Tumor Bed Delineation for Partial Breast and Breast Boost Radiotherapy Planned in the Prone Position: What Does MRI Add to X-ray CT Localization of Titanium Clips Placed in the Excision Cavity Wall?

Purpose: To compare tumor bed (TB) volumes delineated using magnetic resonance imaging plus computed tomography and clips (MRCT) with those delineated using CT and clips (CT/clips) alone in postlumpectomy breast cancer patients positioned prone and to determine the value of MRCT for planning partial breast irradiation (PBI). Methods and Materials: Thirty women with breast cancer each had 6 to 12 titanium clips secured in the excision cavity walls at lumpectomy. Patients underwent CT imaging in the prone position, followed by MRI (T1-weighted [standard and fat-suppressed] and T2-weighted sequences) in the prone position. TB volumes were delineated separately on CT and on fused MRCT datasets. Clinical target volumes (CTV) (where CTV = TB + 15 mm) and planning target volumes (PTV) (where PTV = CTV + 10 mm) were generated. Conformity indices between CT- and MRCT-defined target volumes were calculated (ratio of the volume of agreement to total delineated volume). Discordance was expressed as a geographical miss index (GMI) (where the GMI = the fraction of total delineated volume not defined by CT) and a normal tissue index (the fraction of total delineated volume designated as normal tissue on MRCT). PBI dose distributions were generated to cover CT-defined CTV (CTVCT) with ≥95% of the reference dose. The percentage of MRCT-defined CTV (CTVMRCT) receiving ≥95% of the reference dose was measured. Results: Mean conformity indices were 0.54 (TB), 0.84 (CTV), and 0.89 (PTV). For TB volumes, the GMI was 0.37, and the NTI was 0.09. Median percentage volume coverage of CTVCT was 97.1% (range, 95.3%-100.0%) and of CTVMRCT was 96.5% (range, 89.0%-100.0%). Conclusions: Addition of MR to CT/clip data generated TB volumes that were discordant with those based on CT/clips alone. However, clinically satisfactory coverage of CTVMRCT by CTVCT-based tangential PBI fields provides support for CT/clip-based TB delineation remaining the method of choice for PBI/breast boost radiotherapy planned using tangential fields.