[Dosimetric evaluation of intensity-modulated tangential beam versus conventional tangential irradiation for breast cancer].

BACKGROUND & OBJECTIVE: Breast-conserving therapy has become a widely accepted treatment option in the management of early-stage breast cancer. Postoperative radiotherapy is conventionally delivered with conventional wedged tangential fields. However, intensity modulation in radiotherapy helps to maximize tumor control while minimizing damage to normal tissues. This study was to evaluate the potential dosimetric benefits and optimal indications of intensity-modulated radiotherapy (IMRT) for the intact breast.

METHODS: Ten patients with stage Tis-2N0M0 breast cancer, who received breast-conserving surgery, were selected for this study. A dosimetric comparison of forward planning IMRT with conventional wedged tangential technique was performed on each patient using three-dimensional treatment planning. The total prescribed dose for both plans was 5000 cGy/25 fractions. Dose volume histograms (DVH) were used to compare the planning target volume (PTV) and organs at risks (OARs) such as ipsilateral lung, contralateral breast, contralateral lung for all patients, and coronary arteries, heart for left sided patients, and liver for right sided patients.

RESULTS: The PTV coverage in IMRT plan was similar to that of the conventional plan (97.7% vs. 98.3%). A better dose uniformity throughout the whole breast was achieved by IMRT plan. The percentage of PTV receiving less than 95% prescribed dose and more than 103% prescribed dose (inhomogeneity index, IHI) decreased from 29.9% to 2.9%; the percentage of PTV receiving more than 105% prescribed dose (V105%) decreased from 28.2% to 0.6%. A better amelioration of IHI and reduction of V105% in IMRT plans were observed in the relatively large PTV subgroup. Obvious reduction in the doses to OARs was achieved by IMRT plan. The maximum dose (D(max)) of coronary artery decreased from 5057.1 cGy to 4832.9 cGy, and the mean dose (D(mean)) of heart decreased from 629.8 cGy to 450.7 cGy; the Dmean of liver decreased from 283.9 cGy to 172.0 cGy for right sided patients; the Dmean and percentage of volume receiving more than 20 Gy (V20) of ipsilateral lung decreased from 925.2 cGy to 765.9 cGy, and from 16.0% to 15.3%, respectively; the Dmean and V20 of different central lung distance (CLD) subgroups decreased by 14.7% and 20.9%, 7.0% and 12.9%, respectively; the Dmean of contralateral breast decreased from 75.4 cGy to 20.3 cGy; the Dmean of contralateral lung decreased from 30.9 cGy to 16.1 cGy.

CONCLUSION: Forward planning IMRT based on a standard tangential beam arrangement significantly improves the dose homogeneity throughout the target volume of intact breast, and reduces the dose to OARs, especially in patients with large breast volumes or exceeded CLD, who might be proposed as candidates of IMRT for intact breast.