Intra-fraction displacement of the prostate bed during post-prostatectomy radiotherapy

Intra-fractional geometric and dose/volume metric variations of magnetic resonance imaging-guided stereotactic radiotherapy of prostate bed after radical prostatectomy

Background and purpose

Magnetic Resonance Imaging (MRI)-guided Stereotactic body radiotherapy (SBRT) treatment to prostate bed after radical prostatectomy has garnered growing interests. The aim of this study is to evaluate intra-fractional anatomic and dose/volume metric variations for patients receiving this treatment.

Materials and methods

Nineteen patients who received 30-34 Gy in 5 fractions on a 0.35T MR-Linac were included. Pre- and post-treatment MRIs were acquired for each fraction (total of 75 fractions). The Clinical Target Volume (CTV), bladder, rectum, and rectal wall were contoured on all images. Volumetric changes, Hausdorff distance, Mean Distance to Agreement (MDA), and Dice similarity coefficient (DSC) for each structure were calculated. Median value and Interquartile range (IQR) were recorded. Changes in target coverage and Organ at Risk (OAR) constraints were compared and evaluated using Wilcoxon rank sum tests at a significant level of 0.05.

Results

Bladder had the largest volumetric changes, with a median volume increase of 48.9 % (IQR 28.9-76.8 %) and a median MDA of 5.1 mm (IQR 3.4-7.1 mm). Intra-fractional CTV volume remained stable with a median volume change of 1.2 % (0.0-4.8 %). DSC was 0.97 (IQR 0.94-0.99). For the dose/volume metrics, there were no statistically significant changes observed except for an increase in bladder hotspot and a decrease of bladder V32.5 Gy and mean dose. The CTV V95% changed from 99.9 % (IQR 98.8-100 %) to 99.6 % (IQR 93.9-100 %).

Conclusion

Despite intra-fractional variations of OARs, CTV coverage remained stable during MRI-guided SBRT treatments for the prostate bed.

Evaluating the geometric and dosimetric impact of applying anisotropic CTV to PTV margins in image-guided post-prostatectomy radiation therapy

INTRODUCTION

Guidelines for clinical target volume (CTV) to planning target volume (PTV) margins in post-prostatectomy radiation therapy (PPRT) are varied and often not clearly defined. Assessment of appropriateness of margins is commonly measured on prevalence of geographic miss.

METHODS

Cone-beam CT (CBCT) images (n = 92) for 10 PPRT patients were incorporated to provide on-treatment information on the appropriateness of six different CTV expansion margins in terms of geographic miss and change in dose-volume statistics for CTV, rectum and bladder. Uniform margins included 10 mm, 5 mm, 10 mm + 5 mm posteriorly and 5 mm + 3 mm posteriorly. In addition, two anisotropic margins were evaluated by separating the superior and inferior portions of the CTV before expansion. Treatment plans were created for each PTV retrospectively.

RESULTS

The frequency of geographic miss was the smallest for the large uniform expansions but resulted in the highest organ-at-risk (OAR) doses. Geographic miss in the smaller uniform and anisotropic PTVs was more prevalent but commonly to a small volume < 1% of CTV. When averaged over all CBCT fractions, V95% dose for all CTV margins remained > 99%. The anisotropic expansions generated smaller irradiated target volumes and consequently saw up to 7.3% reduction in bladder dose when compared with similar uniform expansion margins.

CONCLUSION

Supplementing the incidence of geographic miss with dosimetric information on target coverage and OAR doses provides more informed assessment of the appropriateness of different CTV expansion margins. Our study extends the evaluation of anisotropic margins for PPRT.

The Role of Adaptive Planning in Margin-Reduced, MRI-Guided Stereotactic Body Radiotherapy to the Prostate Bed Following Radical Prostatectomy: Post-hoc Analysis of a Phase II Clinical Trial

BACKGROUND AND PURPOSE

We examined the interfractional variations of clinical target volumes (CTVs), planning target volumes (PTVs), and organs-at-risk (OARs) in patients receiving MRI-guided stereotactic body radiotherapy (SBRT) to the prostate bed and evaluated the potential role of adaptive planning.

MATERIALS AND METHODS

31 patients received 30-34 Gy in five fractions to the prostate bed on a phase II clinical trial. OARs, CTVs, and PTVs were retrospectively contoured on daily pretreatment MRIs (n=155). Geometric comparisons were made between initial planning contours and daily pretreatment contours. Predicted treatment plans for each fraction were evaluated using the following constraints: CTV V95%>93%, PTV V95%>90%, bladder Dmax<36.7Gy, bladder V32.5Gy<35%, rectum Dmax<36.7Gy, rectum V27.5Gy<45%, rectum 32.5Gy<30%, and rectal wall V24Gy<50%. Adaptive planning was simulated for all fractions that failed to meet these criteria. Plans were then re-evaluated.

RESULTS

Median change in volume was 0.48% for CTV, -24.5% for bladder, and 6.95% for rectum. Median DSC was 0.89 for CTV, 0.79 for bladder, and 0.76 for rectum. 145/155 fractions (93.5%) met CTV V95%>93%. 75/155 fractions (48.4%) failed at least one OAR dose constraint. Overall, 83/155 fractions (53.5%) met criteria for adapting planning. This affected 24/31 patients (77.4%). Following adaptive planning, all fractions met CTV V95%>93% and PTV V95%>90% and 120/155 fractions (77.4%) met all OAR constraints.

CONCLUSION

Due to significant interfractional variations in anatomy, a majority of fractions failed to meet both target volume and OAR constraints. However, adaptive planning was effective in overcoming these anatomic changes. Adaptive planning should be routinely considered in prostate bed SBRT. This was a post-hoc analysis of the following registered clinical trial: NCT03541850.

Clinical Practice Evolvement for Post-Operative Prostate Cancer Radiotherapy-Part 2: Feasibility of Margin Reduction for Fractionated Radiation Treatment with Advanced Image Guidance

Purpose: Planning target volume (PTV) expansion for post-prostatectomy radiotherapy is typically ≥5 mm. Recent clinical trials have proved the feasibility of a reduced margin of 2−3 mm for treatments on MRI-linac. We aim to study the minimum PTV margin needed using iterative cone-beam CT (iCBCT) as image guidance on conventional linacs. Materials/Methods: Fourteen patients who received post-prostatectomy irradiation (8 with an endorectal balloon and 6 without a balloon) were included in this study. Treatment was delivered with volumetric modulated radiation therapy (VMAT). Fractional dose delivery was evaluated in 165 treatment fractions. The bladder, rectal wall, femoral heads, and prostate bed clinical tumor volume (CTV) were contoured and verified on daily iCBCT. PTV margins (0 mm, 2 mm, and 4 mm) were evaluated on daily iCBCT. CTV coverage and OAR dose parameters were assessed with each PTV margin. Results: CTV D100% was underdosed with a 0 mm margin in 32% of fractions in comparison with 2 mm (6%) and 4 mm (6%) PTV margin (p ≤ 0.001). CTV D95% > 95% was met in 93−94% fractions for all PTV expansions. CTV D95% > 95% was achieved in more patients with an endorectal balloon than those without: 0 mm—90/91 (99%) vs. 63/74 (85%); 2 mm—90/91 (99%) vs. 65/75 (87%); 4 mm—90/90 (100%) vs. 63/73 (86%). There was no difference in absolute median change in CTV D95% (0.32%) for 0-, 2-, and 4 mm margins. The maximum dose remained under 108% for 100% (0 mm), 97% (2 mm), and 98% (4 mm) of images. Rectal wall maximum dose remained under 108% for 100% (0 mm), 100% (2 mm), and 98% (4 mm) of images. Conclusions: With high-quality iCBCT image guidance, PTV margin accounting for inter-fractional uncertainties can be safely reduced for post-prostatectomy radiotherapy. For fractionated radiotherapy, an isotropic expansion of 2 mm and 4 mm may be considered for margin expansion with and without the endorectal balloon. Future application for margin reduction needs to be further evaluated and considered with the advent of shorter post-prostatectomy radiation courses.

Evaluation of internal margins for prostate for step and shoot intensity-modulated radiation therapy and volumetric modulated arc therapy using different margin formulas

PURPOSE

This feasibility study evaluated the intra-fractional prostate motion using an ultrasound image-guided system during step and shoot intensity-modulated radiation therapy (SS-IMRT) and volumetric modulated arc therapy (VMAT). Moreover, the internal margins (IMs) using different margin formulas were calculated.

METHODS

Fourteen consecutive patients with prostate cancer who underwent SS-IMRT (n = 5) or VMAT (n = 9) between March 2019 and April 2020 were considered. The intra-fractional prostate motion was observed in the superior-inferior (SI), anterior-posterior (AP), and left-right (LR) directions. The displacement of the prostate was defined as the displacement from the initial position at the scanning start time, which was evaluated using the mean ± standard deviation (SD). IMs were calculated using the van Herk and restricted maximum likelihood (REML) formulas for SS-IMRT and VMAT.

RESULTS

For SS-IMRT, the maximum displacements of the prostate motion were 0.17 ± 0.18, 0.56 ± 0.86, and 0.18 ± 0.59 mm in the SI, AP, and LR directions, respectively. For VMAT, the maximum displacements of the prostate motion were 0.19 ± 0.64, 0.22 ± 0.35, and 0.14 ± 0.37 mm in the SI, AP, and LR directions, respectively. The IMs obtained for SS-IMRT and VMAT were within 2.3 mm and 1.2 mm using the van Herk formula and within 1.2 mm and 0.8 mm using the REML formula.

CONCLUSIONS

This feasibility study confirmed that intra-fractional prostate motion was observed with SS-IMRT and VMAT using different margin formulas. The IMs should be determined according to each irradiation technique using the REML margin.