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Achieving a Uniform Breast Junction with the Elekta Agility

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E Letourneau

E Letourneau1*, F Girard2, (1) Centre Integre de Cancerologie de Laval, Laval, Quebec, (2) Centre Integre de Cancerologie de Laval, Laval, Quebec

SU-E-T-330 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
Assessment of a clinical method that insures a uniform junction when treating four field breast cases on Elekta Synergy linacs outfitted with the new Agility 160 leaf multileaf collimator.
Methods:
Numerous breast patient step-and-shoot plans were generated using Philips Pinnacle³ treatment planning system. Calculated doses were compared with measurements done using Gafchromic EBT3 film inserted in a plastic water phantom.
The linac beam spot position was measured according to one of the recently published methods by Balazs et al. (2012). The jaws and MLC leaves were calibrated using the portal imager following the calibration workflow integrated into the linac console. The resulting junction was evaluated using two half blocked fields on XV film at gantry 0°.
Results:
Initial measurements showed slight variations in beam spot position between all three linacs. These variations were the main factor causing inconsistent and cold junctions. The beam spot adjustment followed by the collimator calibration workflow improved field junctions on the XV film to within tolerance for all linacs.
Patient plans in Pinnacle³ showed that a beam overlap was needed to achieve adequate coverage of the treatment region on the CT slices located close to the junction. In order to avoid overlapping fields at the skin, only the external tangent and the posterior field were overlapped.
Gafchromic film measurements showed that a 2 mm beam overlap for a minimum of 5% of the monitor units of both overlapping beams was required to produce a uniform junction within 5% of Pinnacle³ dose calculation.
Conclusion:
In order to achieve a uniform junction, fine tuning of the beam spot position was necessary. A dosimetric method was developed to achieve uniform dose distribution in the junction area when treating four field breast cases on an Elekta Synergy linac with the new Agility 160 leaf multileaf collimator.

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