ESTRO 38 Abstract book

S620 ESTRO 38

ventral bony anatomy. Online, CBCT’s and tangential MV images were acquired for verification of adequate positioning of the lymph node region and breast tissue. Offline, the CBCT’s and MV portal images were used to evaluate the misalignment in lymph node regions, thoracic wall, breast and humeral head after the online match. Accurate quantification of these misalignments was obtained by performing separate CBCT matches offline, focusing on each of these structures individually. Results For lymph nodes level I-II, 2D-2D kV setup lead to adequately positioning in all sessions. Levels II-IV were adequately positioned in 46 out of 47 sessions. In only one session setup lead to a 6mm misalignment in vertical direction which was larger than the CTV-PTV margin. Average misalignments were 0.2, 0.19 and 0.13 in vertical, longitudinal and lateral directions respectively(fig.1) Verification with CBCT and tangential MV images shows that misalignment of the thoracic wall was less than 8mm in all sessions. The breast contour deviated more than 8mm in 5 out of 47 sessions. This was related to seroma change. With the tangential field approach, breast contour deviations had a minimal effect on the breast dose coverage. Misalignment of the humeral head exceeded the limit of 8mm in 5 of the 47 sessions, but none resulted misalignment of the lymph nodes larger than 5mm.

Purpose or Objective During the treatment of nasopharyngeal carcinoma (NPC), the use of intensity modulated radiotherapy (IMRT) showed positive therapeutic effects on irradiated patients by reducing xerostomia. We proposed in this work to evaluate the dosimetric impact of parotids anatomic variation during the IMRT for the NPC. Material and Methods Patients with NPC receiving IMRT treatment were included. For each patient, a second dosimetric CT was performed at a dose of 38 Gy, which was fused with the initial planning dosimetric CT. Manual contouring of the parotids was performed on the second scanner. We recorded the volume and the isocenter position in the three planes of space (X, Y, Z), and the average dose at parotids (right and left) on both scanners. We calculated the volume percent change. Statistical mean differences were calculated using Wilcoxon signed-rank’ test and SPSS 20 software was used for data analysis. Results Twenty consecutive patients were enrolled in this study. We observed a significant decrease of right parotid volume of 28 % [5%-48%] with p = 10¯³, and a decrease of 27.9% for the left parotid [0-54%] with p =10¯³. Parotid migration averages in the medial direction (X) were of 2.1 mm [-2,3_15](right) and 2,2 mm [-60_2,8] (left). Moreover, It was only 0.9 mm (right) and 0.2 mm (left) in the postero-anterior direction (Y) , also in cranio-caudal direction with 0.9 mm (right) and 0.4 mm (left). Regarding dosimetry results, we observed an increase in the mean dose. This increase was significant for the left parotid with an average value of 3.23 Gy more (p = 0.02), and no significant for the right parotid with an average value of 0.82 Gy more (p = 0.35). Conclusion The moving of the parotid after nodal tumor falling, and also the decrease of its volume cause the migration of parotid to the higher dose areas. This can explain the dose increase noted on the scanner during treatment. It seems that a careful adaptation of the treatment plan should be considered during therapy. PO-1118 Verification of new respiratory gating device for clinical use in proton therapy wobbler method I. Maeshima 1 1 Suwa Red Cross Hospital, Radiation Therapy Center, Fujimi, Japan Purpose or Objective In recent years, a new respiratory synchronizer (AbchesET) has been developed that allows the patient's own respiratory control to be more easily performed. By using AbchesET, respiratory control by the patient himself can be easily and accurately reproduced by directly seeing indication of rotation angle of pointer displayed on a small monitor. In this study, we compared the conventional respiration synchronizer (ANZAI) as a comparative subject and made various investigations on the usefulness of respiratory synchronous irradiation of proton beam wobbler method using a new respiratory synchronizer (AbchesET). Material and Methods Verification of proton beam characteristics and delay time at respiratory synchronized irradiation was performed for each of ANZAI and AbchesET. With regard to verification of beam characteristics, verification of synchronism with no synchronization (stopped state) (Gate width: 12%, 25% when maximum expiration is 0%) is performed using a moving body phantom and a two dimensional detector for a rectangular radiation field. We evaluated each beam characteristics (Flatness, Symmetry, Penumbra, Field size and Dose) of the irradiation field. We also evaluated the dose distribution by gamma analysis. The breathing waveform of the moving body phantom was evaluated with respect to three types of respiration rates (Breath per

Fig 1. Misalignment of lymphnodes level III-IV after 2D/2D kV setup with additional setup instructions. Conclusion For locoregional breast irradiation, this study has shown that 2D-2D kV setup with additional setup instructions results in adequate positioning of the lymph nodes having a 5 mm CTV-PTV margin. Accurate positioning of bony anatomy in the lymph node region and the humeral head remains important, and misalignments of more than 5mm resp 8mm should result in repositioning or CBCT verification of the lymph nodes.

Poster: RTT track: Motion management and adaptive strategies

PO-1117 Dosimetric effect of parotid glands geometric modifications during the IMRT for NPC W. Mnejja 1 , H. Daoud 1 , L. Farhat 1 , N. Fourati 1 , T. Sahnoun 1 , W. Siala 1 , J. Daoud 1 1 Hbib Bourguiba Hospital, radiotherapy, sfax, Tunisia