ESTRO 36 Abstract Book

S194 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective The clinical use of intraoperative radiotherapy (IORT) is steadily increasing based on novel applications like breast and brain cancer and spinal column metastases. Traditionally, IORT has been eye and hand guided without treatment planning and inhomogeneity correction. This limits the precision of the application and the precise documentation of the location and the deposited dose in the tissue. Kypho-IORT is a novel treatment option for patients with spinal column metastases in which a minimally invasive kyphoplasty is combined with a sterilising dose of IORT. Here we present a set-up where we use image guidance by intraoperative cone beam CT (CBCT) for precise online Monte Carlo treatment planning including inhomogeneity correction. Material and Methods During kyphoplasty a working cannula is used to insert a balloon catheter in the vertebra. The same cannula is used to insert a x-ray source with a dedicated Needle Applicator (Carl Zeiss Surgical GmbH, Oberkochen, Germany) to perform the IORT. For treatment planning an intraoperative cone beam CT (CBCT) was performed with the Needle Applicator in place. This CBCT was registered with a preoperative CT (pre-op CT) in Velocity (Varian, California, USA). The spinal cord and the metastasis were contoured on the pre-op CT and the applicator tip was contoured on the CBCT and transferred to the pre-op CT. On both CTs the treatment planning was then performed in Radiance (GMV, Madrid, Spain) using a hybrid Monte Carlo algorithm simulating dose in homogeneous (MCwater) and heterogeneous medium (MChet). Dose distributions on CBCT and pre-op CT were compared with each other (figure 1).

shifts of 1, 3 and 5mm in both the IN/OUT direction and 2D shifts in both IN/OUT and SUP/INF directions were introduced. Results Fig1b shows the 95% (of 7Gy) dose distribution of the combined BT and SBRT treatments. Analysis using 2%/2mm gamma criterion resulted in 99% agreement. Isodose line matching and a cross profile between measured and planned doses are shown in Fig1c-d. Fig2 shows the effect of 1D and 2D isocenter shift on D 98% , D 90% and D 2cc metrics of the CTV. A threshold of +10% was used as a gauge to compare dose values after shift with the zero shift baseline. The most changes in dose were for D 98% and D 90% , both exceeded the threshold for 3mm shifts and almost reached -20% for the 5mm shifts.

Conclusion Using the applicator as a guide, SBRT and BT for cervical cancer can be delivered on the same day. The isodos e gradient from BT is used to create dose shells needed to deliver doses by SBRT. 2D shifts were shown to affect D 98% the most and a positioning accuracy of 2mm results in dose variations within +10% from expected. OC-0362 Precision IORT – image guided IORT in cluding online CBCT based Monte Carlo treatment pl anning F. Schneider 1 , L.D. Jimenez 1 , F. Bludau 2 , A. Jahnke 1 , C. Illana 3 , J. Fleckenstein 1 , S. Clausen 1 , U. Obertacke 2 , F. Wenz 1 1 University Medical Center Mannheim, Department of Radiation Oncology, Mannheim, Germany 2 University Medical Center Mannheim, Department for Orthopaedics and Trauma Surgery, Mannheim, Germany 3 GMV, Innovating Solutions, Madrid, Spain

Figure 1: from top to bottom: dose distribution on CBCT (MCwater), pre-op CT (MCwater), CBCT (MChet), pre-op

(MChet) Results

The MCwater calculations showed a spherical dose distribution as expected. The resulting treatment times for the prescription of 8Gy in 13mm distance (in water) from isocenter were within ± 5% of the described treatment time of the INTRABEAM ® system. Due to the