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?, International Journal of Radiation Oncology Biology Physics

Purpose: To compare tumour bed (TB) volumes delineated using MRI plus CT/clips (MRCT) with those delineated using CT/clips-alone in post-lumpectomy breast cancer patients positioned prone, and to determine the value of MRCT for planning partial breast radiotherapy (PBI). Materials/ Methods: 30 women with breast cancer each had 6-12 titanium-clips secured in the excision cavity walls at lumpectomy. Patients underwent prone CT-imaging followed by prone MRI (T1-weighted (standard & fat-suppressed) and T2-weighted sequences). TB volumes were delineated separately on CT and fused-MRCT datasets. Clinical (CTV=TB+15mm) and planning target volumes (PTV=CTV+10mm) were generated. Conformity indices between CTand MRCT-defined target volumes were calculated (ratio of volume-of-agreement to total-delineatedvolume). Discordance was expressed as a geographical-miss index (GMI=fraction of totaldelineated-volume not defined by CT), and a normal-tissue index (NTI=fraction of totaldelineated-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, GMI was 0.37 and 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 MRto CT/clip-data generated TB-volumes that were discordant to those based on CT/clips-alone. However, clinically satisfactory coverage of CTVMRCT by CTVCTbased 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. MRCT tumor bed delineation for partial breast RT Kirby page 3