A 10-year-old female spayed Labrador retriever was presented to a surgeon at a referral center for resection of a large lung tumor. Three years previously, the patient had been diagnosed with an undefined hepatopathy. During diagnostics performed at that time, a small lung nodule was found. Further diagnostics (including FNA) were discussed but declined due to her other medical issues at that time. The patient was free of clinical signs for 3 years until her owner noticed occasional coughing. Since that time, the cough increased in frequency and intensity.
Other pertinent medical history includes a history of allergies as well as infected hygroma. She had been on Apoquel chronically, but this was discontinued several months prior to her presentation to the surgery service. She had also received 2 Cytopoint injections. At the time of evaluation, she was receiving Denamarin as well as flea/tick preventative.
A CT scan was performed to aid in surgical planning. This revealed that there was a large irregularly margined soft tissue mass expanding and effacing the right middle lung lobe. The mass measured 11.4 cm H x 7.8 cm W x 10.3 cm L and had heterogeneous patchy contrast enhancement throughout. The pulmonary mass was causing leftward displacement of the heart. The caudal vena cava ran along the medial margin of the mass and was moderately compressed by the mass. The mass was also immediately adjacent to and partially surrounding several bronchi. The cranial margin of the right caudal lung lobe was poorly defined which may have indicated the local invasion of the right middle lobe mass into the right caudal lobe. Primary differentials included pulmonary carcinoma and histiocytic sarcoma. Histiocytic sarcoma was less likely given the reported long duration with which the pulmonary lesion has been present.
A fine needle aspirate was performed and results were consistent with a primary pulmonary carcinoma. Due to the invasiveness of the tumor and proximity to vital organs, the tumor was deemed non-resectable. Options for therapy at this point included chemotherapy and radiation therapy.
The patient’s owner elected to pursue CyberKnife radiation therapy to treat the lung tumor. CyberKnife radiation therapy is a form of stereotactic radiation therapy which allows radiation to be delivered with submillimeter accuracy. The implantation of small gold beads into the tumor via ultrasound guidance, called fiducial markers, allow the CyberKnife to track the movement of the lung tumor as it moves with respiration, thus allowing accurate radiation delivery. Using this technique, large doses of radiation can be delivered to the tumor quickly with minimal dose delivery to normal, healthy lung tissue.
Radiation treatment planning was completed based on the CT images generated for the patient. The tumor was identified and contoured as well as the healthy organs, or the organs at risk (OAR), which included the right and left lungs, heart, spinal cord, and liver. Radiation delivery to the tumor was calculated based on the intended total dose goal and the tolerance levels of the OAR (see figure 1).
Figure 1 shows the final radiation treatment plan. The top picture is the axial CT scan showing the tumor (outlined in red) as well as a surrounding OAR. The green crosshairs are centered over one of the fiducial markers placed in the tumor. The bottom picture is the sagittal image at the same level within the tumor. The prescribed dose of radiation is being delivered to the areas inside the orange line.
For the radiation treatments, the patient was anesthetized and placed in dorsal recumbency in a vacuum-deformable mattress. Air was removed from the mattress, securing the patient into a position that would be repeatable for future treatments. No other rigid immobilization was necessary. The patient was intubated and ventilated using mechanical ventilation. Orthogonal kilovoltage radiographs were taken in order to localize the fiducial markers and to move the patient into the appropriate position for treatment using a robotically controlled treatment couch. The patient’s respiration was tracked using Synchrony cameras, which allow the CyberKnife to track the movement of the tumor in the x, y, and z planes and to move with the patient respiration, thereby delivering the radiation in a dynamic fashion and with the same level of accuracy as if the tumor were in a static position. Once all quality assurance processes were completed, orthogonal radiographs were repeated every 2 minutes during the treatment to ensure continued accuracy of radiation delivery.
The radiation treatment was delivered in 3 fractions, or treatments, over a one-week time period. The total radiation dose delivered was 30 Gy. The treatment was well tolerated by the patient and there were no complications or side effects noted in the weeks after radiation was completed. Repeat radiographs were taken at the patient’s primary care veterinarian approximately 2 months after radiation treatment documented a partial tumor response.
In the veterinary literature, no studies exist evaluating the treatment of lung tumors in dogs or cats with radiation. Historically, treatment of lung tumors has been focused on surgery and chemotherapy, however, cases that are not surgically resectable are left with few options.1-6 Before stereotactic radiation therapy was developed, more conventional forms of radiation therapy were unable to deliver radiation in a conformal fashion, therefore, concerns about normal tissue toxicity prevented the use of this therapy. Stereotactic radiation therapy is becoming increasingly available at academic institutions and private referral centers; however, a board-certified radiation oncologist should always be consulted about the most appropriate treatment type and delivery of radiation and should ideally be on-site for all treatments. Trained personnel involved in treatment delivery should also include radiation therapists, medical physicists and dedicated veterinary anesthesia nurses. Prospective studies are needed in order to determine the tolerability and efficacy of radiation therapy for lung tumors in dogs and cats.
Submitted by Siobhan Haney, VMD, MS, DACVR (RO), MBA
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