ESTRO 38 Abstract book

S1019 ESTRO 38

EP-1877 Proton vs photon deep inspiration breath- hold planning study for left-sided breast cancer patients K. Czerska 1 , P. Winczura 2 , J. Wejs-Maternik 2 , A. Blukis 2 , M. Antonowicz-Szydlowska 2 , A. Rucinski 3 , P. Olko 3 , R. Kopec 1 , A. Badzio 2 1 Institute of Nuclear Physics PAN, Cyclotron Center Bronowice, Krakow, Poland ; 2 Radiotherapy Center Elblag, Radiotherapy, Elblag, Poland ; 3 Institute of Nuclear Physics PAN, Proton Radiotherapy Group, Krakow, Poland Purpose or Objective Deep inspiration breath-hold (DIBH) is an effective approach to reduce the dose to the heart and coronary vessels in left-sided breast radiotherapy. The aim of this study was to compare doses to organs at risk between 3D photon DIBH technique and an Intensity Modulated Proton Therapy (IMPT) planning using a deep inspiration and free breathing (FB). Material and Methods Six patients with early left-sided breast cancer were planned on a computed tomography datasets acquired for FB and DIBH, with a 3D photon (6 MeV tangential fields with additional lower weight fields to ensure homogeneity of the plan) and IMPT proton technique (with the application of 3 anterior-oblique fields). All cases were planned to the whole breast only. Prescribed dose was 50 Gy for photons and 50 Gy(RBE) for protons in 25 fractions. Main goal was to achieve acceptable target coverage with dose reduction to the organs at risk, which could reflect to the clinical satisfactory plans. Dosimetric comparison was made by using a paired, two-tailed Student’s t-test and Wilcoxon signed rank test. Results With regard to doses to the heart and coronary vessels, there was no difference between DIBH and FB for proton plans. However proton FB technique, as compared to the photon DIBH, ensured significant dose reduction to the organs at risk. Mean heart dose was lower in proton plans comparing to photons (0,5 Gy(RBE) vs 1,6 Gy; p=0,0096). Also the mean dose to the ipsilateral lung was reduced from 9,0 Gy to 5,4 Gy(RBE); (p=0,0006) for photons and protons, respectively. Several parameters were taken into account to depict the benefit for the Left Anterior Descending Artery (LAD): mean, maximum dose, D max 0,2cm 3 and V45Gy. We found profound decrease of the D max 0,2cm 3 by a factor of 3 for proton plans comparing to photons (8,2 Gy(RBE) vs 22,0 Gy; p=0,0687). The volume of LAD receiving dose of 45 Gy was very low for both techniques. Detailed dosimetric comparison between both techniques is presented in table 1.

Conclusion Although SIB technique is more convenient due to the shorter treatment time and well tolerated in previous study, we found that lung dose was higher than in SEQ technique. More efforts should be made when using this technique. EP-1876 Level II volume and parotid doses during nasopharyngeal radiotherapy: what relationship? Z. Fessi 1 , N. Fourati 1 , W. Mnejja 1 , L. Farhat 1 , T. Sahnoun 1 , W. Siala 1 , J. Daoud 1 1 Hopital Habib Bourguiba, Radiation Oncology, Sfax, Tunisia Purpose or Objective High dose to parotid gland may increase the risk of xerostomia during radiotherapy for nasopharyngeal carcinoma (NPC). This dose is essentially correlated to the proximity of the target volume, mainly linked to the level II lymph node invasion. The aim of this study was to evaluate the relationship between parotid doses and level Retrospective study of 45 patients with NPC treated by intensity modulated radiotherapy (IMRT) in our institution during 2 years (2016-2018). The 2 parotid glands were delineated as a single structure and we noted the mean doses (Dmean) received at their level. We then retrospectively delineated Level II lymph node separately to determine its volume. The results were analyzed by SPSS v20. Pearson Test was used to determine the correlation between Dmean and Level II lymph node volume. We then determine the function that links the 2 variables according to a linear model: Dmean = a + b Level II lymph node volume. Results Median Dmean dose was 35.54 Gy (23.36-53.34). Median Level II lymph node volume was 9.8 cc (0-93 cc). The correlation between the two variables showed a proportional relationship with a Pearson r coefficient of 5.23 (p <0.001). According to the linear model, the relationship between Dmean and level II lymph node volume was as follows: Dmean = 33.56 + 0.15 Level II lymph node volume. Conclusion Level II lymph node involvement is common in NPC and is found in more than 80% of cases. The results of our study found that parotid mean doses, which represent a significant risk organ during IMRT planning, are correlated with level II lymph node volume. It is therefore difficult to respect parotid doses in the presence of a level II invasion. In this situation, the choice would be between excluding the part of the target volume extending at the parotid and taking into account only the superficial lobe of the parotid. II lymph node volume. Material and Methods

Conclusion Breath-hold approach for photon planning already ensures a significant dose reduction to the heart and coronary vessels. This study shows that the application of proton FB technique leads to better dose reduction to the organs at risk. No further benefit was achieved with proton DIBH technique. Proton FB radiotherapy could be a promising solution for patients with more demanding anatomies, these with serious cardiac comorbidities or patients unable to perform deep inspiration breath-hold.