ESTRO 2021 Abstract Book

S1339

ESTRO 2021

Conclusion Our study revealed good agreement between the measured EBT – XD film GRID dose map and the TPS GRID dose distribution. The geometry was accurately depicted on the film. The valley-to-peak ratios calculated from both the plan and film are considered clinically acceptable. Both ratios are in accordance with the literature presented values. However, the MLC interleaf leakage may contribute substantially to the final GRID dose distribution and it could potentially lead to non-acceptable valley to peak ratios. According to our results, EBT – XD film can be considered suitable for GRID dosimetry but further studies should be carried out, in order to compare both the TPS planned dose distribution and the EBT – XD film dose map with water phantom measurements.

PO-1617 Assessing the impact of treatment delivery uncertainty using DVHs N. Papanikolaou 1 , C. Kabat 1 , H. Parenica 1 , S. Stathakis 1 1 UT Health San Antonio MD Anderson Cancer Center, Radiation Oncology, San Antonio, USA

Purpose or Objective Plan evaluation commonly involves the use of Dose Volume Histograms (DVH), which are generated based on the CT simulation and the dosimetric model of the linear accelerator. Patient movement, variation in daily setup and alignment, and fluctuations in machine delivery parameters give rise to an uncertainty in the dose delivered to the patient. Quantifying how those uncertainties impact the delivered plan’s quality can help us generate a more realistic DVH. This study assesses the differences in DVHs that are reconstructed from fractions delivered over the course of the patient’s treatment. Materials and Methods Log files produced from three Elekta Versa HD linacs were collected from ten unique head and neck patients. Each patient received at least 30 fractions of the same treatment. Each log file was used to generate a new radiation plan (RP) file that mirrored dose rates, gantry and collimator angles, and MLC locations. New RP files were used in conjunction with the original patient CT and structures to recalculate dose distributions and DVHs for each fraction of a patient’s treatment delivery using the Monaco treatment planning system. Dose distributions were exported and interpreted in a MATLAB code, based on the original plan structures. Results Normal tissue volumes were found to have an average DVH variation of 25 cGy, and target volumes had a variation of 60.5 cGy. Deviations occurred most often for structures that received higher doses to a lower normalized volume. There was no correlation between total structure volume and DVH variance. No specific region (i.e., PTV or OAR) exhibited significant deviations between planned and delivered dose as evaluated through the DVH reconstruction analysis.

Figure 1: (Left) displays a DVH generated for one of the HN patients for 30 delivered fractions. DVHs were plotted for each fraction, allowing them to stack up over time and produce a spread. Figure 2: (Right) shows that the dose distribution is mostly random, and there are no significant portions of the delivery or anatomy that contribute significantly to the deviations in the plotted DVHs. There were no significant differences in dose distributions and a max value of ± 1 Gy was observed in any single voxel. Conclusion DVHs generated from treatment delivery log files on the Elekta Versa HD linear accelerator were shown to have good delivery reproducibility. All deviations were within 1% of their expected value. The DVHs generated in this study provide valuable insight into how delivered dose correlates to a planned dose based on machine reproducibility. We plan to incorporate daily CBCTs for dose calculation to study how alignment variability and machine delivery impact treatment quality.

PO-1618 Validation of the Monte Carlo calculation in the presence of high-density materials using gafchromic

B. Pawalowski 1,3 , A. Ryczkowski 2,4 , K. Graczyk 5 , M. Kruszyna- Mochalska 5,4 , H. Szweda 5 , T. Piotrowski 5,4

1 Greater Poland Cancer Centre, Medial Physics Department, Poznan, Poland; 2 Greater Poland Cancer Centre,