Author: Yussuf A. Abdelal, Sebastien A. Gros, Luke Layman, Anil Sethi 👨🔬
Affiliation: Stritch School of Medicine, Cardinal Bernardin Cancer Center, Loyola University Chicago, Department of Radiation Oncology, Stritch School of Medicine, Loyola University Chicago 🌍
Purpose:
End-to-end (E2E) test is a vital QA check of the overall radiotherapy process from simulation imaging to treatment delivery. Although clinical passing criteria are available, guidelines on the test itself are scarce. E2E test suffers from two major drawbacks: absence of test sensitivity on treatment parameters and lack of transparency between dosimetric results and geometric uncertainties.
Methods:
Fifty-seven E2E plans were prepared on a test phantom equipped with ion chamber (IC) and radio-chromic films. Prescription doses were 6Gy to GTV (IC volume) and 4Gy to three target expansions of 2mm, 5mm and 10mm (PTV2, PTV5 & PTV10). Systematic 6-D linear and angular shifts of 1-5mm and 1-5° respectively were introduced to evaluate sensitivity of E2E test on target margins and geometric uncertainties. Plans were compared via IC dose and various plan comparison indices (dose profiles, dose difference, γ-max/mean values and γ-pass rates for various DD/DTA criteria: 1%/1mm - 5%/5mm). Dose vs. shift data were registered to recover geometric uncertainties from dosimetric analysis.
Results:
Compared to reference E2E plan, shifted plans showed a wide range of dosimetric changes. For PTV2, a 5mm-shift resulted in 13.4% & 42.3% reduction in IC Dmean and Dmin respectively. Whereas for PTV10, a 5mm shift had a less pronounced impact: 1.8% and 2.9% decrease in Dmean and Dmin respectively. PTV5 had intermediate dose results. Gamma analysis failed for all PTV2 plans that were subject to a 5mm shift with γ-pass rates of 42.4%(1%/1mm) to 91.6%(5%/5mm). However, PTV2 plans with 1mm shift passed all gamma criteria except 1%/1mm (82.2%).
Conclusion:
Correct selection of target margins and passing criteria is critical for a robust E2E test. Results of E2E sensitivity may be combined with dosimetric data to evaluate treatment uncertainties and setup appropriate target margins.