ESTRO 38 Abstract book

S1184 ESTRO 38

were consistent and independent on contouring methods.

M. Szlag 1 , A. Cholewka 1 , P. Wojcieszek 2 , S. Kellas- Ślęczka 2 , M. Stąpór-Fudzińska 1 , T. Krzysztofiak 2 , K. Ślosarek 1 1 Cancer Center and Institute of Oncology Gliwice Branch, Radiotherapy Planning Department, Gliwice, Poland; 2 Cancer Center and Institute of Oncology Gliwice Branch, Brachytherapy Department, Gliwice, Poland Purpose or Objective Imaging modalities, routinely used for 3D dose calculation for HDR prostate brachytherapy, are transrectal ultrasound (US) and computed tomography (CT). Depend on the imaging modalities, contours of the rectum contain: the whole rectum (when CT is used) or anterior rectal wall (for US based treatment plans). Contouring and rectum deformation by the transrectal probe modify the dose delivered to rectum. For different imaging types, the caution should be paid when using the same treatment plan evaluation protocol since some of the dosimetric parameters may be inconsistent. In this study we defined DVH regions where dosimetric parameters of the rectum and rectal wall differs significantly. The aim of this study was to specify dose–volume parameters for reporting the dose delivered to rectum and rectal wall during image guided brachytherapy of prostate and to indicate parameters which can be used for both imaging methods and these which should be used with caution. Clinical date are in relation to the set of physical dose parameters obtained from planning systems. Validation of dosimetric protocol is required to properly predict the clinical outcomes when new modalities are introduced to planning. Material and Methods In our department prostate brachytherapy is performed in a real-time based on transrectal ultrasound (US) or computed tomography (CT). For the purpose of this study 33 randomly selected CT-based plans (OncentraBrachy ELEKTA) were compared with 33 US-based plans (OncetraProstate ELEKTA). Plans were calculated for patients treated with brachytherapy as a monotherapy with total dose of 26Gy delivered in two fractions. DVH parameters for prostate, rectum and rectal wall were selected for analysis (Table1). We evaluated statistically significant difference between mean values of dosimetric parameters for rectum and rectal wall.

Conclusion Dose delivered to rectal wall in the low- and intermediate- DVH region is underestimated in comparison to the dose delivered to the whole rectum. Only high dose region was comparable for both types of plan. CT and US plans should be evaluated with the use of different evaluation protocol for low and intermediate rectum doses. EP-2142 Analysis of dose distribution between TG-43 and TG-186 in lung cancer M. Sawicki 1 , Ł. Jarosław 2 1 University of Rzeszów, Faculty of Medicine, Rzeszów, Poland; 2 Subcarpathian Cancer Center, Department of Brachytherapy, Brzozów, Poland Purpose or Objective Standard treatment planning and dose calculation in brachytherapy HDR is based on water mass density. Human tissues has a different mass density than water. In the case of cancer located in the lung, the dose distribution is calculated mainly in the area of lung and critical organs (heart, esophagus, spinal cord). The OARs in lung, for example heart mass density is 1.05 which is very close to water density (1.0) and it is seems correct to treat this area like a area of homogeneous water, but in case of lung there are such as areas like soft tissue and air. For lung (inflated) mass density is 0.2. in the area of lung cancer. The dose calculation algorithm should take into account the heterogeneity of the area, otherwise the calculated dose distribution will differ from the actual dose distribution in patient’s body. Material and Methods The material for this study included 15 patients with advanced inoperable non-small cell lung cancer treated with HDR Ir-192. The study group consisted 24 treatment plans with bronchial applicators (1-3 applicators). For each treatment fraction, two treatment plans were created. One for TG-43 and second one for TG-186. TP was created in the Oncentra MasterPlan Brachy 4.5.3 ® system. Dose distributions were compared in both treatment plans. First, the value of dose was compared at points at constant and same distance from the axis of the applicators (range 0.1-2.0 cm;). Then, the dose in the Target and critical organs (heart, oesophagus, spinal cord, healthy lung, lung) would have been evaluated. Parameters analysed for PTV are maximum and minimum value of median and V 85, V 100 and V 115. . In cases of the heart, spinal cord and oesophagus, the examined dosage equaled D 0.1 cm 3 , D 1 cm 3 and D 2 cm 3 for each of the structures.

Results Table 1 presents dosimetry of prostate, rectum and rectal wall. Two groups of treatment plans were similar when comparing the PTV volume and PTV coverage, significant differences were observed for dose delivered to the 10%, 50% (intermediate-dose region) and 90% (low-dose region) of the rectum delineated based on CT and US. In the relative mode (Fig 1) rectal wall received higher dose in the low- and intermediate-dose regions but in absolute scale the dose delivered to the rectal wall covers larger volume when the whole rectum is delineated (Fig 1). The high dose regions for rectum and rectal wall (D0.01ccm, D2ccm) were comparable in US and CT plans and they