Chin Med J (Taipei) 1998;61:S69.

Three-Dimensional Measurement of Dose Distribution Produced by a Intensity Modulated Radiation Therapy System Using Magnetic Resonance of BANG Polymer Gel Dosimeters

Ping S. Wong, Ph.D., Chung Y. Lin, M.S., Richard L. Cardenas, M.S., Kelvin K. Cheng, Ph.D.

Department of Radiation Oncology, Southwest Cancer Center, Texas Tech University Medical Center, Lubbock, Texas

Abstract

Purpose/Objective. A intensity modulated radiotherapy (IMRT) system¹, capable of moving each of the 40 leaves in and out of the beam during the gantry rotation, produces complex dose distributions which are difficult and impractical to measure with conventional dosimetry instrumentation. The recently developed BANG^2,3,4 polymer gel dosimetry system provides accurate, high resolution and three dimensional dose distributions data and is ideally suited for the task described above. In this study, the polymer gels were used for imaging the dose distributions produced by IMRT system.

Materials & Methods. The dosimeter materials consist of 2-liter BANG-15 polymer gels² in a spherical, clear glass flask (14cm diameter with 12cm neck), closed with a ground glass stopper. Leaf patterns were generated for a single target (3cm diameter) and a dual target (3cm and 1.5cm diameter) plans using the Peacock inverse planning system. The gels were irradiated using a 6 MV Linac with a multileaf intensity modulating collimator(MIMic). A total of 10 Gy(85% of maximum dose) was delivered to the target volume by three couch indexes using I cm mode. The gels were then imaged by MRI (GE Signa Horizon LX8.1) at 1.5T, using a series of Hahn spin echoes of TR=2s, TE=20,100,200,400 ms. Transverse relaxation rate(R2) maps were constructed from those multiple images using the non-linear least-squares Lavenberg-Marquardt algorithm and a data analysis and display program "Dosemap"², which was written using the scientific computational program MATLAB⁵. R2 maps were converted to dose maps using R2-to-dose calibration curve. Dosernaps and isodose curves were then compared with corresponding data from the treatment planning computer software.

Results. The dose dependence of the NMR transverse relation rate R2, is reproducible (less then 3% variation) and is linear up to about 15 Gy, with a slope of 1.1s^-1 Gy^-1 at 1.5 Tesla. Isodose curves (for both single and dual targets) in three orthogonal (axial, saggital and coronal) planes show good agreement between the gel data with those calculated by the Peacock treatment planning system.

Conclusion. The tissue-equivalent polymer dosimeter provides a unique and reliable approach for visualization and validation of three dimensional dose distributions produced by the intensity modulated conformal radiotherapy system.

1. NOMOS Corporation, Sewickley, PA
2. MGS Research,lnc. Guilford, CT
3. Maryanski, M.J. et al, US Pat. No. 5,321,357,1994
4. Maryanski, M.J. et al, Phys.Med.Biol.39,pp. 1437-1455,1994
5. The Math Works, Inc.,Natick, Mass.

[Chin Med J (Taipei) 1998;61:S69.]