Daily electronic portal imaging of implanted gold seed fiducials in patients undergoing radiotherapy after radical prostatectomy

PURPOSE: The aim of this study was to measure interfraction prostate bed motion, setup error, and total positioning error in 10 consecutive patients undergoing postprostatectomy radiotherapy. METHODS AND MATERIALS: Daily image-guided target localization and alignment using electronic portal imaging of gold seed fiducials implanted into the prostate bed under transrectal ultrasound guidance was used in 10 patients undergoing adjuvant or salvage radiotherapy after prostatectomy. Prostate bed motion, setup error, and total positioning error were measured by analysis of gold seed fiducial location on the daily electronic portal images compared with the digitally reconstructed radiographs from the treatment-planning CT. RESULTS: Mean (+/- standard deviation) prostate bed motion was 0.3 +/- 0.9 mm, 0.4 +/- 2.4 mm, and -1.1 +/- 2.1 mm in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) axes, respectively. Mean set-up error was 0.1 +/- 4.5 mm, 1.1 +/- 3.9 mm, and -0.2 +/- 5.1 mm in the LR, SI, and AP axes, respectively. Mean total positioning error was 0.2 +/- 4.5 mm, 1.2 +/- 5.1 mm, and -0.3 +/- 4.5 mm in the LR, SI, and AP axes, respectively. Total positioning errors >5 mm occurred in 14.1%, 38.7%, and 28.2% of all fractions in the LR, SI, and AP axes, respectively. There was no significant migration of the gold marker seeds. CONCLUSIONS: This study validates the use of daily image-guided target localization and alignment using electronic portal imaging of implanted gold seed fiducials as a valuable method to correct for interfraction target motion and to improve precision in the delivery of postprostatectomy radiotherapy.     

Daily electronic portal imaging for morbidly obese men undergoing radiotherapy for localized prostate cancer

PURPOSE: We summarize our experience with a series of morbidly obese men treated using daily online portal imaging and implanted gold markers to guide external beam radiation therapy (EBRT). METHODS AND MATERIALS: Three consecutive morbidly obese men were treated with EBRT for localized prostate cancer. Daily electronic portal imaging was used to verify patient position. The magnitude and direction of patient positioning error were documented for each fraction. RESULTS: The absolute magnitude of positioning error was greatest in the left-right direction with a mean of 11.4 mm/fraction (median, 8 mm; range, 0-42 mm). Mean error in the superior-inferior direction was also substantial at 7.2 mm/fraction (median, 5 mm; range, 0-47 mm). Anteroposterior error was the least problematic with a mean value of 2.6 mm/fraction (median, 2.5 mm; range, 0-8 mm). CONCLUSIONS: Daily electronic portal imaging combined with gold fiducial markers dramatically improves the precision of EBRT in the treatment of morbidly obese men with prostate cancer. Setup error rather than organ motion appears to be the dominant force in positioning error in obese men.     

(Non)-migration of radiopaque markers used for on-line localization of the prostate with an electronic portal imaging device

PURPOSE: Radiopaque gold markers can be implanted in the prostate to visualize its position on portal images during radiation therapy. This procedure assumes that the markers do not move within the prostate. In this work we test this assumptiom. METHODS AND MATERIALS: Three markers were implanted transrectally in the prostate of patients undergoing external radiation therapy. An orthogonal pair of portal images was acquired periodically throughout the course of radiation therapy with an a-Si electronic portal imaging device (EPID). The marker coordinates were determined, and the distances between the implanted markers were recorded. The distance time trend is used to evaluate the magnitude of marker migration. RESULTS: The average standard deviation (SD) of the distances between markers was 1.3 mm (range 0.44 to 3.04 mm). Three of the 11 patients show a SD larger than 2 mm. For these patients, all three distances show a simultaneous reduction with time, compatible with a shrinking of the prostate. All had been treated with neoadjuvant hormone therapy. For 1 of the 3 patients, this reduction in volume was confirmed with a repeat computed tomographic scan. CONCLUSION: None of the 33 markers studied migrated significantly. The implantation of three radiopaque gold markers enables accurate and precise on-line verification of the prostate position during external beam radiation therapy. The use of three markers provides a tool to monitor prostate position and volume changes that can occur over time due to hormone or radiation therapy.     

The effect of an endorectal balloon and off-line correction on the interfraction systematic and random prostate position variations: a comparative study

PURPOSE: To investigate the effect of an endorectal balloon (ERB) and an off-line correction protocol on the day-to-day, interfraction prostate gland motion, in patients receiving external beam radiotherapy for prostate cancer. METHODS AND MATERIALS: In 22 patients, irradiated with an ERB in situ (ERB group) and in 30 patients without an ERB (No-ERB group), prostate displacements were measured daily in three orthogonal directions with portal images. Implanted gold markers and an off-line electronic portal imaging correction protocol were used for prostate position verification and correction. Movie loops were analyzed to evaluate prostate motion and rectal filling variations. RESULTS: The off-line correction protocol reduced the systematic prostate displacements, equally for the ERB and No-ERB group, to 1.3-1.8 mm (1 SD). The mean 3D displacement was reduced to 2.8 mm and 2.4 mm for the ERB and No-ERB group, respectively. The random interfraction displacements, relative to the treatment isocenter, were not reduced by the ERB and remained nearly unchanged in all three directions: 3.1 mm (1 SD) left-right, 2.6 mm (1 SD) superior-inferior, and 4.7 mm (1 SD) for the anterior-posterior direction. These day-to-day prostate position variations can be explained by the presence of gas and stool beside the ERB. CONCLUSIONS: The off-line corrections on the fiducial markers are effective in reducing the systematic prostate displacements. The investigated ERB does not reduce the interfraction prostate motion. Although the overall mean displacement is low, the day-to-day interfraction motion, especially in anterior-posterior direction, remains high compared with the systematic displacements.     

Measurements of intrafraction motion and interfraction and intrafraction rotation of prostate by three-dimensional analysis of daily portal imaging with radiopaque markers

PURPOSE: To measure the interfraction and intrafraction motion of the prostate during the course of external beam radiotherapy using a video electronic portal imaging device and three-dimensional analysis. METHODS AND MATERIALS: Eighteen patients underwent implantation with two or three gold markers in the prostate before five-angle/11-field conformal radiotherapy. Using CT data as the positional reference, multiple daily sets of portal images, and a three-dimensional reconstruction algorithm, intrafraction translations, as well as interfraction and intrafraction rotations, were analyzed along the three principal axes (left-right [LR], superoinferior [SI], and AP). The overall mean values and standard deviations (SDs), along with random and systematic SDs, were computed for these translations and rotations. RESULTS: For 282 intrafraction translational displacements, the random SD was 0.8 mm (systematic SD, 0.2) in the LR, 1.0 mm (systematic SD, 0.4) in the SI, and 1.4 mm (systematic SD, 0.7) in the AP axes. The analysis of 348 interfraction rotations revealed random SDs of 6.1 degrees (systematic SD, 5.6 degrees ) around the LR axis, 2.8 degrees (systematic SD, 2.4 degrees ) around the SI axis, and 2.0 degrees (systematic SD, 2.2 degrees ) around the AP axis. The intrafraction rotational motion observed during 44 fractions had a random SD of 1.8 degrees (systematic SD, 1.0 degrees ) around the LR, 1.1 degrees (systematic SD, 0.8 degrees ) around the SI, and 0.6 degrees (systematic SD, 0.3 degrees ) around the AP axis. CONCLUSION: The interfraction rotations observed were more important than those reported in previous studies. Intrafraction motion was generally smaller in magnitude than interfraction motion. However, the intrafraction rotations and translations of the prostate should be taken into account when designing planning target volume margins because their magnitudes are not negligible.     

Detection of fiducial gold markers for automatic on-line megavoltage position verification using a marker extraction kernel (MEK)

Purpose: In this study automatic detection of implanted gold markers in megavoltage portal images for on-line position verification was investigated.

Methods and Materials: A detection method for fiducial gold markers, consisting of a marker extraction kernel (MEK), was developed. The detection success rate was determined for different markers using this MEK. The localization accuracy was investigated by measuring distances between markers, which were fixed on a perspex template. In order to generate images comparable to images of patients with implanted markers, this template was placed on the skin of patients before the start of the treatment. Portal images were taken of lateral prostate fields at 18 MV within 1–2 monitor units (MU).

Results: The detection success rates for markers of 5 mm length and 1.2 and 1.4 mm diameter were 0.95 and 0.99 respectively when placed at the beam entry and 0.39 and 0.86 when placed at the beam exit. The localization accuracy appears to be better than 0.6 mm for all markers.

Conclusion: Automatic marker detection with an acceptable accuracy at the start of a radiotherapy fraction is feasible. Further minimization of marker diameters may be achieved with the help of an a-Si flat panel imager and may increase the clinical acceptance of this technique.     

Assessment of prostatic fiducial marker introduction: patient morbidity, staff satisfaction and improved treatment field placement

Introduction: Increased accuracy when using fiducial markers for prostate localisation is well documented. This project aimed to establish the improvement in accuracy when using gold markers for daily prostate localisation, to assess patient satisfaction and morbidity from the transrectal implantation of gold seed markers and establish staff attitudes towards the newly introduced processes. Methods: Twenty patients with prostate cancer had three gold seeds implanted into the base, apex and central zone of the prostate transrectally using ultrasound guidance. Surveys were conducted to assess staff and patient satisfaction with the process of gold seed localisation. The gold markers were used to localise the prostate on a daily basis using megavoltage electronic portal imaging. Measurements were taken to establish the increase in accuracy when using gold fiducial markers compared with using the surrounding bony anatomy. Results: Inter‐fraction motion (1 standard deviation (SD)) of the fiducial markers was 2.20, 4.28 and 4.27 mm in the LR, SI and AP directions, respectively. Intra‐fraction prostate motion (1 SD) was measured as 0.8 mm LR, 1.1 mm SI and 2.0 mm AP. The patient survey showed that the insertion and associated side effects were acceptable, with 5% of patients stating that the seed insertion was worse than the prostate biopsy, and 23.1% of patients experienced short duration (1–2 days) haematuria. The staff survey showed that daily online image guidance was achievable without affecting patient throughput. Thirty percent of treatment staff believed that performing online daily localisation did not add any extra time to a standard treatment, and the remaining 70% thought that the added time was minimal (2–4 min). Conclusions: Gold fiducial markers are an accurate, reliable and tolerable method of daily prostate localisation.     

X-ray-assisted positioning of patients treated by conformal arc radiotherapy for prostate cancer: comparison of setup accuracy using implanted markers versus bony structures

PURPOSE: The aim of this study was to compare setup accuracy of NovalisBody stereoscopic X-ray positioning using implanted markers in the prostate vs. bony structures in patients treated with dynamic conformal arc radiotherapy for prostate cancer. METHODS AND MATERIALS: Random and systematic setup errors (RE and SE) of the isocenter with regard to the center of gravity of three fiducial markers were measured by means of orthogonal verification films in 120 treatment sessions in 12 patients. Positioning was performed using NovalisBody semiautomated marker fusion. The results were compared with a control group of 261 measurements in 15 patients who were positioned with NovalisBody automated bone fusion. In addition, interfraction and intrafraction prostate motion was registered in the patients with implanted markers. RESULTS: Marker-based X-ray positioning resulted in a reduction of RE as well as SE in the anteroposterior, craniocaudal, and left-right directions compared with those in the control group. The interfraction prostate displacements with regard to the bony pelvis that could be avoided by marker positioning ranged between 1.6 and 2.8 mm for RE and between 1.3 and 4.3 mm for SE. Intrafraction random and systematic prostate movements ranged between 1.4 and 2.4 mm and between 0.8 and 1.3 mm, respectively. CONCLUSION: The problem of interfraction prostate motion can be solved by using implanted markers. In addition, the NovalisBody X-ray system performs more accurately with markers compared with bone fusion. Intrafraction organ motion has become the limiting factor for margin reduction around the clinical target volume.     

Technical aspects of daily online positioning of the prostate for three-dimensional conformal radiotherapy using an electronic portal imaging device

PURPOSE: To develop a real-time electronic portal imaging device (EPID) procedure to identify intraprostatic gold markers and correct daily variations in target position during external beam radiotherapy for prostate cancer. METHODS AND MATERIALS: Pretherapy electronic portal images (EPIs) were acquired with a small portion of the therapeutic 18-MV dose from an orthogonal pair of treatment fields. The position of the intraprostatic gold markers on the EPIs was aligned with that on the simulation digitally reconstructed radiographs. If the initial three-dimensional target displacement (3DI) exceeded 5 mm or rotations exceeded 3 degrees, the beam was realigned before the remainder of the dose was delivered. Field-only EPIs were then acquired for all fields and offline analysis was performed to determine the final 3D target placement (3DF). RESULTS: Twenty patients completed protocol-specified treatment, and all markers were identified on 99.6% of the pretherapy EPIs. Overall, 53% of treatment fractions were realigned. The mean 3DI was 5.6 mm in all patients (range 3.7-9.3), and the mean 3DF was 2.8 mm (range 1.6-4.0), which was statistically significant (p < 0.001). Rotational corrections were made on 15% of treatments. Mean treatment duration was 1.4 min greater for protocol patients than for similar patients in whom localization was not performed. CONCLUSIONS: Frequent field misalignment occurs when external fiducial marks are used for patient alignment. Misalignments can be readily and rapidly identified and corrected with an EPID-based online correction procedure that integrates commercially available equipment and software.     

On-line aSi portal imaging of implanted fiducial markers for the reduction of interfraction error during conformal radiotherapy of prostate carcinoma

PURPOSE: An on-line system to ensure accuracy of daily setup and therapy of the prostate has been implemented with no equipment modification required. We report results and accuracy of patient setup using this system. METHODS AND MATERIALS: Radiopaque fiducial markers were implanted into the prostate before radiation therapy. Lateral digitally reconstructed radiographs (DRRs) were obtained from planning CT data. Before each treatment fraction, a lateral amorphous silicon (aSi) portal image was acquired and the position of the fiducial markers was compared to the DRRs using chamfer matching. Couch translation only was used to account for marker position displacements, followed by a second lateral portal image to verify isocenter position. Residual displacement data for the aSi and previous portal film systems were compared. RESULTS: This analysis includes a total of 239 portal images during treatment in 17 patients. Initial prostate center of mass (COM) displacements in the superior, inferior, anterior, and posterior directions were a maximum of 7 mm, 9 mm, 10 mm and 11 mm respectively. After identification and correction, prostate COM displacements were <3 mm in all directions. The therapists found it simple to match markers 88% of the time using this system. Treatment delivery times were in the order of 9 min for patients requiring isocenter adjustment and 6 min for those who did not. CONCLUSIONS: This system is technically possible to implement and use as part of an on-line correction protocol and does not require a longer than standard daily appointment time at our center with the current action limit of 3 mm. The system is commercially available and is more efficient and user-friendly than portal film analysis. It provides the opportunity to identify and accommodate interfraction organ motion and may also permit the use of smaller margins during conformal prostate radiotherapy. Further integration of the system such as remote table control would improve efficiency.     

A randomized comparison of interfraction and intrafraction prostate motion with and without abdominal compression

BACKGROUND AND PURPOSE: To quantify inter- and intrafraction prostate motion in a standard VacLok (VL) immobilization device or in the BodyFix (BF) system incorporating a compression element which may reduce abdominal movement. MATERIALS AND METHODS: Thirty-two patients were randomly assigned to VL or BF. Interfraction prostate motion >3 mm was corrected pre-treatment. EPIs were taken daily at the start and end of the first and last treatment beams. Interfraction and intrafraction prostate motion were measured for centre of mass (COM) and individual markers. RESULTS: There were no significant differences in interfraction (p0.002) or intrafraction (p0.16) prostate motion with or without abdominal compression. Median intrafraction motion was slightly smaller than interfraction motion in the AP (7.0 mm vs. 7.6 mm) and SI direction (3.2 mm vs. 4.7 mm). The final image captured the maximal intrafraction displacement in only 40% of fractions. Our PTV incorporated >95% of total prostate motion. CONCLUSIONS: Intrafraction motion became the major source of error during radiotherapy after online correction of interfraction prostate motion. The addition of 120 mbar abdominal compression to custom pelvic immobilization influenced neither interfraction nor intrafraction prostate motion.     

Positioning errors and prostate motion during conformal prostate radiotherapy using on-line isocentre set-up verification and implanted prostate markers.

PURPOSE: To evaluate treatment errors from set-up and inter-fraction prostatic motion with port films and implanted prostate fiducial markers during conformal radiotherapy for localized prostate cancer. METHODS: Errors from isocentre positioning and inter-fraction prostate motion were investigated in 13 men treated with escalated dose conformal radiotherapy for localized prostate cancer. To limit the effect of inter-fraction prostate motion, patients were planned and treated with an empty rectum and a comfortably full bladder, and were instructed regarding dietary management, fluid intake and laxative use. Field placement was determined and corrected with daily on-line portal imaging. A lateral portal film was taken three times weekly over the course of therapy. From these films, random and systematic placement errors were measured by matching corresponding bony landmarks to the simulator film. Superior-inferior and anterior-posterior prostate motion was measured from the displacement of three gold pins implanted into the prostate before planning. A planning target volume (PTV) was derived to account for the measured prostate motion and field placement errors. RESULTS: From 272 port films the random and systematic isocentre positioning error was 2.2 mm (range 0.2-7.3 mm) and 1.4 mm (range 0.2-3.3 mm), respectively. Prostate motion was largest at the base compared to the apex. Base: anterior, standard deviation (SD) 2.9 mm; superior, SD 2.1 mm. Apex: anterior, SD 2.1 mm; superior, SD 2.1 mm. The margin of PTV required to give a 99% probability of the gland remaining within the 95% isodose line during the course of therapy is superior 5.8 mm, and inferior 5.6 mm. In the anterior and posterior direction, this margin is 7.2 mm at the base, 6.5 mm at the mid-gland and 6.0 mm at the apex. CONCLUSIONS: Systematic set-up errors were small using real-time isocentre placement corrections. Patient instruction to help control variation in bladder and rectal distension during therapy may explain the observed small SD for prostate motion in this group of patients. Inter-fraction prostate motion remained the largest source of treatment error, and observed motion was greatest at the gland base. In the absence of real-time pre-treatment imaging of prostate position, sequential portal films of implanted prostatic markers should improve quality assurance by confirming organ position within the treatment field over the course of therapy.     

A magnetic resonance imaging study of prostate deformation relative to implanted gold fiducial markers

PURPOSE: To describe prostate deformation during radiotherapy and determine the margins required to account for prostate deformation after setup to intraprostatic fiducial markers (FM). METHODS AND MATERIALS: Twenty-five patients with T1c-T2c prostate cancer had three gold FMs implanted. The patients presented with a full bladder and empty rectum for two axial magnetic resonance imaging (MRI) scans using a gradient recalled echo (GRE) sequence capable of imaging the FMs. The MRIs were done at the time of radiotherapy (RT) planning and a randomly assigned fraction. A single observer contoured the prostate surfaces. They were entered into a finite element model and aligned using the centroid of the three FMs. RESULTS: During RT, the prostate volume decreased by 0.5%/fraction (p = 0.03) and the FMs in-migrated by 0.05 mm/fraction (p < 0.05). Prostate deformation was unrelated to differential bladder and bowel filling, but was related to a transurethral resection of the prostate (TURP) (p = 0.003). The standard deviation for systematic uncertainty of prostate surface contouring was 0.8 mm and for FM centroid localization was 0.4 mm. The standard deviation of random interfraction prostate deformation was 1.5 mm and for FM centroid variability was 1.1 mm. These uncertainties from prostate deformation can be incorporated into a margin recipe to determine the total margins required for RT. CONCLUSIONS: During RT, the prostate exhibited: volume decrease, deformation, and in-migration of FMs. Patients with TURPs were prone to prostate deformation.     

Daily organ tracking in intensity-modulated radiotherapy of prostate cancer using an electronic portal imaging device with a dose saving acquisition mode.

BACKGROUND AND PURPOSE: Daily use of conventional electronic portal imaging devices (EPID) for organ tracking is limited due to the relatively high dose required for high quality image acquisition. We studied the use of a novel dose saving acquisition mode (RadMode) allowing to take images with one monitor unit per image in prostate cancer patients undergoing intensity-modulated radiotherapy (IMRT) and tracking of implanted fiducial gold markers. PATIENTS AND METHODS: Twenty five patients underwent implantation of three fiducial gold markers prior to the planning CT. Before each treatment of a course of 37 fractions, orthogonal localization images from the antero-posterior and from the lateral direction were acquired. Portal images of both the setup procedure and the five IMRT treatment beams were analyzed. RESULTS: On average, four localization images were needed for a correct patient setup, resulting in four monitor units extra dose per fraction. The mean extra dose delivered to the patient was thereby increased by 1.2%. The procedure was precise enough to reduce the mean displacements prior to treatment to < o =0.3 mm. CONCLUSIONS: The use of a new dose saving acquisition mode enables to perform daily EPID-based prostate tracking with a cumulative extra dose of below 1 Gy. This concept is efficiently used in IMRT-treated patients, where separation of setup beams from treatment beams is mandatory.     

Implanted gold markers for position verification during irradiation of head-and-neck cancers: a feasibility study

PURPOSE: To assess the toxicity and reliability of the use of implanted gold markers for position verification during irradiation of head-and-neck cancer. METHODS AND MATERIALS: Ten patients with localized head-and-neck tumors received two gold markers in the parapharyngeal region. The acute and late radiation-related toxicity were scored prospectively using the Common Toxicity Criteria. The patients were immobilized during irradiation using a five-point mask. The marker location was detected in portal images taken with an a-Si flat panel imager. The intermarker distance, as well as the interfraction motion of the markers, was determined for all patients. RESULTS: No acute major complications were observed. The acute toxicity grade was not greater than normally detected. The markers were visible in all images. On average, the projected intermarker distance varied 0.8 mm (1 standard deviation). A small time trend was observed in the intermarker distance for 3 patients. For these patients, at least one marker was located in the mucosa or pharyngeal constrictor muscle. Deeper-seated gold markers did not show a time trend in the intermarker distance. The random positioning uncertainty determined using the markers was on average 1.1 and 1.4 mm (1 SD) in the craniocaudal and AP direction, respectively. CONCLUSION: The use of implanted gold markers for position verification during radiotherapy for head-and-neck cancer patients seems safe and feasible. To avoid any chance of migration, markers should be placed in deep muscular compartments.     

Feasibility of fully automated detection of fiducial markers implanted into the prostate using electronic portal imaging: a comparison of methods

PURPOSE: To investigate the feasibility of fully automated detection of fiducial markers implanted into the prostate using portal images acquired with an electronic portal imaging device. METHODS AND MATERIALS: We have made a direct comparison of 4 different methods (2 template matching-based methods, a method incorporating attenuation and constellation analyses and a cross correlation method) that have been published in the literature for the automatic detection of fiducial markers. The cross-correlation technique requires a-priory information from the portal images, therefore the technique is not fully automated for the first treatment fraction. Images of 7 patients implanted with gold fiducial markers (8 mm in length and 1 mm in diameter) were acquired before treatment (set-up images) and during treatment (movie images) using 1MU and 15MU per image respectively. Images included: 75 anterior (AP) and 69 lateral (LAT) set-up images and 51 AP and 83 LAT movie images. Using the different methods described in the literature, marker positions were automatically identified. RESULTS: The method based upon cross correlation techniques gave the highest percentage detection success rate of 99% (AP) and 83% (LAT) set-up (1MU) images. The methods gave detection success rates of less than 91% (AP) and 42% (LAT) set-up images. The amount of a-priory information used and how it affects the way the techniques are implemented, is discussed. CONCLUSIONS: Fully automated marker detection in set-up images for the first treatment fraction is unachievable using these methods and that using cross-correlation is the best technique for automatic detection on subsequent radiotherapy treatment fractions.     

Comparison of prostate set‐up accuracy and margins with off‐line bony anatomy corrections and online implanted fiducial‐based corrections

The aim of the study was to determine prostate set‐up accuracy and set‐up margins with off‐line bony anatomy‐based imaging protocols, compared with online implanted fiducial marker‐based imaging with daily corrections. Eleven patients were treated with implanted prostate fiducial markers and online set‐up corrections. Pretreatment orthogonal electronic portal images were acquired to determine couch shifts and verification images were acquired during treatment to measure residual set‐up error. The prostate set‐up errors that would result from skin marker set‐up, off‐line bony anatomy‐based protocols and online fiducial marker‐based corrections were determined. Set‐up margins were calculated for each set‐up technique using the percentage of encompassed isocentres and a margin recipe. The prostate systematic set‐up errors in the medial–lateral, superior–inferior and anterior–posterior directions for skin marker set‐up were 2.2, 3.6 and 4.5 mm (1 standard deviation). For our bony anatomy‐based off‐line protocol the prostate systematic set‐up errors were 1.6, 2.5 and 4.4 mm. For the online fiducial based set‐up the results were 0.5, 1.4 and 1.4 mm. A prostate systematic error of 10.2 mm was uncorrected by the off‐line bone protocol in one patient. Set‐up margins calculated to encompass 98% of prostate set‐up shifts were 11–14 mm with bone off‐line set‐up and 4–7 mm with online fiducial markers. Margins from the van Herk margin recipe were generally 1–2 mm smaller. Bony anatomy‐based set‐up protocols improve the group prostate set‐up error compared with skin marks; however, large prostate systematic errors can remain undetected or systematic errors increased for individual patients. The margin required for set‐up errors was found to be 10–15 mm unless implanted fiducial markers are available for treatment guidance.     

Prostate position relative to pelvic bony anatomy based on intraprostatic gold markers and electronic portal imaging

PURPOSE: To describe the relative positions and motions of the prostate, pelvic bony anatomy, and intraprostatic gold fiducial markers during daily electronic portal localization of the prostate. METHODS AND MATERIALS: Twenty prostate cancer patients were treated supine with definitive external radiotherapy according to an on-line target localization protocol using three or four intraprostatic gold fiducial markers and an electronic portal imaging device. Daily pretherapy and through-treatment electronic portal images (EPIs) were obtained for each of four treatment fields. The patients' pelvic bony anatomy, intraprostatic gold markers, and a best visual match to the target (i.e., prostate) were identified on simulation digitally reconstructed radiographs and during daily treatment setup and delivery. These data provided quantitative inter- and intrafractional analysis of prostate motion, its position relative to the bony anatomy, and the individual intraprostatic fiducial markers. Treatment planning margins, with and without on-line localization, were subsequently compared. RESULTS: A total of 22,266 data points were obtained from daily pretherapy and through-treatment EPIs. The pretherapy three-dimensional (3D) average displacement of the fiducial markers, as a surrogate for the prostate, was 5.6 mm, which improved to 2.8 mm after use of the localization protocol. The bony anatomy 3D average displacement was 4.4 mm both before and after localization to the prostate (p = 0.46). Along the superior-inferior (SI), anterior-posterior (AP), and right-left (RL) axes, the average prostate displacement improved from 2.5, 3.7, and 1.9 mm, respectively, before localization to 1.4, 1.6, and 1.1 mm after (all p < 0.001). The pretherapy to through-treatment position of the bony landmarks worsened from 1.7 to 2.5 mm (p < 0.001) in the SI axis, remained statistically unchanged at 2.8 mm (p = 0.39) in the AP axis, and improved from 2.0 to 1.2 mm in the RL axis (p < 0.001). There was no significant intrafractional displacement of prostate position or bony anatomic landmarks. An intermarker distance was identified for all fiducial markers, and 96 were followed daily. Seventy-nine percent had a standard deviation of <1 mm, and 96% were <1.5 mm. Margins were 5.1, 7.3, and 5.0 mm in the SI, AP, and RL axes, respectively, before localization and 2.7, 2.9, and 2.8 mm after localization. CONCLUSIONS: Significant interfractional motion exists for patients' prostate and pelvic bony anatomy. However, these move independently, so the pelvic bony anatomy should not be used as a surrogate for prostate motion. Fiducial markers are stable within the prostate and allow significant margin reduction when used for on-line localization of the prostate.     

Application of the No Action Level (NAL) protocol to correct for prostate motion based on electronic portal imaging of implanted markers

PURPOSE: To evaluate the efficacy of the No Action Level (NAL) off-line correction protocol in the reduction of systematic prostate displacements as determined from electronic portal images (EPI) using implanted markers. METHODS AND MATERIALS: Four platinum markers, two near the apex and two near the base of the prostate, were implanted for localization purposes in patients who received fractionated high dose rate brachytherapy. During the following course of 25 fractions of external beam radiotherapy, the position of each marker relative to the corresponding position in digitally reconstructed radiographs (DRRs) was measured in EPI in 15 patients for on average 17 fractions per patient. These marker positions yield the composite displacements due to both setup error and internal prostate motion, relative to the planning computed tomography scan. As the NAL protocol is highly effective in reducing systematic errors (recurring each fraction) due to setup inaccuracy alone, we investigated its efficacy in reducing systematic composite displacements. The analysis was performed for the center of mass (COM) of the four markers, as well as for the cranial and caudal markers separately. Furthermore, the impact of prostate rotation on the achieved positioning accuracy was determined. RESULTS: In case of no setup corrections, the standard deviations of the systematic composite displacements of the COM were 3-4 mm in the craniocaudal and anterior-posterior directions, and 2 mm in the left-right direction. The corresponding SDs of the random displacements (interfraction fluctuations) were 2-3 mm in each direction. When applying a NAL protocol based on three initial treatment fractions, the SDs of the systematic COM displacements were reduced to 1-2 mm. Displacements at the cranial end of the prostate were slightly larger than at the caudal end, and quantitative analysis showed this originates from left-right axis rotations about the prostate apex. Further analysis revealed that significant time trends are present in these prostate rotations. No significant trends were observed for the prostate translations. CONCLUSIONS: The NAL protocol based on marker positions in EPI halved the composite systematic displacements using only three imaged fractions per patient, and thus allowed for a significant reduction of planning margins. Although large rotations of the prostate, and time trends therein, were observed, the net impact on the measured displacements and on the accuracy obtained with NAL was small.     

Clinical application of a repositioning scheme, using gold markers and electronic portal imaging.

BACKGROUND AND PURPOSE: To implement an on-line correction scheme based on implanted markers to reduce treatment margins in external beam radiation therapy (EBRT) of carcinoma of the prostate. In turn reduction in treatment margins reduces irradiated volumes and offers the possibility of reduced normal tissue complications or escalated target dose. PATIENTS AND METHODS: Five or six gold markers were implanted in 10 patients treated for prostate carcinoma using EBRT. All patients were enlisted in an IRB-approved protocol. Before each fraction two portal images were obtained using a low dose (2MU). Positions of the markers were calculated from these images using an in-house developed program. Corrections were applied with a threshold of 2mm displacement. After correction the procedure was repeated. RESULTS: Overall systematic errors were reduced from 7.45, 1.29, and 5.12 mm to 0.65, 0.11, and 0.46 mm in, respectively, the antero-posterior, lateral, and cranio-caudal directions. Likewise, the overall SD were reduced from 5.99, 5.34, and 4.44 mm to 2.82, 2.64, and 2.22 mm, respectively. All reductions were highly significant (P < 0.01) using a t-test for systematic and an F-test for random errors. On an individual level all but three patients showed significant improvements in all directions for the random errors. All patients improved in at least one direction. Systematic errors were significantly lower in all patients. Simulated correction schemes using this data suggest that margin reduction using off-line reduction does not benefit substantially from on-line corrections in the first few fractions. CONCLUSIONS: Use of marker-based correction improves the patient position. Factors influencing the accuracy were: (1) number of seeds usable for correction, (2) distribution of markers throughout the volume of interest, and (3) objective instructions for patient realignment.