Cone-beam CT-based adaptive planning improves permanent prostate brachytherapy dosimetry: An analysis of 1266 patients

Technical evaluation of the cone-beam computed tomography imaging performance of a novel, mobile, gantry-based X-ray system for brachytherapy

Purpose

A novel, mobile cone‐beam computed tomography (CBCT) system for image‐guided adaptive brachytherapy was recently deployed at our hospital as worldwide first site. Prior to the device's clinical operation, a profound characterization of its imaging performance was conducted. This was essential to optimize both the imaging workflow and image quality for achieving the best possible clinical outcomes. We present the results of our investigations.

Methods

The novel CBCT‐system features a ring gantry with 121 cm clearance as well as a 43.2 × 43.2 cm² flat‐panel detector, and is controlled via a tablet‐personal computer (PC). For evaluating its imaging performance, the geometric reproducibility as well as imaging fidelity, computed tomography (CT)‐number accuracy, uniformity, contrast‐noise‐ratio (CNR), noise characteristics, and spatial resolution as fundamental image quality parameters were assessed. As dose metric the weighted cone‐beam dose index (CBDI_w) was measured. Image quality was evaluated using standard quality assurance (QA) as well as anthropomorphic upper torso and breast phantoms. Both in‐house and manufacturer protocols for abdomen, pelvis, and breast imaging were examined.

Results

Using the in‐house protocols, the QA phantom scans showed altogether a high image quality, with high CT‐number accuracy (R² > 0.97) and uniformity (<12 Hounsfield Unit (HU) cupping), reasonable noise and imaging fidelity, and good CNR at bone–tissue transitions of up to 28:1. Spatial resolution was strongly limited by geometric instabilities of the device. The breast phantom scans fulfilled clinical requirements, whereas the abdomen and pelvis scans showed severe artifacts, particularly at air/bone–tissue transitions.

Conclusion

With the novel CBCT‐system, achieving a high image quality appears possible in principle. However, adaptations of the standard protocols, performance enhancements in image reconstruction referring to artifact reductions, as well as the extinction of geometric instabilities are imperative.

Low dose rate brachytherapy for primary treatment of localized prostate cancer: A systemic review and executive summary of an evidence-based consensus statement

PURPOSE

The purpose of this guideline is to present evidence-based consensus recommendations for low dose rate (LDR) permanent seed brachytherapy for the primary treatment of prostate cancer.

METHODS AND MATERIALS

The American Brachytherapy Society convened a task force for addressing key questions concerning ultrasound-based LDR prostate brachytherapy for the primary treatment of prostate cancer. A comprehensive literature search was conducted to identify prospective and multi-institutional retrospective studies involving LDR brachytherapy as monotherapy or boost in combination with external beam radiation therapy with or without adjuvant androgen deprivation therapy. Outcomes included disease control, toxicity, and quality of life.

RESULTS

LDR prostate brachytherapy monotherapy is an appropriate treatment option for low risk and favorable intermediate risk disease. LDR brachytherapy boost in combination with external beam radiation therapy is appropriate for unfavorable intermediate risk and high-risk disease. Androgen deprivation therapy is recommended in unfavorable intermediate risk and high-risk disease. Acceptable radionuclides for LDR brachytherapy include iodine-125, palladium-103, and cesium-131. Although brachytherapy monotherapy is associated with increased urinary obstructive and irritative symptoms that peak within the first 3 months after treatment, the median time toward symptom resolution is approximately 1 year for iodine-125 and 6 months for palladium-103. Such symptoms can be mitigated with short-term use of alpha blockers. Combination therapy is associated with worse urinary, bowel, and sexual symptoms than monotherapy. A prostate specific antigen <= 0.2 ng/mL at 4 years after LDR brachytherapy may be considered a biochemical definition of cure.

CONCLUSIONS

LDR brachytherapy is a convenient, effective, and well-tolerated treatment for prostate cancer.

Prostate brachytherapy intraoperative dosimetry using a combination of radiographic seed localization with a C-arm and deformed ultrasound prostate contours

PURPOSE

The purpose of the study was to assess the feasibility of performing intraoperative dosimetry for permanent prostate brachytherapy by combining transrectal ultrasound (TRUS) and fluoroscopy/cone beam CT [CBCT] images and accounting for the effect of prostate deformation.

METHODS AND MATERIALS

13 patients underwent TRUS and multiview two-dimensional fluoroscopic imaging partway through the implant, as well as repeat fluoroscopic imaging with the TRUS probe inserted and retracted, and finally three-dimensional CBCT imaging at the end of the implant. The locations of all the implanted seeds were obtained from the fluoroscopy/CBCT images and were registered to prostate contours delineated on the TRUS images based on a common subset of seeds identified on both image sets. Prostate contours were also deformed, using a finite-element model, to take into account the effect of the TRUS probe pressure. Prostate dosimetry parameters were obtained for fluoroscopic and CBCT-dosimetry approaches and compared with the standard-of-care Day-0 postimplant CT dosimetry.

RESULTS

High linear correlation (R² > 0.8) was observed in the measured values of prostate D_90%, V_100%, and V_150%, between the two intraoperative dosimetry approaches. The prostate D_90% and V_100% obtained from intraoperative dosimetry methods were in agreement with the postimplant CT dosimetry. Only the prostate V_150% was on average 4.1% (p-value <0.05) higher in the CBCT-dosimetry approach and 6.7% (p-value <0.05) higher in postimplant CT dosimetry compared with the fluoroscopic dosimetry approach. Deformation of the prostate by the ultrasound probe appeared to have a minimal effect on prostate dosimetry.

CONCLUSIONS

The results of this study have shown that both of the proposed dosimetric evaluation approaches have potential for real-time intraoperative dosimetry.

Phase II study of intraoperative dosimetry for prostate brachytherapy using registered ultrasound and fluoroscopy

PURPOSE

To assess the performance of a system of intraoperative dosimetry and obtain estimates of dosimetry outcomes achieved when utilizing the system in a Phase II clinical trial.

METHODS AND MATERIALS

Forty-five patients undergoing permanent Pd-103 seed implantation for prostate cancer were prospectively enrolled. Seed implantation was performed and dose was tracked intraoperatively using intraoperative registered ultrasound and fluoroscopy (iRUF). Three-dimensional seed locations were computed from X-rays and registered to ultrasound for intraoperative dosimetry, followed by adaptive plan modification to achieve prostate V100 ≥95% and ≥95% D90. Time required for iRUF was recorded. Postoperative CT/MRI scans were performed 1 day after the implantation and used as reference for dosimetric analysis. Dosimetric parameters for the prostate and urethra were compared between standard ultrasound-based dosimetry (USD), iRUF, and postoperative CT/MRI.

RESULTS

Mean total time for iRUF was <30 min. A mean of four seeds (0-12) were added per implant to correct cold spots discovered by iRUF. Day 1 CT/MRI prostate V100 was ≥95% for 44/45 patients; 1 patient had Day 1 V100 93%. No patient had rectal V100 exceeding 1 cc. Compared to CT/MRI, iRUF dosimetry had significantly smaller mean differences and higher correlations for all prostate and urethral dosimetric parameters examined than USD. Both USD and iRUF tended to overestimate dose, but with less bias in iRUF than USD.

CONCLUSIONS

Intraoperative dosimetry utilizing iRUF was associated with acceptable increase in procedure time and enabled very high rates of achieving excellent prostate dose coverage. iRUF intraoperative dosimetry approximated postoperative CT/MRI dosimetry to a greater degree than USD for the prostate and urethra.

An uncertainty metric to evaluate deformation vector fields for dose accumulation in radiotherapy

Background and purpose

In adaptive radiotherapy, deformable image registration (DIR) is used to propagate delineations of tumors and organs into a new therapy plan and to calculate the accumulated total dose. Many DIR accuracy metrics have been proposed. An alternative proposed here could be a local uncertainty (LU) metric for DIR results.

Materials and methods

The LU represented the uncertainty of each DIR position and was focused on deformation evaluation in uniformly-dense regions. Four cases demonstrated LU calculations: two head and neck cancer cases, a lung cancer case, and a prostate cancer case. Each underwent two CT examinations for radiotherapy planning.

Results

LU maps were calculated from each DIR of the clinical cases. Reduced fat regions had LUs of 4.6 ± 0.9 mm, 4.8 ± 1.0 mm, and 4.5 ± 0.7 mm, while the shrunken left parotid gland had a LU of 4.1 ± 0.8 mm and the shrunken lung tumor had a LU of 3.7 ± 0.7 mm. The bowels in the pelvic region had a LU of 10.2 ± 3.7 mm. LU histograms for the cases were similar and 99% of the voxels had a LU < 3 mm.

Conclusions

LU is a new uncertainty metric for DIR that was demonstrated for clinical cases. It had a tolerance of <3 mm.

A cold spot compensation technique using a combination of trans-rectal ultrasonography and intraoperative computed tomography for interstitial permanent prostate brachytherapy: a single-arm prospective trial

Purpose

To evaluate the efficacy of a cold spot compensation technique using a combination of trans-rectal ultrasonography (TRUS) and computed tomography (CT) for permanent interstitial prostate brachytherapy.

Material and methods

Sixty-five patients were treated with the cold spot compensation technique using TRUS-CT fusion. The prescribed dose was set at 145 Gy. The dose to 90% of prostate volume (D₉₀) was planned to be within 195 Gy (134%) and 205 Gy (141%). After implantation using the conventional technique, additional seeds were implanted if cold spots were detected on TRUS-CT fusion images.

Results

Cold spots were detected in 32 of 65 patients (49%) and were compensated by additional seeds. Median number of additional seeds was 3 (range, 1-5). A CT scan 1 month later revealed that the percentage of patients receiving an undesirably low D₉₀ (160-180 Gy) was significantly reduced in the examination arm compared to historical controls. However, mean operation time was significantly longer in the examination arm (64 min) than in historical controls (49 min, p < 0.001). With median follow-up of 18 months (range, 9-24 months), no grade 3 or worse toxicity was encountered.

Conclusion

The cold spot compensation technique using TRUS-CT fusion appears effective for patients receiving permanent interstitial prostate brachytherapy.