ESTRO 2023 - Abstract Book

S1642

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ESTRO 2023

motion, translation. However, large intra-fraction liver tumour rotation up to 15º has been observed [2-5]. As technological innovation evolves towards delivering increasingly precise radiotherapy, it is important to investigate the dosimetric impact of intra-fraction tumour rotation in liver SBRT treatments. In this study, we quantified the dosimetric impact of intra- fraction liver tumour rotation for seven liver cancer patients treated in the TROG 17.03 LARK clinical trial [6]. Materials and Methods Patients received SBRT in one of three treatment schedules: 3x17 Gy, 5x10 Gy, 5x6.5 Gy. The patients were treated at breath-hold using 6FFF VMAT plans with a 5 mm gross tumour volume (GTV) to planning target volume (PTV) margin. The intra-fraction tumour translation and rotation was measured with Kilovoltage Intrafraction Monitoring (KIM) [7]. A gating threshold of 3 mm for 5 sec or 5 mm for 5 sec was used in KIM. The residual translation is denoted as T res . Intra-fractional rotation (R uncorr ) remained uncorrected during treatment. The delivered dose was reconstructed including intra-fractional dynamic (a) translation and rotation (T res +R uncorr ) and (b) only translation (T res ) using the DoseTracker software [8-9]. The differences from the planned dose including translation, T res and rotation, R uncorr ( Δ D(T res +R uncorr )) and including only translation, T res ( Δ D(T res )) were determined for GTV D100 and PTV D95. Results The mean and standard deviation of translation (T res ) and rotation (R uncorr ) in left-right, superior-inferior and anterior- posterior directions were 0.5±1.0, 0.3±1.6, -0.6±1.4 mm and 0.0±2.6º, 1.4±2.4º, 1.6±2.1º respectively over all treatment fractions for all patients, the minimum and maximum rotation angles being -21.9º and 17.9º respectively. Δ D(T res +R uncorr ) was >5% in 3 fractions (11%) for GTV and in 7 fractions (25%) for PTV out of 28 treatment fractions (Fig.1). Δ D(T res ) was always within 5% of the planned dose in all treatment fractions for both GTV and PTV. The cumulative dose ( Σ D(T res +R uncorr ) and Σ D(T res )) over all treatment fractions for all patients was always within 5% of the planned dose for both GTV and PTV.

Conclusion The dosimetric impact of intra-fraction liver tumour rotation during liver SBRT treatments was quantified for the first time in this study. These results suggest that while dose errors > 5% exist in certain fractions due to residual translation (T res ) and uncorrected rotation (R uncorr ), the cumulative dose for every patient remains within 5% of the planned dose. Residual dose errors due to uncorrected liver rotation exist which may be corrected using different treatment adaptation strategies [10-12] to further improve the dosimetric accuracy.