Siobhan Haney, VMD, MS, DACVR (RO), MBA
A 33.7 kg, 10 year-old neutered male mixed breed presented to his local emergency service for acute onset of anorexia, restlessness and mild dyspnea. At presentation, he had evidence of abdominal fluid and CBC/chemistry panel revealed hypocalemia of 7.2 (reference range 9-12.2 mg/dL) and hypoproteinemia of 4.7 (reference range 5.5-7.6 g/dL). Ultrasonographic examination of his abdomen and thorax revealed a 6.9 x 5.2 cm solid mass along the right cranial aspect of the heart along with a moderate amount of free fluid in the pleural space (see figure 1). The dog was prescribed Lasix 40 mg twice a day after abdominocentesis was performed. Thoracocentesis was not performed. Given the solid nature of the mass and the location at the heart base, a presumptive diagnosis of a chemodectoma was made. Other differentials in this case were hemangiosarcoma, thyroid tumor, lymphoma or hemangiosarcoma. Because of the proximity of the mass to normal heart and lung tissue, the recommendation was made for stereotactic radiation therapy.
Figure 1. A still image from the original thoracic ultrasound. The four small white “x” mark the edges of the mass along the heart base. The mass initially measured 6.9 x 5.2 cm. Image courtesy of Dr. Anna Caceres.
Stereotactic radiation therapy is the use of a few hundred to a few thousand beams of radiation that are precisely target to a tumor, with a steep dose gradient in the few millimeters outside of the defined target volume. Typically, stereotactic radiation therapy treatments will entail 1-5 treatments, or fractions, that are given over a relatively short time period (typically one week or less). Large doses of radiation are prescribed, with emphasis put on highly accurate dose delivery. In cases such as this, the CyberKnife stereotactic radiation therapy unit employs the use of Synchrony, which is a camera system that establishes a pattern of respiration for a patient, thus “tracking” the tumor in real time as it changes position in space with respiratory motion. The Synchrony camera established a respiratory pattern in this patient’s case with the aid of fiducial markers, which are small gold seeds that were placed inside the tumor with ultrasound guidance approximately 1 week prior to treatment with CyberKnife. In conjunction with Synchrony, orthogonal x-ray sources use kilovoltage x-ray images to localize the fiducial markers, therefore allowing submillimeter accuracy for dose delivery.
The dog received 3 fractions of CyberKnife stereotactic radiation therapy. Each individual fraction size was 10 Gy for a total dose of 30 Gy of radiation that was delivered to the PTV (planning target volume) in a one-week time period. In accordance with well-established human guidelines for maximum tolerance of cardiac tissue, the maximum dose that the heart received was 23.8 Gy. These guidelines, although not well-established in veterinary medicine, are useful in radiation therapy in order to avoid unwanted toxicity in normal, healthy tissue that may surround a tumor (see figure 2).
Figure 2. These images are from the radiation treatment plan, which was created with specialized software that utilized these CT images. The top image shows an axial view of the thorax. The green crosshairs are centered on the gold fiducial markers, which were placed in the center of the tumor, outlined in red. A portion of the normal heart can be seen adjacent to the tumor, outlined in light blue. The bottom image is a sagittal view of the tumor outlined in red with fiducial markers and the adjacent heart, outlined in light blue.
No acute side effects of radiation were observed in the weeks following treatment. The dog did experience mild lethargy and inappetance approximately 3 months after his treatment, but these symptoms were considered unrelated. The dog had 2 repeat thoracic ultrasounds after treatment; the first ultrasound which was 2 months after radiation treatment revealed stable disease. The second ultrasound, which was 4 months after treatment, revealed a tumor size of 5.9 x 3.1. The dog’s Lasix was also weaned over a period of several weeks after radiation therapy; the bicavitary effusion previously noted in the thorax and abdomen was not noted in subsequent exams and all presenting clinical signs resolved.
There is a paucity of information in the veterinary literature regarding non-surgical treatment of heart base tumors1,2. The use of radiation therapy in the treatment of these tumors is becoming more widely accepted, especially with the growing availability of stereotactic radiation therapy. It has been reported that dogs with chemodectomas may enjoy several years of survival after treatment with conformal radiation therapy, as the metastatic rate for this tumor is reportedly very low. The reported response time of these tumors after treatment with radiation therapy are typically very slow, over several months1. This case report documents the successful treatment of a heart base tumor with stereotactic radiation therapy.
J Am Vet Med Assoc. August 15, 2012;241(4):472-6.
Nicholas J Rancilio1; Takashi Higuchi; Jerome Gagnon; Elizabeth A McNiel
Semin Vet Med Surg (Small Anim). August 1995;10(3):190-6. 32 Refs
S M LaRue1; S M Gillette; J M Poulson