MERT vs tangential photon beams vs IMRT for breast cancer: a comparative dosimetry study

Abstract

Purpose: Recently, energy- and intensity-modulated electron radiotherapy (MERT) has garnered growing interest for the treatment of superficial targets. In MERT, small electron beams (beamlets) of different angles, energies and intensities are optimized to achieve dose conformity both laterally and in the depth direction. The potential of MERT for breast cancer is to improve dose uniformity in the treatment volume while reducing dose to normal tissues such as the lung, heart and contralateral breast. In this work, we carried out a comparative dosimetry study to evaluate MERT, photon beam intensity-modulated radiation therapy (IMRT) and conventional tangential photon beams for the treatment of breast cancer. We also included the effects of respiration (organ motion) on each treatment modality. Materials and methods: A MERT system has been investigated, which consists of a set of software tools to perform accurate dose calculation, treatment optimization, leaf sequencing and plan analysis. A prototype electron multileaf collimator (eMLC) for accurate MERT beam delivery has also been developed. For this study, we have compared breast treatment plans (with and without nodal involvement) generated using MERT, IMRT and conventional tangential photon beams. The MERT plans were derived using our own treatment optimization software with up to 3 gantry angles and 5 nominal energies (6, 9, 12, 16, 20 MeV beams from a Varian Clinac 2100C accelerator). The tangential photon treatment plans were derived using the CMS FOCUS 3D treatment planning system with 6 MV wedged photon beams. A 6 MV anterior-posterior field was also used if there was nodal involvement. The IMRT plans were derived using the NOMOS CORVUS treatment optimization system with the same tangential beam angles of 6 MV x-rays. To remove any inconsistencies between the dose calculation algorithms used in the treatment planning system, all the plans were recalculated using the Monte Carlo method. Our Monte Carlo code system also took into account the effects of organ motion due to respiration and the effects of photon and electron leakage through the MLC and scattering off the MLC leaves. Results: In general, MERT provides better or similar target dose coverage compared with conventional wedged tangential photon beams and intensity-modulated tangential beams. Our results confirmed the findings of previous investigators that IMRT could reduce the dose to the lung, heart and contralateral breast compared to conventional tangential wedged beams (up to 5 Gy in the maximum dose). However, MERT reduces the maximum dose to the lung by up to 20 Gy and to the heart by up to 35 Gy compared to conventional tangential wedged beams. When respiration motion was considered with a 1 cm chest wall movement significant dose heterogeneities were found in regions where the tangential photon beams abut the anterior-posterior supraclav photon beams. The effect of respiratory motion on MERT dose distributions was very small since the modulated electron beams were incident normally or at relatively small oblique angles. Conclusion: Because of its superior capabilities to achieve dose conformity both laterally and in the depth direction, MERT can be developed into a useful modality for superficial targets, especially for the treatment of breast cancer. The advantages of MERT over the currently used treatment techniques include uniform target coverage and significantly reduced normal tissue toxicity. Software and hardware development is ongoing with the hope for clinical testing of this new beam modality in early 2002. Further investigations are being carried out for other treatment sites such as head and neck, lung (mesothelioma), and palliative spine cases. This work was supported in part by NIH: CA78331 and by DOD US Army Breast Cancer Research Program: BC971292.