Po27

Purpose Optimal placement and correct reconstruction of needles are paramount in achieving high treatment efficacy in interstitial high dose rate (HDR) brachytherapy. Nonetheless, abutting needles are sometimes observed in challenging cases, posing a potential risk for false needle reconstruction and thus error-prone dose calculation. This study aims to investigate the dosimetric impact of false reconstruction of abutting needles. Materials and Methods Syed HDR cases (n=72) were investigated. Needle reconstruction as well as clinical plan optimization was done in Oncentra® Brachy. A Matlab-based script was generated to detect abutting needles if their closest distance was smaller than 1.5 times the pixel size in the axial plane. A simulated plan was created simulating false reconstruction of the abutting needle pair with the smallest distance, by switching the needle segments superior to the abutting point location after which the dwell points in the clinical plan were replaced based on the simulated needle tips. Dose calculation for the clinical and simulated plans were conducted using a validated Matlab-based dose engine. Changes in dose volume histogram (DVH) metrics, including the high-risk clinical target volume (HRCTV) D90%, V90%, D100%, V100%, V150%, as well as bladder and rectum D2cc, were compared. Results Abutting needles were detected in 19 out of the 72 (26.4%) investigated cases. Compared to the clinical plans, the simulated plans showed negligible (<±1%) dosimetric changes in 14 cases, whereas significant (>±5%) changes in at least one DVH metric in the other 5 cases. Among all 19 cases, a wide range of DVH changes (%) were observed in HRCTV: V100% (-1.7 [-23.1 1.3]), V90% (-1.5 [-17.6 0.3]), D100% (-3.6 [-40.1 0.7]), D90% (-1.2 [-17.5 13.4]), V150% (-1.1 [-30.9 19.9]). The D2cc of bladder (-1.7 [-23.1 1.3]) and rectum (0.5 [-6.8 10.1]) showed similar changes. Conclusions False reconstruction of abutting needles may lead to significant dose changes to targets and sensitive organs. In case of uncertainty in needle reconstruction, strategies such as light loading of the needle pair and avoiding distinct loading patterns between them may be employed to minimize potential dosimetric errors. Optimal placement and correct reconstruction of needles are paramount in achieving high treatment efficacy in interstitial high dose rate (HDR) brachytherapy. Nonetheless, abutting needles are sometimes observed in challenging cases, posing a potential risk for false needle reconstruction and thus error-prone dose calculation. This study aims to investigate the dosimetric impact of false reconstruction of abutting needles. Syed HDR cases (n=72) were investigated. Needle reconstruction as well as clinical plan optimization was done in Oncentra® Brachy. A Matlab-based script was generated to detect abutting needles if their closest distance was smaller than 1.5 times the pixel size in the axial plane. A simulated plan was created simulating false reconstruction of the abutting needle pair with the smallest distance, by switching the needle segments superior to the abutting point location after which the dwell points in the clinical plan were replaced based on the simulated needle tips. Dose calculation for the clinical and simulated plans were conducted using a validated Matlab-based dose engine. Changes in dose volume histogram (DVH) metrics, including the high-risk clinical target volume (HRCTV) D90%, V90%, D100%, V100%, V150%, as well as bladder and rectum D2cc, were compared. Abutting needles were detected in 19 out of the 72 (26.4%) investigated cases. Compared to the clinical plans, the simulated plans showed negligible (<±1%) dosimetric changes in 14 cases, whereas significant (>±5%) changes in at least one DVH metric in the other 5 cases. Among all 19 cases, a wide range of DVH changes (%) were observed in HRCTV: V100% (-1.7 [-23.1 1.3]), V90% (-1.5 [-17.6 0.3]), D100% (-3.6 [-40.1 0.7]), D90% (-1.2 [-17.5 13.4]), V150% (-1.1 [-30.9 19.9]). The D2cc of bladder (-1.7 [-23.1 1.3]) and rectum (0.5 [-6.8 10.1]) showed similar changes. False reconstruction of abutting needles may lead to significant dose changes to targets and sensitive organs. In case of uncertainty in needle reconstruction, strategies such as light loading of the needle pair and avoiding distinct loading patterns between them may be employed to minimize potential dosimetric errors.