Corrigendum to "Harmonization of breast cancer radiotherapy treatment planning in the Netherlands" [Tech. Innov. Patient Support Radiat. Oncol. 19 (2021) 26-32].

[This corrects the article DOI: 10.1016/j.tipsro.2021.06.004.].

Local seed displacement from Day 0 to Day 30 in I-125 permanent prostate brachytherapy: A detailed, computed tomography-based analysis.

PURPOSE: To accurately quantify local seed displacement from Day 0 to Day 30 for our brachytherapy procedure. To quantify seed loss/migration and to identify the locations from where seeds are missing. METHODS AND MATERIALS: Seed displacements were analyzed in 62 consecutive patients, who received brachytherapy with stranded I-125 seeds. At the start of the procedure, four fiducial gold markers were implanted. At the end of the implantation procedure an in-room 3D CBCT scan (Day 0) was acquired for accurate seed localization. At Day 30 a regular CT scan was acquired. This CT scan was rigidly registered to the CBCT scan using the fiducials. Subsequently, the Hungarian method was used to find pairs of corresponding seeds. Displacements were calculated and missing seeds were identified. RESULTS: Local seed displacements are smaller than 5 mm for 76.3% of the seeds; 2.3% show displacements larger than 10 mm. The largest seed displacements are seen along the inferior-superior axis: on average 1.0 ± 3.2 mm in superior direction with respect to the intraprostatic fiducials. Largest displacements are seen at the inferior-lateral sides of the prostate. On average, the inferior seeds move 1.0 ± 2.8 mm in anterior and 1.8 ± 3.3 mm in superior direction. The percentage of missing seeds is 0.2% (8 out of 3893 seeds for 5 patients). Most of the missing seeds were also originally implanted at the inferior-lateral sides of the prostate. CONCLUSIONS: Local seed displacements and number of missing seeds are small, and predominantly occur around the apex.

Harmonization of breast cancer radiotherapy treatment planning in the Netherlands.

PURPOSE: The aim was to reach consensus in The Netherlands on which parameters should be used to evaluate breast cancer radiotherapy (RT) plans. MATERIALS AND METHODS: A Benchmark Case with delineated planning target volumes (PTVs) and Organs At Risk (OARs) was sent to all Dutch radiotherapy centres in combination with a questionnaire, with the request to generate RT plans prescribing 15 times 2.67 Gy for four different treatment indications according to the institutional irradiation technique. The plans and accompanying questionnaire answers were analysed using descriptive statistics. These results, together with a harmonisation proposal, were sent to all centres. The proposal was discussed at a meeting of the Dutch Society of Radiation Oncology breast cancer platform. Distinct parameters were accepted if consensus on them was reached. RESULTS: 19 out of 20 Dutch departments participated in this study. PTV coverage varied considerably, with D98% between 63% and 99% for the breast and between 37% and 97% for the internal mammary nodes (IMN). Also substantial OAR dose differences were observed, with e.g. mean heart doses ranging between 1.85 Gy and 5.42 Gy in case the IMN were included in the PTV. For evaluation of the PTVs D98%, D2% and Dmean were chosen to report on, with target values of ≥ 95% (90% for the PTV_IMN), ≤ 107%, and 99-101%, respectively. For OARs, consensus was reached on the parameters to be evaluated, without target values: Dmean of the heart, Dmean and V5% of the lungs, and in case of periclavicular radiotherapy V30Gy of the thyroid gland. For patients younger than 40 years a contralateral mean breast dose of ≤ 1 Gy was agreed upon. CONCLUSION: A new Dutch consensus guideline for evaluation of breast cancer RT plans has been established.

Dosimetric impact of contouring and image registration variability on dynamic 125I prostate brachytherapy.

PURPOSE: The quality of permanent prostate brachytherapy can be increased by addition of imaging modalities in the intraoperative procedure. This addition involves image registration, which inherently has inter- and intraobserver variabilities. We sought to quantify the inter- and intraobserver variabilities in geometry and dosimetry for contouring and image registration and analyze the results for our dynamic 125I brachytherapy procedure. METHODS AND MATERIALS: Five observers contoured 11 transrectal ultrasound (TRUS) data sets three times and 11 CT data sets one time. The observers registered 11 TRUS and MRI data sets to cone beam CT (CBCT) using fiducial gold markers. Geometrical and dosimetrical inter- and intraobserver variabilities were assessed. For the contouring study, structures were subdivided into three parts along the craniocaudal axis. RESULTS: We analyzed 165 observations. Interobserver geometrical variability for prostate was 1.1 mm, resulting in a dosimetric variability of 1.6% for V100 and 9.3% for D90. The geometric intraobserver variability was 0.6 mm with a V100 of 0.7% and D90 of 1.1%. TRUS-CBCT registration showed an interobserver variability in V100 of 2.0% and D90 of 3.1%. Intraobserver variabilities were 0.9% and 1.6%, respectively. For MRI-CBCT registration, V100 and D90 were 1.3% and 2.1%. Intraobserver variabilities were 0.7% and 1.1% for the same. CONCLUSIONS: Prostate dosimetry is affected by interobserver contouring and registration variability. The observed variability is smaller than underdosages that are adapted during our dynamic brachytherapy procedure.

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

PURPOSE: To evaluate adaptive planning for permanent prostate brachytherapy and to identify the prostate regions that needed adaptation. METHODS AND MATERIALS: After the implantation of stranded seeds, using real-time intraoperative planning, a transrectal ultrasound (TRUS)-scan was obtained and contoured. The positions of seeds were determined on a C-arm cone-beam computed tomography (CBCT)-scan. The CBCT-scan was registered to the TRUS-scan using fiducial gold markers. If dose coverage on the combined image-dataset was inadequate, an intraoperative adaptation was performed by placing remedial seeds. CBCT-based intraoperative dosimetry was analyzed for the prostate (D90, V100, and V150) and the urethra (D30). The effects of the adaptive dosimetry procedure for Day 30 were separately assessed. RESULTS: We analyzed 1266 patients. In 17.4% of the procedures, an adaptation was performed. Without the dose contribution of the adaptation Day 30 V100 would be < 95% for half of this group. On Day 0, the increase due to the adaptation was 11.8 ± 7.2% (1SD) for D90 and 9.0 ± 6.4% for V100. On Day 30, we observed an increase in D90 of 12.3 ± 6.0% and in V100 of 4.2 ± 4.3%. For the total group, a D90 of 119.6 ± 9.1% and V100 of 97.7 ± 2.5% was achieved. Most remedial seeds were placed anteriorly near the base of the prostate. CONCLUSION: CBCT-based adaptive planning enables identification of implants needing adaptation and improves prostate dose coverage. Adaptations were predominantly performed near the anterior base of the prostate.

Edema and Seed Displacements Affect Intraoperative Permanent Prostate Brachytherapy Dosimetry.

PURPOSE: We sought to identify the intraoperative displacement patterns of seeds and to evaluate the correlation of intraoperative dosimetry with day 30 for permanent prostate brachytherapy. METHODS AND MATERIALS: We analyzed the data from 699 patients. Intraoperative dosimetry was acquired using transrectal ultrasonography (TRUS) and C-arm cone beam computed tomography (CBCT). Intraoperative dosimetry (minimal dose to 40%-95% of the volume [D40-D95]) was compared with the day 30 dosimetry for both modalities. An additional edema-compensating comparison was performed for D90. Stranded seeds were linked between TRUS and CBCT using an automatic and fast linking procedure. Displacement patterns were analyzed for each seed implantation location. RESULTS: On average, an intraoperative (TRUS to CBCT) D90 decline of 10.6% ± 7.4% was observed. Intraoperative CBCT D90 showed a greater correlation (R(2) = 0.33) with respect to Day 30 than did TRUS (R(2) = 0.17). Compensating for edema, the correlation increased to 0.41 for CBCT and 0.38 for TRUS. The mean absolute intraoperative seed displacement was 3.9 ± 2.0 mm. The largest seed displacements were observed near the rectal wall. The central and posterior seeds showed less caudal displacement than lateral and anterior seeds. Seeds that were implanted closer to the base showed more divergence than seeds close to the apex. CONCLUSIONS: Intraoperative CBCT D90 showed a greater correlation with the day 30 dosimetry than intraoperative TRUS. Edema seemed to cause most of the systematic difference between the intraoperative and day 30 dosimetry. Seeds near the rectal wall showed the most displacement, comparing TRUS and CBCT, probably because of TRUS probe-induced prostate deformation.

An automated, fast and accurate registration method to link stranded seeds in permanent prostate implants.

The geometry of a permanent prostate implant varies over time. Seeds can migrate and edema of the prostate affects the position of seeds. Seed movements directly influence dosimetry which relates to treatment quality. We present a method that tracks all individual seeds over time allowing quantification of seed movements. This linking procedure was tested on transrectal ultrasound (TRUS) and cone-beam CT (CBCT) datasets of 699 patients. These datasets were acquired intraoperatively during a dynamic implantation procedure, that combines both imaging modalities. The procedure was subdivided in four automatic linking steps. (I) The Hungarian Algorithm was applied to initially link seeds in CBCT and the corresponding TRUS datasets. (II) Strands were identified and optimized based on curvature and linefits: non optimal links were removed. (III) The positions of unlinked seeds were reviewed and were linked to incomplete strands if within curvature- and distance-thresholds. (IV) Finally, seeds close to strands were linked, also if the curvature-threshold was violated. After linking the seeds an affine transformation was applied. The procedure was repeated until the results were stable or the 6th iteration ended. All results were visually reviewed for mismatches and uncertainties. Eleven implants showed a mismatch and in 12 cases an uncertainty was identified. On average the linking procedure took 42 ms per case. This accurate and fast method has the potential to be used for other time spans, like Day 30, and other imaging modalities. It can potentially be used during a dynamic implantation procedure to faster and better evaluate the quality of the permanent prostate implant.

Helical 4D CT pitch management for the Brilliance CT Big Bore in clinical practice.

In external beam radiotherapy treatment planning for patients with thoracic malignancies, respiratory-correlated CT (4D CT) is used to obtain high quality studies in the presence of respiratory motion. When helical 4D CT scans are acquired with a Brilliance CT Big Bore, the pitch must meet two conditions. It must be low enough to avoid motion artifacts, and high enough to cover the entire scan length within 120 s to prevent overheating of the X-ray tube. We developed a nomogram that can be used to obtain a suitable pitch satisfying both requirements. We also assessed the effects on the image quality of a pitch that exceeds the maximum pitch, and of a field of view (FOV) reduction. It was shown that, for AVG and MIP reconstructions, the manufacturer's maximum pitch equation yields an underestimation due to its FOV term.

The CT number accuracy of a novel commercial metal artifact reduction algorithm for large orthopedic implants.

Philips Healthcare released a novel metal artifact reduction algorithm for large orthopedic implants (O-MAR). Little information was available about its CT number accuracy. Since CT numbers are used for tissue heterogeneity corrections in external beam radiotherapy treatment planning, we performed a phantom study to assess the CT number accuracy of O-MAR. Two situations were simulated: a patient with a unilateral metallic hip prosthesis and a patient with bilateral metallic hip prostheses. We compared the CT numbers in the O-MAR reconstructions of the simulations to those in the nonO-MAR reconstruction and to those in a metal-free baseline reconstruction. In both simulations, the CT number accuracy of the O-MAR reconstruction was better than the CT number accuracy of the nonO- MAR reconstruction. In the O-MAR reconstruction of the unilateral simulation, all CT numbers were accurate within ± 5 HU (AAPM criterion). In the O-MAR reconstruction of the bilateral simulation, CT numbers were found that differed more than ± 5 HU from the metal-free baseline values. However, none of these differences were clinically relevant.

High-dose-rate prostate brachytherapy based on registered transrectal ultrasound and in-room cone-beam CT images.

PURPOSE: To present a high-dose-rate (HDR) brachytherapy procedure for prostate cancer using transrectal ultrasound (TRUS) to contour the regions of interest and registered in-room cone-beam CT (CBCT) images for needle reconstruction. To characterize the registration uncertainties between the two imaging modalities and explore the possibility of performing the procedure solely on TRUS. METHODS AND MATERIALS: Patients were treated with a TRUS/CBCT-based HDR brachytherapy procedure. For 100 patients, dosimetric results were analyzed. For 40 patients, registration uncertainties were examined by determining differences in fiducial marker positions on TRUS and registered CBCT. The accuracy of needle reconstruction on TRUS was investigated by determining the position differences of needle tips on TRUS and CBCT. The dosimetric impact of reregistration and needle reconstruction on TRUS only was studied for 8 patients. RESULTS: The average prostate V100 was 97.8%, urethra D10 was 116.3%, and rectum D1 cc was 66.4% of the prescribed dose. For 85% of the patients, registration inaccuracies were within 3 mm. Large differences were found between needle tips on TRUS and CBCT, especially in cranial-caudal direction, with a maximum of 10.4 mm. Reregistration resulted in a maximum V100 reduction of 0.9%, whereas needle reconstruction on TRUS only gave a maximum reduction of 9.4%. CONCLUSIONS: HDR prostate brachytherapy based on TRUS combined with CBCT is an accurate method. Registration uncertainties, and consequently dosimetric inaccuracies, are small compared with the uncertainties of performing the procedure solely based on static TRUS images. CBCT imaging is a requisite in our current procedure.

A quality assurance tool for helical tomotherapy using a step-wedge phantom and the on-board MVCT detector.

The purpose of this study was to develop and evaluate filmless quality assurance (QA) tools for helical tomotherapy by using the signals from the on-board megavoltage computed tomography (MVCT) detector and applying a dedicated step-wedge phantom. The step-wedge phantom is a 15 cm long step-like aluminum block positioned on the couch. The phantom was moved through the slit beam and MVCT detector signals were analyzed. Two QA procedures were developed, with gantry fixed at 0°: 1) step-wedge procedure: to check beam energy consistency, field width, laser alignment with respect to the virtual isocenter, couch movement, and couch velocity; and 2) completion procedure: to check the accuracy of a field abutment made by the tomotherapy system after a treatment interruption. The procedures were designed as constancy tool and were validated by measurement of deliberately induced variations and comparison with a reference method. Two Hi-Art II machines were monitored over a period of three years using the step-wedge procedures. The data acquisition takes 5 minutes. The analysis is fully automated and results are available directly after acquisition. Couch speed deviations up to 2% were induced. The mean absolute difference between expected and measured couch speed was 0.2% ± 0.2% (1 standard deviation SD). Field width was varied around the 10 mm nominal size, between 9.7 and 11.1 mm, in steps of 0.2 mm. Mean difference between the step-wedge analysis and the reference method was < 0.01 mm ± 0.03 mm (1 SD). Laser (mis)alignment relative to a reference situation was detected with 0.3 mm precision (1SD). The step-wedge profile was fitted to a PDD in water. The PDD ratio D20/D10, measured at depths of 20 cm and 10 cm, was used to check beam energy consistency. Beam energy variations were induced. Mean difference between step-wedge and water PDD ratios was 0.2% ± 0.3% (1SD). The completion procedure was able to reveal abutment mismatches with a mean error of -0.6 mm ± 0.2 mm (1SD). The trending data over a period of three years showed a mean deviation of 0.4% ± 0.1% (1 SD) in couch speed. The spread in field width was 0.15 mm (1 SD). The sagittal and transverse lasers showed a variation of 0.5 mm (1 SD). Beam energy varied 1.0% (1 SD). A mean abutment mismatch was found of -0.4 mm ± 0.2 mm (1 SD) between interrupted treatments. The on-board MVCT detector, in combination with the step-wedge phantom, is a suitable tool for a QA program for helical tomotherapy. The method allowed frequent monitoring of machine behavior for the past three years.

Objective automated assessment of time trends in prostate edema after (125)I implantation.

PURPOSE: To present an objective automated method to determine time trends in prostatic edema resulting from iodine-125 brachytherapy. METHODS AND MATERIALS: We followed 20 patients, implanted with stranded seeds, with seven consecutive CT scans to establish a time trend in prostate edema. Seed positions were obtained automatically from the CT series. The change in seed positions was used as surrogate for edema. Two approaches were applied to model changes in volume. (1) A cylindrical model: seeds from the compared distribution were linked to the reference distribution of Day 28. After alignment, the compared distribution was scaled in cylindrical coordinates, leading to the changes in radial and craniocaudal directions. The volume changes were calculated using these scaling factors. (2) A spherical model: distances of seeds to the center of gravity of all seeds were used as a measure to model volume changes. RESULTS: With Day 28 as reference, the observed volume changes were smaller than 18% ± 6% (1 standard deviation) for the cylindrical model and 12% ± 7% for the spherical model. One day after implantation, the implanted prostate was less than 10% larger than in the reference scan for both models. Apart from Day 0, both models showed similar volume changes. CONCLUSIONS: We present an objective automated method to determine changes in the implanted prostate volume, eliminating the influence of an observer in the assessment of the prostate size. The implanted volume change was less than 18% ± 7% for the studied group of 20 patients. Edema was 9% ± 5% from 1 day after implantation onward.

Adaptive radiotherapy for prostate cancer using kilovoltage cone-beam computed tomography: first clinical results.

PURPOSE: To evaluate the first clinical results of an off-line adaptive radiotherapy (ART) protocol for prostate cancer using kilovoltage cone-beam computed tomography (CBCT) in combination with a diet and mild laxatives. METHODS AND MATERIALS: Twenty-three patients began treatment with a planning target volume (PTV) margin of 10 mm. The CBCT scans acquired during the first six fractions were used to generate an average prostate clinical target volume (AV-CTV), and average rectum (AV-Rect). Using these structures, a new treatment plan was generated with a 7-mm PTV margin. Weekly CBCT scans were used to monitor the CTV coverage. A diet and mild laxatives were introduced to improve image quality and reduce prostate motion. RESULTS: Twenty patients were treated with conform ART protocol. For these patients, 91% of the CBCT scans could be used to calculate the AV-CTV and AV-Rect. In 96% of the follow-up CBCT scans, the CTV was located within the average PTV. In the remaining 4%, the prostate extended the PTV by a maximum of 1 mm. Systematic and random errors for organ motion were reduced by a factor of two compared with historical data without diet and laxatives. An average PTV reduction of 29% was achieved. The volume of the AV-Rect that received >65 Gy was reduced by 19%. The mean dose to the anal wall was reduced on average by 4.8 Gy. CONCLUSIONS: We safely reduced the high-dose region by 29%. The reduction in irradiated volume led to a significant reduction in the dose to the rectum. The diet and laxatives improved the image quality and tended to reduce prostate motion.

An adaptive off-line procedure for radiotherapy of prostate cancer.

PURPOSE: To determine the planning target volume (PTV) margin for an adaptive radiotherapy procedure that uses five computed tomography (CT) scans to calculate an average prostate position and rectum shape. To evaluate alternative methods to determine an average rectum based on a single delineation. METHODS AND MATERIALS: Repeat CT scans (8-13) of 19 patients were used. The contoured prostates of the first four scans were matched on the planning CT (pCT) prostate contours. With the resulting translations and rotations the average prostate position was determined. An average rectum was obtained by either averaging the coordinates of corresponding points on the rectal walls or by selecting the "best" rectum or transforming the pCT rectum. Dose distributions were calculated for various expanded average prostates. The remaining CT scans were used to determine the dose received by prostate and rectum during treatment. RESULTS: For the prostate of the pCT scan and a 10-mm margin, all patients received more than 95% of the prescribed dose to 95% of the prostate. For the average prostate, a margin of 7 mm was needed to obtain a similar result (average PTV reduction 30%). The average rectum overestimated the mean dose to the rectum by 0.4 +/- 1.6 Gy, which was better than the pCT rectum (2.1 +/- 3.0 Gy) and the alternative average rectums (1.0 +/- 2.6 Gy and 1.4 +/- 3.2 Gy). CONCLUSIONS: Our adaptive procedure allows for reduction of the PTV margin to 7 mm without decreasing prostate coverage during treatment. For accurate estimation of the rectum dose, rectums need to be delineated and averaged over multiple scans.