Advanced Linear Accelerator Newest Weapon in War on Pet Cancer
By Stephania Smith
While no pet owner wants to hear it and no veterinarian wants to diagnose it, estimates are that 50% of dogs and 33% of cats will develop some form of cancer during their lives. Radiation oncologists at North Carolina State University’s College of Veterinary Medicine now have a weapon that enhances their effectiveness in fighting this deadly disease.
The new Varian Novalis Linear Accelerator, which is built on the platform of its predecessor, the Trilogy, features several upgrades that make it one of the most sophisticated radiotherapy units in the world. The Novalis will allow the board certified radiation oncologists Drs. Mike Nolan and Tracy Gieger at NC State to treat more animal tumors with greater precision resulting in fewer side effects and, in the case of some tumors, a shorter treatment period.“The Varian Novalis will be able to treat some types of tumors and cancers that could not be treated with previous machines and technology,” says Dr. Nolan. “The new technology is versatile and we have been able to customize it to meet our specific needs to a degree never possible before.”According to Dr. Nolan, a key to more effective tumor treatment in the new technology’s capabilities in the areas of intensity-modulated, image-guided, and stereotactic radiation therapy.
IMRT – Shaping the beam. IMRT is Intensity Modulated Radiation Therapy. This feature offers the ability to “paint” the tumor with radiation while sparing or lowering the effects on surrounding healthy tissue. The collateral damage to surrounding tissue and organs has long been the challenge of radiation therapy. How can radiation oncologists focus the radiation on the tumor and cancer cells without making things worse by the radiation effects on the healthy areas?
In the past, the radiation beam was like a square that had to be large enough to encompass the entire tumor. Since the tumor was not the same square shape of the radiation beam, there was more area of healthy tissue affected by the beam. With IMRT it is now possible to shape the radiation beam and make it exactly match the irregular shape of the tumor, minimizing the radiation dose delivered to surrounding normal tissues. Now a round or irregularly shaped tumor can be treated with a radiation beam that exactly matches it.
IMRT also allows the radiation beam to be shaped to avoid hitting other vital organs. This allows for the treatment of some types of tumors that could not be treated before. An example is bladder and prostate tumors. In the past, definitive radiation therapy was often not a great option because of the potential damage to the rectum. Now the effects on those surrounding organs can be minimized so that radiation treatment can be a good option for treating prostate cancer. This same principle can be applied to many other areas of the body.
Another feature of IMRT is facilitated by the linear accelerator’s ability to rotate around the patient during treatment. This allows the oncologist to direct multiple beams from many different angles.
Dr. Nolan used the example of a dog with a nasal tumor. In the past, the radiation beam was not well conformed to the tumor and was often directed from about three locations, i.e., above and each side. This resulted in much radiation exposure to the tissues of the muzzle and roof of the mouth. Now, with the new with the Varian Novalis and use of IMRT, more beams can be used from additional directions. Each beam can be a lower intensity which has less effect on the surrounding tissue. But all beams intersect on the tumor resulting in the tumor receiving a high cumulative dose of radiation, while each area of surrounding tissue receives only a small portion of the radiation dose that the tumor receives.
What does that mean for the pet being treated? In the past, the high radiation doses in surrounding tissue could cause a severe sunburn type reaction on the nose and roof of the mouth. The fur and sometimes some skin would fall off and be raw. While the effects were temporary and would heal in a matter of weeks, the dog would be very uncomfortable and it looked so bad to the owner that veterinarians might hesitate to recommend the treatment. Now, with the new linear accelerator, the damage to surrounding tissue is significantly minimized with mild or no side effects at all.
IGRT – identifying the position. IGRT is Image Guided Radiation Therapy. It refers to the ability of the Varian Novalis linear accelerator to identify exactly where the tumor is located each day of treatment. It does this by having various imaging tools incorporated into the machine, known as on-board-imaging (OBI), including a CT scanner. This allows the radiation oncologists, prior to each treatment, to take 3D pictures and account for any changes in the position of the tumor. With traditional radiation therapy, the radiation field had to be expanded around the tumor to account for shifts from one treatment to the next. That resulted in additional radiation to surrounding healthy tissue. Now, with an onboard CT scanner, the radiation field can continue to be tightly conformed to the tumor while accounting for almost any shift in position.
Once the exact position of the tumor in the body is identified, the patient’s position can be shifted to line up perfectly with the radiation beam. To further facilitate the exact alignment, the NC State Veterinary Health Complex purchased an add-on to the linear accelerator, a Civco Protura robotic couch, which can adjust the positioning of the patient with sub-millimeter precision in six different dimensions. This accounts for slight changes in positioning of the patient and its tumor between treatments, and allows dose to the surrounding normal tissues to be a low as possible, thus further decreasing side effects.
Another benefit of IGRT, and the ability to precisely position a patient, is that for many tumors, including nasal tumors, there is no need to shave the animal or put markings on the skin to help with the positioning, as was done in the past.
SRT - SRT is Stereotactic Radiation Therapy. SRT is the use of a high dose treatment of radiation for a shorter treatment period. Sometimes it can be a single radiation treatment such as with some types of brain tumors, or can be treatments for just one to five days rather than over a period of several weeks. It cannot be used for all tumors and requires a well-defined tumor with no cancer cells in the surrounding tissue. Some of the types of tumors that SRT can be used for include osteosarcoma (bone), brain, pituitary, and nasal tumors.
“The bottom line for the Varian is that we can offer radiation therapy for diseases that we have treated historically but now can treat more effectively with far fewer side effects, potentially better tumor control, sometimes with a much shorter course of treatment,” says Dr. Nolan. “And the reality is that we can now also treat tumors that were once inaccessible to us.”
Traditional therapy field around tumor.
IMRT therapy radiation field sculpted tightly around tumor.
Traditional therapy with tumor on Day 1 (yellow) shifting slightly on Day 2 of therapy (blue) resulting in larger radiation field (dashed line) to encompass shift in position.
IGRT uses imaging to adjust for any shift in tumor location so that the radiation field (dashed line) remains tightly conformed to the tumor.
For more information:
Mr. Bear’s Treatment with the Varian Novalis. Radiation Pictures
Stephania Smith is a freelance writer who specializes in veterinary medicine.