Effectiveness of rectal displacement devices during prostate external-beam radiation therapy: A review

Dose-escalated prostate radiotherapy (RT) can improve treatment outcomes, but rectal toxicity is the main limiting factor for introducing dose-escalated RT. Pushing rectal wall away from the prostate reduces the volume of the rectum in high-dose region, which can decrease both short- and long-term rectal toxicities after RT. This review focuses on the literature using different rectal displacement devices such as endorectal balloons, tissue spacers, rectal retractor, and ProSpare during prostate External beam radiotherapy, with regard to dosimetric effects, clinical benefits, prostate motion, and postoperative RT setting.

Adaptive radiotherapy and the dosimetric impact of inter- and intrafractional motion on the planning target volume for prostate cancer patients

Purpose

To investigate the dosimetric influence of daily interfractional (inter) setup errors and intrafractional (intra) target motion on the planning target volume (PTV) and the possibility of an offline adaptive radiotherapy (ART) method to correct larger patient positioning uncertainties in image-guided radiotherapy for prostate cancer (PCa).

Materials and methods

A CTV (clinical target volume)-to-PTV margin ranging from 15 mm in LR (left-right) and SI (superior-inferior) and 5–10 mm in AP (anterior-posterior) direction was applied to all patients. The dosimetric influence of this margin was retrospectively calculated by analysing systematic and random components of inter and intra errors of 31 consecutive intermediate- and high-risk localized PCa patients using daily cone beam computed tomography and kV/kV (kilo-Voltage) imaging. For each patient inter variation was assessed by observing the first 4 treatment days, which led to an offline ART-based treatment plan in case of larger variations.

Results:

Systematic inter uncertainties were larger (1.12 in LR, 2.28 in SI and 1.48 mm in AP) than intra systematic errors (0.44 in LR, 0.69 in SI and 0.80 mm in AP). Same findings for the random error in SI direction with 3.19 (inter) and 2.30 mm (intra), whereas in LR and AP results were alike with 1.89 (inter) and 1.91 mm (intra) and 2.10 (inter) and 2.27 mm (intra), respectively. The calculated margin revealed dimensions of 4–5 mm in LR, 8–9 mm in SI and 6–7 mm in AP direction. Treatment plans which had to be adapted showed smaller variations with 1.12 (LR) and 1.72 mm (SI) for Σ and 4.17 (LR) and 3.75 mm (SI) for σ compared to initial plans with 1.77 and 2.62 mm for Σ and 4.46 and 5.39 mm for σ in LR and SI, respectively.

Conclusion

The currently clinically used margin of 15 mm in LR and SI and 5–10 mm in AP direction includes inter and intra uncertainties. The results show that offline ART is feasible which becomes a necessity with further reductions in PTV margins.

Electronic supplementary material

The online version of this article (10.1007/s00066-020-01596-x) contains supplementary material, which is available to authorized users.

In vivo dosimetry with an electronic portal imaging device for prostate cancer radiotherapy with an endorectal balloon

Electronic portal imaging device-based in vivo dosimetry with a commercial product was performed for 10 prostate cancer patients treated with an air-filled endorectal balloon. With the conventional in vivo method the verification results were outside of our clinical acceptance criteria for all patients. The in aqua vivo method, originally developed for lung cancer treatments, proved to be a practical solution to this problem. On average the percentage of points within γ agreement of 3% and 3 mm significantly improved from 90.9% ± 2.5% (1SD) for the conventional in vivo method to 99.0% ± 1.0% (1SD) for the in aqua vivo method.

Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?

Background

This is a dosimetric comparative study intended to establish appropriate low-to-intermediate dose-constraints for the rectal wall (R_wall) in the context of a randomized phase-II trial on urethra-sparing stereotactic body radiotherapy (SBRT) for prostate cancer. The effect of plan optimization on low-to-intermediate R_wall dose and the potential benefit of an endorectal balloon (ERB) are investigated.

Methods

Ten prostate cancer patients, simulated with and without an ERB, were planned to receive 36.25Gy (7.25Gyx5) to the planning treatment volume (PTV) and 32.5Gy to the urethral planning risk volume (uPRV). Reference plans with and without the ERB, optimized with respect to PTV and uPRV coverage objectives and the organs at risk dose constraints, were further optimized using a standardized stepwise approach to push down dose constraints to the R_wall in the low to intermediate range in five sequential steps to obtain paired plans with and without ERB (Vm₁ to Vm₅). Homogeneity index for the PTV and the uPRV, and the Dice similarity coefficient (DSC) for the PTV were analyzed. Dosimetric parameters for R_wall including the median dose and the dose received by 10 to 60% of the R_wall, bladder wall (B_wall) and femoral heads (F_Heads) were compared. The monitor units (MU) per plan were recorded.

Results

Vm₄ reduced by half D_30%, D_40%, D_50%, and D_med for R_wall and decreased by a third D_60% while HI_PTV, HI_uPRV and DSC remained stable with and without ERB compared to Vm_ref. HI_PTV worsened at Vm₅ both with and without ERB. No statistical differences were observed between paired plans on R_wall, B_wall except a higher D_2% for F_heads with and without an ERB.

Conclusions

Further optimization to the R_wall in the context of urethra sparing prostate SBRT is feasible without compromising the dose homogeneity to the target. Independent of the use or not of an ERB, low-to-intermediate doses to the R_wall can be significantly reduced using a four-step sequential optimization approach.

The effect of endorectal balloon on anorectal dose during postoperative volumetric arc radiotherapy of prostate cancer

PURPOSE

To evaluate the impact of endorectal balloon (ERB) on anorectal dose during postoperative VMAT of prostate cancer.

METHODS

In ten patients referred for salvage radiotherapy CTs were obtained without ERB and with air-filled ERB of 50ml and 100ml. CTs were repeated weekly (4-6 control CTs) and registered to the respective planning CT. For each planning CT, a VMAT plan was made with defined anorectal dose constraints and propagated on the respective control CTs. The dose volumes V40Gy, V60Gy and V65Gy of the rectal and anal wall (Rwall and Awall, respectively) and the ERB position were obtained from each plan.

RESULTS

In plans with ERB, the mean Rwall dose volumes V40Gy, V60Gy and V65Gy were higher by 8%, 5% and 2% (ERB 50ml) and 2%, 3% and 3% (ERB 100ml) in comparison to plans without ERB. The respective Awall dose volume differences were 2%, 0%, -1% (ERB 50ml), and -3%, -2%, -2% (ERB 100ml). The dose volume variability of the Rwall was comparable with and without ERB, but was slightly reduced by ERB for the Awall. The mean ERB position variability was >2mm in anterior-posterior and inferior-superior directions.

CONCLUSION

The use of ERB during post-operative VMAT has no advantages for anorectal dose.

Two-dimensional in vivo rectal dosimetry using an endorectal balloon with unfoldable radiochromic film during prostate cancer radiotherapy

BACKGROUND AND PURPOSE

The present study aims to investigate the feasibility of two-dimensional (2D) in vivo rectal dosimetry using an endorectal balloon for the radiotherapy of prostate cancer.

MATERIALS AND METHODS

The endorectal balloon was equipped with an unfoldable radiochromic film. The film was unrolled as the balloon was inflated, and rolled as it was deflated. Its mechanical and imaging properties were tested, and the dosimetric effectiveness was evaluated in clinical photon and proton beams.

RESULTS

The size of the endorectal balloon including the film was linearly proportional to the volume of water filled in the balloon, and its position could be identified by X-ray radiography. The loss of dose information due to film cutting was within ±1mm from the cutting line. Applying linear interpolation on cut film, the gamma passing rate was more than 95% for 2%/2mm criteria. The measured dose profiles agreed with the plan within 3% and 4% for the photon and proton beams, respectively. A dose-volume histogram of the anterior rectal wall could be obtained from the measured dose distribution in the balloon, which also agreed well with the plan.

CONCLUSIONS

2D in vivo rectal dosimetry is feasible using the endorectal balloon with a radiochromic film in the radiotherapy of prostate cancer.

The impact of stool and gas volume on intrafraction prostate motion in patients undergoing radiotherapy with daily endorectal balloon

PURPOSE

The aim of this study was to quantify the impact of rectal stool/gas volumes on intrafraction prostate motion for patients undergoing prostate radiotherapy with daily endorectal balloon (ERB).

METHODS

Total and anterior stool/gas rectal volumes were quantified in 30 patients treated with daily ERB. Real-time intrafraction prostate motion from 494 treatment sessions, at most 6 min in length, was evaluated using Calypso(®) tracking system.

RESULTS

The deviation of prostate intrafraction motion distribution was a function of stool/gas volume, especially when stool/gas is located in the anterior part of the rectum. Compared to patients with small anterior stool/gas volumes (<10 cm(3)), those with large volume (10-60 cm(3)) had a twofold increase in 3D prostate motion and interquartile data range within the 6th minute of treatment time. The 10% of the overall CBCT session where large anterior rectal volumes were observed demonstrated larger percentage of time at displacement greater than our proposed internal margin 3 mm.

CONCLUSION

Volume and location of stool/gas can directly impact the ERB's intrafraction immobilization ability. Although our patient preparation protocol and the 100 cm(3) daily ERB effectively stabilized prostate motion for 90% of the fractions, a larger-sized ERB may improve prostate fixation for patients with greater and/or variable daily rectal volume.

Endorectal balloons in the post prostatectomy setting: do gains in stability lead to more predictable dosimetry?

PURPOSE

To perform a comparative study assessing potential benefits of endorectal-balloons (ERB) in post-prostatectomy patients.

METHOD AND MATERIALS

Ten retrospective post-prostatectomy patients treated without ERB and ten prospective patients treated with the ERB in situ were recruited. All patients received IMRT and IGRT using kilovoltage cone-beam computed tomography (kVCBCT). kVCBCT datasets were registered to the planning dataset, recontoured and the original plan recalculated on the kVCBCTs to recreate anatomical conditions during treatment. The imaging, structure and dose data were imported into in-house software for the assessment of geometric variation and cumulative equivalent uniform dose (EUD) in the two groups.

RESULTS

The difference in location (ΔCOV) for the bladder between planning and each CBCT was similar for each group. The range of mean ΔCOV for the rectum was 0.15-0.58 cm and 0.15-0.59 cm for the non-ERB and ERB groups. For superior-CTV and inferior-CTV the difference between planned and delivered D95% (mean ± SD) for the non-ERB group was 2.1 ± 6.0 Gy and -0.04 ± 0.20 Gy. While for the ERB group the difference in D95% was 8.7 ± 12.6 Gy and 0.003 ± 0.104 Gy.

CONCLUSIONS

The use of ERBs in the post-prostatectomy setting did improve geometric reproducibility of the target and surrounding normal tissues, however no improvement in dosimetric stability was observed for the margins employed.