ESTRO 37 Abstract book

S1203

ESTRO 37

Purpose or Objective MR only workflow is based on treatment planning solely from MRI, hence excluding the CT. A synthetic CT (sCT) image is generated from MRI data, replacing the conventional CT image. Using the kV-cone beam CT (CBCT) system we could compare dose distributions calculated on both the sCT and the CBCT images. The aim of this study was to investigate the possibility of using the CBCT image to dosimetrically verify the sCT image of male pelvis. Material and Methods Stability in Hounsfield Units (HUs) over time for one kV- CBCT system as well as variation in HU between six kV- CBCT systems were measured. Phantom measurements were carried out with CIRS Model 062M Electron Density Phantom on the kV-CBCT systems (Varian On-Board Imager TM TrueBeam TM ). The HUs of one kV-CBCT system was also compared to HUs from a Siemens Somatom Definition AS+ CT system. Using 28 CBCT image sets from seven prostate cancer patients, a HU to relative electron density (RED) table was created. Four patients were used for treatment planning and sCT images was generated using the commercial MriPlanner TM software. The CT and CBCT images was rigidly registered towards the sCT, resampled and reshaped. An original treatment plan (RapidArc) were calculated on sCT image using the standard HU-RED table. The plan was transferred onto the CT and CBCT images and recalculated using the same HU-RED table. For the CBCT image, an additional calculation was done using the HU-RED table developed for CBCT (CBCT HU-RED). The difference between the dose distributions was evaluated using clinical dose volume histogram (DVH) criteria. Results The phantom measurements showed that the kV-CBCT system was stable in HU over time (SD < 40 HU for all density plugs in the range of 0.190 - 1.695 RED). All six kV-CBCT systems generated comparable HU values (SD < 70 HU). The variation of HUs between CT and CBCT images was < 60 HU. The CBCT images exhibited larger variation across the field of view compared to CT images. Dose calculation based on CBCT images showed a mean dose difference to PTV of 0.0% (HU-RED CT) and -0.8% (HU-RED CBCT) compared to dose calculations based on sCT image (Fig. 1).

dose calculations on CBCT images, indicating that the variation in HUs between one CBCT system and one CT system had no clinical impact. The compared dose distributions based on CBCT and sCT images showed good agreement in terms of absorbed dose accuracy, regardless of which HU-RED table used. This shows that CBCT image can be used to dosimetrically verify the sCT image in an MR only workflow for male pelvis. EP-2174 Fabrication of three-dimensional printed customized bolus for the irregular shape of the outer ear M. Baeza Trujillo 1 , G. Gomez 2 , J.C. Mateos 3 , J.A. Rivas 2 , S. Velazquez 3 , J. Simon 3 , D. Mesta Ortega 4 , M.A. Flores Carrión 4 , M.J. Ortiz Gordillo 4 , T. Gómez-Cía 5 , J.L. Lopez Guerra 4 1 Hospital Universitario Virgen del Rocío, Servicio de Radiofísica Hospitalaria, Seville, Spain 2 University Hospital Virgen del Rocio, Group of Technological Innovation, Seville, Spain 3 University Hospital Virgen del Rocio, Radiation Physics, Seville, Spain 4 University Hospital Virgen del Rocio, Radiation Oncology, Seville, Spain 5 University Hospital Virgen del Rocio, Plastic Surgery, Seville, Spain Purpose or Objective The skin-sparing effect of megavoltage-photon beams in radiotherapy reduces the target coverage of superficial tumours. Consequently, a bolus is widely used to enhance the target coverage for superficial targets. Commercial bolus cannot easily be applied on irregular surfaces. A three-dimensional (3D)-printed customized bolus (3D bolus) can be used for radiotherapy application to irregular surfaces. This study examines the possibility of the fabrication of a 3D customized bolus for an irregular surface. Material and Methods We fabricated a bolus using a computed tomography (CT) scanner and evaluated its efficacy. The head of an Alderson Rando phantom was scanned with a CT scanner. A 3D bolus of 5-mm thickness designed to fit onto the ear was printed with the use of a Stratasys Objet260 Connex3 with the use of PolyJet technology (Stratasys Ltd., Eden Prairie, MN, USA) with the malleable `rubber-like' printing material, Agilus (Stratasys Ltd.). CT simulations (Figure 1) of the Rando phantom with and without the 3D and a commercial high density bolus (eXaSkin©) were performed to evaluate the dosimetric properties of the 3D bolus. The ear was delineated as the target (15 cc). Radiotherapy plans with two beams were generated in the Oncentra Masterplan v4.1 radiotherapy treatment planning system with the use of the enhanced collapse cone algorithm. The prescription dose was normalized for 95% of the prescribed dose to cover 90% of the target volume in the plan without bolus. The plans with the bolus were normalized to ensure that the target dose lie within 95% and 107% of the prescription dose. The following dosimetric parameters were estimated for all cases: maximal dose (Dmax), mean dose (Dmean), minimum dose (Dmin), V95% (volume receiving at least 95% of the prescription dose), V90%, and homogeneity index (HI) proposed in ICRU-83.

Conclusion Phantom measurements showed that the kV-CBCT systems were stable in HU over time and relative to each other. Results obtained for one system can therefore be transferred onto all the systems. A correction of HUs was not necessary to obtain sufficiently accurate absorbed