Patient-specific quality assurance at the Heidelberg Ion Beam Therapy Center: 10 years experience in treatment plan
Abdallah Qubala1,2, Christian P Karger2,3, Julian Horn1,2,4
1Heidelberg Ion Beam Therapy Center (HIT), Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany.
Background:
To ensure accurate, safe, and reproducible patient treatments, it is essential to have precise knowledge and a solid understanding of patient-specific quality assurance (PSQA). For many years, the delivery of doses to all patients has been verified using dosimetric measurements. However, these measurements require substantial work, and the reasons for the occasional deviations are unclear. For these reasons, alternative methods such as independent dose calculations (IDCs) and analysis of beam-monitor log files are increasingly discussed in the particle therapy community. Nevertheless, before replacing dose-verification measurements with other methods, existing measurement data should be thoroughly analyzed to determine what can be learned from them and how they compare with potential alternatives. These alternative methods are mentioned in this work only to provide context and to outline possible directions for future studies.
Purpose:
To evaluate the dosimetric accuracy and efficiency of PSQA using a water phantom (WP) over a 10-year period at the Heidelberg Ion Beam Therapy Center (HIT).
Methods:
Between 2016 and 2025, 23014 treatment fields with protons, carbon, or helium ions were verified using a WP equipped with 24 pinpoint ionization chambers. The patient treatment plans were recalculated in the water phantom geometry and compared to measured absolute doses. The data were categorized by treatment room, ion species, treatment planning systems (TPS), range shifter (RaShi) use, indication, depth, and target volume, excluding measurements with human errors. Statistical analysis compared measured and calculated doses, focusing on mean, maximum, and minimum dose deviations. Furthermore, the workflow efficiency was assessed based on the beam time required for dosimetric verification, as well as the total time needed for preparation and analysis.
Results:
Mean dose deviations were in general slightly negative (t-test, p < 0.01), within ±1 % across all categories (total mean ± SD = -0.50 ± 0.90 %), with 91 % of fields passing institutional ±5 % tolerances. Further, significant differences (p < 0.01) were also observed between treatment rooms, ion species, TPS platforms, and RaShi settings. Additionally, the RayStation TPS showed lower deviations than the Syngo TPS, and helium ions had the smallest deviations. Moreover, repeated verifications reduced variability but without significant improvement. Correlations with target depth or volume were statistically significant but clinically negligible. Less than 1 % of maximum and minimum dose measurements exceeded ±7 % annually. Finally, over 4308 h of beam time, preparation, and analysis were spent on PSQA during the 10-year period.
Conclusions:
PSQA at HIT demonstrated high dosimetric accuracy and delivery stability. Integration of IDCs and log file analysis may improve efficiency and allow to omit verification measurements in well-established cases without compromising patient safety and treatment quality, if the extensive machine QA program is maintained.
