Clinical implementation of a wide-field electron arc technique with a scatterer for widespread Kaposi’s sarcoma in the distal extremities

Abstract

A novel wide-field electron arc technique with a scatterer is implemented for widespread Kaposi's sarcoma (KS) in the distal extremities. Monte Carlo beam modeling for electron arc beams was established to achieve <2% deviation from the measurements, and used for dose calculation. MCbased electron arc plan was performed using CT images of a foot and leg mimicking phantom and compared with in-vivo measurement data. We enrolled one patient with recurrent KS on the lower extremities who had been treated with photon radiation therapy. The 4-and 6-MeV electron arc plans were created, and then compared to two photon plans: two opposite photon beam and volumetric modulated arc with bolus. Compared to the two photon techniques, the electron arc plans resulted in superior dose saving to normal organs beneath the skin region, although it shows inferior coverage and homogeneity for PTV. The electron arc treatment technique with scatterer was successfully implemented for the treatment of widespread KS in the distal extremities with lower radiation exposure to the normal organs beyond the skin lesions, which could be a treatment option for recurrent skin cancer in the extremities. Kaposi's sarcoma (KS) is a superficial tumor that typically occurs in the distal extremities of the hands and feet. Surgical excision is not a recommended treatment method for KS because this tumor is spread sparsely on the skin. Systemic chemotherapy is not indicated for the treatment of multiple superficial tumors without distant metastasis, and it is challenging to treat multiple superficial tumors with local cryotherapy 1. Previous studies have reported that KS responds well to radiation therapy, and so radiation therapy is used to provide palliation with minimal side-effects in all forms of KS 2-5. Megavoltage photon beams are generally used to treat widespread skin cancer. These beams have a build-up characteristic near the surface before reaching electronic equilibrium. To treat superficial skin tumors with a megavoltage photon beam, a layer of scattering material that imitates skin called a bolus is used to increase the radiation dose at the skin surface 6. A previous study has suggested that a simple photon beam technique with various tissue-equivalent boluses such as water or rice could be used in the treatment of skin cancer 7. This technique has the advantage that it can deliver a uniform dose of radiation to widespread lesions. However, megavoltage photon beam treatment will inevitably deliver as much as the prescribed dose to deep-seated normal tissues. This increases the potential for complications such as fibrosis, edema, and joint contracture as a result of increased radiation exposure to underlying normal tissues such as skin, muscles, vasculature, and bones 3,8. Electron beam treatments that use beam spoilers could help overcome this issue. Previous studies have confirmed that this technique is useful in treating flat surfaces because the spoiler increases the surface dose and reduces the electron beam's penetration range 9. Howev...