Achieving a Uniform Breast Junction With the 160 Leaf Multi Leaf Collimator

Abstract

Assessment of a clinical method that insures a uniform junction when treating four field breast cases on a linac outfitted with a new 160 leaf multileaf collimator. This work was undertaken because of inconsistent junction measurements between three matched linacs. Numerous breast patient step-and-shoot plans were generated using a treatment planning system using a collapse cone convolution superposition dose calculation algorithm. Each plan was composed of two parallel-opposed beams for the supraclavicular-axillary region and two tangents for the breast gland using a single isocenter technique. 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. It was centered by adjusting the beam trajectory parameters. 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°. 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 the treatment planning system 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 the calculated dose. 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 with the new 160 leaf multileaf collimator.