ESTRO 37 Abstract book

S1072

ESTRO 37

heart, ipsilateral lung, contralateral lung, contralateral breast. 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.

Conclusion During the process of hippocampus sparing, HAR_D mean decreased linearly as well. However, dosimetry variation of PTV presented three different regions: plateau region (> 20Gy), gradient region (~20 to ~10Gy) and falling region (<10Gy). There’s almost no significant impact on the PTV dosimetry factors when hippo_D mean was not lower than 15Gy. With the hippocampus dose decreasing, CI decreased linearly and slightly, however, HI was impaired severely in the falling region. Plan delivery duration time was significantly prolonged during the hippocampus sparing process (from 13min to 23 min). EP-1969 The impact of isocenter movement on treatment plan delivery – a planning study L.J.M. Wack 1 , O.A. Sauer 1 1 Klinik und Poliklinik für Strahlentherapie- Universitätsklinikum Würzburg, Medizinische Physik, Würzburg, Germany Purpose or Objective Switching patients between linacs is a common practice in many hospitals during machine maintenance and repair. Commonly, if the linac models are identical, the same treatment plan is used without re-planning. Nonetheless, machines, even of the same type, have slightly different mechanical uncertainties which are not accounted for in the planning system. Here, we investigate the possible impact of isocenter shifts on RT delivery. Material and Methods Isocenter shifts were determined for three Elekta Synergy linacs using the ISO-CBCT module as distributed by QualiFormeD (La Roche sur Yon, France). Shifts were determined for 6 gantry angles, as well as the distance between treatment beam (MV) and cone-beam CT (CBCT) isocenters. Five head-and-neck intensity-moduled radiotherapy (IMRT) plans with two prescribed dose levels and five lung stereotactic (STX) treatment plans were calculated in Pinnacle 9.10 (Philips, The Netherlands), using the CC convolution algorithm on a 1mm dose grid. For each machine, a copy of the plan was created and a new isocenter was assigned to each beam to account for the off-set between CBCT and MV and the isocenter movements during gantry rotation. If the beam angle was not measured in QualiFormeD, the isocenter coordinates were linearly interpolated. Resulting dose volume histograms were compared to determine dose changes in planned target volume (PTV) and boost volume. For the head-and-neck IMRT cases, doses to three organs at risk (OAR; spinal canal, parotis, and larynx) were also determined. Results Distances between CBCT and MV isocenters ranged from 0.83 to 1.52 mm. Under gantry rotation, the movement of isocenter position along the sagittal and horizontal axes ranged from 0 to 0.25mm. In longitudinal direction, the isocenter position changed by up to 1. 3mm.