Online correction of catheter movement using CT in high-dose-rate prostate brachytherapy

Development of patient and catheter specific error thresholds for high dose rate prostate brachytherapy

BACKGROUND

In-vivo source tracking has been an active topic of research in the field of high-dose rate brachytherapy in recent years to verify accuracy in treatment delivery. Although detection systems for source tracking are being developed, the allowable threshold of treatment error is still unknown and is likely patient-specific due to anatomy and planning variation.

PURPOSE

The purpose of this study was to determine patient and catheter-specific shift error thresholds for in-vivo source tracking during high-dose-rate prostate brachytherapy (HDRPBT).

METHODS

A module was developed in the previously described graphical processor unit multi-criteria optimization (gMCO) algorithm. The module generates systematic catheter shift errors retrospectively into HDRPBT treatment plans, performed on 50 patients. The catheter shift model iterates through the number of catheters shifted in the plan (from 1 to all catheters), the direction of shift (superior, inferior, medial, lateral, cranial, and caudal), and the magnitude of catheter shift (1-6 mm). For each combination of these parameters, 200 error plans were generated, randomly selecting the catheters in the plan to shift. After shifts were applied, dose volume histogram (DVH) parameters were re-calculated. Catheter shift thresholds were then derived based on plans where DVH parameters were clinically unacceptable (prostate V100 < 95%, urethra D0.1cc > 118%, and rectum Dmax > 80%). Catheter thresholds were also Pearson correlated to catheter robustness values.

RESULTS

Patient-specific thresholds varied between 1 to 6 mm for all organs, in all shift directions. Overall, patient-specific thresholds typically decrease with an increasing number of catheters shifted. Anterior and inferior directions were less sensitive than other directions. Pearson's correlation test showed a strong correlation between catheter robustness and catheter thresholds for the rectum and urethra, with correlation values of -0.81 and -0.74, respectively (p < 0.01), but no correlation was found for the prostate.

CONCLUSIONS

It was possible to determine thresholds for each patient, with thresholds showing dependence on shift direction, and number of catheters shifted. Not every catheter combination is explorable, however, this study shows the feasibility to determine patient-specific thresholds for clinical application. The correlation of patient-specific thresholds with the equivalent robustness value indicated the need for robustness consideration during plan optimization and treatment planning.

Vaginal Dose Reduction by Changing the Ovoid Loading Pattern in Image Guided Intracavitary Brachytherapy of Cervix

Aim

Locally advanced cervical cancer is frequently treated using a combination of external beam radiotherapy and brachytherapy. Radiotherapy often leads to vaginal morbidity, which poses a significant problem. This study aims to analyze the impact of reducing ovoid loading on dosimetry.

Materials and methods

We analyzed forty-five CT-based intracavitary brachytherapy plans from fifteen patients. Three plan sets were created for the 45 applications: a standard loading plan (A), a plan with reduced ovoid loading (B), and a tandem-only loading plan (C). We generated Dose-Volume Histograms and recorded dose volume parameters for the three plan sets.

Results

The D90 for the Clinical Target Volume (CTV) did not show significant differences among the three plan sets (p = 0.20). The average D90 values for plans A, B, and C were 8.15 Gy, 8.16 Gy, and 7.4 Gy, respectively. No statistically significant differences were observed in D2cc bladder (p = 0.09) (average values: 6.8 Gy, 6.5 Gy, and 5.9 Gy for plans A, B, and C, respectively) and D2cc sigmoid (p = 0.43) (average values: 2.8 Gy, 2.6 Gy, and 2.4 Gy, respectively) among the three plan sets. However, there was a statistically significant difference in D2cc rectum (p < 0.001) (average values: 4 Gy, 3.3 Gy, and 1.8 Gy, respectively), as well as in vaginal dose points (p < 0.001).

Conclusion

Reducing ovoid loading significantly decreased the doses to vaginal dose points and the rectum without compromising the dose to the Clinical Target Volume (CTV). Therefore, in carefully selected cases, the adoption of tandem-only loading or reduced ovoid loading could be considered to minimize vaginal morbidity following high dose rate intracavitary brachytherapy.

In vivo dosimetry in pelvic brachytherapy

ADVANCES IN KNOWLEDGE

This paper describes the potential role for in vivo dosimetry in the reduction of uncertainties in pelvic brachytherapy, the pertinent factors for consideration in clinical practice, and the future potential for in vivo dosimetry in the personalisation of brachytherapy.

Prostate volume and implant configuration during 48 hours of temporary prostate brachytherapy: limited effect of oedema

BACKGROUND

In pulsed-dose rate prostate brachytherapy the dose is delivered during 48 hours after implantation, making the treatment sensitive to oedematic effects possibly affecting dose delivery. The aim was to study changes in prostate volume during treatment by analysing catheter configurations on three subsequent scans.

METHODS

Prostate expansion was determined for 19 patients from the change in spatial distribution of the implanted catheters, using three CT-scans: a planning CT (CT1) and two CTs after 24 and 48 hours (CT2, CT3). An additional 4 patients only received one repeat CT (after 24 hours). The mean radial distance (MRD) of all dwell positions to the geometric centre of all dwell positions used was calculated to evaluate volume changes. From three implanted markers changes in inter-marker distances were assessed. The relative shifts of all dwell positions were determined using catheter- and marker-based registrations. Wilcoxon signed-rank tests were performed to compare the results from the different time points.

RESULTS

The MRDs measured on the two repeat CTs were significantly different from CT1. The mean prostate volume change derived from the difference in MRD was +4.3% (range -9.3% to +15.6%) for CT1-CT2 (p < .05) and +4.4% (range -7.5% to +16.3%) for CT1-CT3 (p < .05). These values represented a mean increase of 1.2 cm(3) in the first 24 hours and 1.5 cm(3) in the subsequent 24 hours. There was no clear sign of prostate expansion from the change in inter-marker distance (CT1-CT2: 0.2 ± 1.8 mm; CT1-CT3: 0.6 ± 2.2 mm). Catheter configuration remained stable; shifts in catheter positions were largest in the C-C direction: 0 ± 1.8 mm for CT1-CT2 and 0 ± 1.4 mm for CT2-CT3.

CONCLUSIONS

The volume changes derived from catheter displacements were small and therefore considered clinically insignificant. Implant configuration remains stable during 2 days of treatment, confirming the safety of this technique.