The Hospital of St. Raphael is zeroing in on the harder part of cancer treatment: destroying a malignancy without damaging surrounding healthy tissue, or the entire body.
This dilemma is especially critical in radiation therapy, which uses X‑rays to bombard and kill cancer cells.
Unlike a scalpel and other surgical tools, radiation is invisible, and the doctor cannot see what is actually being irradiated. Consequently, the beam has to be aimed as accurately as possible.
Saint Raphael’s has installed a new system of computerized lasers that can pinpoint a target to within a fraction of an inch. Because doctors and radiologists know that few nearby cells will be damaged, they are able to deliver a higher, more effective, dose.
St. Raphael’s recently installed an arch of three lasers adjacent to a scanner that performs both X‑ray and positron emission scans. The two images are used to define the target and distinguish between cancer and scar tissue.
The data is sent to a computer that controls the lasers, which can define the center of a three-dimensional object. Freckle-like tattoos are injected under the laser points, so that when the patient and his cancer are in precisely the right position for treatment in another laser-equipped room.
“The patient is lined up with the lasers. Then he gets a PET scan,” said Jamie Sheehan, manager of nuclear medicine and PET scans at the hospital’s Father Michel J. McGivney Center for Cancer Care.
Peggy Black, lead technician for radiation oncology at St. Raphael’s, said the laser navigation system is employed with all forms of radiation therapy, including intensity modulated radiation therapy, and stereotactic therapy — two types of therapy designed to minimize radiation expose to healthy cells.
Radiation disrupts and damages the DNA in cancer cells, preventing them from reproducing.
An incoming patient is first immobilized in as comfortable way as possible, Sheehan said.
Otherwise, patients have a tendency to move, making precise therapy impossible.
A new system to keep people still uses plastic bags full of soft foam. When the patient is placed in exactly the correct position, air is pumped out of the bags, leaving a rigid cast of the patient’s body, Black said.
The patient then undergoes two scans from a dual-purpose Siemens Biograph Sensation 16 PET/CT scanner. (Black andSheehan are pictured at the top of the story under the laser bridge next to the scanner.)
During the CT, or computed tomography scan, the patient is slowly advanced through a large circular device that takes X‑ray pictures all the way around the patient. The scan produces a detailed image of the patient’s chest, abdomen, pelvis, or other area of interest.
The CT scan also reveals the size and location of the tumor. The patient is then injected with a radioactive sugar-like compound. As the radioisotope decays, it emits positrons, which are like electrons, only positively charged.
When positrons encounter electrons, the particles annihilate and beam two gamma rays in opposite directions on the same plane. Many of the rays escape undetected, but enough are captured by sensors ringing the patient. This information is converted into an image, too.
Unlike the CT scan, the PET scan reveals cells that are absorbing the sugary tracer. That includes cancer cells. By superimposing the CT and PERT images, radiologists can see exactly where the cancer is, and how to target the tumor.
Ultimately, all of this data flows into a computer that calculates the optimal dose of radiation.
The patient is then relocated in a treatment room, and lined up with lasers on the walls and ceiling.
Many weaker beams of radiation may be set to intersect at the tumor, or the therapy machine may be equipped with an aperture that changes the rate of radiation as the machine circles the patient.
“If we localize the cancer we can use a higher dose and cause less collateral damage,” Black said.