Comparison of three irradiation ways with Monte Carlo calculation for the feline lymphoma with orthovoltage radiation therapy

Although orthovoltage radiation therapy remains widely used in veterinary medicine in Japan due to its affordability and operational simplicity, accurate calculation of dose distribution is challenging. Treatment planning varies between practitioners and tends to rely on empirical methods. This study aimed to compare the effects of three different orthovoltage beam directions on dose distribution in feline nasal lymphoma using Monte Carlo simulations. CT images from five clinical cases were analyzed using three irradiation plans: Plan A (beam directed perpendicularly to the hard palate), Plan B (perpendicularly to the nasal bridge), and Plan C (from the oral cavity). Each plan used a 4 × 4 cm field and delivered a 10 Gy dose at 280 kVp. Doses to the tumor, brain, and both eyes were calculated. Mean dose for the tumor did not significantly differ among the three plans. Plan B resulted in significantly higher doses to the ipsilateral eye compared to Plan C, but delivered the lowest brain dose. Depth-dose analysis revealed a peak just above the bone at the beam entrance, followed by a sharp attenuation in all plans. These findings suggest that beam direction substantially affects dose distribution in orthovoltage therapy, particularly for organs at risk including superficial bones and overlying skin. Meticulous planning is essential to ensure sufficient tumor dose coverage while mitigating and distributing adverse effects, especially in complex regions like the feline nasal cavity.