Monte carlo modelling and dosimetric evaluation of cobalt-60 teletherapy in advanced radiation oncology

Abstract

Numerous external beam radiotherapy innovations have emerged, and these are biased to linear accelerators. A very few computational studies have been undertaken concerning these in cobalt-60 teletherapy. The influence of the lower energy and penetration of a cobalt-60 beam compared with linear accelerator beams in negligible for intensity-modulated radiotherapy. Recent innovations include the incorporation of magnetic resonance imaging guidance in cobalt-60 units. The aim of this research was to investigate source head fluence modulation in cobalt-60 teletherapy by using a three-dimensional physical compensator and secondary collimator jaw motion. This was validated by Monte Carlo simulations of three different source diameters. The Oncentra treatment planning system was used to develop three hypothetical plans by secondary collimator jaw motion. A design was made of a three-dimensional physical compensator. A clinical MDS Nordion Equinox 80 cobalt-60 teletherapy unit was used to acquire conventional water phantom beam characteristics. Central-axis depth dose curves and off-axis lateral beam profiles were generated from the water phantom scans. Fluence modulation experiments were executed at 5.0 cm depth in a PTW universal IMRT verification phantom using calibrated GafChromic EBT2 and RTQA2-1010 film batches. Gafchromic EBT2 film was used to sample intensity maps generated by secondary collimator jaw motion, yet GafChromic RTQA2-1010 film sampled maps from the three-dimensional physical compensator. The SSDs used were 75.0 cm and 74.3 cm for the GafChromic EBT2 and GafChromic RTQA2-1010 film measurements, respectively. Isodose contour printouts imported in DICOM format from the treatment planning system were used to generate the corresponding lateral beam profiles. A two-dimensional gamma index analysis was coded to compare EBT2 film measurements with DICOM data. This analysis was also used to verify film measurements versus Monte Carlo simulations. Lateral beam profiles generated from water phantom measurements were also used to establish source head fluence modulation on the film measurements. The source head fluence of a cobalt-60 teletherapy beam could be modulated by secondary collimator jaw motion and using a three-dimensional physical compensator. Sharply-defined beamlets similar to those of linear accelerators could be produced by a shift to smaller source diameters. Radiochromic film was a viable tool for verifying the dose distributions in intensity-modulated cobalt-60 teletherapy beams. There is thus a potential to modulate the source head fluence of a cobalt-60 beam by using secondary collimator jaw motion and a three-dimensional physical compensator. The Monte N-Particle eXtended radiation transport code can be a viable tool for the treatment planning of the anticipated fluence maps in cobalt-60 teletherapy.