397P - Comparison of different dose accumulation techniques to determine out-of-field accuracy for multiple lesions in lung SBRT

Background

The aim of the study is to determine out of field and isocenter dose accuracy for multiple lesions in lung SBRT of different dose accumulation techniques.

Methods

In the study stereotactic dose verification phantom with heterogeneity insert to simulate lung lesions used. A1SL ion chamber was placed in the phantom and the CT images were obtained with a slice thickness of 0.1 mm. The images were imported into the Monaco planning system and 3 pairs of lesions were created 2 cm, 5 cmand 8 cm distance from each other as ionization chamber placed at equal distances between the lesions. The electron densities of the lesions were corrected to 0.8 gr/cm³. The cavity of the ion chamber is also defined in the planning system. For two lesions located 2 cm apart, firstly individual plans were performed, and a sum plan was created to obtain total cavity dose. Secondly, a bias plans were created for two lesions and finally both two lesions optimized simultaneously, and cavity doses were obtained for all plans at two photon energies, 6 MV FFF and 10 MV FFF. This procedure was also performed for lesions 5 and 8 cm apart. All plans were transferred to the Elekta Versa HD and absolute point dose measurements were performed for out of field. The ionization chamber’s mean cavity dose values were compared with the calculated doses.

Results

It was determined that the measurements were higher than the planning values for all techniques between 0,51% and 9,87%. In the bias technique, the greatest difference in the 6 FFF and 10 FFF plans were found in lesions at 5 cm apart which is smaller than 10%. For the 2 cm and 8 cm apart lesions the difference between measurement and calculated were smaller than 2% for 6 FFF, 5% for 10 FFF. For the sum plan technique, the 2 cm and 5 cm apart lesions values were smaller than 10% both for 6 FFF and 10 FFF. It was found that the 8 cm apart lesions calculated, and measurement dose differences were smaller than 2%. When the simultaneous optimization technique was examined, it was revealed that the differences between measurement and calculation for both energies were less than 5%.

Conclusions

It is figured out that it would be more reliable to use the simultaneous optimization technique in appropriate cases as all results are below 5%.

Funding

Has not received any funding

Disclosure

All authors have declared no conflicts of interest.