ESTRO 37 Abstract book

S1136

ESTRO 37

conditions were selected for highlighting dose variation between reference and deformed phantom state. Dose 'deformed” value obtained after DIR application was compared with detectors measurement.

The mean D95% bladder coverage was found to be 98.3% with a range from 58.9% to 111.1%. S values were found to vary from -13.5 to 4.2 with a mean of -1.6. S values were found to have individual ranges between 2.5 and 14.2. Eight out of ten patients had negative mean S values ranging from -0.63 to -5.8. The remaining two patients’ S values were 0.17 and 0.02. No plans were found to be over the S value limits. Conclusion The variation in Dice coefficients throughout the patients’ course of treatment gives weight to the concept of adaptive bladder treatments. As the synthetic CTs take into account the day to day changes in anatomical shape and target location the trend for negative S values is indicative of sub-optimal plans being used. However, considering the discrete nature of plan selection, no better plan could have been chosen for any fraction without the possibility of compromising target coverage. This indicates that further consideration could be given to creating the expansion margins on which the initial plans are produced. [1] S Webb. The physics of three dimensional radiation therapy: Conformal radiotherapy, radiosurgery and treatment planning. IoP 1993 p20 EP-2069 Effects of MVCT scan methodology on setup verification and adaptive dose calculation in TomoTherapy J. Zhu 1 , T. Bai 1 , J. Gu 1 , Z. Sun 2 , Y. Wei 3 , B. Li 3 , Y. Yin 1 1 Shandong Cancer Hospital affiliated to Shandong University, Department of Radiation Oncology Physic and Technology, Jinan, China 2 Affiliated Hospital of Shandong Academy of Medical Sciences, Medical Department, Jinan, China 3 Shandong Cancer Hospital affiliated to Shandong University, Department of Radiation Oncology, Jinan, China Purpose or Objective To evaluate the effect of pretreatment megavoltage computed tomographic (MVCT) scan methodology on setup verification and adaptive dose calculation in helical TomoTherapy. Material and Methods Both anthropomorphic heterogeneous chest and pelvic phantoms were planned with virtual targets by TomoTherapy Physicist Station and were scanned with TomoTherapy megavoltage image-guided radiotherapy (IGRT) system consisted of six groups of options: three different acquisition pitches (APs) of ‘fine’, ‘normal’ and ‘coarse’ were implemented by multiplying 2 different corresponding reconstruction intervals (RIs). In order to mimic patient setup variations, each phantom was shifted 5 mm away manually in three orthogonal directions respectively. The effect of MVCT scan options was analyzed in image quality (CT number and noise), adaptive dose calculation deviations and positional correction variations. Results MVCT scanning time with pitch of ‘fine’ was approximately twice of ‘normal’ and 3 times more than ‘coarse’ setting, all which will not be affected by different RIs. MVCT with different APs delivered almost identical CT numbers and image noise inside 7 selected regions with various densities. DVH curves from adaptive dose calculation with serial MVCT images acquired by varied pitches overlapped together, where as there are no significant difference in all p values of intercept & slope of emulational spinal cord (p=0.761 & 0.277), heart (p=0.984 & 0.978), lungs (p=0.992 & 0.980), soft tissue (p=0.319 & 0.951) and bony structures (p=0.960 & 0.929) between the most elaborated and the roughest serials of MVCT. Furthermore, gamma index analysis shown that, compared to the dose distribution calculated on MVCT of ‘fine’, only 0.2% or 1.1% of the points analyzed on MVCT

Results On numerical phantoms, average Dice values were 0.83 (± 2σ = 0.1) and 1.8mm for MDC for tested deformation scenarios. For vector fields comparison, the criterion 95% voxels not exceeding 2mm was respected. A reduced voxel size improved results. The markers position comparison between reference frame 'no deformed – deformed” and 'no deformed – deformed after DIR” gives us an uncertainty associated with voxels mapping of 10.3% on average. In terms of dose, uncertainty associated with voxels mapping is 4.3% for MOSFET's and 10.4% for TLD's. Conclusion ART software commissioning requires images deformation and dose warping validation. Results with numerical and physical phantoms make it possible to consider implementation of the first clinical applications. EP-2068 Adaptive bladder: Are we adapting well? M. Anderson 1 , E. Miguel 1 , S. Currie 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Glasgow, United Kingdom Purpose or Objective The bladder is capable of great size variations in a relatively short period of time. The adaptive treatment process involves producing three plans with the aim of accounting for day to day differences in bladder filling. The patient is given a cbCT prior to each fraction and the most suitable plan is chosen based on bladder volume on the day including a small margin between bladder and 90% isodose volume. The aim of this investigation was to determine if the most suitable plan was indeed chosen. This data could be used to refine the process for future patients. Material and Methods Data from ten patients previously treated with 55 Gy in 20 fractions were analysed using Velocity (Varian, Palo Alto, USA, v 3.2.1) and Eclipse (Varian, Palo Alto, USA, v 15.5). Synthetic CTs were created in Velocity for each pre-treatment cbCT for each patient. The chosen plan of the day was then calculated on the synthetic CTs. The Dice coefficient, bladder size difference, and D95% coverage was recorded for each fraction. Using a volume of regret (VoR), calculated as the difference between 90% isodose volume and bladder volume, a factor was created for each fraction using the equation: S = 10*(1-V d /V p ) [1] Where V d is the VoR for the delivered plan and V p is the VoR that was planned. These factors were assessed for each fraction and limiting values were calculated for each patient to indicate where a better plan could have been chosen. Results The Dice coefficients were found to vary from 0.48 to 0.93 with a mean of 0.75. The Dice coefficients were found to vary by between 0.12 and 0.44 for individual patients across all fractions. Bladder size was found to vary from -348.8 cc to 220.6 cc and a mean of -35.2 cc.