18 Background 19 High -dose-rate (HDR) brachytherapy has been recognized as a popular treatment for patients with 20 intermediate - and high -risk prostate cancer, despite its side-effects such as edema, incontinence, 21 and impotence . Dose escalation to dominant intrap rostatic lesions (DILs) results in improved 22 cancer control. However, its application is limited due to the risk of excessive irradiation to organs - 23 at-risk (OARs) such as the urethra, bladder, rectum . 24 Methods 25 This work introduces curvilinear catheter implan tation for dose escalation to the DIL, while 26 reducing toxicity to OARs . Curvilinear catheters were first pre -planned for an anonymized patient 27 using Oncentra treatment planning system ( TPS) and hybrid inverse planning optimization ( HIPO ) 28 algorithm . The cur ved needles were then analyzed using MATLAB to extract their radius of 29 curvature. Tendon -driven active needles were then used to implant curvilinear catheter s inside a 30 patient -specific phantom tissue. 31 Results 32 Proposed curvilinear catheter implantation resulted in significant improvement in dosimetric 33 plans . Tendon -driven active needles were shown to be capable of realizing required pre -planned 34 curvatures inside prostate. Accuracy of needle implantation was evaluated and reported. 35 Conclusio ns 36 This work demonstrates the feasibility of using tendon -driven active needles in curvilinear catheter 37 implantation inside prostate to improve the outcomes of HDR -BT via dose escalation to the DILs 38 and radiation dose reduction to the OARs. 39 3 Keywords: 40 Curvi linear catheter implantation , dose escalation , dominant intraprostatic lesions , reduced 41 toxicity to organs at risk , improved clinical outcome 42 1. Introduction 43 Prostate cancer is the second most common cancer in the U.S. with an estimated 288,300 new 44 cases and 34,700 deaths in 2023 [1]. High -dose-rate (HDR) brachytherapy (BT), which entails 45 delivery of temporary high -dose radiation to the prostate gland to eradiate cancer [2], has been 46 identified as one of the most effective treatment modalities [3]. HDR -BT involves implantation of 47 15 to 20 straight catheters inside prostate to provide access channels to place a radiation source 48 (e.g., iridium -192 capsule is welded at the end of a steel wire ) at desired positions (known as dwell 49 positions). In current practice, the ca theter implantation is done manually using transrectal 50 ultrasound (TRUS) guidance. Upon catheter implantation , a CT scan is taken to provide a 3D 51 image of the implanted catheters with respect to the prostate and the organs at risk (OARs) such 52 as urethra, bladder, and rectum. As an alternative, both the catheter placement and treatment can 53 be done with TRUS guidance, i.e., without the CT images. Dosimetric planning is done at this 54 point to determine dwell positions and estimate dwell time (radiation period) based on the actual 55 position of the catheters and prescribed radiation dose. An afterloader device is then used to 56 position the radiation source at dwell positions for t heir respective dwell time based on the 57 dosimetric plan. The afterloader device retracts the radiation source from the patient after the 58 treatment delivery . 59 Conventional r igid needles and templates only allow insertions only in a straight trajectory, with 60 no conformity to the patient’s specific anatomy. Studies have reported that the existing BT 61 4 procedures can result in side -effects such as edema, incontinence, and impotence resulted by an 62 excessive dose to the OARs [4]. Imprecise catheter impla ntation often causes insufficient dose to 63 the cancer and/or inadvertent radiation of the OARs . The former causes failure of treatment, while 64 the latter results in adverse side effects like rectal ulceration, incontinence, and dysuria (painful 65 urination). Studies have also shown improved outcomes (e.g., cancer control 8-year biochemical 66 failure free survival ) upon boost doses to dominant intraprostatic lesions (DILs) [5,6] . Prostate BT 67 provides unique opportunity to escalate dose to the DILs, however, bounded due to the high risk 68 of intoxicating the OARs. 69 The curvilinear catheters implantation, which is conformal to the patient -specific anatomy, 70 provides access channels away from the OARs and closer to th e DILs to optimize dosimetry and 71 expand the limits for dose escalation to the DILs. In a previous study with 20 patients [7], we 72 showed that the curvilinear catheter implantation yield s significantly improved dosimetric plan s 73 with a substantial reduction in number of needles compared to conventional rectilinear (straight) 74 catheter implantation . The study showed a high potential to improve the clinical outcomes for 75 prostate cancer patients. This work presents the feasibility o f curvilinear catheter implantation 76 method using tendon -driven active needles , along with a framework to realize the plan inside a 77 patient -specific phantom tissue. 78 This work is organized as follows: Section 2 introduces a framework for curvilinear catheter 79 implantation . Section 2.1 .1 pre-plans a curvilinear catheter implantation and a dosimetric plan for 80 an anonymized patient, while Section 2.1.2 evaluate s the dosimetric plan . Section 2.2.1 analyzes 81 the catheters’ curvatur es for the implantation . Section 2.2.2 introduces a new design of tendon - 82 driven active needles to realize the required curvatures . Section 2.2.3 presents our robotic system 83 to insert and actuate the active needles. Section 2.3.1 presents a patient -specific phantom tissue, 84 5 specifically developed to evaluate the active needles as explained in Section 2.3.2. Section 3.1 85 presents the dosimetric benefits of using curvilinear catheters in HDR BT. Sections 3.2 and 3.3 86 demonstrate the feasibility of curvilinear cat heter implantation inside prostate via evaluations in 87 air and inside a phantom tissue, respectively. 88 2. Materials and Methods 89 Figure 1 shows a framework to realize curvilinear catheter implantation inside prostate gland for 90 HDT BT that includes: (i) identifi cation of patient -specific organs of patient from MRI and/or CT 91 scans , (ii) manual mapping of curved trajectories inside the prostate gland conformal to the prostate 92 boundaries and away from the OARs , (iii) estimation of dosimetric benefits, (iv) identification of 93 best planar polynomial fit to the curved trajectories , (v) design and development of a tendon -driven 94 active needle to realize the objective curvatures , (vi) design and development of a custom template 95 for proper angulation of the needles, and (vii) evaluation of the plan in air and inside patient - 96 specific phantom tissue with actual geometric relationships . 97 Figure 1. A framework to realize the curvilinear catheter implantation for optimized HDR BT. 6 2.1. Curvilinear catheter implantation 98 This section presents methods to preplan a curvilinear catheter configuration conformal to the 99 patient -specific anatomy followed by a dose plan . 100 2.1.1. Preplanning needle configuration for an anonymized patient 101 T2-weigthed MRIs of a n anonymized patient from our practice were first obtained . The p rostate, 102 and the OARs, i.e., urethra, rectum, and bladder were then manually contoured (Figure 2a) to 103 construct 3D model of the patient -specific organs . The 3D models (volumes) of the organs were 104 then exported as STL files for further analysis . Curved trajectories were manually mapped inside 105 the prostate gland , conformal to the prostate capsule , closer to the peripheral zone of the prostate, 106 and away from the OARs. Using Oncentra HDR TPS and HIPO optimizer, a dosimetric plan was 107 generated using curvilinear catheters that overlayed on the curved trajectories . The plan generated 108 ten curved catheters , as shown in Figure 2 b, to host the radiation source for treatment . 109 (a) (b) Figure 2. (a) Contour s of prostate, urethra, bladder, and rectum to identify and model the organs, (b) ten curved trajectories mapped inside prostate gland conformal to the boundaries for curvilinear catheter implantation. 110 7 2.1.2. Dosimetric evaluation of the plan with curvilinear catheter implantation 111 Dwell positions and dwell time for the radiation source were planned using Oncentra software 112 based on the dose prescription for the anonymized patient . Data were collected for D90, V100, V150, 113 V200; for prostate (target); Dmax, D 2cc, V12Gy; for rectum; Dmax, D 10, V100, for urethra, and Dmax for 114 bladder from dose-volume histogram (DVH ) were collected . Section 3.1 describes the dosimetric 115 benefits of the curvilinear catheter implantation. 116 2.2. Tendon -driven active needles 117 2.2.1. Curvature analyses – 2D planar bending 118 The curved trajectories, generated in the previous step using Oncentra, were imported into 119 MATLAB as 3D coordinate text file s, identifying several points along the ir trajectories (see Figure 120 2b). The trajectories displayed curvatures in different planes. To represent each trajectory as a 2D 121 planar curvature, a second order polynomial was fitted to the data points . Table 1 lists the catheters 122 with the best fit polynomial, the radius of curvature ( R), and the curvature ( K) for the ten 123 trajectories . The averag e, maximum, and minimum radius of curvature (R) for all ten catheters 124 were 56.48, 165.50, 15.19mm, respectively , with a standard deviation of 44.55mm. The average, 125 maximum, and minimum curvature ( K) for all ten catheters were 0.0 3, 0.0 7, 0.0 1 mm-1, 126 respect ively , with a standard deviation of 0.02mm-1, covering a wide range of curvatures for the 127 catheters. The average, maximum, and minimum bending angle of all ten catheters were 22. 36, 128 49.99, 4.59 deg, respectively, with a standard deviation of 16.4 1 deg. 129 130 8 Table 1. Polynomial fit to the curvilinear catheters preplanned using Oncentra. Catheter x2 x int Radius of Curvature R (mm) Curvature, K, 1/R (1/mm) Angular bending (deg) 1 -0.0067 0.4567 30.0045 74.42 0.01 10.21 2 -0.0094 0.7743 21.8162 53.10 0.02 14.30 3 -0.0030 0.2478 30.5477 165.50 0.01 4.59 4 -0.0142 1.3510 -13.8872 35.21 0.03 21.57 5 0.0114 -1.1749 12.8432 43.67 0.02 17.39 6 -0.0329 2.9523 -44.2359 15.19 0.07 49.99 7 0.0076 -0.9477 15.6304 65.46 0.02 11.60 8 0.0067 -0.7836 32.1606 75.18 0.01 10.10 9 -0.0317 2.8600 -46.9309 15.80 0.06 48.09 10 -0.0235 2.1377 -36.6467 21.28 0.05 35.70 131 2.2.2. Tendon -driven active needle design to realize required curvatures 132 Tendon -driven active needles have been used to bend inside tissue for path tracking and accurate 133 targeting in different applications [8–10]. The tendon -driven active needle features a flexure 134 section (Figure 3a) inclusive to several notches that are carve d on the needle’s Nitinol tube for 135 improved flexibility . The flexure section is actuated via two internal tendons, shown in the figure, 136 for bidirectional planar bending. The length of the flexure section as well as the height and depth 137 of the notches and t he number of notches could alternate to facilitate different curvatures (angular 138 bending) . The tendon -driven active needle and the angular bending are shown in Figure 3b. The 139 flexure section of the active needle is insulated using a biocompatible and ultra thin (wall thickness 140 of 76 μm) medical -grade heat shrink tubing to prevent tissue penetration inside the needle tube. 141 142 9 Figure 3. (a) Flexure section of the tendon -driven active needle , and (b) the fabricated prototype with an insulating sheath. 143 The following equations are used to estimate the bending angle (and consequently the radius of 144 curvature) of the flexure section. For the notches schematically show n in Figure 3a, the neutral 145 bending axis can be calculated using the following equation [11]: 146 𝑦̄=4(𝑟𝑜3𝑠𝑖𝑛3(𝜙𝑜)−𝑟𝑖3𝑠𝑖𝑛3(𝜙𝑖)) 3(𝑟𝑜2(2𝜙𝑜−𝑠𝑖𝑛2𝜙𝑜)−𝑟𝑖2(2𝜙𝑖−𝑠𝑖𝑛2𝜙𝑖)) (1) 147 𝑟𝑖=𝐼𝐷 2,𝑟𝑜=𝑂𝐷 2 (2) 148 where 𝑦̄ is the distance moved from the center of the tube, OD, ID, 𝑟𝑜, and 𝑟𝑖 are the outer and 149 inner diameters, outer and inner radii of the needle tube, respectively, and 𝜙𝑜 and 𝜙𝑖 are found by 150 the following equations: 151 θtotal Insulating sheath (a) (b) 10 𝜙𝑜=𝑎𝑟𝑐𝑐𝑜𝑠 (𝑑−𝑟𝑜 𝑟𝑜) (3) 152 𝜙𝑖=𝑎𝑟𝑐𝑐𝑜𝑠 (𝑑−𝑟𝑜 𝑟𝑖) (4) 153 where d is the notch depth. The bending angle for each notch can be estimated as: 154 𝜃𝑖=ℎ 𝑟𝑜+𝑦̄ (5) 155 where h is the height of the neutral axis for each, which in this work is equal to the height of the 156 cut (t). 157 The flexure section of the active needle was designed for the smallest radius of curvature among 158 the ten catheters listed in Table 1 (i.e., Catheter 3). The bending angle for the sharpest curved 159 trajectory was 50.11 degrees. 160 Assuming that the total deflection is distributed equ ally among the notches, the bending angle of 161 the whole flexible section (θtotal) could be found by multi plying the bending angle of each notch 162 (𝜃𝑖) by the number of notches. 163 Table 2. The dimensions of the notches for the active needle. Units are in mm. l 𝑟𝑖 (𝐼𝐷) 𝑟𝑜 (𝑂𝐷) 𝑦̄ 𝑑 𝑡 13.280 0.745 (1.49) 0.965 (1.93) 1.450 1.680 0.460 164 Dimensions of each notch for the active needles are listed in Table 2. Using the equations above, 165 the angular deflection of each notch was 14.76 degrees. The overall angular deflection of the 166 flexure sections ( eight notches) 118.08 degrees, which surpass es the required angular deflection 167 of 50 .11 degrees for the active needle . 168 11 Next section evaluates the capability of the tendon -driven active needle , with the dimensions 169 shown in Figure 3 and listed in Table 2 , to realiz e the required curvatures listed in Table 1. 170 2.2.3. Tendon actuation to curve needles - bending evaluations in air 171 The needle insertion and actuation system , shown in Figure 4, was used to push the needle forward 172 and to pull the tendons to realize the curvatures on each catheter . The system consists of stepper 173 motors, gear boxes, and lead screws to insert and bend the catheters for proper implantation inside 174 prostate . The motors were programmed to pull the internal tendons. The experimental setup for 175 automated control of the robotic tool consists of two NEMA 11 stepper motors (ToAuto, 176 Shenzhen, China) attached to two 100mm linear rails. Each motor is attached to one tendon that is 177 responsible for tightening or loosening the tendons to perpetuate bending at the active needle . The 178 motors are wired to DM542 T drivers (StepperOnline Inc., New York, NY), which are run through 179 Arduino Uno control boards. Serial communications are established between the Arduino boards 180 and a Python script on a computer to control the tendon displacement. 181 Figure 4. Needle insertion and actuation system to push the needle forward and bend the needle to realize curvatures. 182 12 2.3. Feasibility study 183 This section demonstrates the feasibility of using tendon -driven active needles to realize a 184 curvilinear catheter implantatio n in phantom tissues. 185 2.3.1. Patient -specific phantom tissue 186 An injection molding process was used here to develop a patient -specific phantom tissue. The 187 mold (Figure 5a) was designed using the prostate model (Figure 5b), extracted from the MRI of 188 the patient, to make a real scale phantom. Plastic softener and hardener were injected into the mold, 189 solidified, and extracted to form a prostate -shaped phantom. The prostate -shaped phantom was 190 then embedded inside a larger phantom (Figure 5c) for catheter implantation . Figure 5d is the 191 ultrasound image of the phantom tissue showing the boundaries of the patient ’s prostate. 192 (a) (b) (c) (d) Figure 5. Patient -specific phantom tissue developed using (a) injection molding, where the mold was made using (b) the 3D model of the patient’s prostate, (c) embedded inside a larger phantom, with (d) the ultrasound image for catheter implantation. 193 2.3.2. Curvilinear implantation inside a patient -specific phantom tissue 194 The actuation system, described in Section 2.2.3 was used to insert and bend the catheters 195 sequentially inside the patient -specific phantom tissue explained in Section 2.3.1. A clamp was 196 designed, and 3D printed to secure the tendons and keep the needles’ curvatures inside the prostate. 197 13 3. Results 198 3.1. Improved dosimetric plans via curvilinear catheter implantation 199 This section evaluates dosimetric benefits when HDR BT is performed using curvilinear catheter 200 implantation . An additional dose plan was generated using conventional rectilinear catheter 201 implantation method for direct comparison with the plan generated Section 2.1.1 . Table 3 lists the 202 dosimtric data for the two plans generated with curvilinear as well as rectilinear catheter 203 implantation. The rectilinear catheter implantation failed to meet the dose constraint for the rectum 204 (12Gy < 0.6cc) , while satisfactory with the curvilinear catheter implantation . The number of 205 needles required to meet the dosimetric constraints for the prostate (target) and the OARs (urethra, 206 rectum, and bladder) was significantly lower (22 vs. 10 catheters) for the curvilinear compared to 207 the rectilinear catheter implantation approach. 208 Table 3. Dosimetric plan using curvilinear and rectilinear catheter implantation . Curvilinear catheter implantation Prostate Urethra vol (cc) D90 (Gy) V100 (%) V150 (%) V200 (%) D10 (Gy) D10 (% Rx) V100 (%) Max dose (cGy) Max (% Rx) 64.8 16.2 95.2 37.3 13.8 18.0 119.9% 92.3 18.4 122.7% Rectum Bladder Needles Vol of 12 Gy (cc) D_2cc (cGy) Max Dose (cGy) Max Dose (cGy) Curved 0.54 10.9 13.3 18.4 10 Rectilinear catheter implantation Prostate Urethra vol (cc) D90 (Gy) V100 (%) V150 (%) V200 (%) D10 (Gy) D10 (% Rx) V100 (%) Max dose (cGy) Max (% Rx) 64.8 16.0 95.6 29.8 11.1 17.8 118.6% 92.3 18.3 122.2% Rectum Bladder Needles Vol of 12 Gy (cc) D_2cc (cGy) Max Dose (cGy) Max Dose (cGy) Straight 0.90 11.2 13.4 19.9 22 209 14 Figure 6 shows the heat map of the prostate due to the radiation received in HDR BT when the 210 radiation source is positions inside the curvilinear catheter s along with the DVH diagram. Using 211 this method, larger dose has been delivered to the peripheral zone of the prostate, where the cancer 212 is normally located. 213 (a) (b) Figure 6. Radiation heat map: (a) dose distribution inside the prostate gland, and (b) dose volume histogram (DVH). 214 3.2. Curvature demonstration and evaluations in air 215 The curvatures of the ten catheters , listed in Table 1, were realized by the tendon -driven active 216 needle s using the actuation system shown in Figure 4 . The curvatures were 3D printed and placed 217 behind the bent catheters for visual comparison. 218 219 15 Figure 7. Tendon -driven active needles actuated to realize the desired curvatures of the curved trajectories as planned in Section 2.2.1 . Top and bottom row, left to right correcponds to Catheters 1 to 10 from Table 1. The required curvatures are 3D printed in black color and placed behind the active needle for visual comparison. 220 The capability of the active needle , presented in Section 2.2.2, to realize desired curvature s in air 221 was evaluated by direct ly compar ing the simulated curvature s with the curvature s of the active 222 needle s realized upon actuation . Table 4 lists the simulated and the actual radii of curvature and 223 bending an gles of the catheters resulted from curve fitting and realized by the active needle s, 224 respectively. The deviation s from the desired radi i of curvature and the bending angle s were 225 estimated and the errors were reported. The average error in realizing the desired radius of 226 curvature and the bending angle was 18.28 and 24.55 %, respectively. Larger errors are associated 227 with the dimensions of the flexure section of the active needle (e.g., length of the section, depth 228 and height of each notch, and the number of notches) that are not designed for a specific desired 229 curvature. Improved accuracy could be achieved by designing a few active needles with different 230 flexure sections to cover the wide range of desired curvatures (which wa s the case in this work) . 231 16 The active needle , presented in this work, was able to represent most of the desired curvatures with 232 reasonable accuracy. 233 Table 4. Simulated and realized radius of curvature and bending angle of the active needles shown in Figure 7. Units are mm and deg for radi i of curvature and bending angle s, respectively. Radius of curvature (simulation) Bending angle (simulation) Radius of curvature (actual) Bending angle (actual) Error in realizing radius of curvature (%) Error in realizing bending angle (%) 74.42 10.21 64.10 11.85 13.86 16.09 53.10 14.30 44.64 17.01 15.93 18.95 165.50 4.59 192.31 3.95 16.19 13.94 35.21 21.57 20.83 36.46 40.83 69.01 43.67 17.39 43.86 17.32 0.43 0.43 15.19 49.99 10.96 69.27 27.83 38.56 65.46 11.60 47.17 16.10 27.94 38.77 75.18 10.10 75.76 10.03 0.77 0.76 15.80 48.09 13.30 57.12 15.81 18.79 21.28 35.70 16.34 46.48 23.20 30.21 234 The tendon pulling force of the active needle was estimated using the equation below for the lead 235 screw [12], 236 𝑇=𝐹𝑝𝑑𝑚 2(𝑙+𝜋𝑓𝑑𝑚𝑠𝑒𝑐𝛼 𝜋𝑑𝑚−𝑓𝑙𝑠𝑒𝑐𝛼) (6) where 𝑇 is the applied torque by the Maxon motor to pull the tendons, 𝑑𝑚 is the mean diameter 237 of a single thread lead screw, 𝐹𝑝 is the tendon pulling force to be found, 𝑙 is the tendon 238 displacement, 𝑓 is the coefficient of friction for threaded pairs of a s teel and dry screw, which is 239 0.25 here, and 𝛼 is the thread angle, which is zero for a square thread [12]. 240 The applied torque ( T) was estimated using the motor’s current, nominal current, and torque 241 constant using the following e quations. 242 17 Torque actual value (per mil)=𝐶𝑢𝑟𝑟𝑒𝑛𝑡 (𝑚𝐴) Nominal current (mA)∗1000 (7) 𝑇 (𝑚𝑁𝑚 )=(Torque actual value (per mil) 1000)∗(𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 1000) ∗𝑇𝑜𝑟𝑞𝑢𝑒 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (𝑚𝑁𝑚 /𝐴) (8) To find the bending angle 𝜃 in each step of insertion, the following equation could be use, 243 𝜃=tan−1𝑢𝑡(𝑖)−𝑢𝑡(0) 𝑧(𝑖)−𝑧(0) (9) where 𝑢𝑡(𝑖) and 𝑢𝑡(0) are the needle tip paths at moment 𝑖 and at the initial moment, 244 respectively, and 𝑧(𝑖) and 𝑧(0) are the insertion depths at moment 𝑖 and at the initial moment, 245 respectively . 246 (a) (b) Figure 8. Tip displacement and angular bending of the t endon -driven active needle via (a) tendon displacement, and (b) tendon force actuations. 247 Figures 8a and 8b show the needle tip displacement and angular bending with respect to tendon 248 displacement and tendon pulling force, respectively. Figure 9 shows th e needle tip displacement, 249 radius of curvature, and angular bending of the tendon -driven active needle via tendon 250 18 displacement actuation using the actuation system (Figure 4) for the ten catheters planned for 251 curvilinear implantation (listed in Table 1). 252 Figure 9. Tendon displacement, needle tip displacement, radius of curvature, and angular bending for the ten catheters listed in Table 1, realize by the tendon -driven prototype and the actuation system. 253 A custom -made template (shown in Figure 10) was designed and fabricated to properly angulate 254 the catheters. Insertion angles of the catheters are defined on the template to facilitate insertion 255 and bending conformal to the prostate gland boundaries. Figure 10a and 10b demonstrate 256 curvilinear catheter implantation c onsisting of the ten catheters planned previous ly in Section 257 2.1.1. The tendons were pulled using the actuation system, and then secured using 3D clamps, to 258 keep the desired curvatures. The tendons were actuated sequentially using the same actuation 259 system . 260 19 (a) (b) Figure 10. Curvilinear catheter implantation: (a) 3D model of the assembly, and (b) configuration demonstrated in air. 261 3.3. Evaluating curvilinear catheter implantation in patient -specific phantom 262 tissue 263 The tendon -driven active needles were inserted into the phantom tissue using the actuation system. 264 The active needles were gradually inserted and bent inside the phantom to realize the required 265 curvatures. Figure 11a shows the catheters implanted inside th e phantom conformal to the prostate 266 boundaries. A transverse ultrasound image was obtained to show the position of all ten curvilinear 267 catheters inside the phantom tissue. The cross section of the catheters appears in the ultrasound 268 image, as marked in the figure with red circles. 269 20 (a) (b) Figure 11. Curvilinear catheter implantation: (a) inside a patient -specific phantom tissue, and (b) transverse ultrasound image of the implant showing the position of all catheters in red circles . 270 4. Discussion 271 This work introduces curvilinear catheter implantation approach to replace the conventional 272 rectilinear catheter implantation method in prostate HDR brachytherapy. The c urvilinear catheter 273 implantation improves dosimetric plans inclusion coverage to the prostate (target) as well as dose 274 constraints to the OARs (urethra, rectum, and bladder) , with a substantial reduction in number of 275 needles . Specifically designed and developed t endon -driven active needle s were used to curve 276 insid e the prostate to facilitate curvilinear catheter implantation. The flexure section of the active 277 needle was designed based on the required curvatures to conform to the patient -specific anatomy. 278 A curvilinear catheter implantation was then demostrated usin g the active needles (operated 279 robotically using an actuation system) in air and inside a patient -specific phantom tissue to show 280 the feasibility of this approach . 281 This work concludes that tendon -driven active needles could specifically designed and develo ped 282 to replace conventional straight needles in HDR prostate BT, enabling curvilinear catheter 283 21 implantation with improved conformity to the patient’s specific anatomy. Arrangement of the 284 curvilinear catheters closer to the areas of interests provides uniqu e opportunity for dose escalation 285 to the dominant intraprostatic lesions . 286 Any imprecision in curvilinear catheter implantation (if exists) is compensated at the replanning 287 stage of HDR BT procedure after actual implantation of the catheters and using actu al positions of 288 the catheters inside the prostate (obtained by additional CT scan). 289 5. Acknowledgement 290 Research reported in this publication was supported by the National Institute Of Biomedical 291 Imaging And Bioengineering of the National Institutes of Health under Award Number 292 K25EB030562. The content is solely the responsibility of the authors and does not necessarily 293 represent the official views of the National Institutes of Health. 294 6.