Multi-centre evaluation of variation in cumulative dose assessment in reirradiation scenarios.

BACKGROUND AND PURPOSE: Safe reirradiation relies on assessment of cumulative doses to organs at risk (OARs) across multiple treatments. Different clinical pathways can result in inconsistent estimates. Here, we quantified the consistency of cumulative dose to OARs across multi-centre clinical pathways. MATERIAL AND METHODS: We provided DICOM planning CT, structures and doses for two reirradiation cases: head & neck (HN) and lung. Participants followed their standard pathway to assess the cumulative physical and EQD2 doses (with provided α/ß values), and submitted DVH metrics and a description of their pathways. Participants could also submit physical dose distributions from Course 1 mapped onto the CT of Course 2 using their best available tools. To assess isolated impact of image registrations, a single observer accumulated each submitted spatially mapped physical dose for every participating centre. RESULTS: Cumulative dose assessment was performed by 24 participants. Pathways included rigid (n = 15), or deformable (n = 5) image registration-based 3D dose summation, visual inspection of isodose line contours (n = 1), or summation of dose metrics extracted from each course (n = 3). Largest variations were observed in near-maximum cumulative doses (25.4 - 41.8 Gy for HN, 2.4 - 33.8 Gy for lung OARs), with lower variations in volume/dose metrics to large organs. A standardised process involving spatial mapping of the first course dose to the second course CT followed by summation improved consistency for most near-maximum dose metrics in both cases. CONCLUSION: Large variations highlight the uncertainty in reporting cumulative doses in reirradiation scenarios, with implications for outcome analysis and understanding of published doses. Using a standardised workflow potentially including spatially mapped doses improves consistency in determination of accumulated dose in reirradiation scenarios.

Radiation dosimetry changes in radiotherapy treatment plans for adult patients arising from the selection of the CT image reconstruction kernel.

OBJECTIVE: The reconstruction kernel used for a CT scan strongly influences the image quality. This work investigates the changes in Hounsfield units (HUs) which can arise when altering the image reconstruction kernel for planning CT images and the associated changes in dose in the radiotherapy treatment plan if the treatment planning system (TPS) is not re-calibrated. METHODS: Head and neck, prostate and lung CT images from four centres were used. For a specific scan, the base image was acquired using the original reconstruction kernel (used when the TPS was calibrated) and the treatment plan produced. The treatment plan was applied to all images from the other reconstruction kernels. Differences in dose-volume metrics for the planning target volume (PTV) and organs at risk (OARs) were noted and HU differences between images measured for air, soft tissue and bone. RESULTS: HU change in soft tissue had the greatest influence on dose change. When within ±20 HU for soft tissue and ±50 HU for bone and air the dose change in the PTV and OAR was within ±0.5% and ±1% respectively. CONCLUSIONS: When imaging parameters were changed, if HU change was within ±20 HU for soft tissue and ±50 HU for bone and air, the change in the PTV and OAR doses was below 1%. ADVANCES IN KNOWLEDGE: The degree of dose change in the treatment plan with HU change is demonstrated for current TPS algorithms. This adds to the limited evidence base for recommendations on HU tolerances as a tool for radiotherapy CT protocol optimization.

Evaluation and clinical implementation of in vivo dosimetry for kV radiotherapy using radiochromic film and micro-silica bead thermoluminescent detectors.

PURPOSE: kV radiotherapy treatment calculations are based on flat, homogenous, full-scatter reference conditions. However, clinical treatments often include surface irregularities and inhomogeneities, causing uncertainty. Therefore, confirmation of actual delivered doses in vivo is valuable. The current study evaluates, and implements, radiochromic film and micro silica bead TLD for in vivo kV dosimetry. METHODS: The kV energy and dose response of EBT3 film and silica bead TLD was established and uncertainty budgets determined. In vivo dosimetry measurements were made for a consecutive series of 30 patients using the two dosimetry systems. RESULTS: Energy dependent calibration factors were required for both dosimetry systems. The standard uncertainty estimate for in vivo measurement with film was 1.7% and for beads was 1.5%. The mean measured dose was -2.1% for film and -2.6% for beads compared to prescription. Deviations up to -9% were found in cases of large surface irregularity, or with underlying air cavities or bone. Dose shielding by beads could be clinically relevant at low kV energies and superficial depths. CONCLUSIONS: Both film and beads may be used to provide in vivo verification of delivered doses in kV radiotherapy, particularly for complex situations that are not well represented by standard reference condition calculations.

A multicentre 'end to end' dosimetry audit of motion management (4DCT-defined motion envelope) in radiotherapy.

PURPOSE: External dosimetry audit is valuable for the assurance of radiotherapy quality. However, motion management has not been rigorously audited, despite its complexity and importance for accuracy. We describe the first end-to-end dosimetry audit for non-SABR (stereotactic ablative body radiotherapy) lung treatments, measuring dose accumulation in a moving target, and assessing adequacy of target dose coverage. MATERIALS AND METHODS: A respiratory motion lung-phantom with custom-designed insert was used. Dose was measured with radiochromic film, employing triple-channel dosimetry and uncertainty reduction. The host's 4DCT scan, outlining and planning techniques were used. Measurements with the phantom static and then moving at treatment delivery separated inherent treatment uncertainties from motion effects. Calculated and measured dose distributions were compared by isodose overlay, gamma analysis, and we introduce the concept of 'dose plane histograms' for clinically relevant interpretation of film dosimetry. RESULTS: 12 radiotherapy centres and 19 plans were audited: conformal, IMRT (intensity modulated radiotherapy) and VMAT (volumetric modulated radiotherapy). Excellent agreement between planned and static-phantom results were seen (mean gamma pass 98.7% at 3% 2 mm). Dose blurring was evident in the moving-phantom measurements (mean gamma pass 88.2% at 3% 2 mm). Planning techniques for motion management were adequate to deliver the intended moving-target dose coverage. CONCLUSIONS: A novel, clinically-relevant, end-to-end dosimetry audit of motion management strategies in radiotherapy is reported.

Treatment planning study of the 3D dosimetric differences between Co-60 and Ir-192 sources in high dose rate (HDR) brachytherapy for cervix cancer.

PURPOSE: To evaluate whether Co-60 is equivalent to Ir-192 for HDR cervical brachytherapy, through 3D-DVH dose comparisons in standard and optimised plans. Previous studies have only considered 2D dosimetry, point dose comparisons or identical loading. Typical treatment times and economics are considered. MATERIAL AND METHODS: Plans were produced for eight cervix patients using Co-60 and Ir-192 sources, CT imaging and IU/two-channel-ring applicator (Eckert Ziegler BEBIG). The comparison was made under two conditions: (A) identical dwell positions and loading, prescribed to Point A and (B) optimised source dwells, prescribed to HR-CTV. This provided a direct comparison of inherent differences and residual differences under typical clinical plan optimisation. The DVH (target and OAR), ICRU reference points and isodose distributions were compared. Typical treatment times and source replacement costs were compared. RESULTS: Small differences (p < 0.01) in 3D dosimetry exist when using Co-60 compared to Ir-192, prescribed to Point A with identical loading patterns, particularly 3.3% increase in rectum D2cc. No significant difference was observed in this parameter when prescribing to the HR-CTV using dwell-time optimisation. There was no statistically significant difference in D90 between the two isotopes. Co-60 plans delivered consistently higher V150% (mean +4.4%, p = 0.03) and V400% (mean +11.6%, p < 0.01) compared to Ir-192 in optimised plans. Differences in physical source properties were overwhelmed by geometric effects. CONCLUSIONS: Co-60 may be used as an effective alternative to Ir-192 for HDR cervix brachytherapy, producing similar plans of equivalent D90, but with logistical benefits. There is a small dose increase along the extension of the source axis when using Co-60 compared to Ir-192, leading to small rectal dose increases for identical loading patterns. This can be eliminated by planning optimisation techniques. Such optimisation may also be associated with increases in the overdose volume (V150-V400) with Co-60 compared to Ir-192.

Use of Maximum Intensity Projections (MIPs) for target outlining in 4DCT radiotherapy planning.

INTRODUCTION: Four-dimensional computed tomography (4DCT) is currently being introduced to radiotherapy centers worldwide, for use in radical radiotherapy planning for non-small cell lung cancer (NSCLC). A significant drawback is the time required to delineate 10 individual CT scans for each patient. Every department will hence ask the question if the single Maximum Intensity Projection (MIP) scan can be used as an alternative. Although the problems regarding the use of the MIP in node-positive disease have been discussed in the literature, a comprehensive study assessing its use has not been published. We compared an internal target volume (ITV) created using the MIP to an ITV created from the composite volume of 10 clinical target volumes (CTVs) delineated on the 10 phases of the 4DCT. METHODS: 4DCT data was collected from 14 patients with NSCLC. In each patient, the ITV was delineated on the MIP image (ITV_MIP) and a composite ITV created from the 10 CTVs delineated on each of the 10 scans in the dataset. The structures were compared by assessment of volumes of overlap and exclusion. RESULTS: There was a median of 19.0% (range, 5.5-35.4%) of the volume of ITV_10phase not enclosed by the ITV_MIP, demonstrating that the use of the MIP could result in under-treatment of disease. In contrast only a very small amount of the ITV_MIP was not enclosed by the ITV_10phase (median of 2.3%, range, 0.4-9.8%), indicating the ITV_10phase covers almost all of the tumor tissue as identified by MIP. Although there were only two Stage I patients, both demonstrated very similar ITV_10phase and ITV_MIP volumes. These findings suggest that Stage I NSCLC tumors could be outlined on the MIP alone. In Stage II and III tumors the ITV_10phase would be more reliable. CONCLUSIONS: To prevent under-treatment of disease, the MIP image can only be used for delineation in Stage I tumors.

Repeatability and reproducibility of corneal thickness measurements by optical coherence tomography.

PURPOSE: To assess the repeatability and interoperator and intersession reproducibility of central corneal thickness (CCT) measurements made by a commercially available optical coherence tomography (OCT) system. METHODS: Intersession and interoperator reproducibility and repeatability were defined according to the guidelines of the British Standards Institution and examined in a control group of 14 normal subjects. An in-house computer program was used to evaluate central corneal thickness from these scans. RESULTS: The coefficient of interoperator reproducibility was 0.18%, whereas that for intersession reproducibility was 1.11%. Wilcoxon analysis (5% level of significance) showed that there was no statistically significant difference between scans acquired during different sessions or by different operators. Coefficients of repeatability were all less than 3%. The average CCT was 526 +/- 28 microm (SD) and the range of normal CCT between 5th and 95th percentiles was 498 to 576 microm. CONCLUSIONS: Although the commercially available OCT scanner was designed for retinal imaging, with a few minor modifications, the system may be used to image the anterior segment. Previous studies have shown that OCT measurements correlate well with those from conventional techniques, and it has the added advantage of being a noncontact technique. This study further demonstrates that the OCT measurements show a high degree of repeatability and reproducibility. Thus, OCT is emerging as a promising tool for evaluation of CCT in the clinical setting.

Repeatability and reproducibility of macular thickness measurements with the Humphrey OCT system.

PURPOSE: To assess the accuracy, precision, repeatability, and reproducibility of measurements made by the Humphrey optical coherence tomography (OCT) system (Humphrey-Zeiss Medical Systems, San Leandro, CA). METHODS: The performance of the system was first investigated by scanning a test object containing an air gap of known size. Measurements were repeated with water or glycerin in the gap. In the clinical setting, macular thickness measurements were obtained from a control group of 20 normal subjects. For analysis, these scans were divided into eight sections, each containing 10 A-scans. RESULTS: The average gap thickness was found to be close to the true value in all cases. The overall coefficients of intersession reproducibility were less than 1% for the test object and 1.51% for the control group. There was no significant difference between scans acquired during different sessions. The overall coefficients of repeatability for the test object were between 0.2% and 1.1% and between 1% and 2% for the control group. The range of normal retinal thickness in terms of the 5th and 95th percentiles was 222 to 248 microm in women and 234 to 257 microm in men. CONCLUSIONS: Measurements made from OCT scans were found to be accurate and precise. Introducing water or glycerin into the test object resulted in considerable degradation of the signal, but measurements of gap thickness were still shown to be accurate, precise, reproducible, and repeatable. Retinal thickness measurements in the macular area were repeatable and reproducible. This demonstrates that OCT is a useful tool in the monitoring of patients with conditions that affect macular thickness, even when there is considerable degradation of the OCT signal.