ESTRO 36 Abstract Book

S908 ESTRO 36 _______________________________________________________________________________________________

2UDepartment of Mathematical Physics and of Fluids, Madrid, Spain

Purpose or Objective Assessment of the differences between CT and CBCT based dose calculation for a volumetric modulated arc therapy (VMAT) in head and neck radiotherapy treatment. Material and Methods CBCT images of an Alderson RANDO phantom with tissue- equivalent material were acquired in a Varian`s On-Board Imager OBI (v1.5) installed on a Varian DHX accelerator, using its standard-dose head protocol (100 kV and 2.5 slice thickness). On the other hand, planning CT images were acquired in a Toshiba Aquilion LB using the same phantom and with our own clinical head and neck protocol (120 kV and 3 mm slice thickness). Different OAR (Body, spinal cord, parotids, mandible, oropharynx, dermis, an inner ring with 1 cm thickness and shoulder) and a PTV were delimitated. Additionally, reference points were inserted over all these structures. All defined structures and points were registered with the CBCT images by means of the Varian rigid registration software. Both the delimitation of volumes and the design of the treatment plan have taken into account the limited field of view of CBCT (length 16 cm, diameter 25 cm). A head and neck VMAT plan has been calculated in Eclipse (v10) using both sets of images. For CT images, we only used a standard calibration curve and 3 different calibration sets of curves for CBCT images, i.e., standard, measured with a CATPHAN 504 phantom and measured with a CIRS 062M head phantom placed between head and neck RANDO slices. Dose and HU were calculated in all reference points as well as dose-volume-histograms for the anatomical locations for both CT and CBCT. A gamma analysis was used for HVD comparison. Results The mean HU differences are less than 50 UH and the relative dose differences are less than 3% for all the calibration curves (Table 1) on every reference point over all the structures. The gamma (2%, 2 mm) DHV analysis shows an excellent agreement for almost all the structures (>95%) (Image1). Ring and dermis have gamma >85% .The non-pass regions correspond to very low dose regions. The worst gamma (>50%) corresponds to the left parotid because it is a very small structure (10 cc) into a high gradient dose zone. Furthermore, there is a difference of 1.8% on its volume as measured on the CBCT and the CT images, probably due to interpolation errors. These results are similar for all the calibration curves analysed.

Conclusion CBCT images for a head and neck VMAT treatment provide accurate dose calculation in adaptive radiotherapy, making them suitable for the assessment of possible changes over the original treatment planning for all the calibration curves analyzed. EP-1669 Assessment of the clinical value of off-line adaptive strategies for tomotherapy treatments D. Dumont 1 , X. Geets 2 , M. Coevoet 2 , E. Sterpin 1 1 Université catholique de Louvain, MIRO, woluwe-saint- lambert, Belgium 2 Cliniques Universitaires Saint-Luc, radiotherapy, woluwe-saint-lambert, Belgium Purpose or Objective This study assessed the clinical potential of offline adaptive strategies based on the dose computed on daily MVCTs (Tomotherapy). We defined clinical indicators that were subsequently used to identify the percentage of plans that should have been adapted due to significant dose deviations to TVs or OARs. Only the consistency of the initial plan throughout the treatment was addressed. Thus, dose was reported to constants TVs and deformed OARs. Material and Methods Cumulative doses were calculated from daily MVCT for 41 lung, 50 prostate and 21 H&N patients, using research versions of off-line adaptive solutions from Accuray and 21 st century Oncology. All deformed contours were checked by an experienced radiation oncologist, while all dose calculations were crosschecked using our in-house Monte Carlo model (TomoPen). The clinical indicators