Control charts for evaluation of quality of low-dose-rate brachytherapy for prostate cancer.

Purpose: Variations in dosimetric outcomes among patients treated with low-dose-rate brachytherapy for prostate cancer exist, even when implants are within dose constraints. Here, we used control charts to investigate reasons for intra-patient dosimetric variability. Material and methods: Univariate and multivariate control charts for prostate V100 (percentage of prostate volume that received 100% of prescribed radiation dose), D90 (radiation dose to 90% of prostate volume), and RV100 (rectal wall volume that received 100% of prescribed radiation dose) were generated for 212 consecutive prostate cancer patients implanted with iodine-125 (125I) radioactive seeds at the Princess Margaret Cancer Centre. Control limits were calculated based on the first fifty implants. Data points that were out of control were identified, and their pre-treatment and post-treatment dosimetric and clinical parameters were compared to data points that were in-control, using Student's t-test. Results: All implants were clinically acceptable. Twelve data points exceeded multivariate control limits. Ten of those points fell below the lower control limit of V100 control chart. Average prostate edema in the 10 out-of-control patients on both multivariate and V100 charts was 8.3%, as compared to 0.4% for in-control patients (p < 0.04). Two patients were observed to be out-of-control on multivariate control chart, but not on V100 control chart, and were found to have a reduction in prostate volume of 19.1% and 20.1% at one month after seed implant, compared to prostate volumes of pre-implantation evaluations. Conclusions: Control charts helped in identifying cases with out-of-control variability in post-plan prostate dosimetry. Post-treatment prostatic edema and contraction are important factors predicting variability in patients treated with 125I permanent seed brachytherapy.

Auto-segmentation of normal and target structures in head and neck CT images: a feature-driven model-based approach.

PURPOSE: Intensity modulated radiation therapy (IMRT) allows greater control over dose distribution, which leads to a decrease in radiation related toxicity. IMRT, however, requires precise and accurate delineation of the organs at risk and target volumes. Manual delineation is tedious and suffers from both interobserver and intraobserver variability. State of the art auto-segmentation methods are either atlas-based, model-based or hybrid however, robust fully automated segmentation is often difficult due to the insufficient discriminative information provided by standard medical imaging modalities for certain tissue types. In this paper, the authors present a fully automated hybrid approach which combines deformable registration with the model-based approach to accurately segment normal and target tissues from head and neck CT images. METHODS: The segmentation process starts by using an average atlas to reliably identify salient landmarks in the patient image. The relationship between these landmarks and the reference dataset serves to guide a deformable registration algorithm, which allows for a close initialization of a set of organ-specific deformable models in the patient image, ensuring their robust adaptation to the boundaries of the structures. Finally, the models are automatically fine adjusted by our boundary refinement approach which attempts to model the uncertainty in model adaptation using a probabilistic mask. This uncertainty is subsequently resolved by voxel classification based on local low-level organ-specific features. RESULTS: To quantitatively evaluate the method, they auto-segment several organs at risk and target tissues from 10 head and neck CT images. They compare the segmentations to the manual delineations outlined by the expert. The evaluation is carried out by estimating two common quantitative measures on 10 datasets: volume overlap fraction or the Dice similarity coefficient (DSC), and a geometrical metric, the median symmetric Hausdorff distance (HD), which is evaluated slice-wise. They achieve an average overlap of 93% for the mandible, 91% for the brainstem, 83% for the parotids, 83% for the submandibular glands, and 74% for the lymph node levels. CONCLUSIONS: Our automated segmentation framework is able to segment anatomy in the head and neck region with high accuracy within a clinically-acceptable segmentation time.

A method to analyze the cord geometrical uncertainties during head and neck radiation therapy using cone beam CT.

We developed a method to analyze quantitatively the residual cord geometrical uncertainties after image registration during head and neck radiation therapy by using sequential cone beam CT (CBCT). The geometrical centroid line of cervical spinal canal was computed to serve as a cord surrogate. We found that the cord motions were non-uniform from C1 to C6, and that the patterns of motion were variable across patients. This method has potential applications in monitoring cord setup accuracy and in designing treatment margins.

The importance of obtaining conjugate views on renographic evaluation of large hydronephrotic kidneys: an in vitro and ex vivo analysis.

PURPOSE: It is postulated that significant obstruction leads to decreased ipsilateral renal function. However, maintained or even increased differential renal function is often seen in patients with large hydronephrotic kidneys. The reason for such a phenomenon is unclear. We designed an in vitro and ex vivo experimental model that permits the controlled orientation of thinned renal parenchyma and background activity during gamma camera detection. MATERIALS AND METHODS: A 5-step experimental study was designed with balloons or reservoirs containing known amounts of radioisotope. A balloon was incrementally compressed between 2 Plexiglas sheets (step 1). Simultaneous measurement of radio emission was done of 5 glass beakers of varying diameters filled with the same amount of water and (99m)Tc (step 2). A single beaker containing diluted (99m)Tc was interfaced with air (step 3) or water (step 4) at different distances from the gamma camera. Pig kidneys previously injected with dimercapto-succinic acid were removed and scanned (step 5). One of the kidneys was then progressively sliced, thinning its parenchyma, and sliced and nonsliced kidneys were simultaneously scanned again. RESULTS: Progressively increased counts were detected as the projected surface area of the radioactive balloons (step 1) and beakers (step 2) increased. Counts detected were inversely proportional to the amount of water interfaced between the container and the gamma camera. Significantly more radiation was detectable in sliced kidneys containing dimercapto-succinic acid than in intact control kidneys. CONCLUSIONS: These findings suggest that the artifactual supranormal differential renal function that is observed during renographic evaluation of large hydronephrotic kidneys actually exists. Parenchymal proximity and distribution in relation to the pelvis are critical determinants. These findings suggest that the conjugate view technique may be more suitable for renographic evaluation of large hydronephrotic kidneys.

Statistical process control for IMRT dosimetric verification.

Patient-specific measurements are typically used to validate the dosimetry of intensity-modulated radiotherapy (IMRT). To evaluate the dosimetric performance over time of our IMRT process, we have used statistical process control (SPC) concepts to analyze the measurements from 330 head and neck (H&N) treatment plans. The objectives of the present work are to: (i) Review the dosimetric measurements of a large series of consecutive head and neck treatment plans to better understand appropriate dosimetric tolerances; (ii) analyze the results with SPC to develop action levels for measured discrepancies; (iii) develop estimates for the number of measurements that are required to describe IMRT dosimetry in the clinical setting; and (iv) evaluate with SPC a new beam model in our planning system. H&N IMRT cases were planned with the PINNACLE treatment planning system versions 6.2b or 7.6c (Philips Medical Systems, Madison, WI) and treated on Varian (Palo Alto, CA) or Elekta (Crawley, UK) linacs. As part of regular quality assurance, plans were recalculated on a 20-cm-diam cylindrical phantom, and ion chamber measurements were made in high-dose volumes (the PTV with highest dose) and in low-dose volumes (spinal cord organ-at-risk, OR). Differences between the planned and measured doses were recorded as a percentage of the planned dose. Differences were stable over time. Measurements with PINNACLE3 6.2b and Varian linacs showed a mean difference of 0.6% for PTVs (n=149, range, -4.3% to 6.6%), while OR measurements showed a larger systematic discrepancy (mean 4.5%, range -4.5% to 16.3%) that was due to well-known limitations of the MLC model in the earlier version of the planning system. Measurements with PINNACLE3 7.6c and Varian linacs demonstrated a mean difference of 0.2% for PTVs (n=160, range, -3.0%, to 5.0%) and -1.0% for ORs (range -5.8% to 4.4%). The capability index (ratio of specification range to range of the data) was 1.3 for the PTV data, indicating that almost all measurements were within +/-5%. We have used SPC tools to evaluate a new beam model in our planning system to produce a systematic difference of -0.6% for PTVs and 0.4% for ORs, although the number of measurements is smaller (n=25). Analysis of this large series of H&N IMRT measurements demonstrated that our IMRT dosimetry was stable over time and within accepted tolerances. These data provide useful information for assessing alterations to beam models in the planning system. IMRT is enhanced by the addition of statistical process control to traditional quality control procedures.

Image registration assessment in radiotherapy image guidance based on control chart monitoring.

Image guidance with cone beam computed tomography in radiotherapy can guarantee the precision and accuracy of patient positioning prior to treatment delivery. During the image guidance process, operators need to take great effort to evaluate the image guidance quality before correcting a patient's position. This work proposes an image registration assessment method based on control chart monitoring to reduce the effort taken by the operator. According to the control chart plotted by daily registration scores of each patient, the proposed method can quickly detect both alignment errors and image quality inconsistency. Therefore, the proposed method can provide a clear guideline for the operators to identify unacceptable image quality and unacceptable image registration with minimal effort. Experimental results demonstrate that by using control charts from a clinical database of 10 patients undergoing prostate radiotherapy, the proposed method can quickly identify out-of-control signals and find special cause of out-of-control registration events.

Intraobserver and interobserver variability in GTV delineation on FDG-PET-CT images of head and neck cancers.

PURPOSE: To determine if the addition of fluorodeoxyglucose positron emission tomography (FDG-PET) data changes primary site gross tumor volumes (GTVs) in head and neck cancers. METHODS AND MATERIALS: Computed tomography (CT), contrast-enhanced CT, and FDG-PET-CT scans were obtained in 10 patients with head and neck cancers. Eight experienced observers (6 head and neck oncologists and 2 neuro-radiologists) with access to clinical and radiologic reports outlined primary site GTVs on each modality. Three cases were recontoured twice to assess intraobserver variability. The magnitudes of the GTVs were compared. Intra- and interobserver variability was assessed by a two-way repeated measures analysis of variance. Inter- and intraobserver reliability were calculated. RESULTS: There were no significant differences in the GTVs across the image modalities when compared as ensemble averages; the Wilcoxon matched-pairs signed-rank test showed that CT volumes were larger than PET-CT. Observers demonstrated the greatest consistency and were most interchangeable on contrast-enhanced CT; they performed less reliably on PET-CT. CONCLUSIONS: The addition of PET-CT to primary site GTV delineation of head and neck cancers does not change the volume of the GTV defined by this group of expert observers in this patient sample. An FDG-PET may demonstrate differences in neck node delineation and in other disease sites.

Quantitative PET comparing gated with nongated acquisitions using a NEMA phantom with respiratory-simulated motion.

UNLABELLED: This study evaluated the use of gated versus nongated PET acquisitions for absolute quantification of radioisotope concentration (RC) in a respiratory motion-simulated moving phantom filled with radioactive spheres and background for both 2-dimensional (2D) and 3-dimensional (3D) acquisitions. METHODS: An image-quality phantom with all 6 spheres filled with the same (18)F RC (range, 19-62 kBq/mL) was scanned with PET/CT at rest and in motion with and without gating. The background was filled with (18)F solution to yield sphere-to-background ratios of approximately 5, 10, 15, and 20 to 1. Both 2D and 3D acquisitions were used for all combinations. Respiratory motion was simulated by using a motor-driven plastic platform to move the phantom periodically with a displacement of 2 cm and a cycle time of 5.8 s. For gated acquisitions, the phantom was tracked using a real-time position management system. Images were reconstructed, and regions of interest with the same sizes as the actual spheres were manually placed on axial slices to determine maximum and mean pixel RC. A threshold method (70% and 94% for 2D and 3D modes) was also used to determine a mean voxel RC. All values were compared with the expected RC; percentage differences were calculated for each sphere. To reduce partial-volume effects, only data for the 4 largest spheres were analyzed. RESULTS: The mean pixel method was the only method with linear responses for all 3 scan types, enabling direct comparisons. The ranges of RC percentage differences were underestimated for all scan types (using the mean pixel method). The overall mean percentage differences were 37, 49, and 41 in 2D mode and 40, 51, and 41 in 3D mode for static, nongated, and gated acquisitions, respectively. Gated acquisitions improved quantification (by reducing underestimation) over nongated acquisitions by 8% and 10% for 2D and 3D modes. CONCLUSION: In the presence of motion, the use of gated PET acquisitions appears to improve quantification accuracy over nongated acquisitions, almost restoring the results to those observed when the phantom is static.

Spinal cord planning risk volumes for intensity-modulated radiation therapy of head-and-neck cancer.

PURPOSE: To assess planning organ at risk volume (PRV) margins of the spinal cord in intensity-modulated radiotherapy (IMRT) of oropharyngeal cancers, by modeling the effect of geometric uncertainties to estimate the probability of the spinal cord receiving a particular dose. METHODS AND MATERIALS: Five patients with oropharyngeal cancer were treated by IMRT with simultaneous doses of 66 Gy (gross disease) and 54 Gy (subclinical disease) in 30 fractions. Spinal cord doses were limited to 45 Gy. The probability, due to random and systematic patient positioning uncertainties (3-mm standard deviation), of the cord receiving a particular dose was determined. The effect of an on-line setup correction protocol was also modeled. RESULTS: The mean probability of a maximum spinal cord dose of 45 Gy was 1%, with a 6-mm PRV margin. The mean probability of a maximum dose exceeding 40 Gy was 37% (range, 13-77%); this probability is reduced with a setup correction protocol. CONCLUSION: A spinal cord PRV generated with a 6-mm margin leads to a 99% probability of maintaining the maximum spinal cord dose below 45 Gy. The application of an on-line setup correction protocol reduces the cord dose by approximately 5 Gy.

Technical challenges of sparing infrahyoid swallowing organs at risk in oropharynx squamous cell cancer treated with IMRT.

This study reports clinical performance in the sparing of infrahyoid swallowing organs at risk (SWOARs) in oropharynx cancer intensity-modulated radiation therapy (IMRT) plans. Rates of meeting dose-volume planning goals are reported and compared with geometry-based estimates of what is achievable. This study also develops 3 measures of target-SWOAR geometry and tests their usefulness in providing geometry-based dose-volume planning goals. A total of 50 oropharynx cancer IMRT plans were reviewed. Success rates in meeting institutional dose-volume goals were determined for the glottic larynx (G), postcricoid pharynx (P), and esophagus (E). The following 3 measures of target-SWOAR geometry were investigated as methods of identifying geometry-based planning goals: presence of gross disease in neck levels 3 to 4, target-SWOAR overlap, and a 3-dimensional (3D) measure of target-SWOAR geometry. Locally advanced disease was predominant in this patient population with target volumes overlapping SWOARs in 68% to 98% of cases. Clinical rates of success in meeting dose-volume goals varied by SWOAR (16% to 82%) but compared well with estimated potentially achievable rates in most cases (14% average difference between clinical and potential). Cases grouped by the presence of levels 3 to 4 neck nodes or target-SWOAR overlap did not have significantly different SWOAR doses. Cases grouped using a 3D measure of target-SWOAR geometry differed significantly, providing useful geometry-based planning goals (e.g., mean Glottis dose <45Gy was achieved 19%, 44%, or 81% of the time in each of 3 groups). This study describes the technical challenge of sparing SWOARs and investigates several potential methods for grouping cases to assist with treatment plan evaluation. Quantifying the 3-D relationship between the targets and SWOARs is a promising way of approaching this complex problem. Data presented in this paper may be useful to evaluate treatment plans using objective geometry-based goals.

A facility for magnetic resonance-guided radiation therapy.

Magnetic resonance (MR) imaging is routinely employed in the design of radiotherapy (RT) treatment plans for many disease sites. It is evident that tighter integration of MR imaging into the RT process would increase confidence in dose placement and facilitate the integration of new MR imaging information (including anatomical and functional imaging) into the therapy process. To this end, a dedicated MR-guided RT (MRgRT) facility has been created that integrates a state-of-the-art linear accelerator delivery system, high-dose rate brachytherapy afterloader, and superconducting MR scanner to allow MR-based online treatment guidance, adaptive replanning, and response monitoring while maintaining the clinical functionality of the existing delivery systems. This system is housed within a dedicated MRgRT suite and operates in a coordinated fashion to assure safe and efficient MRgRT treatments.

A Practice-based Taxonomy for Radiation Treatment Errors.

PURPOSE: An absence of a common language for incident classification limits knowledge sharing within and between organizations in the radiotherapy community. This challenge provided the motivation to develop a clinically relevant taxonomy for radiotherapy errors. MATERIALS AND METHODS: This was a multicenter, prospective study that consisted of three phases: (1) an initial version of the taxonomy was developed based on the World Health Organization Conceptual Framework for the International Classification for Patient Safety and taxonomy models from radiotherapy and other industries; (2) the taxonomy was evaluated using actual incident data from a single practitioner and revised; and (3) face validity testing of the taxonomy was performed by two additional practitioners from different radiotherapy centers using simulated incident cases. RESULTS: The taxonomy consisted of seven classes: incident nature, impact, incident type, stage of origin, stage of discovery, contributing factors, and preventative strategies. Each class was divided into subcategories containing increasingly detailed information. A total of 191 consecutive incidents were classified in phase 2 to ensure no further revision to the taxonomy was required. In phase 3, low interobserver agreement (<60%) was obtained for most classes of the taxonomy in the first face validity test. After revisions were made to the taxonomy based on practitioners' feedback, a second face validity test yielded a high degree of agreement (70%-93%) for all classes. CONCLUSIONS: Our multiphase, iterative approach has yielded a workable and multidimensional set of incident classifiers that can be scaled to accommodate local, regional and discipline-specific requirements. Opportunities exist to implement this taxonomy in institutional and national incident databases to facilitate incident learning within and between institutions.

The Effect of Registration Volume Extent on Residual Errors Assessed Using Cone-Beam Computed Tomography in Radiation Treatment of Head and Neck Cancer.

PURPOSE: The objective of this study was to investigate the effect of the varying extent of cone-beam computed tomography (CBCT) registration volumes (RVs) on setup errors for head and neck (H&N) radiotherapy. METHODS AND MATERIALS: Daily CBCT images for 31 patients receiving H&N intensity-modulated radiotherapy (IMRT) were reviewed. Registrations using anatomically defined RVs with a fixed superior border at base of sella and varying inferior extent were used retrospectively to evaluate patient setup. The inferior extent was defined as the number of cervical bodies included, from none (C0) to six (C6). The frequency of residual displacements at four landmarks (clivus, vertebral bodies C5-C6, manubrium-sterni, and anterior body of mandible) was assessed. RESULTS: Expansion of the RVs inferiorly reduced the occurrence of residual displacements for the C5-C6 vertebral bodies (from 57% to 93% of fractions with residual displacements ≤ 3 mm) and increased the rate of simultaneous positioning of C5-C6 and clivus (from 41% to 76%). Maximum residual displacements for mandible (48%-64% ≤ 3 mm) and manubrium (73%-81% ≤ 3 mm) varied somewhat by the inferior extent of the RV. Residual displacements for clivus were small (88%-96% ≤ 3 mm) in all cases. Random and systematic errors were clinically acceptable for a 5-mm planning margin around the clinical targets. CONCLUSIONS: In conclusion, expansion of the RV inferiorly to include C6 will improve the positioning of structures in the C5-C6 region (adjacent nodal zones 3 and 4) without compromising clival positioning. Insufficient inferior extent of the RV reduces reliability of low neck positioning. Substantial variability can occur for structures not included in the RV. Based on these data, we use the C6 RV except in cases with planning concerns outside this volume.

Audit of an automated checklist for quality control of radiotherapy treatment plans.

PURPOSE: To assess the effect of adding an automated checklist to the treatment planning process for head and neck intensity-modulated radiotherapy. METHODS: Plans produced within our treatment planning system were evaluated at the planners' discretion with an automated checklist of more than twenty planning parameters. Plans were rated as accepted or rejected for treatment, during regular review by radiation oncologists and physicists as part of our quality control program. The rates of errors and their types were characterised prior to the implementation of the checklist and with the checklist. RESULTS: Without the checklist, 5.9% of plans were rejected; the use of the checklist reduced the rejection rate to 3.1%. The checklist was used for 64.7% of plans. Pareto analysis of the causes of rejection showed that the checklist reduced the number of causes of rejections from twelve to seven. CONCLUSIONS: The use of an automated checklist has reduced the need for reworking of treatment plans. With the use of the checklist, most rejections were due to errors in prescription or inadequate dose distributions. Use of the checklist by planners must be increased to maximise improvements in planning efficiency.

Cone-beam CT assessment of interfraction and intrafraction setup error of two head-and-neck cancer thermoplastic masks.

PURPOSE: To prospectively compare setup error in standard thermoplastic masks and skin-sparing masks (SSMs) modified with low neck cutouts for head-and-neck intensity-modulated radiation therapy (IMRT) patients. METHODS AND MATERIALS: Twenty head-and-neck IMRT patients were randomized to be treated in a standard mask (SM) or SSM. Cone-beam computed tomography (CBCT) scans, acquired daily after both initial setup and any repositioning, were used for initial and residual interfraction evaluation, respectively. Weekly, post-IMRT CBCT scans were acquired for intrafraction setup evaluation. The population random (sigma) and systematic (Sigma) errors were compared for SMs and SSMs. Skin toxicity was recorded weekly by use of Radiation Therapy Oncology Group criteria. RESULTS: We evaluated 762 CBCT scans in 11 patients randomized to the SM and 9 to the SSM. Initial interfraction sigma was 1.6 mm or less or 1.1 degrees or less for SM and 2.0 mm or less and 0.8 degrees for SSM. Initial interfraction Sigma was 1.0 mm or less or 1.4 degrees or less for SM and 1.1 mm or less or 0.9 degrees or less for SSM. These errors were reduced before IMRT with CBCT image guidance with no significant differences in residual interfraction or intrafraction uncertainties between SMs and SSMs. Intrafraction sigma and Sigma were less than 1 mm and less than 1 degrees for both masks. Less severe skin reactions were observed in the cutout regions of the SSM compared with non-cutout regions. CONCLUSIONS: Interfraction and intrafraction setup error is not significantly different for SSMs and conventional masks in head-and-neck radiation therapy. Mask cutouts should be considered for these patients in an effort to reduce skin toxicity.