ESTRO 38 Abstract book

S536 ESTRO 38

compared to a library of plans approach (LOP), which is currently the state of the art. However, intrafraction anatomical changes set a lower limit on the required PTV margin. The aim of this study is a dosimetric comparison of MRI-guided strategies with a LOP strategy taking intrafraction anatomical changes into account. Material and Methods The 14 patients included in this study were treated with chemoradiation at our institute and received weekly MRIs after informed consent. The weekly MRIs were considered to represent day-to-day anatomical changes that occur during the treatment. For the LOP strategy the plans were based on interpolations of the cervix-uterus on pretreatment full and empty bladder scans. The two MRI- guided strategies (MRI_3mm and MRI_5mm) consisted of treatment plans created on the weekly sagittal MRIs with 3 mm and 5 mm PTV margin around the cervix-uterus. All plans were VMAT dual arc plans with a prescribed dose of 45 Gy in 25 fractions. For the dosimetric evaluation of the different strategies targets and OARs were delineated on each weekly transversal MRI, which was acquired on average 10 minutes after the sagittal MRI. This way also the effect of intrafraction motion was taken into account. To enable comparison with DVH parameters for the whole treatment, for each weekly MRI the fraction dose was multiplied by 25, the total number of fractions. For the delineations on each transversal MRI the following DVH parameters were calculated: cervix-uterus CTV D 98% , high- risk CTV D 98% , bowel bag V 40Gy and V 30Gy , and bladder and rectum D mean and V 40Gy . Results For the MRI_5mm strategy D 98% of the high-risk CTV was at least 95% for all weekly MRIs of all patients, while for the LOP and MRI_3mm strategy this requirement was not satisfied for at least one weekly MRI for 1 and 3 patients, respectively. In Figure 1 an example is given where intrafraction anatomical changes resulted in insufficient coverage of the cervix-uterus CTV for the MRI_3mm strategy. As compared to the LOP strategy the bowel bag V 40Gy was on average reduced by 148 cm 3 and 135 cm 3 for the MRI_3mm and MRI_5mm strategy, respectively, while for V 30Gy this was 136 cm 3 and 129 cm 3 (Figure 2). Bladder D mean was reduced by 2.7 Gy and 1.8 Gy, for the MRI_3mm and MRI_5mm strategy, respectively, while bladder V 40Gy was reduced by 24% and 17% (percentage points). Rectum D mean was reduced by 14.0 Gy and 11.9 Gy, while V 40Gy was reduced by 53% and 47% (percentage points). All differences were significant (p < 0.05).

Conclusion With online MRI-guided radiotherapy of cervical cancer considerable sparing of OARs can be achieved. If a new treatment plan can be generated and delivered within 10 minutes, an online MRI-guided strategy with a 5 mm PTV margin for the CTV of the cervix-uterus is sufficient to account for intrafraction anatomical changes. PO-0981 Role of on-table plan adaptation in MR- guided ablative radiation therapy for central lung tumors T. Finazzi 1 , M. Palacios 1 , F. Spoelstra 1 , C. Haasbeek 1 , A. Bruynzeel 1 , B. Slotman 1 , F. Lagerwaard 1 , S. Senan 1 1 VU University Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective As patients with centrally located lung tumors are at increased risk of toxicity with SABR, we performed stereotactic MR-guided adaptive radiation therapy (or SMART) for such patients. We retrospectively analyzed the benefits of daily on-table plan adaptation. Material and Methods Twenty-four patients with central tumors underwent video-assisted, respiration-gated SMART on the MRIdian (ViewRay, Inc.). Tumors were sub-grouped as moderately central (PTV within 2 cm of proximal bronchial tree; n=18), ultracentral (PTV overlapping trachea / main stem