Abstract Book

S90

ESTRO 37

Material and Methods We propose a novel delivery system for IMBT which can dynamically control the rotation of shielded catheters. To modulate the intensity of the source, the device combines a custom made 169 Yb source with thin platinum shields (maximum thickness of 0.8 mm) within the catheter. The source has an active core with a diameter of 0.6 mm and a length of 3 mm. The device can be connected to any commercial afterloader as an add-on device, and is compatible with both interstitial and intracavitary applicators. The device is divided in three main systems: a rotating system, a link assembly and a shield assembly (Fig. 1). The rotation of the shield is controlled through a series of moving panels with an interlock system. Each panel is connected to a stepper motor which handles the rotation of a subset of needles. The shielded needles are connected to the rotating mechanism through flexible locking luers that will allow the opportunity to implant the needles at an angle. As in conventional brachytherapy, the afterloader is responsible for the motion of the source through the source guide and within the catheter. Controller sensors will read the actual position of the shield and provide feedback to the stepper motor system. As a proof of principle, an IMBT treatment plan for a prostate cancer case was simulated using a Monte Carlo based treatment planning system to show the potential advantages of IMBT. Results Validation of the functionality of the delivery system, the stability of the connection between the source guide and the shielded catheters, and the resistance to wear of the plastic needle during use has been completed. The platinum shield can reduce the dose on the shielded side by 75 % compared to the dose on the unshielded side at a radial distance of 1 cm from the source. For the same minimum dose to the hottest 90 % of the planning target volume (PTV), the IMBT plan resulted in a reduction (expressed as percentage of prescription dose) in the bladder D 2cc , rectum D 2cc , and urethral D 10 by 13%, 21%, and 10%, respectively, while improving the homogeneity within the target (PTV V 150 reduced by 25 %). Conclusion For many cancer sites the optimal dose cannot be delivered to the tumor due to normal tissue toxicity. IMBT enables dose escalation in the tumor volume while protecting OARs. Possibility to increase the dose in the target and reduce the dose spillage to OARs has the potential to improve brachytherapy treatment outcomes significantly. In addition, it will broaden the use of brachytherapy as a treatment modality on cancer sites.

Conclusion High-intensity ISD materials, e.g. ZnSe:O and CsI:Tl, make it possible to develop low-cost in vivo dosimetry systems with 1 mm-size detector volumes that exhibit large signal-to-noise ratios and negligible stem and photoluminescence backgrounds. The new ISD system makes it possible to precisely monitor BT treatments at a low cost and can therefore facilitate dissemination of real-time treatment verification technology for BT. OC-0173 Intensity modulated brachytherapy system for dynamic modulation of shielded catheters G. Famulari 1 , S.A. Enger 1,2,3 1 McGill University, Medical Physics Unit, Montreal, Canada 2 McGill University Health Centre, Research Institute of the McGill University Health Centre, Montreal, Canada Conventional brachytherapy often results in les s than ideal tumor dose conformity due to the non-symmetrical shape of the tumors, resulting in dose spillage to radiation sensitive organs at risk (OARs). Intensity modulated brachytherapy (IMBT) can dynamically direct the radiation towards the tumor and away from OARs by incorporating metallic shields inside within brachytherapy needles/applicators. 3 McGill University, Oncology, Montreal, Canada Purpose or Objective