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.