A Descriptive Study Comparing the 95% and 100% Dose Volume Coverage at the Match Line in a Four-Field Monoisocentric Breast Phantom Plan in Eclipse by Opening Different Degrees of MLCs under the Collimator Jaws vs Closed

Introduction A monoisocenteric technique is a common technique used to treat breast cancer with regional nodal involvement. MIT has a single isocenter that is placed at the match line of the supraclavicular and tangential fields. The upper and lower portions of the fields are blocked by asymmetric jaws. The main disadvantage in a monoisocentric technique is underdosing at the match line region due to the tongue-and-groove effect. The tongue of one of the leaves travels across the same space as the groove of its neighboring leaf. This overlap gives rise to a region of low dose to the abutting fields. By removing the tongue-and-groove effect and opening different degrees of MLCs under the jaws could possibly give a better conformal dose to that area. This study aims to compare if there is an increase in the 95% and 100% volume coverage at the match line in a four field monoisocentric plan with different degrees of MLCs opened under the collimator jaws vs. closed. Methods This was a descriptive study based on a test patient that mimicked a monoisocenteric four-field breast plan created in the Varian Eclipse treatment planning system (TPS) (Version 13.1). The study included ten external beam 3D conformal photon plans using the same test patient for each plan but had two different energies (6MV,10MV). The degree of MLC was altered in each plan under the collimator jaws at the match line. The results were interpreted using IBM SPSS Statistics 24. These results were analyzed using a summary statistics chart displaying the mean, standard deviation, median, minimum, and maximum. A bar chart was also used to represent the comparison between the data. Results The results of this study demonstrated that as the MLC degree retraction increased, the magnitude of the 95% and 100% relative improvement dose volume coverage at the match line increased for 6MV and 10MV. The percentage of leakage radiation that was calculated for each field was between 1-2% for all plans. Conclusion When treating a whole breast or chest wall with a monoisocentric technique, there is always some concern about accuracy and dose uniformity at the junction of the match line due to the tongue-and-groove effect from the MLC. Since the dose volume at the match line has insufficient coverage, this results in the ability to effectively treat all sites of disease. By removing the tongue-and-groove effect and retracting the MLCs under the jaws, this study showed an increase in the 95% and 100% dose volume coverage at the match line in a monoisocentric breast plan