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Abutment region dosimetry for the monoisocentric three-beam split field technique in the head and neck region using asymmetrical collimators

K Abdel-Hakim, MD 1 T Nishimura, MD 1 M Takai, PhD 2 S Suzuki, RT 1 and H Sakahara, MD 1

Departments of 1 Radiology and 2 Informatics, Hamamatsu University School of Medicine, 1-20-1 Handayama, Hamamatsu, 431-3192 Japan



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  		Figure 1. Field arrangement and film positioning with respect to a solid-water phantom.

 


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  		Figure 2. Verification film for one asymmetric jaw to be set exactly at the beam central axis using a collimator rotation of 180°.

 


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  		Figure 3. Film dosimetry of the three-beam split field (setting 1) shows consistent underdose at the junction owing to consistent collimator overlap at the zero position.

 


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  		Figure 4. Film dosimetry of the three-beam split field (setting 2) shows a nearly uniform dose at the junction after offsetting the collimator +1 mm from the supraclavicular side.

 


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  		Figure 5. Film dosimetry of the three-beam split field (setting 3) shows the same result as in Figure 2Go after offsetting the collimator +1 mm from the lateral side.

 


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  		Figure 6. Film dosimetry of the three-beam split field (setting 4) shows overdose at the junction after offsetting the collimator +1 mm from both sides, indicating too much collimator offsetting.

 


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  		Figure 7. Profiles of the three-beam split field show approximately 15% underdose (setting 1), nearly uniform dose (settings 2 and 3) and more than 10% overdose (setting 4) for the zero position, +1 mm offsetting from one side and +1 mm offsetting from both sides, respectively.

 


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  		Figure 8. Profiles of the two-beam split field show approximately 25% underdose (setting 1), 8% underdose (settings 2 and 3) and more than 15% overdose (setting 4) for the zero position, 1 mm offsetting from one side and 1 mm offsetting from both sides, respectively.

 


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  		Figure 9. Film dosimetry of the two-beam split field with a collimator rotation of 180°, shows a nearly homogeneous dose at the junction.

 


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  		Figure 10. Film dosimetry of the three-beam split field with matched collimators, shows an overdose at the junction.

 


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  		Figure 11. Profiles of the three-beam split and two-beam split fields with matched collimators. Note an 8% overdose at the junction owing to variation in the abutting technique.

 




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