ESTRO 2023 - Abstract Book

S1768

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ESTRO 2023

RGSC. DIBH effectively reduced mean heart dose by 33% as compared to FB technique. DIBH also reduced the mean dose to left ventricle by 37.2% and mean dose to LAD by 34%. The individual segment reduction was greatest and significant in the distal segment of LAD by absolute number of 807cGy and relative reduction of 36.5% as compared to FB technique. Among the segments of the left ventricle, the most pronounced dose reduction was in the apical segment. DIBH reduced the absolute mean dose by 426 cGy and relatively by 34.5%. The degree of lung expansion positively correlated with reduced mean dose to left lung, left lung volume receiving >5Gy(V5Gy%), >10Gy(V10Gy%), >20Gy(V20Gy%) was statistically significant and also with reduced mean doses to whole heart, left ventricle, LAD. Limitations small cohort, no prior cardiac function assesment Our study confirms the ability of the DIBH technique either voluntary or with respiratory gating, to reduce dose to the heart (MHD) and its substructures which includes left ventricle and coronaries in left breast cancer radiotherapy after either BCS or MRM surgery. The largest absolute reduction was found in distal LAD and apical LV segments. DIBH RT should be considered for suitable patients and all the substructures including LAD and its all segments, LV & it’s segments, LMCA, LCA, RCA should be contoured and dosimetrically assessed as these structures have shown to receive high dose and injury to them can result in cardiac events in future. Conclusion 1 Institut Salah Azaïz, Radiation Oncology , Tunis, Tunisia; 2 Institut Salah Azaïz , Radiation Oncology , Tunis, Tunisia; 3 Institut Salah AZIEIZ, Radiation Oncology, Tunis, Tunisia; 4 Institut Salah Azaïz Radiation Oncology , Radiation Oncology , Tunis, Tunisia; 5 Institut Salah Azaïz, Medical Physics , Tunis, Tunisia; 6 Institut Salah Azaïz, Medical Physics, Tunis, Tunisia; 7 Institut Salah Azaïz, Radiation Oncology, Tunis, Tunisia Purpose or Objective There are different options in the treatment planning of stereotactic radiotherapy for brain metastases. The objective was to compare the dosimetric parameters obtained by VMAT with those by conformal dynamic arc therapy (DCA). Materials and Methods we included 20 patients treated with stereotactic radiotherapy for brain metastases. The mean dose prescribed was 25.9 Gy (21–30 Gy) in 3 to 5 fractions. Treatment planning was performed on the Eclipse TPS (AXB13.7 and PO13.7 algorithm, Varian Medical Systems, PA,CA), using the VMAT technique (1 Arc and 2 non-coplanar pArc).The DCA plan consists of 3-5 non-coplanar arcs. The dosimetric analysis relating to the coverage of the planning target volume (PTV) included the parameters and indices recommended by the IRCU91, namely the Conformity Index (CI) and the Gradient Index (GI). The constraints of the healthy-brain (V21, V24) were evaluated according to the recommendations of QUANTEC. A comparison of the number of Motor Units (MU) was also carried out. Results The mean volume of the (PTV) was 12.75 cm3 (1.4–51.5 cm3). The mean relative minimum dose was 24.4 Gy (19 - 29.14Gy) for the VMAT and 23.5 Gy (17.46–28.16Gy) for the DCA plan. The mean relative maximum dose was 32.57 Gy (28.9–37.31Gy) and 33.45 Gy (28.8–38.6Gy) for respectively the VMAT and DCA. The CI was 1.31 (1.11-1.61) and 1.34 (1.2– 1.56) for VMAT and DCA respectively. The mean GI was 0.65 (0.51–1.31) in VMAT versus 0.59 (0.46-1.25) in DCA. However this difference in GI value was not significant (p=0.4). The V21 Gy was respected in 95% of cases (<20.9 cm3) with a mean of 8.15 cm3 (1.6–38 cm3) and 8.22 cm3 (1.6-34 cm3) respectively for the VMAT and the DCA. The V24 Gy was also respected (< 16.8 cm3) in 95% of cases with a mean of 4.51 cm3 (0.63–21.2 cm3) and 4.85 cm3 (0.65–20 cm3) respectively for the VMAT plan and the DCA plan. There was no significant difference in bivariate analysis between the two techniques in terms of respect of all parameters evaluated in this study. Otherwise, the number of MU was lower for the conformal dynamic arc therapy technique with a mean of 1398 (948–2764) versus 2318 (1487–3351) for the VMAT technique. Conclusion the homogeneity and the conformity, while sparing healthy brain tissue, was similar in this dosimetric analysis between these two treatment techniques. However, The VMAT technique allows a rapid delivery of the dose and makes a potential limitation of intra-fraction movements which can help treating effectively much more patients especially in the condition of lack of sources in our country. PO-1994 Stereotactic Radiotherapy For Brain Metastases: a dosimetric analysis between two techniques A. Essadok 1 , A. Mousli 2 , S. Masssoudi 3 , W. Kaabia 4 , A. Ghorbel 4 , R. Abidi 1 , L. Ben Salem 5 , M. Besbes 6 , C. Nasr 7

PO-1995 Impact of range uncertainties on robust mixed electron-photon beam radiation therapy

V.J. Heng 1 , M. Serban 2 , M. Renaud 3 , J. Seuntjens 2

1 McGill University, Medical Physics Unit, Montreal, Canada; 2 Princess Margaret Cancer Centre, Department of Medical Physics, Toronto, Canada; 3 Gray Oncology Solutions, ., Montreal, Canada Purpose or Objective Mixed Electron-Photon Beam Radiation Therapy (MBRT) is an emerging technique that consists of delivering a combination of both electrons and photons in a single treatment. MBRT aims to leverage the steep dose fall-off and high superficial dose of electron beams while maintaining the high target conformity of photon treatments. As electron dose distributions are