SU‐E‐T‐186: Treatment Planning Dose Accuracy of Whole Breast Irradiation Using Field‐In‐Field Technique

Purpose: To evaluate the planned dose accuracy of whole breast irradiation using field‐in‐field technique by comparing with 2D array detector measurements and Monte Carlo simulations. Methods: The ion chamber based Octavius 2D‐array detector (PTW, Freiburg, Germany) was used to measure the absolute dose and dose distributions of tangential field‐in‐field breast irradiation. A commercially available treatment planning system (Eclipse V10.0, Varian, Palo Alto CA USA) was used to calculate the absolute dose and dose distributions. Phantom geometry was simplified for the measurements and for Monte Carlo simulations, but its size and shape were similar to typical breast. The dose accuracy of the treatment planning system was evaluated and the desirable points for monitor unit calculation were recommended based on the results of the study. Results: The discrepancy between measured and planned dose distribution depended on the location of the monitor unit calculation point. The differences were larger when monitor unit calculation point was closer to the irradiated edges of phantom. The doses calculated by the treatment planning system near the center of the irradiated field were about 2% higher than measured doses. There was good agreement between the measured doses and those predicted by Monte Carlo methods. Conclusion: Because of inadequate treatment of scatter in the irradiation of breast with tangential ports, the dose predictions by a treatment planning system do not always agree with the actual measurements. The treatment planning algorithm evaluated in this study over‐compensates for the loss of scatter in tangential ports glancing the sloping surface of a typical breast. The choice of the prescription point can have significant effect on the dose prediction and the actual dose delivered. Keeping the prescription point away from the field edges will help improve the agreement between the calculated and measured dose.