Evaluation of the dose calculation accuracy in intensity-modulated radiation therapy for mesothelioma, focusing on low doses to the contralateral lung

Our plain-English summary of Evaluation of the dose calculation accuracy in intensity-modulated radiation therapy for mesothelioma, focusing on low doses to the contralateral lung, by L.E. Court, D. Ching, D. Schofield, et al (Journal of Applied Clinical Medical Physics, 2009).

The study

• Authors: L.E. Court, D. Ching, D. Schofield, M. Czerminska, A.M. Allen
• Journal: Journal of Applied Clinical Medical Physics
• Published: 2009;10(2):34-42
• Cited by: 9 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

This study compares Monte Carlo (MC) with conventional treatment planning system (TPS) calculations. The EGS4nrc MC code, BEAMnrc, was commissioned to simulate a Varian 21Ex Linac. The accuracy of the simulations, including points blocked by the jaws, was evaluated by comparing MC with ion chamber and MOSFET measurements. Eight mesothelioma IMRT cases were planned using Eclipse (pencil beam and superposition convolution algorithms). Dose distributions were recalculated using BEAMnrc/DOSxyz, and compared with TPS. MC agreed with experimental results for IMRT fields within 3% (96% of points). For regions blocked by the jaws, average agreement between MC and experiment was better than 5% up to 20 cm from isocenter. The pencil beam algorithm underestimated lung MLD, V20, and V5, compared with MC, by a mean (range) of 16% (11-22%), 9.0% (2.4-30.1%), and 11.8% (2-30%), respectively. The superposition convolution algorithm gave better agreement of 8.5% (0-17%), 4% (0-12%) and 0% (-6-6%). Mean dose to the targets was better than +/- 5% in all cases. In conclusion, there is excellent correlation between TPS and MC calculations for the target doses. The pencil beam algorithm and superposition convolution algorithms both underestimate lung dose parameters, but the superposition convolution dose offers improvements in dose calculation accuracy for these patients.

In plain English

This medical physics study concerned radiation treatment planning for pleural mesothelioma. It did not study asbestos exposure. The authors used CT scans and outlines from four patients previously treated with intensity-modulated radiation therapy (IMRT) after extrapleural pneumonectomy in Boston. They made two research plans per patient, eight in all. They compared Eclipse planning-system doses, from a pencil-beam and a superposition convolution algorithm, with Monte Carlo simulations first checked against measurements. Compared with Monte Carlo, the pencil-beam algorithm underestimated mean contralateral (opposite-side) lung dose by a mean of 16.4% (range 11.5 to 22.5). The superposition convolution algorithm underestimated it by 8.5% (range 0.4 to 17.2). For the lung volume receiving 5 Gy or more, mean differences were 11.8 and minus 0.2 percentage points of total lung volume, respectively. The minus sign meant superposition convolution gave a slightly higher value than Monte Carlo. The average difference in mean target dose was under 1% for both algorithms (standard deviation 2.5%). The authors concluded that both algorithms underestimate important lung dose parameters but accurately calculate dose to the targets. They noted that their accuracy tests did not include significant tissue density differences.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Treatment-planning (dosimetry) comparison study: planning-system dose calculations compared with Monte Carlo simulations, after testing the simulations against phantom measurements
• Who was studied: CT images and contours of four patients (two right-sided, two left-sided) previously treated with IMRT for mesothelioma after extrapleural pneumonectomy at Dana-Farber Cancer Institute / Brigham & Women's Hospital, Boston. Two research IMRT plans were made per patient, giving 8 plans. These were not the plans used to treat the patients.
• Exposure: No asbestos exposure was studied. The study concerned 6 MV X-ray IMRT plans (prescriptions of 54 Gy, 48 Gy and 60 Gy to different target volumes). Doses were calculated by a modified-Batho pencil-beam algorithm and by a superposition convolution algorithm (AAA), and compared with Monte Carlo simulation.
• Main results:
  • Monte Carlo compared with MOSFET measurements on a solid water phantom (81 points): only 3 points (4% of points) agreed worse than plus or minus 3%
  • For regions distant from the collimators up to 20 cm from isocenter, average agreement between Monte Carlo and measurement was 5 plus or minus 1%, 2 plus or minus 5%, and 3 plus or minus 2% for three 10 x 10 cm field set-ups. Differences were generally larger beyond 20 cm (30%, 31% and 1%).
  • Contralateral lung mean dose, pencil-beam algorithm compared with Monte Carlo: mean difference 16.4% (range 11.5 to 22.5), expressed as a percentage change referenced to the Monte Carlo dose. A positive value means Monte Carlo showed a higher dose than the planning system.
  • Contralateral lung mean dose, superposition convolution algorithm compared with Monte Carlo: mean difference 8.5% (range 0.4 to 17.2)
  • Lung V5 (volume receiving 5 Gy or more), pencil-beam: mean difference 11.8 (range 2.4 to 30.1), expressed as a percentage of total lung volume (percentage points)
  • Lung V5, superposition convolution: mean difference minus 0.2 (range minus 6.4 to 5.6) percentage points of total lung volume. A negative value means the planning system showed a slightly higher value than Monte Carlo.
  • Lung V20 (volume receiving 20 Gy or more), pencil-beam: mean difference 9.0 (range 6.5 to 16.4) percentage points of total lung volume
  • Lung V20, superposition convolution: mean difference 4.1 (range 0.0 to 11.5) percentage points of total lung volume
  • The underestimation compared with Monte Carlo was statistically significant at the 5% level (Wilcoxon matched pairs signed rank test) in two sets of measures. For the pencil-beam algorithm these were mean lung dose, lung V5, lung V20 and mean liver dose. For the superposition algorithm they were mean lung dose and lung V20.
  • For the targets (GTV and CTV), the average difference in calculated mean dose was less than 1% for both algorithms, with a standard deviation of 2.5%. The article's abstract states that mean dose to the targets was better than plus or minus 5% in all cases.
• The authors' conclusion: The authors concluded that, for the eight cases evaluated, both the superposition convolution and pencil-beam algorithms underestimate important lung dose parameters in mesothelioma IMRT compared with Monte Carlo. The pencil-beam algorithm gave particularly large differences. Both algorithms accurately calculate dose to the targets.
• Limitations the authors state:
  • The tests of the Monte Carlo code did not include significant density heterogeneities, so they did not test its ability to evaluate the impact of tissue heterogeneities

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Intensity-Modulated Radiation Therapy (IMRT), Mesothelioma.

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