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Does a three-field electron/minitangent photon technique offer dosimetric advantages to a multifield, photon-only
Aminudin R Mydin1, Helene Gaffney, Alanah Bergman
1Vancouver Island Centre, Victoria, British Columbia, Canada.
Purpose:
To determine whether 3-field electron/minitangent photons (mixed-modality) technique improved dosimetry compared with a multifield, photon-only technique for accelerated partial breast irradiation (APBI).
Materials And Methods:
Subjects were 20 breast cancer patients previously treated with photon-only APBI as part of a clinical trial to a dose of 38.5Gy/10 fractions in 5 days. Mixed-modality replans were compared with the photon-only plans with regards to planning target volume (PTV) coverage, conformity, homogeneity, and doses to normal tissues.
Results:
The mixed-modality plans had similar PTV coverage, with more conformality, and reduced the volume of the ipsilateral breast receiving > or =95% and > or =50% doses, but had the pitfalls of less homogeneity and increased exposure of the ipsilateral lung and heart receiving low-dose radiation because of the exit dose of the electron beam. Factors associated with this increase were medial/deep-seated tumor beds, large tumor beds with PTV/Ipsilateral Breast ratio >20%, and use of high energy electrons.
Conclusions:
The three-field electron/minitangent photon APBI technique was more conformal and reduced the dose to the ipsilateral breast but had the disadvantage of exposing increased volumes of heart and ipsilateral lung to low-dose radiation compared with the photon-only technique.
