(P029) A Comparison of CT- and MRI-Defined Lumpectomy Cavity for Radiotherapy Planning of Breast Cancer

Wei Huang, MD, PhD, Adam Currey, MD, Victor Chen, PhD, Frank Wilson, MD, Allen Li, PhD; Medical College of Wisconsin

PURPOSE: To compare lumpectomy (LC) and planning target volume (PTV) that is delineated using magnetic resonance imaging (MRI) and computed tomography (CT) and examine the possibility of replacing CT with MRI for radiotherapy (RT) planning for breast cancer.

MATERIALS AND METHODS: MRI and CT data were acquired for 14 patients with early-stage breast cancer who had undergone lumpectomy at their radiation treatment positions (prone) using a large-bore CT scanner (HighSpeed, GE) and a 3-T large-bore MRI scanner (Vero, Siemens) during RT simulation. All patients had dense breast tissues. The LCs were delineated manually on both CT (LC-CT) and MRI acquired with three sequences (T1, T2, and short TI inversion recovery [STIR]) (LC-T1, LC-T2, and LC-STIR, respectively) by a radiation oncologist and verified by another radiation oncologist. The PTV (PTV-MRI) was created by expanding a uniform 15-mm margin from the union of the LC-T1, LC-T2, and LC-STIR and was compared with those from CTs (PTV-CT). Differences were measured in terms of cavity visualization score (CVS), volume, Dice coefficient (DC), and distance between centers of mass (COMs).

RESULTS: The mean CVSs for T1-, T2-, STIR-, and CT-defined LCs were 3.36, 3.36, 3.79, and 2.50, respectively, implying that LC is mostly visible with the MR STIR sequence. For most cases (12/14), the LC volumes or PTVs from T1, T2, and STIR were smaller than those from CT. The mean reductions of LCs for T1, T2, and STIR from those for CT were 20%, 44%, and 36%, respectively. However, the differences between the volumes of PTV-MRI (the union of three sequences) and PTV-CT were smaller. The DCs between CT- and MRI (union of T1, T2, and STIR)-defined volumes were 0.60 ± 0.15 for LCs and 0.85 ± 0.08 for PTVs. The COM shifts from PTV-MRI to PTV-CT were 0.31 ± 0.25, 0.35 ± 0.39, and 0.36 ± 0.33 cm in the x-, y- and z-axis, respectively. The average PTV-MRI:PTV-CT volume ratio was 1.11±0.22, ranging from 1.00 to 1.25 for most cases (10/14). In 12 of 14 cases, MRI-defined LC included extra regions that would not be visible from CT.

CONCLUSIONS: MRI substantially improves the visibility and accuracy of lumpectomy cavity definition as compared with CT. Although the LC and PTV volumes that are delineated from an individual MRI sequence are generally smaller as compared with those from CT, the volumes, shapes, and locations for the PTV-MRI, defined by the union of T1, T2, and STIR, were comparable with PTV-CT for most of the cases studied. It is feasible to use MRI to replace CT in RT simulations for breast-conserving RT.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org