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Improving dose calculations on tomotherapy MVCT images.

Frederik Crop¹, Antoine Bernard, Nick Reynaert

¹Department of Medical Physics, Centre Oscar Lambret, 3, Rue Frédéric Combémale, 59020 Lille, France. F-Crop@O-Lambret.fr

Journal of Applied Clinical Medical Physics

|November 15, 2012

Summary

This study introduces a new protocol for precise dose calculations in tomotherapy using megavoltage CT (MVCT) images. The method corrects for daily variations in image value to density tables (IVDT), significantly reducing dose calculation errors for adaptive and StatRT planning.

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Area of Science:

Medical Physics
Radiation Oncology
Image-Guided Therapy

Background:

Megavoltage CT (MVCT) images are crucial for patient positioning and adaptive radiotherapy planning.
Hounsfield Unit (HU) to physical density conversion using Image Value to Density Tables (IVDT) is essential for accurate dose calculations.
Variations in measured HUs over time and due to technical factors introduce dose calculation errors, impacting treatment precision.

Purpose of the Study:

To develop a novel, precise protocol for dose calculations on MVCT images in tomotherapy.
To address and minimize dose calculation errors arising from daily IVDT variations in adaptive and StatRT planning.
To improve the accuracy of daily dose checks and adaptive planning by refining the HU-to-density conversion.

Main Methods:

A calibration model (HU = bρc - 1020) was derived using weighted least squares inverse prediction, accounting for heteroscedasticity.
A simplified daily protocol involving an airscan-like procedure with two inserts was proposed to determine the 'b' parameter and thus the IVDT curve.
A dose rate (DR) model was developed to correct for DR variations throughout the day, ensuring IVDT accuracy.

Main Results:

The proposed protocol significantly reduces dose calculation variations from up to 3% to 0.4% for pelvic and head-and-neck cases.
The 'b' parameter, dependent on dose rate, was identified as the major variable affecting IVDT accuracy.
The constancy of the 'c' parameter was demonstrated, simplifying the calibration process.

Conclusions:

The developed protocol enables accurate reconstruction of the daily IVDT using a simple morning measurement and dose rate correction.
This method enhances the reliability of daily dose checks and adaptive tomotherapy planning.
The simplified daily protocol with DR correction offers a practical solution to minimize dosimetric variations in clinical practice.