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Variability of Low-Z Inhomogeneity Correction in IMRT/SBRT: A Multi-Institutional Collaborative Study
Poonam Yadav1, Colleen M DesRosiers2, Raj K Mitra3
1Department of Radiation Oncology, Northwest Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA.
Journal of Clinical Medicine
|February 11, 2023
Summary
Pencil beam algorithms show significant dose calculation errors in lung cancer radiation therapy, especially for small fields. Collapsed cone algorithms offer better accuracy but still exhibit variations across treatment planning systems.
Area of Science:
- Medical Physics
- Radiation Oncology
- Computational Dosimetry
Background:
- Accurate dose calculation is crucial for effective radiation therapy, with outcomes varying between treatment planning systems (TPS).
- Modern algorithms employ detailed 3D beam modeling for precise dose calculations, particularly in heterogeneous tissues and small fields common in intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT).
- The dosimetric accuracy of these algorithms in lung environments remains incompletely understood.
Purpose of the Study:
- To evaluate and compare the accuracy of Type A (pencil beam) and Type B (collapsed cone) dose-calculation algorithms.
- To analyze calculated doses in lung patients and validate algorithms through phantom measurements.
- To assess the impact of clinically used dose algorithms on target and organ at risk (OAR) coverage.
Main Methods:
- CT scans of a lung patient and a custom-built lung-equivalent phantom were used for treatment planning at nine institutions.
- Dose measurements were performed in the phantom at various depths and field sizes, with and without inhomogeneity correction.
- Calculated and measured doses were compared to evaluate algorithm validity.
Main Results:
- Significant dosimetric variations were observed in patient plans for target and OAR coverage across different TPS.
- Pencil beam algorithms exhibited substantial discrepancies (≈70% variation) between measured and calculated correction factors, especially in small fields.
- Collapsed cone algorithms showed better agreement with measurements (within ±10% for Type B) but still presented inter-system variability.
Conclusions:
- Pencil beam algorithms are not recommended for small fields in IMRT/SBRT due to inaccurate dose calculations.
- Type B collapsed cone algorithms provide superior accuracy compared to pencil beam but require further standardization across TPS.
- Variations among TPS highlight the need for rigorous algorithm validation and quality assurance in clinical practice.
