Dosimetric comparison of utilizing saline or air-filled endorectal balloons in MR linac-based prostate SBRT

In prostate cancer Stereotactic Body Radiation Therapy (SBRT), endorectal balloons are commonly used to reduce rectal exposure to excessive radiation. This study investigates the magnetic field-induced dose effects associated with air-filled endorectal balloons in MR-linac treatments and compares the dosimetric metrics and radiobiological outcomes between air-filled and saline-filled balloons. A retrospective analysis was conducted on 20 prostate cancer patients treated with a 1.5-Tesla MR-linac using a saline-filled rectal balloon. Each patient received a total prescription dose of 36.25 Gy to the Planning Target Volume (PTV) over 5 fractions. The simulation scans and treatments were performed with an 80 mL saline-filled balloon in place. To simulate the use of air-filled balloons, Intensity-Modulated Radiation Therapy (IMRT) plans were generated, adjusting the balloon density to represent air, followed by re-optimization to preserve the target dose. Dosimetric and radiobiological metrics for targets and organs at risk (OARs) were compared between the 2 scenarios. Rectal wall toxicity was assessed using the Lyman-Kutcher-Burman (LKB) normal tissue complication probability (NTCP) model. The introduction of air-filled endorectal balloons during MR-linac treatments intensifies material heterogeneity, leading to dose perturbations within the treatment field. Specifically, the V40 Gy hot spot on the rectal wall increased from 0.55% to 1.51% due to the electron return effect (ERE). A more pronounced impact was observed in patients receiving prostate-only irradiation compared to those with pelvic lymph node involvement, likely due to the reduced number of beam angles. Additionally, undesired dose deposition on the rectal wall outside the treatment field was noted, attributed to increased scatter and the electron streaming effect (ESE), where secondary electrons deflected by the transverse magnetic field deposited energy on the surfaces they encountered. The mean maximal out-of-field rectal wall dose was 4 Gy, ranging from 2 to 7.4 Gy. Consequently, the cohort's rectal wall NTCP increased with the use of air-filled balloons. This study highlights that the use of air-filled endorectal balloons can introduce hot spots to the rectal wall and cause unwanted ESE-related dose depositions outside the treatment field. In contrast, saline-filled balloons provide superior dosimetric performance and better protect the rectum from radiobiological damage in prostate SBRT delivered with MR-linac. These findings suggest that saline-filled balloons may be preferable for this type of therapy to minimize potential adverse effects on the rectum.