Session Item

In CT-planned HDR prostate brachytherapy the time delay of ~2 to 3 hours between the insertion of catheters and treatment delivery may bring about inaccuracies to the treatment delivery due to catheter displacement. The catheter position in the scanned image, used to plan a treatment, may not match the true positions in the patient during treatment. In such cases, the catheters may need to be repositioned and the procedure repeated. This prolongs the treatment procedure and may lower the quality of the treatment.  This study involves evaluating the impact of the catheter displacement in single fraction HDR prostate brachytherapy treatment.

20 patients had simulated catheter displacement introduced on Oncentra Brachy. The catheter shifts were simulated in two ways, synchronous and selective catheter shift. For the former, for each treatment plan, all catheters were displaced uniformly through known values. For the latter, catheters were selectively displaced, based on the segment of the planning target volume (PTV) where they were positioned; anterior, middle and posterior segment as shown in Figure 1. The dose coverage to the PTV, urethra, bladder and rectum were then evaluated using the parameters of dose coverage to 90% of the PTV volume, D90, dose accumulated by 30% of the urethra volume, D30, and the volume of the bladder and rectum receiving 70% (10.5 𝐺𝑦) of prescribed dose, V70.

The treatment planning goal for D90 is to deliver a minimum of 90% of the prescribed dose. Majority of treatment plans for synchronous catheter shift simulations show PTV dose coverage meeting this clinical constraint in the range of −3𝑚𝑚 to 6𝑚𝑚. Increase in D30 of the urethra is seen as the catheters are shifted inferiorly; bladder V70 increases as the catheters shift superiorly; and as the catheters shift inferiorly, more of the rectal V70 accumulates. For selective catheter shift simulation, a significant dose increase to the PTV was detected at 3𝑚𝑚 superiorly upon shifting the catheters in the middle segment for all patient groups. For urethral D30, a distinct dose increase is observed in the results for the ‘anterior segment’ and ‘posterior segment’ catheter shift as the shifts increase inferiorly. However, simulating the inferior shifts for the ‘middle segment’ catheters resulted in a random dose fall-off to the urethra. There is an increase in V70 for the bladder as the catheters shift superiorly, and for the rectum, dose increase is observed when catheters in the posterior segment shift inferiorly. To estimate the needle shift tolerance, the normalised dose coverage drop to the PTV is plotted using the data from the selective catheter shift simulation, see Figure 2. 

Approximately 90% of all treatment plans experience a dose decrease of less than and equal to 5% when the catheters were displaced by 3𝑚𝑚. To ensure PTV dose coverage is maintained at an optimum level and to reduce organ toxicity, the needle shift tolerance has been concluded to be 3𝑚𝑚.