Differences in effective target volume between various techniques of accelerated partial breast irradiation

Area of Science:

• Oncology
• Medical Physics
• Radiotherapy

Background:

• Clinical target volume (CTV) definition for accelerated partial breast irradiation (APBI) varies between balloon brachytherapy (1 cm margin) and three-dimensional conformal radiotherapy (3D-CRT) (1.5 cm margin).
• Previous research suggested equivalence between these CTV margins.
• Deformable registration offers a method to precisely assess treated volumes in radiotherapy.

Purpose of the Study:

• To evaluate the effective clinical target volume (CTV) treated by MammoSite balloon brachytherapy using deformable registration.
• To compare the effective MammoSite CTV with CTVs generated by standard 3D-CRT planning margins.
• To investigate the impact of cavity-balloon conformance on the effective CTV margin.

Main Methods:

• Ten patients treated with MammoSite balloon brachytherapy were retrospectively analyzed.
• Two CT scans (pre- and post-implantation) were obtained for each patient.
• In-house deformable registration software was used to map the MammoSite CTV onto a balloonless CT dataset, with validation using anatomical landmarks.

Main Results:

• The effective MammoSite CTV was, on average, 7% larger than 1 cm 3D-CRT CTVs but 38% smaller than 1.5 cm 3D-CRT CTVs.
• The average effective CTV margin was 1.0 cm, matching the actual MammoSite margin, but was non-uniform, ranging from 5 to 15 mm.
• Poor cavity-balloon conformance resulted in effective margins less than 1 cm; balloon size did not significantly correlate with effective margin.

Conclusions:

• The 1.0-cm MammoSite CTV margin treated a significantly smaller effective volume compared to the 1.5-cm 3D-CRT CTV.
• The effective margin for MammoSite balloon brachytherapy is non-uniform and influenced by cavity-balloon conformance.
• This study suggests a potential discrepancy in target volume coverage between MammoSite and 3D-CRT APBI techniques.