ESTRO 2024 - Abstract Book

S5946

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

were performed on patient. GTV has been delineated in all 10 phases to identify the iTV. After rigid registration, iTV was transferred on the near exhaled CT. PTV is described as iTV + 5mm margins. Prescriptions are based on the referential center for lung SBRT: 3 fractions of 20Gy or 18 Gy, 4 fractions of 15Gy and 5 fractions of 12Gy normalized on the median GTV dose [1][3]. For the CyberKnife® we used X-Sight-Spine® tracking. Dosimetric optimization was performed with the same goals. We optimize homogenous fluences on the 90% isodose, recalculated with respectively Monte Carlo and Accuros® and re-prescribe to the median GTV dose in the medium[7]. To ensure homogenous fluence optimization in Precision® we made the optimization with Ray Tracing and recalculated with Monte Carlo [1] [2] [4]. In Eclipse® we replaced the density of PTV by water equivalent and we recalculated with Accuros®. On the CIRS 4D phantom we simulated a respiratory motion equal to the motion of the GTV on the iTV and the exact respiratory frequency measured during 4D CT. Inside the CIRS we use calibrated gafchromic films cut on the same mask as the XLT CyberKnife® test on the sagittal direction. 3 patients were tested with Eclipse and 2 with CyberKnife® in 0-view spine tracking mode same as Eclipse patients.

Results:

For simple and complex geometry on the homogenous phantom, Ray Tracing and Monte Carlo are an accordance less than 2% and Accuros® less than 3%. Heterogeneous phantom made in evidence the high differences between Ray Tracing and Monte carlo for the Precision® TPS as in ref [5]. We found a good accordance with measurements for Monte Carlo (less than 1.5%) and an unacceptable more than 7% of under estimation for Ray Tracing. Accuros® is very close to Monte Carlo with only 2% differences on measurements. For measurements without motion for 4D CIRS: Accuros® on the Truebeam® is only -0.7% as already published [6] and for synchrony XLT test with monte carlo for CyberKnife® difference is less than 0.5% [5]. The dose comparison between calculated dose on the GTV and measured dose on the film during patient QA with motion are between -4% and -0% for Accuros® and 0.9% to 2.3% for 0-view CyberKnife®.

Conclusion:

Monte Carlo on CyberKnife® and Accuros® on Eclipse are in good agreement for dose calculation in low density conditions. Using an adequate dose optimization ensures dose accuracy for iTV treatments on free breathing method with a CyberKnife® on all type of lung treatments and with conventional LINAC. This result confirms the equivalence of dose prescription for these both SBRT techniques with this optimization and algorithms.

Keywords: LUNG, MonteCarlo, Accuros

References:

[1] GTV-based prescription in SBRT for lung lesions using advanced dose calculation algorithms [Lacornerie T et al 2014]

[2] On the pitfalls of PTV in lung SBRT using types-B dose engine : An analysis of PTV and worst case scenario concepts for treatment plan optimization. [Leung R et al, 2020]

[3] Improving inter-institutional and inter-technology consistency of pulmonary SBRT by dose prescription to the mean ITV dose [L. Wilke et al, 2021]