Clinical feasibility of a commercially available MRI-only method for radiotherapy treatment planning of the brain.

BACKGROUND: Advancements in deep-learning based synthetic computed tomography (sCT) image conversion methods have enabled the development of magnetic resonance imaging (MRI)-only based radiotherapy treatment planning (RTP) of the brain. PURPOSE: This study evaluates the clinical feasibility of a commercial, deep-learning based MRI-only RTP method with respect to dose calculation and patient positioning verification performance in RTP of the brain. METHODS: Clinical validation of dose calculation accuracy was performed by a retrospective evaluation for 25 glioma and 25 brain metastasis patients. Dosimetric and image quality of the studied MRI-only RTP method was evaluated by a direct comparison of the sCT-based and computed tomography (CT)-based external beam radiation therapy (EBRT) images and treatment plans. Patient positioning verification accuracy of sCT images was evaluated retrospectively for 10 glioma and 10 brain metastasis patients based on clinical cone-beam computed tomography (CBCT) imaging. RESULTS: An average mean dose difference of Dmean = 0.1% for planning target volume (PTV) and 0.6% for normal tissue (NT) structures were obtained for glioma patients. Respective results for brain metastasis patients were Dmean = 0.5% for PTVs and Dmean =1.0% for NTs. Global three-dimensional (3D) gamma pass rates using 2%/2 mm dose difference and distance-to-agreement (DTA) criterion were 98.0% for the glioma subgroup, and 95.2% for the brain metastasis subgroup using 1%/1 mm criterion. Mean distance differences of <1.0 mm were observed in all Cartesian directions between CT-based and sCT-based CBCT patient positioning in both subgroups. CONCLUSIONS: In terms of dose calculation and patient positioning accuracy, the studied MRI-only method demonstrated its clinical feasibility for RTP of the brain. The results encourage the use of the studied method as part of a routine clinical workflow.

Long-term outcome of biologically guided dose-escalated radiotherapy of localized prostate cancer.

BACKGROUND: Biologically created subvolumes enable non-uniform dose distributions in prostate cancer radiotherapy (RT) thus potentially improving therapeutic ratio and reducing toxicity. We present the long-term outcome of men receiving focal boosting of carbon-11 acetate (ACE) PET-CT metabolically active areas in prostate carcinoma. MATERIAL AND METHODS: Thirty men with hormone naïve localized prostate carcinoma underwent ACE PET/CT for RT planning. There were five low-, 17 intermediate-, and eight high-risk patients. Based on thresholding of the standardized uptake values (SUVs) metabolic target volumes (MTVs) corresponding to intraprostatic lesions (IPLs) were contoured. Two planning target volumes (PTVs) were applied i.e., PTVlow-risk for the whole prostate with 8-10 mm margin and PTVhigh-risk for the MTV. Pelvic nodes were not irradiated. Late toxicity of biologically guided RT was reviewed after a median of 63 months and outcome after a median follow-up of 124 months. RESULTS: Median doses to PTVlow-risk, PTVhigh-risk, prostate, and MTV were 72.9 Gy, 79.4 Gy, 76.6 Gy, and 80.4 Gy, respectively, in 38 fractions. The 10-year cancer-specific survival was 86% and the biochemical failure-free ratio 68%, respectively. The median biochemical progression-free survival (PFS) was 37, 108, and 119 months in the high, intermediate, and low-risk groups, respectively, the difference being significant between high and intermediate-risk groups (p = 0.02). One patient (3%) presented with locoregional and 5 (17%) with distant nodal metastases. Five patients (17%) had a biochemical relapse. A larger MTV was associated with shorter PFS (r = -0.41, p = 0.02), but had no influence on OS. No other statistically significant differences in the dose painting parameters were observed between recurrence-free and recurring patients. CONCLUSIONS: Biological guidance for dose-escalated prostate RT is feasible with ACE PET/CT. Since a larger MTV may be associated with a higher risk for progression, we encourage further study of dose-escalation to ACE-positive lesions considering the low toxicity of our protocol.

Repeatability of hypoxia dose painting by numbers based on EF5-PET in head and neck cancer.

BACKGROUND: Hypoxia dose painting is a radiotherapy technique to increase the dose to hypoxic regions of the tumour. Still, the clinical effect relies on the reproducibility of the hypoxic region shown in the medical image. 18F-EF5 is a hypoxia tracer for positron emission tomography (PET), and this study investigated the repeatability of 18F-EF5-based dose painting by numbers (DPBN) in head and neck cancer (HNC). MATERIALS AND METHODS: Eight HNC patients undergoing two 18F-EF5-PET/CT sessions (A and B) before radiotherapy were included. A linear conversion of PET signal intensity to radiotherapy dose prescription was employed and DPBN treatment plans were created using the image basis acquired at each PET/CT session. Also, plan A was recalculated on the image basis for session B. Voxel-by-voxel Pearson's correlation and quality factor were calculated to assess the DPBN plan quality and repeatability. RESULTS: The mean (SD) correlation coefficient between DPBN prescription and plan was 0.92 (0.02) and 0.93 (0.02) for sessions A and B, respectively, with corresponding quality factors of 0.02 (0.002) and 0.02 (0.003), respectively. The mean correlation between dose prescriptions at day A and B was 0.72 (0.13), and 0.77 (0.12) for the corresponding plans. A mean correlation of 0.80 (0.08) was found between plan A, recalculated on image basis B, and plan B. CONCLUSION: Hypoxia DPBN planning based on 18F-EF5-PET/CT showed high repeatability. This illustrates that 18F-EF5-PET provides a robust target for dose painting.

A multi-institutional analysis of a general pelvis continuous Hounsfield unit synthetic CT software for radiotherapy.

PURPOSE: To validate a synthetic computed tomography (sCT) software with continuous HUs and large field-of-view (FOV) coverage for magnetic resonance imaging (MRI)-only workflow of general pelvis anatomy in radiotherapy (RT). METHODS: An sCT software for general pelvis anatomy (prostate, rectum, and female pelvis) has been developed by Philips Healthcare and includes continuous HUs assignment along with large FOV coverage. General pelvis sCTs were generated using a two-stack T1-weighted mDixon fast-field echo (FFE) sequence with a superior-inferior coverage of 36 cm. Seventy-seven prostate, 43 rectum, and 27 gynecological cases were scanned by three different institutions. mDixon image quality and sCTs were evaluated for soft tissue contrast by using a confidence level scale from 1 to 5 for bladder, prostate/rectum interface, mesorectum, and fiducial maker visibility. Dosimetric comparison was performed by recalculating the RT plans on the sCT after rigid registration. For 12 randomly selected cases, the mean absolute error (MAE) between sCT and CT was calculated to evaluate HU similarity, and the Pearson correlation coefficients (PCC) between the CT- and sCT-generated digitally reconstructed radiographs (DRRs) were obtained for quantitative comparison. To examine geometric accuracy of sCT as a reference for cone beam CT (CBCT), the difference between bone-based alignment of CBCT to CT and CBCT to sCT was obtained for 19 online-acquired CBCTs from three patients. RESULTS: Two-stack mDixon scans with large FOV did not show any image inhomogeneity or fat-water swap artifact. Fiducials, Foley catheter, and even rectal spacer were visible as dark signal on the sCT. Average visibility confidence level (average ± standard deviation) on the sCT was 5.0 ± 0.0, 4.6 ± 0.5, 3.8 ± 0.4, and 4.0 ± 1.1 for bladder, prostate/rectum interface, mesorectum and fiducial markers. Dosimetric accuracy showed on average < 1% difference with the CT-based plans for target and normal structures. The MAE of bone and soft tissue between the sCT and CT are 120.9 ± 15.4 HU, 33.4 ± 4.1 HU, respectively. Average PCC of all evaluated DRR pairs was 0.975. The average offset between CT and sCT as reference was (LR, AP, SI) = (0.19 ± 0.35, 0.14 ± 0.60, 0.44 ± 0.54) mm. CONCLUSIONS: The continuous HU sCT software-generated realistic sCTs and DRRs to enable MRI-only planning for general pelvis anatomy.

Repeatability of tumour hypoxia imaging using [18F]EF5 PET/CT in head and neck cancer.

PURPOSE: Hypoxia contributes to radiotherapy resistance and more aggressive behaviour of several types of cancer. This study was designed to evaluate the repeatability of intratumour uptake of the hypoxia tracer [18F]EF5 in paired PET/CT scans. METHODS: Ten patients with newly diagnosed head and neck cancer (HNC) received three static PET/CT scans before chemoradiotherapy: two with [18F]EF5 a median of 7 days apart and one with [18F]FDG. Metabolically active primary tumour volumes were defined in [18F]FDG images and transferred to co-registered [18F]EF5 images for repeatability analysis. A tumour-to-muscle uptake ratio (TMR) of 1.5 at 3 h from injection of [18F]EF5 was used as a threshold representing hypoxic tissue. RESULTS: In 10 paired [18F]EF5 PET/CT image sets, SUVmean, SUVmax, and TMR showed a good correlation with the intraclass correlation coefficients of 0.81, 0.85, and 0.87, respectively. The relative coefficients of repeatability for these parameters were 15%, 17%, and 10%, respectively. Fractional hypoxic volumes of the tumours in the repeated scans had a high correlation using the Spearman rank correlation test (r = 0.94). In a voxel-by-voxel TMR analysis between the repeated scans, the mean of Pearson correlation coefficients of individual patients was 0.65. The mean (± SD) difference of TMR in the pooled data set was 0.03 ± 0.20. CONCLUSION: Pretreatment [18F]EF5 PET/CT within one week shows high repeatability and is feasible for the guiding of hypoxia-targeted treatment interventions in HNC.

Magnetic resonance-only simulation and dose calculation in external beam radiation therapy: a feasibility study for pelvic cancers.

BACKGROUND: The clinical feasibility of using pseudo-computed tomography (pCT) images derived from magnetic resonance (MR) images for external bean radiation therapy (EBRT) planning for prostate cancer patients has been well demonstrated. This paper investigates the feasibility of applying an MR-derived, pCT planning approach to additional types of cancer in the pelvis. MATERIAL AND METHODS: Fifteen patients (five prostate cancer patients, five rectal cancer patients, and five gynecological cancer patients) receiving EBRT at Turku University Hospital (Turku, Finland) were included in the study. Images from an MRCAT (Magnetic Resonance for Calculating ATtenuation, Philips, Vantaa, Finland) pCT method were generated as a part of a clinical MR-simulation procedure. Dose calculation accuracy was assessed by comparing the pCT-based calculation with a CT-based calculation. In addition, the degree of geometric accuracy was studied. RESULTS: The median relative difference of PTV mean dose between CT and pCT images was within 0.8% for all tumor types. When assessing the tumor site-specific accuracy, the median [range] relative dose differences to the PTV mean were 0.7 [-0.11;1.05]% for the prostate cases, 0.3 [-0.25;0.57]% for the rectal cases, and 0.09 [-0.69;0.25]% for the gynecological cancer cases. System-induced geometric distortion was measured to be less than 1 mm for all PTV volumes and the effect on the PTV median dose was less than 0.1%. CONCLUSIONS: According to the comparison, using pCT for clinical EBRT planning and dose calculation in the three investigated types of pelvic cancers is feasible. Further studies are required to demonstrate the applicability to a larger cohort of patients.

¹¹C-ORM-13070, a novel PET ligand for brain α2C-adrenoceptors: radiometabolism, plasma pharmacokinetics, whole-body distribution and radiation dosimetry in healthy men.

PURPOSE: (11)C-labelled 1-[(S)-1-(2,3-dihydrobenzo[1,2]dioxin-2-yl)methyl]-4-(3-methoxy-methylpyridin-2-yl)-piperazine ((11)C-ORM-13070) is a novel PET tracer for imaging of α2C-adrenoceptors in the human brain. Brain α2C-adrenoceptors may be therapeutic targets in several neuropsychiatric disorders, including depression, schizophrenia and Alzheimer's disease. To validate the use of (11)C-ORM-13070 in humans, we investigated its radiometabolism, pharmacokinetics, whole-body distribution and radiation dose. METHODS: Radiometabolism was studied in a test-retest setting in six healthy men. After intravenous injection of (11)C-ORM-13070, blood samples were drawn over 60 min. Plasma samples were analysed by radio-HPLC for intact tracer and its radioactive metabolites. Metabolite-corrected plasma time-activity curves were used for calculation of pharmacokinetics. In a separate group of 12 healthy men, the whole-body distribution of (11)C-ORM-13070 and radiation exposure were investigated by dynamic PET/CT imaging without blood sampling. RESULTS: Two radioactive metabolites of (11)C-ORM-13070 were detected in human arterial plasma. The proportion of unchanged (11)C-ORM-13070 decreased from 81 ± 4 % of total radioactivity at 4 min after tracer injection to 23 ± 4 % at 60 min. At least one of the radioactive metabolites penetrated into red blood cells, while the parent tracer remained in plasma. The apparent elimination rate constant and corresponding half-life of unchanged (11)C-ORM-13070 in arterial plasma were 0.0117 ± 0.0056 min(-1) and 73.6 ± 35.8 min, respectively. The organs with the highest absorbed doses were the liver (12 µSv/MBq), gallbladder wall (12 µSv/MBq) and pancreas (9.1 µSv/MBq). The mean effective dose was 3.9 µSv/MBq, with a range of 3.6 - 4.2 µSv/MBq. CONCLUSION: (11)C-ORM-13070 was rapidly metabolized in human subjects after intravenous injection. The effective radiation dose of (11)C-ORM-13070 was in the same range as that of other (11)C-labelled brain receptor tracers. An injection of 500 MBq of (11)C-ORM-13070 would expose a subject to 2.0 mSv of radiation. This supports the use of (11)C-ORM-13070 in repeated PET scans, for example, in receptor occupancy trials with novel drug candidates.

Isocentric integration of intensity-modulated radiotherapy with electron fields improves field junction dose uniformity in postmastectomy radiotherapy.

BACKGROUND: In postmastectomy radiotherapy (PMRT), the dose coverage of the planning target volume (PTV) with additional margins, including the chest wall, supraclavicular, interpectoral, internal mammary and axillar level I-III lymph nodes, is often compromised. Electron fields may improve the medial dose coverage while maintaining organ at risk (OAR) doses at an acceptable level, but at the cost of hot and cold spots at the electron and photon field junction. To improve PMRT dose coverage and uniformity, an isocentric technique combining tangential intensity-modulated (IM)RT fields with one medial electron field was implemented. MATERIAL AND METHODS: For 10 postmastectomy patients isocentric IMRT with electron plans were created and compared with a standard electron/photon mix and a standard tangent technique. PTV dose uniformity was evaluated based on the tolerance range (TR), i.e. the ratio of the standard deviation to the mean dose, a dice similarity coefficient (DSC) and the 90% isodose coverage and the hot spot volumes. OAR and contralateral breast doses were also recorded. RESULTS: IMRT with electrons significantly improved the PTV dose homogeneity and conformity based on the TR and DSC values when compared with the standard electron/photon and tangent technique (p < 0.02). The 90% isodose coverage improved to 86% compared with 82% and 80% for the standard techniques (p < 0.02). Compared with the standard electron/photon mix, IMRT smoothed the dose gradient in the electron and photon field junction and the volumes receiving a dose of 110% or more were reduced by a third. For all three strategies, the OAR and contralateral breast doses were within clinically tolerable limits. CONCLUSION: Based on these results two-field IMRT combined with an electron field is a suitable strategy for PMRT.

68Ga-Citrate PET of Healthy Men: Whole-Body Biodistribution Kinetics and Radiation Dose Estimates.

68Ga-citrate has one of the simplest chemical structures of all 68Ga-radiopharmaceuticals, and its clinical use is justified by the proven medical applications using its isotope-labeled compound 67Ga-citrate. To support broader application of 68Ga-citrate in medical diagnosis, further research is needed to gain clinical data from healthy volunteers. In this work, we studied the biodistribution of 68Ga-citrate and subsequent radiation exposure from it in healthy men. Methods: 68Ga-citrate was prepared with an acetone-based radiolabeling procedure compliant with good manufacturing practices. Six healthy men (age 41 ± 12 y, mean ± SD) underwent sequential whole-body PET/CT scans after an injection of 204 ± 8 MBq of 68Ga-citrate. Serial arterialized venous blood samples were collected during PET imaging, and the radioactivity concentration was measured with a γ-counter. Urinary voids were collected and measured. The MIRD bladder-voiding model with a 3.5-h voiding interval was used. A model using a 70-kg adult man and the MIRD schema was used to estimate absorbed doses in target organs and effective doses. Calculations were performed using OLINDA/EXM software, version 2.0. Results: Radioactivity clearance from the blood was slow, and relatively high radioactivity concentrations were observed over the whole of the 3-h measuring period. Although radioactivity excretion via urine was rather slow (biologic half-time, 69 ± 24 h), the highest decay-corrected concentrations in urinary bladder contents were measured at the 90- and 180-min time points. Moderate concentrations were also seen in kidneys, liver, and spleen. The source organs showing the largest residence times were muscle, liver, lung, and heart contents. The heart wall received the highest absorbed dose, 0.077 ± 0.008 mSv/MBq. The mean effective dose (International Commission on Radiological Protection publication 103) was 0.021 ± 0.001 mSv/MBq. Conclusion: PET imaging with 68Ga-citrate is associated with modest radiation exposure. A 200-MBq injection of 68Ga-citrate results in an effective radiation dose of 4.2 mSv, which is in the same range as other 68Ga-labeled tracers. This suggests the feasibility of clinical studies using 68Ga-citrate imaging in humans and the possibility of performing multiple scans in the same subjects across the course of a year.

Recurrence of head and neck squamous cell carcinoma in relation to high-risk treatment volume.

BACKGROUND: Locoregional recurrence remains a major cause of failure in head and neck squamous cell carcinoma (HNSCC). Human papilloma virus (HPV)-associated HNSCCs generally have a good prognosis but may recur even after standard photon radiotherapy (RT). Another incentive in observing patterns of recurrence is increased use of highly conformal techniques such as proton therapy. We therefore studied geographic distribution of recurrent tumors in relation to the high-risk treatment volume in a cohort of patients with HNSCC receiving combined modality therapy. METHODS: Medical records of 508 patients diagnosed with HNSCC in 2010-2015 were reviewed. We identified a subgroup that had local and/or regional recurrence at hybrid positron emission tomography (PET)/computed tomography (CT) and/or magnetic resonance imaging (MRI). We adapted p16 as a surrogate marker for HPV-positivity and only patients with known p16 status were eligible for a detailed analysis where recurrent tumor was copied on the planning CT and the dose received by the recurrent tumor volume was determined using dose-volume histograms. RESULTS: Twenty-five patients who had received either cisplatin (n = 23) or cetuximab-enhanced (n = 2) RT were identified. 31 locoregional recurrent tumors were detected among 18 p16 negative and 7 p16 positive patients. Of recurrent tumors 14 (45%) were classified as in-field, 5 (16%) as marginal miss, and 12 (39%) as true miss. p16 positive patients had 4 in-field, 2 marginal, and 1 true miss. By contrast, p16 negative patients had 10 in-field, 3 marginal, and 11 true miss recurrences. CONCLUSIONS: Both p16 positive and negative HNSCC recur in high-risk treatment volume despite the common view of high radiosensitivity of the former. Biomarkers predicting radioresistance should be characterized in p16 positive tumors before widely embarking on de-escalated CRT protocols. Another concern is how to decrease the number of true or marginal misses in p16 negative cases despite multimodality imaging-based target delineation.

First-in-Humans Study of 68Ga-DOTA-Siglec-9, a PET Ligand Targeting Vascular Adhesion Protein 1.

Sialic acid-binding immunoglubulinlike lectin 9 (Siglec-9) is a ligand of vascular adhesion protein 1. A 68Ga-labeled peptide of Siglec-9, 68Ga-DOTA-Siglec-9, holds promise as a novel PET tracer for imaging of inflammation. This first-in-humans study investigated the safety, tolerability, biodistribution, and radiation dosimetry of this radiopharmaceutical. Methods: Six healthy men underwent dynamic whole-body PET/CT. Serial venous blood samples were drawn from 1 to 240 min after intravenous injection of 162 ± 4 MBq of 68Ga-DOTA-Siglec-9. In addition to γ-counting, the plasma samples were analyzed by high-performance liquid chromatography to detect intact tracer and radioactive metabolites. Radiation doses were calculated using the OLINDA/EXM software, version 2.2. In addition, a patient with early rheumatoid arthritis was studied with both 68Ga-DOTA-Siglec-9 and 18F-FDG PET/CT to determine the ability of the new tracer to detect arthritis. Results:68Ga-DOTA-Siglec-9 was well tolerated by all subjects. 68Ga-DOTA-Siglec-9 was rapidly cleared from the blood circulation, and several radioactive metabolites were detected. The organs with the highest absorbed doses were the urinary bladder wall (0.38 mSv/MBq) and kidneys (0.054 mSv/MBq). The mean effective dose was 0.022 mSv/MBq (range, 0.020-0.024 mSv/MBq). Most importantly, however, 68Ga-DOTA-Siglec-9 was comparable to 18F-FDG in detecting arthritis. Conclusion: Intravenous injection of 68Ga-DOTA-Siglec-9 was safe and biodistribution was favorable for testing of the tracer in larger group of patients with rheumatoid arthritis, as is planned for the next phase of clinical trials. The effective radiation dose of 68Ga-DOTA-Siglec-9 was within the same range as the effective radiation doses of other 68Ga-labeled tracers. Injection of 150 MBq of 68Ga-DOTA-Siglec-9 would expose a subject to 3.3 mSv. These findings support the possible repeated clinical use of 68Ga-DOTA-Siglec-9, such as in trials to elucidate the treatment efficacy of novel drug candidates.

MRI-based IMPT planning for prostate cancer.

PURPOSE: Treatment planning for proton therapy requires the relative proton stopping power ratio (RSP) information of the patient for accurate dose calculations. RSP are conventionally obtained after mapping of the Hounsfield units (HU) from a calibrated patient computed tomography (CT). One or multiple CT are needed for a given treatment which represents additional, undesired dose to the patient. For prostate cancer, magnetic resonance imaging (MRI) scans are the gold standard for segmentation while offering dose-less imaging. We here quantify the clinical applicability of converted MR images as a substitute for intensity modulated proton therapy (IMPT) treatment of the prostate. METHODS: MRCAT (Magnetic Resonance for Calculating ATtenuation) is a Philips-developed technology which produces a synthetic CT image consisting of five HU from a specific set of MRI acquisitions. MRCAT and original planning CT data sets were obtained for ten patients. An IMPT plan was generated on the MRCAT for each patient. Plans were produced such that they fulfill the prostate protocol in use at Massachusetts General Hospital (MGH). The plans were then recomputed onto the nominal planning CT for each patient. Robustness analyses (±5 mm setup shifts and ±3.5 % range uncertainties) were also performed. RESULTS: Comparison of MRCAT plans and their recomputation onto the planning CT plan showed excellent agreement. Likewise, dose perturbations due to setup shifts and range uncertainties were well within clinical acceptance demonstrating the clinical viability of the approach. CONCLUSIONS: This work demonstrate the clinical acceptability of substituting MR converted RSP images instead of CT for IMPT planning of prostate cancer. This further translates into higher contouring accuracy along with lesser imaging dose.

Validation of automated magnetic resonance image segmentation for radiation therapy planning in prostate cancer.

BACKGROUND AND PURPOSE: Magnetic resonance imaging (MRI) is increasingly used in radiation therapy planning of prostate cancer (PC) to reduce target volume delineation uncertainty. This study aimed to assess and validate the performance of a fully automated segmentation tool (AST) in MRI based radiation therapy planning of PC. MATERIAL AND METHODS: Pelvic structures of 65 PC patients delineated in an MRI-only workflow according to established guidelines were included in the analysis. Automatic vs manual segmentation by an experienced oncologist was compared with geometrical parameters, such as the dice similarity coefficient (DSC). Fifteen patients had a second MRI within 15 days to assess repeatability of the AST for prostate and seminal vesicles. Furthermore, we investigated whether hormonal therapy or body mass index (BMI) affected the AST results. RESULTS: The AST showed high agreement with manual segmentation expressed as DSC (mean, SD) for delineating prostate (0.84, 0.04), bladder (0.92, 0.04) and rectum (0.86, 0.04). For seminal vesicles (0.56, 0.17) and penile bulb (0.69, 0.12) the respective agreement was moderate. Performance of AST was not influenced by neoadjuvant hormonal therapy, although those on treatment had significantly smaller prostates than the hormone-naïve patients (p < 0.0001). In repeat assessment, consistency of prostate delineation resulted in mean DSC of 0.89, (SD 0.03) between the paired MRI scans for AST, while mean DSC of manual delineation was 0.82, (SD 0.05). CONCLUSION: Fully automated MRI segmentation tool showed good agreement and repeatability compared with manual segmentation and was found clinically robust in patients with PC. However, manual review and adjustment of some structures in individual cases remain important in clinical use.

Quality assurance measurements of geometric accuracy for magnetic resonance imaging-based radiotherapy treatment planning.

BACKGROUND: Using magnetic resonance imaging (MRI) as the only imaging method for radiotherapy treatment planning (RTP) is becoming more common as MRI-only RTP solutions have evolved. The geometric accuracy of MR images is an essential factor of image quality when determining the suitability of MRI for RTP. The need is therefore clear for clinically feasible quality assurance (QA) methods for the geometric accuracy measurement. MATERIALS AND METHODS: This work evaluates long-term stability of geometric accuracy and the validity of a 2D geometric accuracy QA method compared to a prototype 3D method and analysis software in routine QA. The long-term follow-up measurements were conducted on one of the 1.5 T scanners over a period of 19 months using both methods. Inter-scanner variability of geometric distortions was also evaluated in three 1.5 T and three 3 T MRI scanners from a single vendor by using the prototype 3D QA method. RESULTS: The geometric accuracy of the magnetic resonance for radiotherapy (MR-RT) platform remained stable within 2 mm at distances of <250 mm from isocenter. All scanners achieved good geometric accuracy with mean geometric distortions of <1 mm at <150 mm and <2 mm at <250 mm from the isocenter. Both measurement methods provided relevant information about geometric distortions. CONCLUSIONS: Geometric distortions are often considered a limitation of MRI-only RTP. Results indicate that geometric accuracy of modern scanners remain within acceptable limits by default even after many years of clinical use based on the 3D QA evaluation.

Assessment of dosimetric and positioning accuracy of a magnetic resonance imaging-only solution for external beam radiotherapy of pelvic anatomy.

BACKGROUND AND PURPOSE: The clinical feasibility of synthetic computed tomography (sCT) images derived from magnetic resonance imaging (MRI) images for external beam radiation therapy (EBRT) planning have been studied and adopted into clinical use recently. This paper evaluates the dosimetric and positioning performance of a sCT approach for different pelvic cancers. MATERIALS AND METHODS: Seventy-five patients receiving EBRT at Turku University Hospital (Turku, Finland) were enrolled in the study. The sCT images were generated as part of a clinical MRI-simulation procedure. Dose calculation accuracy was assessed by comparing the sCT-based calculation with a CT-based calculation. In addition, we evaluated the patient position verification accuracy for both digitally reconstructed radiograph (DRR) and cone beam computed tomography (CBCT) -based image guidance using a subset of the cohort. Furthermore, the relevance of using continuous Hounsfield unit values was assessed. RESULTS: The mean (standard deviation) relative dose difference in the planning target volume mean dose computed over various cancer groups was less than 0.2 (0.4)% between sCT and CT. Among all groups, the average minimum gamma-index pass-rates were better than 95% with a 2%/2mm gamma-criteria. The difference between sCT- and CT-DRR-based patient positioning was less than 0.3 (1.4) mm in all directions. The registrations of sCT to CBCT produced similar results as compared with CT to CBCT registrations. CONCLUSIONS: The use of sCT for clinical EBRT dose calculation and patient positioning in the investigated types of pelvic cancers was dosimetrically and geometrically accurate for clinical use.

Dosimetric Comparison and Evaluation of 4 Stereotactic Body Radiotherapy Techniques for the Treatment of Prostate Cancer.

PURPOSE: The aim of this study was to compare dosimetric characteristics, monitor unit, and delivery efficiency of 4 different stereotactic body radiotherapy techniques for the treatment of prostate cancer. METHODS: This study included 8 patients with localized prostate cancer. Dosimetric assets of 4 delivery techniques for stereotactic body radiotherapy were evaluated: robotic CyberKnife, noncoplanar intensity-modulated radiotherapy, and 2 intensity-modulated arc therapy techniques (RapidArc and Elekta volumetric-modulated arc therapy). All the plans had equal treatment margins and a prescription dose of 35 Gy in 5 fractions. RESULTS: Statistically significant differences were observed in homogeneity index and mean doses of bladder wall and penile bulb, all of which were highest with CyberKnife. No significant differences were observed in the mean doses of rectum, with values of 15.2 ± 2.6, 13.3 ± 2.6, 13.1 ± 2.8, and 13.8 ± 1.6 Gy with CyberKnife, RapidArc, volumetric-modulated arc therapy, and noncoplanar intensity-modulated radiotherapy, respectively. The highest dose conformity was realized with RapidArc. The dose coverage of the planning target volume was lowest with noncoplanar intensity-modulated radiotherapy. Treatment times and number of monitor units were largest with CyberKnife (on average 34.0 ± 5.0 minutes and 8704 ± 1449 monitor units) and least with intensity-modulated arc therapy techniques (on average 5.1 ± 1.1 minutes and 2270 ± 497 monitor units). CONCLUSION: Compared to CyberKnife, the RapidArc, volumetric-modulated arc therapy, and noncoplanar intensity-modulated radiotherapy produced treatment plans with similar dosimetric quality, with RapidArc achieving the highest dose conformity. Overall, the dosimetric differences between the studied techniques were marginal, and thus, the choice of the technique should rather focus on the delivery accuracies and dose delivery times.

Radiotherapy volume delineation using dynamic [18F]-FDG PET/CT imaging in patients with oropharyngeal cancer: a pilot study.

PURPOSE: Delineation of gross tumour volume in 3D is a critical step in the radiotherapy (RT) treatment planning for oropharyngeal cancer (OPC). Static [18F]-FDG PET/CT imaging has been suggested as a method to improve the reproducibility of tumour delineation, but it suffers from low specificity. We undertook this pilot study in which dynamic features in time-activity curves (TACs) of [18F]-FDG PET/CT images were applied to help the discrimination of tumour from inflammation and adjacent normal tissue. METHODS: Five patients with OPC underwent dynamic [18F]-FDG PET/CT imaging in treatment position. Voxel-by-voxel analysis was performed to evaluate seven dynamic features developed with the knowledge of differences in glucose metabolism in different tissue types and visual inspection of TACs. The Gaussian mixture model and K-means algorithms were used to evaluate the performance of the dynamic features in discriminating tumour voxels compared to the performance of standardized uptake values obtained from static imaging. RESULTS: Some dynamic features showed a trend towards discrimination of different metabolic areas but lack of consistency means that clinical application is not recommended based on these results alone. CONCLUSIONS: Impact of inflammatory tissue remains a problem for volume delineation in RT of OPC, but a simple dynamic imaging protocol proved practicable and enabled simple data analysis techniques that show promise for complementing the information in static uptake values.

Somatostatin receptor subtype 2 in high-grade gliomas: PET/CT with (68)Ga-DOTA-peptides, correlation to prognostic markers, and implications for targeted radiotherapy.

BACKGROUND: High-grade gliomas (HGGs) express somatostatin receptors (SSTR), rendering them candidates for peptide receptor radionuclide therapy (PRRT). Our purpose was to evaluate the potential of (68)Ga-DOTA-1-Nal(3)-octreotide ((68)Ga-DOTANOC) or (68)Ga-DOTA-Tyr(3)-octreotide ((68)Ga-DOTATOC) to target SSTR subtype 2 (SSTR2) in HGGs, and to study the association between SSTR2 expression and established biomarkers. METHODS: Twenty-seven patients (mean age 52 years) with primary or recurrent HGG prospectively underwent (68)Ga-DOTA-peptide positron emission tomography/computed tomography (PET/CT) before resection. Maximum standardized uptake values (SUVmax) and receptor binding potential (BP) were calculated on PET/CT and disruption of blood-brain barrier (BBB) from contrast-enhanced T1-weighted magnetic resonance imaging (MRI-T1-Gad). Tumor volume concordance between PET and MRI-T1-Gad was assessed by Dice similarity coefficient (DC) and correlation by Spearman's rank. Immunohistochemically determined SSTR2 status was compared to receptor imaging findings, prognostic biomarkers, and survival with Kruskal-Wallis, Pearson chi-square, and multivariate Cox regression, respectively. RESULTS: All 19 HGGs with disrupted BBB demonstrated tracer uptake. Tumor SUVmax (2.25 ± 1.33) correlated with MRI-T1-Gad (r = 0.713, P = 0.001) although DC 0.41 ± 0.19 suggested limited concordance. SSTR2 immunohistochemistry was regarded as positive in nine HGGs (32%) but no correlation with SUVmax or BP was found. By contrast, SSTR2 expression was associated with IDH1 mutation (P = 0.007), oligodendroglioma component (P = 0.010), lower grade (P = 0.005), absence of EGFR amplification (P = 0.021), and longer progression-free survival (HR 0.161, CI 0.037 to 0.704, P = 0.015). CONCLUSIONS: In HGGs, uptake of (68)Ga-DOTA-peptides is associated with disrupted BBB and cannot be predicted by SSTR2 immunohistochemistry. Thus, PET/CT shows limited value to detect HGGs suitable for PRRT. However, high SSTR2 expression portends favorable outcome along with established biomarkers such as IDH1 mutation. TRIAL REGISTRATION: ClinicalTrials.gov NCT01460706.

Plasma pharmacokinetics, whole-body distribution, metabolism, and radiation dosimetry of 68Ga bombesin antagonist BAY 86-7548 in healthy men.

UNLABELLED: This first-in-human study investigated the safety, tolerability, metabolism, pharmacokinetics, biodistribution, and radiation dosimetry of (68)Ga-bombesin antagonist (68)Ga-DOTA-4-amino-1-carboxymethylpiperidine-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (BAY 86-7548). METHODS: Five healthy men underwent dynamic whole-body PET/CT after an intravenous injection of BAY 86-7548 (138 ± 5 MBq). Besides total radioactivity, plasma samples were analyzed by radio-high-performance liquid chromatography for metabolism of the tracer. Dosimetry was calculated using the OLINDA/EXM software. RESULTS: Three radioactive plasma metabolites were detected. The proportion of unchanged BAY 86-7548 decreased from 92% ± 9% at 1 min after injection to 19% ± 2% at 65 min. The organs with the highest absorbed doses were the urinary bladder wall (0.62 mSv/MBq) and the pancreas (0.51 mSv/MBq). The mean effective dose was 0.051 mSv/MBq. BAY 86-7548 was well tolerated by all subjects. CONCLUSION: Intravenously injected BAY 86-7548 is safe, and rapid metabolism is demonstrated. A 150-MBq injection of BAY 86-7548 results in an effective dose of 7.7 mSv, which could be reduced to 5.7 mSv with frequent bladder voids.

Preclinical evaluation of a radioiodinated fully human antibody for in vivo imaging of vascular adhesion protein-1-positive vasculature in inflammation.

UNLABELLED: Vascular adhesion protein-1 (VAP-1) is an endothelial glycoprotein mediating leukocyte trafficking from blood to sites of inflammation. BTT-1023 is a fully human monoclonal anti-VAP-1 antibody developed to treat inflammatory diseases. In this study, we preclinically evaluated radioiodinated BTT-1023 for inflammation imaging. METHODS: Rabbits were intravenously injected with radioiodinated BTT-1023. Distribution and pharmacokinetics were assessed by PET/CT up to 72 h after injection. Human radiation dose estimates for (124)I-BTT-1023 were extrapolated. Additionally, rabbits with chemically induced synovitis were imaged with (123)I-BTT-1023 SPECT/CT. RESULTS: Radioiodinated BTT-1023 cleared rapidly from blood circulation and distributed to liver and thyroid. Inflamed joints were delineated by SPECT/CT. The estimated human effective dose due to (124)I-BTT-1023 was 0.55 mSv/MBq, if blockage of thyroid uptake is assumed. CONCLUSION: The radioiodinated BTT-1023 was able to detect mild inflammation in vivo. Clinical (124)I-BTT-1023 PET studies with injected radioactivity of 0.5-0.7 MBq/kg may be justified.