ESTRO 38 Abstract book

S51 ESTRO 38

Conclusion Two DIBH techniques have now been implemented into clinical practice with no significant difference observed between the treatment delivery times. Preliminary results indicate that both techniques ensure good reproducibility and stability of BH. This study will continue with the inclusion of additional patients. Future work will include the roll out of DIBH to enable IMC treatment but this will require a tightening of imaging tolerances and the evaluation of the dosimetric consequences to heart and left anterior descending artery.

and gated DIBH treatment for left sided breast cancer patients. Material and Methods Patients receiving adjuvant radiotherapy for breast cancer who had a maximum heart distance ≥10mm from a standard free-breathing tangential field treatment plan underwent a second planning CT scan using a coached DIBH technique. All patients underwent whole‐breast or chest wall DIBH RT. For 5 patients, Varian® Real‐time Position Management (RPM™) system was used to monitor respiratory movement and to gate treatment delivery. In the other 5 patients, the vmDIBH technique was used to monitor the patient’s breath-hold (BH) during treatment delivery by observing the marked isocentre in relation to the laser and treatment interrupted if there was concern that BH depth had changed. Daily cine electronic portal images were acquired, when possible, during delivery of the medial treatment field and treatment delivery times recorded. For every cine image acquired, the central lung distance (CLD), central flash distance, inferior central margin was measured at two-second intervals. Stability and reproducibility of BH was examined. Results Ten patients were included in this analysis. Age ranged from 32 to 73 years (gated) and 40 to 55 years (vmDIBH). For all patients the treatment course was delivered as planned in DIBH (40Gy/15f) without interruption. Treatment delivery time per field was comparable between the gated (time= 162 seconds (s)) and vmDIBH groups (t=114s). For each patient, the stability of BH was determined from the difference in CLD relative to reference position in CT digitally reconstructive radiograph (DRR). (Figure 1). All images are within local breast imaging tolerance (≤6mm).

Poster Viewing: Poster viewing 2: Advanced technologies

PV-099 MC simulations and dose measurements of a patient-specific 3D range-modulator for proton therapy Y. Simeonov 1 , U. Weber 2 , C. Schuy 2 , K. Zink 1 1 Technische Hochschule Mittelhessen - IMPS, Lse, Gießen, Germany; 2 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Biophysics division, Darmstadt, Germany Purpose or Objective The purpose of this work was to develop and validate a novel 3D range-modulator for very fast treatment of moving targets. In contrast to the pencil beam scanning technique, where the multiple iso-energy layers are associated with relatively long irradiation times, the 3D modulator uses only one single energy to create a homogeneous and highly conformal 3D dose distribution, decreasing extremely the treatment time. Material and Methods Extending previous extensive research in this field, a 3D modulator was now developed from a real CT patient data, utilizing a ray-tracing algorithm to calculate the necessary information. A lung tumour with a complex 3D contour and irregular distal and proximal edges was deliberately chosen in order to simulate a worst-case scenario and investigate the limitations of the newly proposed technique. The 3D modulator consists of many fine pyramid-shaped structures (pins) with ~ 4 mm 2 base area (Fig.1). By using this pins an additional degree of freedom is introduced, i.e., the height and shape of each single pins can now be independently varied and optimised in such a way that the final 3D shape of the modulator corresponds to the 3D tumour form. When irradiated, the 3D range-modulator should create a quasi-static irradiation field, tightly shaped around the target. The modulator was optimised for 151.77 MeV 1 H and was eventually triangulated and manufactured on a 3D-printer in high-quality rapid prototyping technique. The FLUKA Monte Carlo (MC) package was used to investigate the modulating properties of the range-modulator and calculate the resulting dose distribution. A sophisticated in-house user routine was additionally implemented into FLUKA to enable intensity modulated scanning and take into account the complex geometry contour of the modulator. In order to validate the MC simulation results, dose measurements were conducted at the Marburg Ion-Beam Therapy Centre (MIT). The dose was measured with a 2D ionization chamber array (977 ICs) with 2.5 mm lateral resolution. The measurement depth was varied with a binary range shifter consisting of a set of retractable polyethylene plates. Results There is very good agreement between the measured and simulated dose. Fig 2. shows a homogeneous dose distribution conformed not only to the distal, but also to the proximal edge of the target.

The frequency of thoracic movements over a treatment course are presented in Figure 2 with the highest frequency observed thoracic movement for gated DIBH at 1.5mm and vmDIBH 2mm.

Reproducibility of BH was determined from the difference in thoracic movement on first frame of the cine image relative to the DRR. The population mean for vmDIBH was 1.7mm (SD 3.4mm) and the population mean for gated was 0.4mm (SD 3.7mm), indicating high reproducibility.