Intensity-modulated proton therapy (IMPT) gives doctors more flexibility to treat complex tumors

As sophisticated as Proton Therapy is today for successfully treating cancers with few side effects, proton beam technologies continue to advance to meet the clinical needs of cancer patients.

The latest generation of proton treatment methods is called intensity-modulated proton therapy or IMPT.

IMPT lets radiation oncologists adjust the precision, depth and intensity of a proton beam to the peaks and valleys of complex spiderlike tumors while avoiding healthy tissue. Tumors like these are typically found in the head, neck or spine. IMPT also is well suited for treating tumors that are next to or wrapped around vital organs such as the heart or pancreas.

This new proton therapy method is particularly attractive when treating children with rhabdomyosarcoma, skull base chordoma and Ewing’s sarcoma, said Dr. Anita Mahajan, medical director of the MD Anderson Proton Therapy Center in Houston, Texas.

Dr. Mahajan recently treated a teenage girl with a skull base chordoma. “The location of her tumor, nestled in between the brain stem and spinal cord, was a deciding factor in using IMPT,” she said.

IMPT improves upon a type of proton treatment called Pencil Beam Scanning (PBS), which has been used in recent years by physicians at MD Anderson and at other proton centers.

PBS delivers a beam spot as narrow as 3 millimeters, about the diameter of a smartphone audio jack. PBS has all but eliminated the need for personalized brass apertures, a kind of beam guide cut to the size and shape of each person’s tumor. Because of the limitations of cutting brass apertures, treating complex tumors with protons had been a big challenge — until PBS was devised.

PBS “paints” protons across the full area of a tumor, one layer at a time using a uniform dose for each layer. IMPT treats a small section of the tumor at a time, adjusting the proton beam dose and depth to wider and narrower contours of the tumor, section by section.

This more refined use of a pencil-thin proton beam, however, requires the latest three-dimensional imaging software to obtain highly detailed pictures of the tumor. In order to provide the benefits of IMPT to cancer patients, many proton therapy centers are investing in more sophisticated imaging technology.

Medical physicists and radiation oncologists are also perfecting how they can calibrate IMPT to account for tumors that move as the patient breathes. Doing so would ensure IMPT’s use to treat lung cancers.