Dealing with Tumor Motion
Before imaging commences, Sarah’s team has to deal with the problem of tumor motion during imaging. This is especially critical in cases of lung cancer, where oncologists have tumors moving 1.5 to 2 centimeters (nearly an inch) during respiration.
       To cope with this, Sarah’s oncology team will use Varian’s RPM™ Respiratory Gating System to synchronize the acquisition of CT and PET images with Sarah’s breathing cycle. While setting Sarah up for imaging, the team will place a small plastic cube with reflective markers on Sarah’s chest. A video camera will track the up-and-down movement of the cube. The X-ray beam from the scanner will be synchronized with Sarah’s breathing, so that images are taken only when the lung is in the proper position. Varian’s respiratory gating system will come into play again when Sarah is treated so that treatment can also be synchronized with her respiratory cycle.

Treatment Planning
Once Sarah’s oncology team has the images needed to begin planning her treatment, they will use Varian’s SomaVision™ image processing software to generate three-dimensional views of Sarah’s tumor and the surrounding anatomy. The medical team will use the software to mark, or “contour,” the 3D images, indicating the area to be treated as well as the organs to be protected.
       The next step for Sarah’s team will be to prepare her treatment plan. At this point, the radiation oncologist will prescribe the ideal radiation dose for the tumor, as well as maximum dose limits for the surrounding healthy tissue. To determine how the dose will be delivered, Sarah’s oncology team uses Varian’s Helios™ inverse treatment planning software. Once the dose levels have been entered, Helios goes to work, using its unique algorithms to calculate and devise a detailed treatment plan just as a computer mapping program determines the best route to a destination. The plan includes beam shapes and exposure times as well as electronic instructions that will automate and control the delivery system through 30 to 40 treatment sessions. The next destination for Sarah will be post-planning simulation.

Simulation
Prior to actually treating Sarah, her oncology team will first conduct a dry run using Varian’s new Acuity™ imaging system. This enables the oncology team to properly position Sarah on the table and run through a simulated treatment session.
       Proper patient positioning is critical to ensure that the tightly focused X-ray beams are targeted accurately. Sarah, like most radiotherapy patients, will be tattooed with small marks that will be aligned with lasers in the treatment room, to ensure that she is in precisely the right spot in relationship to the radiotherapy machine. The Acuity system, which mimics the treatment machine, will enable the medical team to take X-ray images of Sarah in her treatment position, and compare them with reference images from the treatment plan. This will enable the team to fine-tune the plan and verify that it will work as intended.

The Treatment Room
Before Sarah begins the next phase of the IMRT process, let’s look at the room in which she will receive her treatment. It measures about 19 feet by 16 feet. In this room is an imposing machine hovering over a futuristic treatment table or couch that might have come from the set of a science fiction film. The machine is a Clinac® medical linear accelerator (linac) manufactured by Varian.
       Linacs are critical to the success of IMRT and all other radiotherapy treatments based on X rays. Reaching tumors deep within the body requires intense penetration power with X rays at energies ranging from 4 to 25 million volts (MV). X-ray tubes, such as those in an X-ray machine being used for diagnostic purposes, typically generate X rays at energies between 60 thousand and 150 thousand volts, far short of what is needed. Linacs, on the other hand, originally developed as a tool for smashing atoms and first adapted to medical applications by Varian in 1960, have no problem meeting the energy requirement.
       When the power and intensity of linac X-ray beams are applied to tumors over a number of treatment sessions, the accumulated radiation dosage is enough to fatally damage cancerous cells.
       To concentrate a dose of radiation on the tumor, Varian outfits its Clinac with a beam-shaping device called a Millennium™ multi-leaf collimator (MLC). An MLC consists of a computer-controlled array of up to 120 parallel and individually adjustable tungsten bars, or “leaves,” that move to shape the aperture through which the radiation passes. It will enable Sarah’s oncology team to precisely and automatically conform their beams to the shape of the tumor in her lung.

Modulating the Dose
With IMRT, Sarah’s doctors can divide the area being treated into thousands of segments as small as 2.5 mm by 5 mm and give each one a specified dose. The adjustable leaves of the MLC are used to control not only the shape of the beam, but also the exposure duration for each segment of the tumor, effectively “modulating” the dose within the treatment area. This way, higher doses can be concentrated in some parts of the tumor while lower doses can be used in other areas where sensitive tissue may need protection.
       Now it is time for Sarah’s first treatment session. She enters the treatment room, which is softly lit and quiet. Her radiation therapist positions her on the treatment table. A small plastic cube is again placed on her chest so the respiratory gating system can again compensate for breathing motion. During treatment, the system will turn the Clinac’s X-ray beam on and off as the tumor on her lung moves in and out of position. If Sarah coughs or moves, the beam switches off, further protecting her healthy tissues.