PSOR04  Presentation Time: 4:45 PM: HDR Prostate Brachytherapy In-Vivo Source Tracking: To Interrupt or Adapt?

Purpose The use of high dose rate brachytherapy (HDR BT) either as monotherapy, or in combination with external beam radiotherapy (EBRT) is a proven technique in the treatment of prostate cancer. The high dose gradients inherent within HDR BT make it an ideal technique for dose escalation, however the same dose gradients also make HDR BT susceptible to dosimetric errors. Treatment verification is therefore imperative, to ensure correct dose delivery in HDR BT of the prostate. One method of performing treatment verification is in-vivo source tracking (IVST), in which the position of the BT source is tracked in real time as it delivers the treatment. Unfortunately, limited data exists on what would be an appropriate error threshold to interrupt prostate HDRT BT treatments when performing IVST. The purpose of this study was two-fold: 1) To determine an appropriate IVST error thresholds for interrupting prostate HDR BT treatments. 2) To examine the feasibility of performing online adaption of prostate HDR BT plans (as an alternative to interrupting), through retrospective simulation of source positioning errors. Materials and Methods Source positioning errors (catheter shifts in 1mm increments in the cranial/caudal, left/right, anterior/posterior directions up to ±6 mm) were simulated in 20 retrospective prostate HDR BT treatment plans. Dose volume histogram (DVH) indices were monitored as errors were introduced into catheters, simulating potential errors during treatment. To derive appropriate IVST error thresholds, the change in DVH indices for simulated errors were used to determine appropriate in-vivo source tracking error thresholds for a total treatment plan. Secondly, to simulate the feasibility of online adaption, whenever DVH indices were outside clinically acceptable criteria (prostate V100% < 95%, urethra D0.1cc > 118%, rectum Dmax > 80%), the plan was adapted via optimisations of the remaining catheters to be delivered (i.e. locking the previously simulated catheters from further optimisation). The final DVH indices were then recorded once all catheters had been deliverd. Results IVST error thresholds to prevent potentially significant changes in prostate (target) DVH metrics ranged from 2 to 5 mm, dependent on the direction of the source positioning error, as well as the relative weight of the dwell position within the plan, and its position relative to the patient anatomy. Source positioning error thresholds to prevent potentially clinically significant changes in urethra and rectum DVH metrics varied significantly, were found to be complex and patient dependent. Through online adaptation, prostate coverage (V100% > 95%) could be maintained for source position errors up to 6 mm. The source position error at which the urethra D0.1cc and rectum Dmax was able to return to clinically acceptable levels using online adaptation varied between 1 mm to 6 mm, depending on the direction of the source position error and patient anatomy. Conclusions IVST error thresholds are highly dependent on the treatment plan and patient anatomy, particularly when using organ at risk dose as the primary endpoint. Online adaptation of prostate HDR BT has potential to prevent underdosage of the target, but improvements in the robustness of treatment plans must be made before it is able to resolve source positioning errors near organs at risk. The use of high dose rate brachytherapy (HDR BT) either as monotherapy, or in combination with external beam radiotherapy (EBRT) is a proven technique in the treatment of prostate cancer. The high dose gradients inherent within HDR BT make it an ideal technique for dose escalation, however the same dose gradients also make HDR BT susceptible to dosimetric errors. Treatment verification is therefore imperative, to ensure correct dose delivery in HDR BT of the prostate. One method of performing treatment verification is in-vivo source tracking (IVST), in which the position of the BT source is tracked in real time as it delivers the treatment. Unfortunately, limited data exists on what would be an appropriate error threshold to interrupt prostate HDRT BT treatments when performing IVST. The purpose of this study was two-fold: 1) To determine an appropriate IVST error thresholds for interrupting prostate HDR BT treatments. 2) To examine the feasibility of performing online adaption of prostate HDR BT plans (as an alternative to interrupting), through retrospective simulation of source positioning errors. Source positioning errors (catheter shifts in 1mm increments in the cranial/caudal, left/right, anterior/posterior directions up to ±6 mm) were simulated in 20 retrospective prostate HDR BT treatment plans. Dose volume histogram (DVH) indices were monitored as errors were introduced into catheters, simulating potential errors during treatment. To derive appropriate IVST error thresholds, the change in DVH indices for simulated errors were used to determine appropriate in-vivo source tracking error thresholds for a total treatment plan. Secondly, to simulate the feasibility of online adaption, whenever DVH indices were outside clinically acceptable criteria (prostate V100% < 95%, urethra D0.1cc > 118%, rectum Dmax > 80%), the plan was adapted via optimisations of the remaining catheters to be delivered (i.e. locking the previously simulated catheters from further optimisation). The final DVH indices were then recorded once all catheters had been deliverd. IVST error thresholds to prevent potentially significant changes in prostate (target) DVH metrics ranged from 2 to 5 mm, dependent on the direction of the source positioning error, as well as the relative weight of the dwell position within the plan, and its position relative to the patient anatomy. Source positioning error thresholds to prevent potentially clinically significant changes in urethra and rectum DVH metrics varied significantly, were found to be complex and patient dependent. Through online adaptation, prostate coverage (V100% > 95%) could be maintained for source position errors up to 6 mm. The source position error at which the urethra D0.1cc and rectum Dmax was able to return to clinically acceptable levels using online adaptation varied between 1 mm to 6 mm, depending on the direction of the source position error and patient anatomy. IVST error thresholds are highly dependent on the treatment plan and patient anatomy, particularly when using organ at risk dose as the primary endpoint. Online adaptation of prostate HDR BT has potential to prevent underdosage of the target, but improvements in the robustness of treatment plans must be made before it is able to resolve source positioning errors near organs at risk.