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Magnetic Particle Imaging May Allow Intraoperative Breast Tumor Margin Assessment

Key findings

  • Up to 66% of women who undergo lumpectomy require a second surgery because positive margins are found postoperatively
  • Researchers at Massachusetts General Hospital are exploring a novel approach to intraoperative margin assessment: magnetic particle imaging (MPI), in which magnetization of iron oxide nanoparticles yields a signal indicating the presence of residual tumor
  • In three different proof-of-concept experiments, a hand-held detector was demonstrated to have clinically relevant sensitivity (it found as little as 100 ng of iron oxide, representing a tumor just 790 microns in diameter)
  • When a small MPI scanner was used to image 3D-printed "lumpectomy specimen" phantoms, it determined tumor location and proximity to margins with high sensitivity in 10.7-second imaging times
  • Repeated use of MPI during breast-conserving surgery might better achieve complete tumor removal and thus a higher likelihood of negative margins

Second surgeries due to positive margins are necessary for 20% to 66% of women with early-stage breast cancer who undergo lumpectomy. After the initial surgery, a pathologist finds residual tumor cells near the edge of the excised tissue, which is a risk factor for cancer recurrence, and the surgery must be repeated to remove more of the tumor.

Many methods have been tried for assessing margins intraoperatively so that surgeons can continue to remove tissue at indicated sites until they're confident all tumor cells are gone. All the modalities have various limitations, though, including lack of depth penetration, long analysis times and the need for additional expertise.

Erica E. Mason, PhD, a postdoctoral fellow in the Magnetic Resonance - Physics and Instrumentation Group in the Martinos Center for Biomedical Imaging at Massachusetts General Hospital and Lawrence L. Wald, PhD, director of the group, and colleagues are exploring a novel approach: magnetic particle imaging (MPI), in which magnetization of iron oxide nanoparticles, which accumulate at tumor tissue, generates a signal that indicates the presence of and can be used to localize the residual tumor.

In Scientific Reports, they show this method was feasible in laboratory simulations of breast-conserving surgery.

Proposed Workflow

The researchers envision that for lumpectomy the MPI workflow would be:

  • Before surgery, the patient would receive an intravenous injection of a superparamagnetic iron oxide nanoparticle (SPIO) tracer, which is known to accumulate in breast tumors
  • During surgery, after the tumor specimen is removed, a hand-held MP detector would be used at the incision site to find SPIOs still in the breast (indicating tumor remaining in the breast)
  • Simultaneously, the tumor specimen would be placed in a small MPI scanner to quickly image the distribution of SPIOs in the removed tissue
  • Having these two types of information, the surgeon would repeat the process as necessary

In this study, the team developed and tested the two devices.

Hand-held MP Detector

The MP detector was mounted on a flexible arm and positioned outside the incision. In three different proof-of-concept experiments, it was able to detect as little as 100 ng of SPIO, the expected accumulation in a tumor just 790 microns in diameter.

MP Scanner

The researchers envision the MP scanner as a portable tabletop or cart-mounted system that would be placed in the operating room and produce fast 3D images. In this study, they tested a small-bore, mechanically rotated 2D field-free line permanent magnet-based imager.

The team developed and 3D-printed three "lumpectomy specimen" phantoms in which the "healthy tissue" was the plastic (no SPIOs) and the "tumor" was a cavity filled with a chosen SPIO concentration. Imaging of the phantoms was accomplished with high sensitivity in 10.7 seconds.

By measuring the distance between the tumor edge and the specimen margin, all three images were classified correctly as to their margin types (positive/negative) and provided visual information about the location of the tumor within the specimen.

Looking Ahead

SPIOs are stable, have a long shelf life and are considered safe injected agents; they are already approved for certain clinical uses. MPI has a number of other promising features:

  • Zero background signal from tissue and positive contrast
  • High sensitivity (SPIO magnetization is 22 million times stronger than the nuclear magnetization in high-field MRI)
  • Direct and quantitative tracer detection
  • No depth penetration limitations (magnetic fields are not attenuated in the human body)
  • Fast temporal resolution

These advantages may make MPI feasible for intraoperative breast cancer margin assessment.

Learn more about the Magnetic Resonance – Physics & Instrumentation Group

Learn more about the Martinos Center for Biomedical Imaging

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