Use of serial CT imaging for the quality assurance of MammoSite therapy

As experience with the MammoSite device for accelerated partial breast irradiation (APBI) has increased, more centers are starting to use three-dimensional (3D) treatment planning to generate plans with multiple nonequally weighted dwell positions. This report presents the use of serial computed tomography (CT) imaging, in addition to planar or ultrasound imaging, for the quality assurance of an APBI treatment using the elliptical MammoSite.

CT images of a patient implanted with a 4cmx6cm elliptical MammoSite balloon were acquired. A treatment plan using multiple, nonequally weighted dwells was generated and delivered on Day 1 of a 10-fraction, twice-daily treatment. Before morning treatments on Days 2-5, the patient was reimaged on CT. Treatment plans on repeat CTs were generated two ways: using the decay-corrected plan from Day 1 (unadapted) vs. modifying the plan to account for changes in implant geometry (adapted). Adapted and unadapted plans on repeat CTs were compared with one another, and to the Day 1 plan.

The use of unadapted plans led to increased doses to normal tissues, particularly the skin. Adaptive planning on the repeat CTs was effective for maintaining acceptable dosimetry throughout treatment.

Serial CT imaging was shown to provide a useful tool for the quality assurance of an elliptical balloon implant during the course of treatment. Serial CT imaging, as opposed to planar or ultrasound imaging, was necessary to evaluate skin dose and to facilitate adaptation of the treatment plan to satisfy limits for skin dose.