  Purpose: To present an efficient NEEdle Position Optimization (NEEPO)
algorithm for prostate rotating shield brachytherapy (RSBT). With RSBT, the
increased flexibility beyond conventional high-dose-rate brachytherapy (HDR-BT)
due to the partially shielded radiation source has been shown by Adams et al.
in 2020 to enable improved urethra sparing (23.1%), enhanced dose escalation
(29.9%), or both, with 20 needles without NEEPO-optimized positions. Within
this regime of improved dosimetry, we propose in this work that the benefits of
RSBT can be maintained while also reducing the number of needles needed for the
delivery. The goal of NEEPO is to provide the capability to further increase
the dosimetric benefit of RSBT and to minimize the number of needles needed to
satisfy a dosimetric goal. Methods: The NEEPO algorithm generates a needle pool
for a given patient and then iteratively constructs a subset of needles from
the pool based on relative needle importance as determined by total dwell times
within needles. The NEEPO algorithm is based on a convex optimization
formulation using a quadratic dosimetric penalty function, dwell time
regularization by total variation, and a block sparsity regularization term to
enable iterative removal of low-importance needles. RSBT treatment plans for 26
patients were generated using single fraction prescriptions with both dose
escalation and urethra sparing goals, and compared to baseline HDR-BT treatment
plans.
