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Dosimetric Effects of Magnetic Field On Co-60 and Yb-169 HDR Brachytherapy Sources

S Ahmad

S Ahmad1,2*, W Song1,2 , (1) Sunnybrook Odette Cancer Centre, Toronto, Ontario, (2) Department of Radiation Oncology, University of Toronto, Toronto, ON


SU-I-GPD-T-40 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate the impact of magnetic field on the dose distribution of Co-60 and Yb-169 high dose rate (HDR) brachytherapy sources.

Methods: HDR brachytherapy sources Co-60 and Yb-169 were simulated in Geant4 in the presence of magnetic field of various strengths ranging between 0T and 6T. The correct physical dimensions of the seeds were acquired from the published articles. Magnetic field was applied both perpendicularly and parallel to the longitudinal axis of the seed. A 10x10x10 cm grid comprising small voxels of uniform 1 mm resolution was used for dose scoring. Comparisons were drawn from the isodose distributions. All the isodose lines were normalized to the dose delivered to a point 1 cm away along the transverse axis as defined in AAPM’s report TG-43.

Results: For Co-60 source the isodose distributions were unaffected by the transverse magnetic field of strength 1.5T in all planes. For 3T transverse magnetic field strength the isodose distributions were tightened (moved closer to the source) by as much as 2 mm in the coronal plane of the seed. The separation between the isodose lines further increased by up to 4 mm when a 6T magnetic field was applied. In case of Yb-169 the magnetic field did not have any effect on the dose distribution around the source.

Conclusion: The dose distribution for a Co-60 HDR brachytherapy source is significantly altered by the application of a transverse magnetic field of strength 3T or higher while that for Yb-169 source is unaffected. The effect of the magnetic field is to tighten the isodose lines for Co-60 source. A combination of a correct source (consequently the correct energy) and the magnetic field can be effectively utilized potentially to generate a more conformal treatment plan by steering the high doses away from the critical organs.

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