ESTRO 36 Abstract Book

S951 ESTRO 36 2017 _______________________________________________________________________________________________

DC and ArcCHECK TM allow volumetric dose comparisons between calculated and measured doses. Moreover DC displays DVHs and isodose lines for the considered structures in the plan while 3D-DVH in ArcCHECK TM is not available for TomoTherapy. DC seems to be a valuable tool for performing patient- specific DQA however, considering the present Pencil Beam algorithm and its known limitations, a verification using film dosimetry and ionization chamber measurements should be done in case of any significant discrepancy. Concerning the beta version for TomoTherapy in RadCalc® software, it seems to be a valid tool for independent treatment time verification, easily incorporable in routine treatment workflow. EP-1752 A simple technique for an accurate shielding of the lungs during total body irradiation H. Mekdash 1 , B. Shahine 1 , W. Jalbout 1 , B. Youssef 1 1 American University of Beirut Medical Center, Radiation Oncology, Beirut, Lebanon Purpose or Objective During total body irradiation (TBI), customized shielding blocks are fabricated and positioned in front of the lungs at a close distance from the patient’s surface to protect the lungs from excessive radiation dose. The difficulty in such treatments is to accurately position the blocks to cover the entire lungs. Any error in the positioning of lung blocks can give higher doses in the lungs than intended and can lead to underdosage in the body/target volume. The conventional technique for the positioning of lung blocks is based on a time-consuming trial and error procedure verified at each trial with radiographic films. A new technique based on CT simulation was developed to determine the exact position of lung blocks prior to treatment for each specific patient. This technique of accurate shield positioning serves the purpose of reducing lung toxicities and most importantly reduces patient’s pain and discomfort by minimizing the length of the overall treatment session. Material and Methods Patients were CT simulated in their lateral recumbent treatment position and lungs were contoured with the aid of a treatment planning system. Horizontal AP/PA fields were designed with MLC aperture conforming to lung contours. The fields were used to project a light field on the patient’s skin representing the extent of the lungs, which was subsequently marked on the patient’s anterior and posterior skin as seen in Figure 1. Prior to each fraction, the lung blocks were positioned with their shadow matching the lungs’ marks. The position of the shielding blocks was radiographically verified prior to the delivery of each beam as per the usual procedure (Figure 2). Three patients underwent TBI with this new technique. Each patient received in total six fractions of AP/PA beams including two fractions with shielded lungs. The lungs received in total 8 Gy and the rest of the body was irradiated with the prescribed dose of 12 Gy. To evaluate the efficiency of this technique, the number of repeated attempts to correctly position the shielding blocks was recorded for each beam.

Results We succeeded in positioning the shielding blocks from the first attempt in 10 out of 12 beams for the three patients. The position of the shielding blocks was adjusted only one time prior to treatment in 2 out of 12 beams. These results are compared to an average of 3 attempts per beam for each patient using the conventional technique of trial and error. The average time of a treatment session was 29 min with a maximum time of 41 min compared to an average of approximately 60 min in past treatments and a maximum of 120 min. Conclusion Most of TBI patients are pediatric patients and it is difficult to keep them immobilized for a long period of time. This new technique succeeded in reducing the length of the overall treatment session of the conventional TBI procedure and hence reduced patient discomfort while ensuring accurate shielding of the lungs. EP-1753 Determining the effect of using lead as electron cutout material compared to low melting point alloy M. Wanklyn 1 , S. Rizkalla 1 , T. Greener 1 1 Guy's and St.Thomas' Hospital NHS Foundation Trust, Radiotherapy Physics, LONDON, United Kingdom Purpose or Objective The aim of this investigation was to determine whether lead cut-outs are suitable for delivering MeV electron treatments on a Varian TrueBeam which have been planned using the eMC algorithm in Eclipse. Due to the eMC algorithm beam data being configured using Cerrobend low melting point alloy as the cut-out material it is important to assess the dosimetric differences between the lead and Cerrobend cut-outs. Material and Methods Unlike the Cerrobend cut-outs which are 1.5cm thick, the lead cut-outs were made to 1cm thickness. This was done to minimise the cost of lead. Lead versions of all the standard Varian cut-outs were made in house (6x6, 10x10, 6x10, 15x15, 20x20 & 25x25cm 2 ). Two regular cut-outs were also made, a 4x8 cm 2 cut-out for the 10x10 cm 2 applicator and a 10x14cm 2 cut-out in a 15x15 cm 2 applicator to determine the out-of- field transmission. Transmission factors through a 10x10 cm 2 closed end plate were calculated for the lead and Cerrobend materials for a range of energies (6, 9, 12, 16, and 18MeV) PDDs in water at 100cm SSD and output factors in solid water at d max at 100cm SSD were measured for the standard applicators with both the lead and Cerrobend inserts for all energies. Cross line and inline profiles at d max were taken in water at 100cm SSD for all energies using the two regular cut- outs. Results As can be seen in Figure 1, the transmission through a closed lead endplate is comparable to that for the Cerrobend.