End-to-end dosimetric evaluation of ITV margins in lung SBRT using a QUASAR phantom

Abstract

Author(s): Adeb A.S.A. Almaamari*, K. Nouni, A. Ait Errouhi, M.A. Youssoufi, M. Driouch, A. Lachgar, H. Elkacemi, T. Kebdani, K. Hassouni

Background: Stereotactic Body Radiotherapy (SBRT) is an advanced technique for treating lung cancer with high precision, but respiratory-induced tumor motion remains a major challenge. To assess the quality and safety of SBRT treatments through an end-to-end test, ensuring treatment reproducibility, accuracy, and patient safety.

Materials and methods: Respiratory tumor motion was simulated using QUASAR and Octavius 4D phantoms, with 4DCT-based SBRT planning using Monaco 5.1. Dosimetric verification was performed using the Anzai system, Semiflex 3D ionization chamber, and Octavius 4D 1600 SRS array.

Results: Based on the results obtained, all measured relative dose deviations remained within the acceptable ± 5% limit recommended by ICRU TG-142, confirming the dosimetric accuracy and reliability of the treatment plans. The 3-arc VMAT plans demonstrated the best overall dosimetric conformity, particularly under irregular and rapid respiratory cycles. Furthermore, the Gamma Index analysis showed excellent agreement (>90%) between measured and calculated doses across all respiratory conditions, supporting the robustness and clinical reliability of the studied plans.

Conclusion: This end-to-end SBRT study demonstrated good agreement between planned and delivered doses across regular, irregular, and rapid respiratory patterns, with both VMAT and DCA achieving acceptable dosimetric accuracy and conformity. Increasing the number of arcs reduced dose deviations and mitigated the respiratory Interplay effect, despite target-motion uncertainties caused by baseline shifts and variations in respiratory amplitude and period.

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