Feasibility for Using Hypofractionated Stereotactic Volumetric Modulated Arc Radiotherapy (Vmat) With Adaptive Planning for Treatment of Thymoma in Rabbits: 15 Cases

Abstract

Thymoma is a relatively common tumor in rabbits. Treatment with surgery, radiation therapy, and chemotherapy alone or in combination has been reported with varying outcomes. Stereotactic volumetric modulated arc radiotherapy delivered in a hypofractionated manner allows high doses of radiation to be delivered to the target volume while allowing sparing of adjacent critical structures. This therapy is ideally suited for thymomas in rabbits given their size, the difficulty of multiple anesthesia episodes and the complexity of the radiotherapy plans required due to the tumor's proximity to the heart, lungs, and mediastinal structures. Fifteen rabbits with thymoma were prospectively recruited for this observational, single institution, single arm clinical study. All rabbits were imaged with both computed tomography (CT) and magnetic resonance imaging (MRI). A total dose of 40 Gy in six fractions was delivered using a single arc over an 11-day period with repeat CT simulation done every other fraction for adaptive planning. Follow-up evaluation was done through repeat CT and MRI imaging and revealed complete responses using the Response Evaluation Criteria in Solid Tumors (RECIST) criteria. Two rabbits had died at 618 and 718 days, 10 were alive and three were lost to follow-up. Observed acute and late effects were graded according to the Veterinary Radiation Therapy Oncology Group (VRTOG) criteria and were found to be minimal.

Frameless stereotactic radiotherapy alone and combined with temozolomide for presumed canine gliomas

“…In veterinary medicine, VMAT technique was successfully used for rabbit thymoma and intracranial meningioma treatment …”

“…The technical difficulties of conformal radiation therapy of canine gliomas, with particular regard to shape irregularity, size and proximity to critical neural structures as well as to markedly heterogeneous structures, were bypassed using VMAT . The putative radiotoxic effects were rare, thus demonstrating the feasibility of this technique.…”

Dosimetric benefit of adaptive radiotherapy in the neoadjuvant management of canine and feline thymoma—An exploratory case series

“…Complete surgical en bloc resection of thymoma is the mainstay of treatment and the completeness of resection a strong prognostic factor in humans, and most likely also in dogs and cats . However, complete excision is not possible in a proportion of cases in humans, dogs, cats and rabbits, and for these patients radiotherapy can be used as neoadjunctive, post‐operative adjunctive or sole therapy . As resectability is closely related to prognosis, a strategy using neoadjuvant treatment with radiotherapy may result in an optimization of resectability of invasive or very large thymomas due to facilitated surgical manipulation after tumour volume reduction and/or improved patient's general condition .…”

High dose hypofractionated frameless volumetric modulated arc radiotherapy is a feasible method for treating canine trigeminal nerve sheath tumors

“…), lesions of complex shape affecting critical neural structures near to organs at risk can now be successfully addressed with potential advantages over past treatment modalities . In particular, volumetric modulated arc radiotherapy, which is a form of dynamic IMRT, has been successfully used in veterinary medicine for the treatment of brachial plexus tumors, presumed gliomas, rabbit thymomas, canine adrenocortical tumors, intracranial meningiomas, and carotid paraganglioma …”