Author: Takahiro Kato, Teiji Nishio, Masataka Oita, Robabeh Rahimi, Yuki Tominaga, Yushi Wakisaka 👨🔬
Affiliation: University of Maryland, Department of Radiological Sciences, School of Health Sciences, Fukushima Medical University, Medical Co. Hakuhokai, Osaka Proton Therapy Clinic, Medical Physics Laboratory, Division of Health Science, Osaka University Graduate School of Medicine, Faculty of Interdisciplinary Science and Engineering in Health Systems, Okayama University 🌍
Purpose: This study evaluated dose verifications and lateral penumbra improvements for scanned proton therapy plans with and without a multi-leaf collimator (MLC) under various air gaps.
Methods: Eighty-six uniform dose plans (38 and 48 plans of with and without a 60 mm range shifter) were generated for a 60×60×54 mm3 cubic target placed at nine different depths in water between 0 and 280 mm. They were created with MLC (MLC+) and without MLC (MLC-), with 50–300 mm air gaps. Forty-eight selected numbers of these plan’s 2D dose distributions were measured at the isocenter using a 2D ionization chamber array and gafchromicTM film. We compared the penumbra of each MLC+ and MLC- plans and average doses of the organs at risk (OARs) region located closely around the target (30 mm in all directions) in each cubic plan. We also compared three representative clinical plans for brain (chordoma), abdomen (liver), and pelvic (prostate) cases between the MLC+ and MLC- plans while maintaining comparable target coverage.
Results: The calculations and measurements were in good agreement, with the penumbra width difference within ±1.5 mm and the averaged gamma scores at 2%/2 mm being 97.4±3.1% for MLC+ and 97.9±2.3% for MLC-. The MLC+ plans reduced the penumbra width and averaged OAR doses by 2.0–10.2 mm and 3.3–13.5%, respectively, compared with MLC- plans even at more than 150 mm depths. All clinical MLC+ plans improved the conformity index covered by 95% and 50% of the prescribed dose by an average of 3.7±2.4% and 13.0±0.4%, respectively. The MLC+ plans also reduced the average OAR doses by at least 2.9%.
Conclusion: The calculated doses agreed with the measurements for both MLC+ and MLC-. We have shown that PBS with MLCs can be useful at more than 150 mm regions depending on the machine.