# Development of Inclined Applicators for Flattened Beam Intraoperative Radiotherapy
## 1 Introduction  
Intraoperative radiotherapy (IORT) is performed immediately after surgical removal of the tumor, providing direct irradiation of the tumor bed while minimizing radiation exposure to normal tissues and delivering a high dose to the tumor/tumor bed [1]. Compared to external radiation therapy (EBERT), IORT offers several advantages, including a shorter interval between surgery and radiotherapy, preventing the repopulation of tumor cells, increased treatment dose due to the positional relationship between the tumor/tumor bed and adjacent normal tissue, and reduced treatment period with local irradiation where necessary [2; 3; 4].  
Moreover, numerous retrospective and randomized trial studies have demonstrated that IORT can be a viable alternative to EBRT. For instance, studies investigating the local recurrence rate and long-term survival rate of breast cancer patients who received IORT and EBRT found that IMRT rates were non-inferior to EBRT rates [5; 6; 7; 8]. In cases whereEBRT was difficult to implement, such as sacral chordoma, locally advanced rectal cancer, pancreatic cancer, cervical cancer, and head and neck cancer with a high local recurrence rate, IORT may be a useful option [9; 10; 11; 12; 13; 14; 15].  
IORTs use either high-voltage electrons or low-kV X-ray systems [16; 17; 18; 19]. The INTRABEAM system (Carl Zeiss Surgical, Oberkochen, Germany) is an IORT device that emits low X-ray energy of 20-50 kV. The main purpose of the applicator in IORT is to block the beam from spreading in all directions and to ensure that it is only incident in the target direction while stably positioning the X-ray generator on the tumor. The beam is focused on the target as much as possible to minimize the dosing incident on the surrounding normal tissue and enable stable treatment. The manufacturer of INTRABEAM offers four types of dedicated applicators: spheres, surfaces, planes, and needles. The spherical applicator shows an isotropic dose distribution and is used to deliver a uniform dose to the inner surface of the breast lumpectomy cavity. The surface and flat applicators exhibit a flat dose distribution due to a flattening filter and are used on the surface of the tumor bed. The needle applicator is designed for the treatment of vertebral metastases and brain tumors [3].  
Although the clinical application range of IORT has been expanded by providing various sizes and types of applicators, two fundamental problems remain. First, the isotropy of the spherical applicator allows high doses to enter a local area. Second, the current applicator is designed in a way that the X-ray generator and affected part meet vertically. However, in many cases, the angle between the tumor and X-ray generator may not be vertical, and it may be difficult to optimize the positioning of the X-ray generator using the current applicator.  
Therefore, to protect normal tissues from the spherical applicator facing the tumor/tumor bed, radiation shielding should be provided for the areas around the target window. The situation where the tumor and X-ray generator are not perpendicular should be considered, and an applicator capable of irradiating the beam at an oblique angle other than 0\({}^{\circ}\) must be used.  
In this study, an applicator dedicated to oblique beams was manufactured using 3D printing, and basic dose evaluations such as leakage dose, dose rate, and percent depth dose were performed. Finally, we used the developed applicator to treat four patients in a clinical setting.