Abstract Book

S1070

ESTRO 37

EP-1968 The capability and characteristic of Tomo at sparing hippocampus in prophylactic cranial irradiation J. Zhu 1 , T. Bai 1 , J. Gu 1 , B. Li 2 , Y. Yin 1 1 Shandong Cancer Hospital affiliated to Shandong University, Department of Radiation Oncology Physic and Technology, Jinan, China 2 Shandong Cancer Hospital affiliated to Shandong University, Department of Radiation Oncology, Jinan, China Purpose or Objective Prophylactic cranial irradiation (PCI) is efficient at avoiding lung cancer intra-cranial metastasis. However, PCI may cause neurocognitive impairment because of the hippocampal toxicities. It is reported that biologically equivalent doses in 2-Gy fractions to 40% of the bilateral hippocampus greater than 7.3 Gy (V 7.3Gy > 40%) is associated with long-term impairment. Considering helical tomotherapy (HT) has smaller sub-field and higher modulation capabilities, this study aims to investigate the capability and characteristic of HT at sparing hippocampus in PCI. Material and Methods Six anonymous patients previously treated with whole- brain radiotherapy with hippocampal sparing were reviewed. Under T1-weighted MRI, the hippocampus was contoured, and hippocampal avoidance regions (HAR) were created using a 5-mm volumetric expansion around the hippocampus. The prescription dose 3.0Gy * 10 fractions was assigned on planning target volume (PTV) which was defined as whole-brain minus HAR and bilateral hippocampus. During the HT plan optimization process, evaluation factors of PTV and other dosimetry factors were recorded whenever the mean dose of hippocampus decreased by every 2Gy. Results The average of six HT plans without hippocampus sparing constraints are as follow: hippo_D mean 27.33±2.22Gy, HAR_D mean 28.62±1.84Gy, PTV_D mean 30.77±0.74Gy, PTV_D 2% 30.93±0.61Gy, PTV_D 98% 29.86±0.34Gy, PTV_V 30Gy 0.97±0.02% and conformal index (CI) 0.85±0.03, homogeneity index (HI) 0.04±0.01, plan delivery duration 13.70±1.92min.The variation trend of each factors during hippocampus sparing were plotted in figures as follow. When V 7.3Gy of bilateral hippocampus achieved 40%, the average dosimetry factors are as follow: hippo_D mean 8.26±0.24Gy, HAR_D mean 12.88±1.27Gy, PTV_D mean 31.00±0.47Gy, PTV_D 2% 31.97±0.63Gy, PTV_D 98% 21.74±5.44Gy, PTV_V 30Gy 0.93±0.02%, CI 0.77±0.08, HI 0.35±0.20, plan delivery duration 21.30±4.97min. Figure1. The variation trends of HAR, PTV_Dmean, PTV_D2% and PTV_D98% while the hippocampus sparing process. Figure2. The variation trends of target coverage percentage (PTV_V30Gy), CI, HI and plan delivery duration time while the hippocampus sparing process.

navigator slices as internal surrogates. The method is not sensitive to the ROI position as inspiration phases were reproducibly detected and respiratory cycle time intervals were automatically defined. Integration into treatment planning system was also possible in order to picture the dynamic behavior of the liver (and so the lesion). EP-1967 VMAT technique on left sided breast focusing on the dose to the heart A. Stravato 1 , V. Palumbo 1 , A. Fogliata 1 , G. Reggiori 1 , P. Mancosu 1 , L. Paganini 1 , F. De Rose 1 , D. Franceschini 1 , S. Tomatis 1 , M. Scorsetti 1 1 Istituto Clinico Humanitas, Department of radiotherapy and radiosurgery, Rozzano Milan, Italy Purpose or Objective This study focuses on the planning of patients with left sided breast cancer. Different volumetric modulated arc (VMAT RapidArc, RA) designs were considered in order to evaluate the dosimetric trade-offs for a clinically acceptable plan, trying to find the optimal balance between heart sparing and target coverage and conformity. Material and Methods Ten patients with left-sided breast cancer treated with VMAT technique were included in the hypofractionated clinical protocol. The daily dose prescription was 3.2 Gy to the boost region in simultaneous integrated boost (SIB) technique and 2.7 Gy to the whole breast for 15 fractions. The patients were simulated in a supine position with the arm abducted (90° or greater) on the disease side. Treatment plans were generated for a 6MV Varian UNIQUE linac, equipped with standard Millennium MLC with 120 leaves. Three different RA designs were optimized for each patient (Fig.1): "2F" : a basic two partial arcs were arranged covering the entire partial arc trajectory with total rotation of about 2x 220° (Fig.1A) "Flip" : each arc from the 2F plan was splitted in two "sub" arcs optimizing for each one the rotation of the collimator for heart dose reduction (Fig.1B) "Flip_Avoid" : the "Flip" plan was re-considered with an anterior avoidance sector of about 50-60 degree to preserve the heart on a couple of fields (Fig.1C) Plan optimization objectives were stringent on dose homogeneity, mean dose to the heart <4Gy, ipsilateral lung <8Gy, contralateral lung <3Gy, contralateral breast <3Gy. An evaluation in terms of DHV comparison was performed with an homemade software based on MatLab. Mean doses, high doses to organs at risk were considered. Homogeneity and conformity indexes were calculated. Results All plans achieved at least 95% of PTV receiving 95% of dose, with V 105% less than 5%. The main differences regarded the low dose distribution. Mean heart doses were significantly lower for the "Flip_Avoid" plans ( p <0.005) when compared to the "2F" plans. No significant differences were seen for the controlateral lung while an improvement was achieved for the V 5Gy and V 20Gy for the ipsilateral lung. The role of the avoidance and the collimator projection is crucial for the plan optimization and dose heart reduction. Conclusion The main differences among plans concerned the doses to the organs at risk, still maintaining an high dose coverage and conformity. "Flip" and "Flip_avoid" plans presented a reduction of the mean doses for all critical structures: heart, ipsilateral lung, contralateral lung, contralateral breast.