By Lyndsay Kubicek, DVM, DACVR (Radiation Oncology)
Volumetric modulated Arc Therapy (VMAT) or RapidArc® Radiotherapy Technology is an innovative form of intensity modulated radiation therapy (IMRT) that delivers precise continuous radiation in a single or multi-arc treatment to the patient. With conventional IMRT techniques like step-and-shoot, the machine must make repeated stops and starts to treat the tumor from a number of different angles generally over 5-10 minutes. In comparison, VMAT can deliver the dose to the entire tumor in a 360-degree rotation, typically under two minutes.
Intensity-modulated radiation therapy is a type of highly conformal radiotherapy that shapes the radiation beam to closely fit the tumor thus minimizing the dose to the adjacent normal tissues. To achieve such conformability, the linear accelerator uses a device called a multileaf collimator (MLC) which consists of thin leaves which move independently and form shapes that fit precisely around the treatment area. For reference, visit angell.org/IMRT.
VMAT is predominantly useful for delivering radiation therapy to tumors adjacent to sensitive body organs, and can be an effective treatment for many types of cancer This is particularly important in terms of late term side effects. As our oncologic treatments are advancing, the potential for long-term survival increases, thus our patients may live long enough to endure such side effects.
In addition to preventing permanent late term side effects, VMAT has the added benefit of faster treatment times. In veterinary medicine our patients are immobilized with heavy sedation or general anesthesia. While all precautions are in place to minimize adverse anesthetic events, short anesthesia times are always advantageous in patients receiving multiple treatments.
This technology can be used for fractionated as well as hypofractionated treatments such as stereotactic body radiation therapy and stereotactic radiosurgery.
Currently there are two reports of VMAT used in veterinary medicine to date. The first was VMAT-SMRT for adrenal tumors with vascular invasion. This study reported on 9 dogs; 3 dogs were affected by cortisol-secreting adrenal tumors and 6 dogs had non-secreting adrenal tumors treated with VMAT SBRT. The median overall survival time was 1030 days, and the overall mean reduction of the diameter and volume were ~32 and 30% respectively. The endocrine profile normalized in two dogs with cortisol-secreting adrenal tumors. Side effects were mild and self-limiting.
The second study reported on 10 patients with brachial plexus tumors and found all treated patients showed a partial response and partial or complete reductions of neurological deficits although local recurrence was observed in 9 of 10 of treated dogs. The mean overall survival of 371 days and mean progression-free survival of 240 were reported which were comparable to reports with surgery alone. No side effects directly related to treatment were observed. This study concluded that VMAT can be a safe and viable alternative to surgery in cases of canine brachial plexus PNSTs involving the proximal nerves and nerve roots.
Other tumor types being treated at our institution as well as other VMAT sites include brain and spinal cord tumors, trigeminal nerve tumors, brachial plexus tumors, adrenal tumors, primary bone tumors, nasal tumors, oral tumors, heart base tumors, urogenital tumors, anal sac tumors and intrathoracic/mediastinal tumors.
As more institutions gain ability to used advanced technologies such as VMAT, more studies will be underway to expand our ever-growing understanding of the best utilization.
We are very fortunate to be within the first 5 veterinary institutions in the world to offer VMAT. If you have any questions regarding the therapy or tumor types treated with VMAT or radiation therapy in general please contact the oncology service at firstname.lastname@example.org.
Picture of a patient that received VMAT-IMRT for a presumed meningioma and an incidental thyroid tumor.
- 1a. Upper left quadrant is the axial images with a color-wash dose distribution (red is goal and blue is lowest dose of radiation). The yellow rings represent the beam rotating 360° around the two tumors.
- 1b. Upper right is the rendering of the patent, skin contoured in pink. The patient is positioning in a bite block device for immobilization. The red rings represent the beams.
- 1c. Lower left is the coronal images, yellow lines represent the beam angles.
- 1d. Lower right is the sagittal images showing the beam angles and color-wash.