7 Gy. Using the Wilcoxon signed rank test, the conformity index and cumulative CTV D98 of the projected and repeated treatment plans were compared. Results Fourteen out of 22 projections were clinically unacceptable. In 8 of those 14 projections, replanning was of added value. In the remaining 6 unacceptable cases, replanning was of limited value as first an intervention would have been necessary to remove air and/or faeces. The figure shows a repeat CT with an unacceptable projection and corresponding replanning. The table summarizes the conformity index and cumulative CTV D98 of the non-adaptive and the adaptive approach. Parameters are presented both for all cases and for all cases excluding those that needed an intervention. Repeat CT-based adaptive HDR-BT resulted in a significantly higher conformity. Conclusion Repeat CT-based adaptive HDR-BT resulted in a more conformal treatment and should be standard practice in radical treatment with HDR-BT in rectal cancer patients.

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

Poster Viewing : Session 6: Imaging

PV-0281 Lymph node MRI in regional breast radiotherapy leads to smaller target volumes and lower OAR dose T. Van Heijst 1 , H.J.G.D. Van den Bongard 1 , N. Hoekstra 1 , M.E.P. Philippens 1 , D. Eschbach 1 , J.J.W. Lagendijk 1 , B. Van Asselen 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective Elective axillary regional radiotherapy (RT) in breast cancer patients is performed with RT-planning CT scans, using delineation guidelines based on anatomical boundaries. In contrast to CT, MRI can directly image axillary lymph nodes (LNs) in RT position [van Heijst et al. 2016, BJR ]. Our MRI linac (MRL) system is designed to be able to treat those LNs precisely. LN-based target volumes