An imrt technique to increase therapeutic ratio of breast irradiation in patients with early-stage left breast cancer: limiting second malignancies

Abstract 

The clinical application of intensity modulated radiotherapy (IMRT) for adjuvant treatment of breast cancer has been the subject of increasing study in recent years. IMRT results in improved target coverage, reduced dose inhomogeneity within the breast, and reduced dose to the heart, lungs, and contralateral breast. However, this has been at the cost of larger volumes of low-dose radiation to these structures, thus increasing the theoretic risk for second malignancies. Our goal was to develop an IMRT beam arrangement that did not result in additional low-dose spill to organs at risk while maintaining equal or better target coverage. Five patients with early-stage left-sided breast cancer, who underwent breast conservation surgery and adjuvant radiation therapy, were chosen for this comparative study. The conventional radiation treatment (CRT) plan was comprised of standard wedged tangential fields. An IMRT plan consisting of 6 tangential beams (3 medial and 3 lateral) was generated by using the gantry, collimator, and table angles of the standard plan used for the CRT plan, and moving the table +10° and −10° on each side. The prescription dose for both CRT and IMRT plans was 45 Gy, 1.8 Gy/fraction, prescribed to the isocenter, which was placed near the center of the breast. IMRT plans provided significantly better coverage of the left breast than the CRT plans (p = 0.03). Although the dose heterogeneity was greater with the IMRT plans, the difference was not significant (p = 0.68). The mean volumes of the heart, lung, and right breast were lower in patients planned with IMRT at all dose levels from 5% to 100% dose (5% increments). This difference was significant for volumes receiving 2.25 Gy for the heart (p = 0.003), and volumes receiving 2.25, 4.5, 6.75, 33.75, 36, 38.25, and 42.75 Gy for the lung (p = 0.014, 0.04, 0.044, 0.05, 0.049, 0.045, and 0.05, respectively). Surprisingly, breast IMRT resulted in significantly lower right breast volumes irradiated at all dose levels compared to CRT. A 6-tangential-field IMRT technique achieved significantly better left breast coverage while maintaining lower doses to risk organs at all dose levels and therefore reduced the potential for induction of a second malignancy.

Key Words: Breast cancer, Radiation therapy, Treatment planning, IMRT, Second malignancies