Online Setup Correction Research Articles

Image-Guided Radiotherapy for Breast Cancer Patients: Surgical Clips as Surrogate for Breast Excision Cavity

To determine the use of surgical clips as a surrogate for localization of the excision cavity and to quantify the stability of the clips' positions during the course of external beam radiotherapy for breast cancer patients, using cone beam computed tomography (CBCT) scans. Twenty-one breast cancer patients with surgical clips placed in the breast excision cavity were treated in a supine position with 28 daily fractions. CBCT scans were regularly acquired for a setup correction protocol. Retrospectively, the CBCT scans were registered to the planning CT scans, using gray-value registration of the excision cavity region and chamfer matching of the clips. Subsequently, residual setup errors (systematic [Σ] and random [σ]) of the excision cavity were estimated relative to the clips' registration. Finally, the stability of the clips' positions were quantified as the movement of each separate clip according to the center of gravity of the excision cavity. When clips were used for online setup corrections, the residual errors of the excision cavity were Σ(left-right) = 1.2, σ(left-right)= 1.0; Σ(cranial-caudal) = 1.3, σ(cranial-caudal)= 1.2; and Σ(anterior-posterior) = 0.7, σ(anterior-posterior)= 0.9 mm. Furthermore, the average distance (over all patients) between the clips and centers of gravity of the excision cavities was 18.8 mm (on the planning CT) and was reduced to 17.4 mm (measured on the last CBCT scan). Clips move in the direction of the center of gravity of the excision cavity, on average, 1.4 mm. The clips are good surrogates for locating the excision cavity and providing small residual errors.

Analysis of Prostate Patient Setup and Tracking Data: Potential Intervention Strategies

To evaluate the setup, interfraction, and intrafraction organ motion error distributions and simulate intrafraction intervention strategies for prostate radiotherapy. A total of 17 patients underwent treatment setup and were monitored using the Calypso system during radiotherapy. On average, the prostate tracking measurements were performed for 8 min/fraction for 28 fractions for each patient. For both patient couch shift data and intrafraction organ motion data, the systematic and random errors were obtained from the patient population. The planning target volume margins were calculated using the van Herk formula. Two intervention strategies were simulated using the tracking data: the deviation threshold and period. The related planning target volume margins, time costs, and prostate position "fluctuation" were presented. The required treatment margin for the left-right, superoinferior, and anteroposterior axes was 8.4, 10.8, and 14.7 mm for skin mark-only setup and 1.3, 2.3, and 2.8 mm using the on-line setup correction, respectively. Prostate motion significantly correlated among the superoinferior and anteroposterior directions. Of the 17 patients, 14 had prostate motion within 5 mm of the initial setup position for ≥91.6% of the total tracking time. The treatment margin decreased to 1.1, 1.8, and 2.3 mm with a 3-mm threshold correction and to 0.5, 1.0, and 1.5 mm with an every-2-min correction in the left-right, superoinferior, and anteroposterior directions, respectively. The periodic corrections significantly increase the treatment time and increased the number of instances when the setup correction was made during transient excursions. The residual systematic and random error due to intrafraction prostate motion is small after on-line setup correction. Threshold-based and time-based intervention strategies both reduced the planning target volume margins. The time-based strategies increased the treatment time and the in-fraction position fluctuation.

Hypofractionated radiotherapy for lung tumors with online cone beam CT guidance and active breathing control

BackgroundTo study the set-up errors, PTV margin and toxicity of cone beam CT (CBCT) guided hypofractionated radiotherapy with active breathing control (ABC) for patients with non-small cell lung cancer (NSCLC) or metastatic tumors in lung.Methods32 tumors in 20 patients were treated. Based on the location of tumor, dose per fraction given to tumor was divided into three groups: 12 Gy, 8 Gy and 6 Gy. ABC is applied for every patient. During each treatment, patients receive CBCT scan for online set-up correction. The pre- and post-correction setup errors between fractions, the interfractional and intrafractional, set-up errors, PTV margin as well as toxicity are analyzed.ResultsThe pre-correction systematic and random errors in the left-right (LR), superior-inferior (SI), anterior-posterior (AP) directions were 3.7 mm and 5.3 mm, 3.1 mm and 2.1 mm, 3.7 mm and 2.8 mm, respectively, while the post-correction residual errors were 0.6 mm and 0.8 mm, 0.8 mm and 0.8 mm, 1.2 mm and 1.3 mm, respectively. There was an obvious intrafractional shift of tumor position. The pre-correction PTV margin was 9.5 mm in LR, 14.1 mm in SI and 8.2 mm in AP direction. After CBCT guided online correction, the PTV margin was markedly reduced in all three directions. The post-correction margins ranged 1.5 to 2.1 mm. The treatment was well tolerated by patients, of whom there were 4 (20%) grade1-2 acute pneumonitis, 3 (15%) grade1 acute esophagitis, 2 (10%) grade1 late pneumonitis and 1 (5%) grade 1 late esophagitis.ConclusionThe positioning errors for lung SBRT using ABC were significant. Online correction with CBCT image guidance should be applied to reduce setup errors and PTV margin, which may reduce radiotherapy toxicity of tissues when ABC was used.

Modeling set-up error by daily MVCT for prostate adjuvant treatment delivered in 20 fractions: Implications for the assessment of the optimal correction strategies

Purpose To optimise image-guided radiotherapy (IGRT) approaches in set-up error correction for hypofractionated adjuvant treatment of prostate cancer using helical tomotherapy (HT). Methods and materials HT unit provides megavoltage – CT (MVCT) imaging capability for daily patient set-up. The intrinsic uncertainty of the available image registration techniques was first investigated on a pelvic phantom. To optimise different set-up correction strategies, MVCT data of 37 patients treated with 58 Gy delivered in 20 fractions were analyzed. 620 MVCT images were globally available for the analysis. Daily on-line set-up correction without any action level was applied for 27/37 patients. The residual and the effective set-up error were assessed. For 10/37 patients an off-line correction strategy with an action level was applied. Results Phantom measurements showed an intrinsic uncertainty in MVCT imaging and registration around 0.6 mm (±0.5 mm) for each of the three main axes. The minimum value of the residual systematic error was found at 7th–10th session, with values between 0.7 and 1.1 mm (1 SD); a systematic residual error <2 mm for all directions was in any case found at 3th–4th session. This result was also confirmed by the effective systematic set-up error analysis, with a minimum value within 1.5 mm nearly at the 4th or 5th fraction. Conclusions Although the minimum systematic residual error is reached at 7th–10th treatment session, the effective systematic set-up error analysis confirmed that an off-line correction at the 4th fraction without any action level may be a robust compromise with a large sparing of time and resources compared to on-line correction.

Target volume shape variation during irradiation of rectal cancer patients in supine position: Comparison with prone position

Purpose To quantify the inter-fraction shape variation of the mesorectum for rectal cancer patients treated with 5 × 5 Gy in supine position and compare it to variation in prone position. Methods and materials For 28 patients a planning CT (pCT) and five daily cone-beam-CT (CBCT) scans were acquired in supine position. The mesorectal part of the CTV (MesoRect) was delineated on all scans. The shape variation was quantified by the distance between the pCT- and the CBCT delineations and stored in surface maps after online setup correction. Data were analyzed for male and female patients separately and compared to prone data. Results A large range of systematic, 1–8 mm (1SD), and random, 1–5 mm, shape variation was found, comparable to prone patients. Random-shape variation was comparable for male and female patients, while systematic variation was 3 mm larger for female patients. Conclusions Shape variation of the MesoRect is substantial, heterogeneous and different between male and female patients. Differences between supine and prone orientation, however, are small. Clinical margins should be differentiated in position along the cranio–caudal axis, in anterior–posterior direction and for gender. Margins should also be increased, even when online setup correction is used. Due to the small margin differences between prone and supine treatments, the setup choice should be determined on dose to the organs at risk.

Stakeholder Insights into Autonomous Setup Correction by Radiation Therapists during High-Dose Prostate Radiotherapy

Although online setup correction is beneficial during high-dose radiotherapy, little is known about the attitudes and concerns of stakeholders directly involved in this process. Therefore, the purpose of this research was to explore radiation oncologists' and therapists' insights on changes in workload, procedures, and professional practice resulting from involvement in therapist-autonomous online setup correction for patients receiving radiotherapy for prostate cancer. This was a single-center study with a qualitative design. All 10 radiation oncologists and 20 therapists involved in the online-autonomous process for prostate radiotherapy were approached for participation. Two specifically designed questionnaires were developed (one for therapists and one for oncologists) using a standard interview-to-pilot process. These were distributed to the participants both by hand and by e-mail. Content analysis methods and descriptive statistics were used to summarize the qualitative responses. Twenty-eight responses to the questionnaire were received. According to the results, the online-autonomous process was considered efficient when left solely in the hands of therapists (27 of 28 responses). Participant confidence with the process was influenced by communication (8/8), education (23/28), and documentation (20/20). Stakeholder perceptions indicated that the process was implemented exclusively to improve patient outcomes (28/28). The respondents experienced no professional resentment or resistance to change (20/20). The assumption of responsibility for online setup correction improved the therapists' perceptions of their role and themselves as professionals. Despite limited generalizability, this study confirms that within a well-established process, radiation oncologists are willing to cede responsibility for autonomous image approval and setup correction to therapists. Despite increases in professional accountability, radiation therapists are prepared to accept that responsibility for the benefit of their patients.

Potentials of on-line repositioning based on implanted fiducial markers and electronic portal imaging in prostate cancer radiotherapy

BackgroundTo evaluate the benefit of an on-line correction protocol based on implanted markers and weekly portal imaging in external beam radiotherapy of prostate cancer. To compare the use of bony anatomy versus implanted markers for calculation of setup-error plus/minus prostate movement. To estimate the error reduction (and the corresponding margin reduction) by reducing the total error to 3 mm once a week, three times per week or every treatment day.Methods23 patients had three to five, 2.5 mm Ø spherical gold markers transrectally inserted into the prostate before radiotherapy. Verification and correction of treatment position by analysis of orthogonal portal images was performed on a weekly basis. We registered with respect to the bony contours (setup error) and to the marker position (prostate motion) and determined the total error. The systematic and random errors are specified. Positioning correction was applied with a threshold of 5 mm displacement.ResultsThe systematic error (1 standard deviation [SD]) in left-right (LR), superior-inferior (SI) and anterior-posterior (AP) direction contributes for the setup 1.6 mm, 2.1 mm and 2.4 mm and for prostate motion 1.1 mm, 1.9 mm and 2.3 mm. The random error (1 SD) in LR, SI and AP direction amounts for the setup 2.3 mm, 2.7 mm and 2.7 mm and for motion 1.4 mm, 2.3 mm and 2.7 mm. The resulting total error suggests margins of 7.0 mm (LR), 9.5 mm (SI) and 9.5 mm (AP) between clinical target volume (CTV) and planning target volume (PTV). After correction once a week the margins were lowered to 6.7, 8.2 and 8.7 mm and furthermore down to 4.9, 5.1 and 4.8 mm after correcting every treatment day.ConclusionProstate movement relative to adjacent bony anatomy is significant and contributes substantially to the target position variability. Performing on-line setup correction using implanted radioopaque markers and megavoltage radiography results in reduced treatment margins depending on the online imaging protocol (once a week or more frequently).

Off-line setup corrections only marginally reduce the number of on-line corrections for prostate radiotherapy using implanted gold markers

Purpose To evaluate the efficiency of combining on-line and off-line corrections for the positioning of patients receiving external beam radiotherapy for prostate cancer. Materials and methods Daily portal images were acquired during the treatment of 102 patients to verify and correct the position of the prostatic gland using implanted gold markers. In addition to an existing off-line procedure, on-line corrections were applied in the anterior–posterior (AP) direction only, to limit the increase in daily workload. The possible increase in workload of the combined correction procedure for on-line corrections in either two or three directions was further investigated by simulating the required position corrections for 500 treatments. Results The combined correction procedure in AP-direction resulted in a systematic dispersion and random variation of 0.3 mm (1 SD) and 1.0 mm (1 SD), respectively. Application of off-line corrections during pre-treatment setup reduced the number of required on-line corrections from 22 ± 4 (1 SD) to 17 ± 4 (1 SD), at the cost of 1.4 ± 1.0 (1 SD) off-line corrections. For on-line corrections in two or three directions, application of a combined on-line/off-line procedure did not noticeably reduce the number of setup corrections. Conclusions The on-line procedure is feasible and significantly improves both systematic and random errors to below 1 mm with a limited impact on the workload and treatment time. The application of off-line setup corrections during pre-treatment patient positioning only marginally reduces the number of on-line setup corrections.

The clinical feasibility and effect of online cone beam computer tomography-guided intensity-modulated radiotherapy for nasopharyngeal cancer

Background and purpose Online adaptive correction in image-guided intensity-modulated radiotherapy appeared to be a promising approach for precision radiation treatment in head and neck tumors. This protocol was designed to evaluate the clinical feasibility and effect of online cone beam computed tomography (CBCT) guidance in IMRT of nasopharyngeal cancer (NPC). Methods and materials The Elekta Synergy system, which integrates an X-ray volumetric imager (XVI), was used to deliver radiation treatment for 22 cases of NPC. The acquired CBCT was registered to the planning CT for online and offline analysis. The systematic and random setup errors, as well as planning target volume (PTV) margin, were calculated at different correction threshold levels. The impact of online setup correction on dosimetry was evaluated by simulation of pre-correction errors. Results The correction-of-setup-errors frequencies for 1, 2 and 3 mm thresholds were 41.3–53.9%, 12.7–21.2% and 6.3–10.3%, respectively. Online correction was effective at the 2 mm threshold level for all three axes. The pre-correction systematic errors for the whole group ranged 1.1–1.3 mm, and the random errors were also 1.1–1.3 mm. After online correction, the systematic and random errors ranged 0.4–0.5 mm and 0.7–0.8 mm, respectively, in the three directions. The PTV margins for the pre-correction, pretreatment and post-treatment positions were 3.5–4.2 mm, 1.6–1.8 mm and 2.5–3.2 mm, respectively, in three directions. Analysis of hypothetical dosimetric change due to a translational isocenter shift of 3 mm showed that if no correction was applied, the mean maximum dose to both the brain stem and spinal cord would be increased by 10 Gy, the mean dose to the left and right parotids would be increased by 7.8 and 8.5 Gy, respectively, and the dose to target volumes would be decreased: 4 Gy for 95% GTV and 5.6 Gy for 95% CTV 60. Conclusions CBCT-based online correction increased the accuracy of IMRT for NPC and reduced irradiated margins, by decreasing both the systematic and random errors. Online CBCT correction reduces the radiation dose to normal tissue and creates room for further dose escalation of tumors.

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.

Assessment of a daily online implanted fiducial marker‐guided prostate radiotherapy process

The aims of this study were to investigate whether intrafraction prostate motion can affect the accuracy of online prostate positioning using implanted fiducial markers and to determine the effect of prostate rotations on the accuracy of the software-predicted set-up correction shifts. Eleven patients were treated with implanted prostate fiducial markers and online set-up corrections. Orthogonal electronic portal images were acquired to determine couch shifts before treatment. Verification images were also acquired during treatment to assess whether intrafraction motion had occurred. A limitation of the online image registration software is that it does not allow for in-plane prostate rotations (evident on lateral portal images) when aligning marker positions. The accuracy of couch shifts was assessed by repeating the registration measurements with separate software that incorporates full in-plane prostate rotations. Additional treatment time required for online positioning was also measured. For the patient group, the overall postalignment systematic prostate errors were less than 1.5 mm (1 standard deviation) in all directions (range 0.2-3.9 mm). The random prostate errors ranged from 0.8 to 3.3 mm (1 standard deviation). One patient exhibited intrafraction prostate motion, resulting in a postalignment prostate set-up error of more than 10 mm for one fraction. In 14 of 35 fractions, the postalignment prostate set-up error was greater than 5 mm in the anterior-posterior direction for this patient. Maximum prostate rotations measured from the lateral images varied from 2 degrees to 20 degrees for the patients. The differences between set-up shifts determined by the online software without in-plane rotations to align markers, and with rotations applied, was less than 1 mm (root mean square), with a maximum difference of 4.1 mm. Intrafraction prostate motion was found to reduce the effectiveness of the online set-up for one of the patients. A larger study is required to determine the magnitude of this problem for the patient population. The inability in the current software to incorporate in-plane prostate rotations is a limitation that should not introduce large errors, provided that the treatment isocentre is positioned near the centre of the prostate.

Radical chemoradiotherapy for adenocarcinoma of the distal oesophagus and oesophagogastric junction: what planning margins should we use?

Distal oesophageal and Type I-II oesophagogastric junctional adenocarcinomas have a poor prognosis. In radical chemoradiotherapy, consensus is lacking on radiotherapy margins. Here, we review the effect of common imaging modalities on the extent of the gross tumour volume (GTV) and the evidence for margins. To do this, papers were identified from PubMed, and geometric uncertainties were combined using the British Institute of Radiology formula. CT and endoscopic ultrasound were best for GTV delineation, but the role of positron emission tomography is uncertain. Evidence suggests 3 cm proximal and 5 cm distal GTV-CTV (clinical target volume) margins (along the mucosa) for advanced tumours, but is lacking for early tumours and radial margins. Nodal spread, present in most pT2 tumours, is strongly prognostic and is initially to regional nodes (not wholly covered by typical radiotherapy). Calculated CTV-PTV (planning target volume) margins for three-dimensional conformal radiotherapy using literature estimates of tumour motion and set-up errors with bony online set-up correction, ignoring delineation errors, are 2.2 cm superiorly (sup) and inferiorly (inf) and 1.2-1.3 cm radially (1.3 cm sup-inf; 0.8 cm radially if the tumour's mid-position is known). As these margins may risk excessive toxicity, we propose treating microscopic disease for potentially curable tumours (cT2N0, some cT3N0), but gross disease only for advanced tumours. Recommended GTV-CTV margins are a maximum of 3 cm proximally and 5 cm distally up to cT2N0; 3 cm proximally and 5 cm distally for cT3N0 if anticipated toxicity allows; and 0 cm for cT4 and most node-positive tumours. The CTV-PTV margins above must be added to this for all stages. Methods of including elective nodal areas close to the GTV should be researched, e.g. nodal maps and intensity-modulated radiotherapy.

Conventionally-fractionated image-guided intensity modulated radiotherapy (IG-IMRT): a safe and effective treatment for cancer spinal metastasis

BackgroundTreatments for cancer spinal metastasis were always palliative. This study was conducted to investigate the safety and effectiveness of IG-IMRT for these patients.Methods10 metastatic lesions were treated with conventionally-fractionated IG-IMRT. Daily kilovoltage cone-beam computed tomography (kV-CBCT) scan was applied to ensure accurate positioning. Plans were evaluated by the dose-volume histogram (DVH) analysis.ResultsBefore set-up correction, the positioning errors in the left-right (LR), superior-inferior (SI) and anterior-posterior (AP) axes were 0.3 ± 3.2, 0.4 ± 4.5 and -0.2 ± 3.9 mm, respectively. After repositioning, those errors were 0.1 ± 0.7, 0 ± 0.8 and 0 ± 0.7 mm, respectively. The systematic/random uncertainties ranged 1.4–2.3/3.0–4.1 before and 0.1–0.2/0.7–0.8 mm after online set-up correction. In the original IMRT plans, the average dose of the planning target volume (PTV) was 61.9 Gy, with the spinal cord dose less than 49 Gy. Compared to the simulated PTVs based on the pre-correction CBCT, the average volume reduction of PTVs was 42.3% after online correction. Also, organ at risk (OAR) all benefited from CBCT-based set-up correction and had significant dose reduction with IGRT technique. Clinically, most patients had prompt pain relief within one month of treatment. There was no radiation-induced toxicity detected clinically during a median follow-up of 15.6 months.ConclusionIG-IMRT provides a new approach to treat cancer spinal metastasis. The precise positioning ensures the implementation of optimal IMRT plan, satisfying both the dose escalation of tumor targets and the radiation tolerance of spinal cord. It might benefit the cancer patient with spinal metastasis.

Quality Assurance of Ultrasound Imaging Systems for Target Localization and Online Setup Corrections

We describe quality assurance paradigms for ultrasound imaging systems for target localization (UISTL). To determine the absolute localization accuracy of a UISTL, an absolute coordinate system can be established in the treatment room and spherical targets at various depths can be localized. To test the ability of such a system to determine the magnitude of internal organ motion, a phantom that mimics the human male pelvic anatomy can be used to simulate different organ motion ranges. To assess the interuser variability of ultrasound (US) guidance, different experienced users can independently determine the daily organ shifts for the same patients for a number of consecutive fractions. The average accuracy for a UISTL for the localization of spherical targets at various depths has been found to be 0.57 +/- 0.47 mm in each spatial dimension for various focal depths. For the phantom organ motion test it was found that the true organ motion could be determined to within 1.0 mm along each axis. The variability between different experienced users who localized the same 5 patients for five consecutive fractions was small in comparison to the indicated shifts. In addition to the quality assurance tests that address the ability of a UISTL to accurately localize a target, a thorough quality assurance program should also incorporate the following two aspects to ensure consistent and accurate localization in daily clinical use: (1) adequate training and performance monitoring of users of the US target localization system, and (2) prescreening of patients who may not be good candidates for US localization.

Evaluation of a Motion-Encompassing Treatment Strategy for Treating Non-Small Cell Lung Cancer

This study analyzes the setup error of a motion-encompassing treatment strategy for treating non-small cell lung cancer. Using data from patients who were treated with daily KV cone-beam CT (CBCT) image-guidance (IG) for setup correction, the residual setup errors for a hybrid off-line IG protocol were assessed. 22 patients with NSCLC were treated with a motion-encompassing daily CBCT IG strategy during 4/2006 to 3/2007. 4D-CT imaging was used for treatment planning. Patients were scanned in the supine position during normal relaxed free-breathing and were immobilized with a wing-board. The internal target volume (ITV) was determined using the maximum-intensity-projection (MiP) CT data to account for organ motion. A 5-mm PTV margin was used to account for setup error. On-line setup correction was based on the daily CBCT imaging prior to treatment, with an action level of 3 mm. The systematic and random errors were analyzed retrospectively. The results were then fit to a hybrid off-line IG strategy where the patient underwent CBCT-IG for the first 4 fractions. The treatment setup was corrected on-line if the setup error exceeded the action level of 3 mm. A mean shift was then applied on the 5th and the subsequent fractions to correct for the systematic error. The residual errors of the hybrid off-line protocols were assessed. The systematic and random errors were found to be 5.2 ± 3.2 mm, 4.4 ± 2.7 mm and 4.4 ± 3.4 mm for anterior-posterior (AP), medial-lateral (ML) and superior-inferior (SI) directions respectively. The 3D displacement vectors (DV) were 9.9 mm ± 4.7 mm (systematic ± random). The residual error was within 2 mm in all directions for the daily IG strategy. For the hybrid off-line IG strategy, the frequency of the residual errors is shown in the figure. The occurrence frequency was 61% for the 5-mm DV, indicating that the hybrid off-line IG strategy was inadequate. The use of a 5 mm PTV margin on the ITV determined by MiP dataset combined with daily CBCT on-line correction is adequate to avoid geometric miss of the tumor for NSCLC patients. However, the hybrid off-line IG strategy is inadequate unless a larger PTV margin is used.

Frameless Stereotactic Treatment of Brain Metastases Using Cone-Beam CT Based Image Guidance

The high precision necessary for single fraction high dose brain treatments is usually attained by the employment of invasive stereotactic frames. However, standard mask fixation used for fractionated brain and head-and-neck treatments does have the potential to provide very good short-term stability. The overall geometrical accuracy will then be limited by the ability to localize the patient on the Linac. We developed a procedure that applies on-line set-up corrections based on bony anatomy registration of cone-beam computed tomography (CBCT) data acquired on the treatment machine, which enables a non-invasive treatment for this patient group. The aim of this study is to determine the accuracy of this approach. Twenty patients were included in the protocol thus far. Standard PETG masks with 3-point fixation were used. Prior to treatment a CBCT scan (Elekta Synergy 3.5) was acquired and matched to the planning CT scan. Deviations larger than 1 mm were corrected by a manual couch shift. Corrections were validated with a second CBCT scan. To assess intra-fraction errors a third scan was acquired after treatment. All scans were registered with in-house developed software that uses a 6-D rigid body transform (3 translations and 3 rotations). The rotation center of the registration was placed in the middle of the PTV to minimize the effect of rotations. The pre-and post-treatment setup errors were computed in terms of a group standard deviation (SD) and overall mean. Matching accuracy was estimated by comparing the registrations of any pair of scans in a “full circle”. Finally, intra-fraction motion was quantified by the difference between the pre- and post-treatment measurements. Sixteen patients were corrected in either one or more directions. The mean and standard deviation of the corrected set-up errors were smaller than 0.1 mm and 1 mm respectively. Intra-fraction motion was smaller than 0.5 mm (SD) in any direction with no significant mean differences (Table). The matching accuracy, as derived from the “full circle” analysis was slightly smaller than the intra-fraction movement. The proposed method is accurate and comparable to methods utilizing an invasive stereotactic frame. The intra-fraction movement was very small, which shows that the rigidity of the PETG mask is sufficient for this type of treatment. Furthermore, registration accuracy is not limiting the total precision of the procedure. In conclusion, the use of CBCT on a linear accelerator to determine and correct set-up errors in combination with standard mask fixation does provide the accuracy needed for frameless stereotactic treatments.Tabled 1Left-right (mm)Cranial-caudal (mm)Anterior-posterior (mm)MeanSDMeanSDMeanSDBefore corr.−0.51.10.82.2−0.41.5After corr.−0.10.50.10.80.10.5Intra frac.0.10.60.20.4−0.10.5 Open table in a new tab

Using an onboard kilovoltage imager to measure setup deviation in intensity‐modulated radiation therapy for head‐and‐neck patients

The purpose of the present study was to use a kilovoltage imaging device to measure interfractional and intrafractional setup deviations in patients with head‐and‐neck or brain cancers receiving intensity‐modulated radiotherapy (IMRT) treatment.Before and after IMRT treatment, approximately 3 times weekly, 7 patients were imaged using the Varian On‐Board Imager (OBI: Varian Medical Systems, Palo Alto, CA), a kilovoltage imaging device permanently mounted on the gantry of a Varian 21EX LINAC (Varian Medical Systems). Because of commissioning of the remote couch correction of the OBI during the study, online setup corrections were performed on 2 patients. For the other 5 patients, weekly corrections were made based on a sliding average of the measured data. From these data, we determined the interfractional setup deviation (defined as the shift from the original setup position suggested by the daily image), the residual error associated with the weekly correction protocol, and the intrafractional setup deviation, defined as the difference between the post‐treatment and pretreatment images. We also used our own image registration software to determine interfractional and intrafractional rotational deviations from the images based on the template‐matching method. In addition, we evaluated the influence of inter‐observer variation on our results, and whether the use of various registration techniques introduced differences. Finally, translational data were compared with rotational data to search for correlations.Translational setup errors from all data were 0.0±0.2 cm, −0.1±0.3 cm, and −0.2±0.3 cm in the right–left (RL), anterior–posterior (AP), and superior–inferior (SI) directions respectively. Residual error for the 5 patients with a weekly correction protocol was −0.1±0.2 cm (RL), 0.0±0.3 cm (AP), and 0.0±0.2 cm (SI). Intrafractional translation errors were small, amounting to 0.0±0.1 cm, −0.1±0.2 cm, and 0.0±0.1 cm in the RL, AP, and SI directions respectively. In the sagittal and coronal views respectively, interfractional rotational errors were −1.1±1.7 degrees and −0.5±0.9 degrees, and intrafractional rotational errors were 0.3±0.6 degrees and 0.2±0.5 degrees. No significant correlation was seen between translational and rotational data.The OBI image data were used to study setup error in the head‐and‐neck patients. Nonzero systematic errors were seen in the interfractional translational and rotational data, but not in the intrafractional data, indicating that the mask is better at maintaining head position than at reproducing it.PACS numbers:87.53.Kn, 87.53.Oq

Investigating the Visibility of Three Types of Surgical Clips as Fiducial Markers in a Breast Phantom

To facilitate online setup correction in breast radiotherapy, we studied the visibility of three types of surgical clips of differing materials using three different image verification methods. Titanium, tantalum and stainless steel clips of identical size were embedded in various orientations in the breast attachments of a tissue-equivalent phantom. Clip visibility was assessed for isocentric megavoltage, isocentric kilovoltage, and floor-mounted kilovoltage imaging. Based upon this phantom study, titanium clips are recommended for isocentric kilovoltage verification imaging, and tantalum clips are recommended for isocentric megavoltage and floor-mounted kilovoltage verification imaging.

Clinical application of image-guided radiotherapy, IGRT (on the Varian OBI platform)

Image-guided radiation therapy (IGRT) can be used to measure and correct positional errors for target and critical structures immediately prior to or during treatment delivery. Some of the most recent available methods applied for target localization are: transabdominal ultrasound, implanted markers with in room MV or kV X-rays, optical surface tracking systems, implantable electromagnetic markers, in room CT such as kVCT on rail, kilovoltage or megavoltage cone-beam CT (CBCT) and helical megavoltage CT. The verification of the accurate treatment position in conjunction with detailed anatomical information before every fraction can be essential for the outcome of the treatment. In this paper we present the on-board imager (OBI, Varian Medical Systems, Palo Alto, CA) that has been in routine clinical use at the Karolinska University Hospital since June 2004. The OBI has been used for on-line set-up correction of prostate patients using internal gold markers. Displacements of these markers can be monitored radiographically during the treatment course and the registered marker shifts act as a surrogate for prostate motion. For this purpose, on-board kV–kV seems to be an ideal system in terms of image quality. The CBCT function of OBI was installed in March 2005 at our department. It focuses on localizing tumors based on internal anatomy, not just on the conventional external marks or tattoos. The CBCT system provides the capacity for soft tissue imaging in the treatment position and real-time radiographic monitoring during treatment delivery.

Efficient on‐line setup correction strategies using plan‐intent functions

With the introduction of image-guided radiation therapy (IGRT) delivery systems on-line set-up correction strategies have gained in popularity. Usually, the correction workload of these strategies is high compared to off-line strategies as daily setup corrections have to be performed based on a predefined action level. In this work, it is proposed that on-line strategies must not only be judged in terms of workload but also in terms of efficacy. While workload can be easily predicted for such strategies, the efficacy must ultimately reflect the efficiency with which the original treatment plan intent is met. The purpose of this work is to investigate the tradeoff between workload and efficacy of three different on-line set-up correction strategies: The common fixed action level strategy and two novel on-line setup correction strategies, i.e., a dose-volume histogram (DVH) constraint and an equivalent uniform dose (EUD) score strategy that aim directly for better compliance with original treatment plan intent. All strategies were reformulated in terms of a score function that reflected treatment plan intent. A retrospective study was conducted on 5 prostate patients (7-field conformal, 79.8 Gy, 42 fractions). PTV margins were 10 mm except in the posterior direction (7 mm). The original treatment plan intent for these patients was defined using a set of DVH constraints. The results show that the on-line setup correction strategy based on a fixed action level of 3 mm resulted in a considerable correction workload. For larger action levels, a dose benefit (in terms of EUD) in the rectum and bladder was observed for all patients which is clinically "fortuitous" but difficult to take advantage of. In contrast, the application of the two novel strategies generally resulted in a controlled decrease of the dose to the rectum and the bladder with a smaller workload. It is concluded that using information about target anatomy and the planned dose distribution allows the design of specific correction strategies that are better tailored to the individual patient and that comply effectively with initial treatment plan intent.