Brachytherapy of Choroidal Melanomas Less Than 10 mm in Largest Basal Diameter: Comparison of 10-mm and 15-mm Ruthenium Plaques.

PURPOSE: To compare tumor control, vision, and complications between patients with a choroidal melanoma of <10 mm in largest basal diameter (LBD) irradiated with 10-mm or 15-mm ruthenium plaques. DESIGN: Retrospective, comparative case series. PARTICIPANTS: One hundred sixty-four consecutive patients with a choroidal melanoma of <10 mm in LBD, 76 and 88 treated with the 10-mm and 15-mm plaque, respectively, from 1998-2014 in a national ocular oncology service. METHODS: Diagnosis was based on growth or high-risk characteristics. The apical dose was 100 to 120 Gy aiming to deliver ≥250 Gy to the sclera. Plaque positioning was modeled retrospectively. An increase of ≥0.3 mm in thickness and ≥0.5 mm in LBD indicated local recurrence. Outcomes were compared with cumulative incidence analysis and Cox regression. Median follow-up time for patients still alive was 8.4 years. MAIN OUTCOME MEASURES: Recurrence rate, low vision, blindness, radiation maculopathy, and optic neuropathy. RESULTS: Melanomas treated with the 10-mm plaque were smaller (median thickness, 1.9 mm vs. 2.6 mm; LBD, 7.1 mm vs. 8.6 mm) and located closer to foveola (median, 2.0 mm vs. 2.8 mm) than those treated with the 15-mm plaque (P < 0.001). The 2 plaques provided a safety margin in 43% versus 40% eyes, provided no safety margin to guard foveola in 17% versus 33%, and did not entirely cover tumor mainly close to the disc in 32% versus 18% of eyes, respectively (P = 0.052). The incidence of a local recurrence was comparable (13% vs. 15% at 10 years; P = 0.31) and associated with plaque positioning (hazard ratio [HR], 2.81 for no safety margin; P = 0.041). At 5 years, the incidence of low vision was 14% versus 24%, and that of blindness was 3% versus 6%. Distance to the foveola was associated with loss of both levels of vision (HR, 0.65 per 1 mm vs. 0.68 per 1 mm; P ≤ 0.001 vs. P = 0.004). The incidence of radiation maculopathy was comparable (19% vs. 18% at 5 years), whereas that of optic neuropathy tended to be higher with the 15-mm plaque (2% vs. 9%; P = 0.054). CONCLUSIONS: The 10-mm ruthenium plaque contributes to better visual preservation, particularly with tumors close to fovea, without increase in local recurrence rate, and may therefore be preferable to the 15-mm plaque.

Multicellular dosimetric chain for molecular radiotherapy exemplified with dose simulations on 3D cell spheroids.

PURPOSE: Absorbed radiation dose-response relationships are not clear in molecular radiotherapy (MRT). Here, we propose a voxel-based dose calculation system for multicellular dosimetry in MRT. We applied confocal microscope images of a spherical cell aggregate i.e. a spheroid, to examine the computation of dose distribution within a tissue from the distribution of radiopharmaceuticals. METHODS: A confocal microscope Z-stack of a human hepatocellular carcinoma HepG2 spheroid was segmented using a support-vector machine algorithm and a watershed function. Heterogeneity in activity uptake was simulated by selecting a varying amount of the cell nuclei to contain 111In, 125I, or 177Lu. Absorbed dose simulations were carried out using vxlPen, a software application based on the Monte Carlo code PENELOPE. RESULTS: We developed a schema for radiopharmaceutical dosimetry. The schema utilizes a partially supervised segmentation method for cell-level image data together with a novel main program for voxel-based radiation dose simulations. We observed that for 177Lu, radiation cross-fire enabled full dose coverage even if the radiopharmaceutical had accumulated to only 60% of the spheroid cells. This effect was not found with 111In and 125I. Using these Auger/internal conversion electron emitters seemed to guarantee that only the cells with a high enough activity uptake will accumulate a lethal amount of dose, while neighboring cells are spared. CONCLUSIONS: We computed absorbed radiation dose distributions in a 3D-cultured cell spheroid with a novel multicellular dosimetric chain. Combined with pharmacological studies in different tissue models, our cell-level dosimetric calculation method can clarify dose-response relationships for radiopharmaceuticals used in MRT.

Effect of calculation method on kidney dosimetry in 177Lu-octreotate treatment.

BACKGROUND: 177Lu-octreotate is an effective treatment modality for patients with metastatic neuroendocrine tumors. The kidney is a critical dose-limiting organ in that modality. We investigated the absorbed doses in the kidney and compared whole kidney volume (WKV) and small (4 cm3) volume of the kidney (SV) methods. We also evaluated a new calculation method that was based on two single photon emission computed tomography/computed tomography (SPECT/CT) scans. METHODS: Absorbed radiation doses in the kidneys were calculated for 24 patients with neuroendocrine tumors. All patients received four cycles of 177Lu-octreotate given at eight-week intervals with a mean activity of 7.1 GBq (range 3.28-8.79 GBq). Absorbed doses and half-lives were calculated by the WKV and SV methods. Dosimetry was determined for the cortex and medulla in the first treatment cycle. RESULTS: The mean absorbed radiation dose was 0.44 ± 0.15 Gy/GBq for the WKV method and, 0.74 ± 0.28 Gy/GBq for the SV method. Three patients had a 20% increase of the absorbed dose over the four treatment cycles for the WKV method compared to eight patients for the SV method. The mean absorbed dose in the medulla was 0.62 ± 0.27 Gy/GBq, whereas the mean absorbed dose in the cortex was 0.41 ± 0.22 Gy/GBq. Both regions had similar half-lives. Patients who received lower activities for medical reasons still had similar absorbed doses to kidneys compared to those who received the full activities. Our study indicates that absorbed doses can be calculated reliably using two SPECT/CT scans, at 24 and 168 hours after each treatment. CONCLUSIONS: Absorbed doses in the kidneys from systemic radionuclide therapy that are measured by the WKV method and SV method cannot be directly compared. There were regional differences within kidneys for the uptake of 177Lu-octreotate. Two SPECT/CTs are sufficient for kidney dosimetry based on our new calculation method.

Prompt gamma and neutron detection in BNCT utilizing a CdTe detector.

In this work, a novel sensor technology based on CdTe detectors was tested for prompt gamma and neutron detection using boronated targets in (epi)thermal neutron beam at FiR1 research reactor in Espoo, Finland. Dedicated neutron filter structures were omitted to enable simultaneous measurement of both gamma and neutron radiation at low reactor power (2.5 kW). Spectra were collected and analyzed in four different setups in order to study the feasibility of the detector to measure 478 keV prompt gamma photons released from the neutron capture reaction of boron-10. The detector proved to have the required sensitivity to detect and separate the signals from both boron neutron and cadmium neutron capture reactions, which makes it a promising candidate for monitoring the spatial and temporal development of in vivo boron distribution in boron neutron capture therapy.

Limiting CT radiation dose in children with craniosynostosis: phantom study using model-based iterative reconstruction.

BACKGROUND: Medical professionals need to exercise particular caution when developing CT scanning protocols for children who require multiple CT studies, such as those with craniosynostosis. OBJECTIVE: To evaluate the utility of ultra-low-dose CT protocols with model-based iterative reconstruction techniques for craniosynostosis imaging. MATERIALS AND METHODS: We scanned two pediatric anthropomorphic phantoms with a 64-slice CT scanner using different low-dose protocols for craniosynostosis. We measured organ doses in the head region with metal-oxide-semiconductor field-effect transistor (MOSFET) dosimeters. Numerical simulations served to estimate organ and effective doses. We objectively and subjectively evaluated the quality of images produced by adaptive statistical iterative reconstruction (ASiR) 30%, ASiR 50% and Veo (all by GE Healthcare, Waukesha, WI). Image noise and contrast were determined for different tissues. RESULTS: Mean organ dose with the newborn phantom was decreased up to 83% compared to the routine protocol when using ultra-low-dose scanning settings. Similarly, for the 5-year phantom the greatest radiation dose reduction was 88%. The numerical simulations supported the findings with MOSFET measurements. The image quality remained adequate with Veo reconstruction, even at the lowest dose level. CONCLUSION: Craniosynostosis CT with model-based iterative reconstruction could be performed with a 20-µSv effective dose, corresponding to the radiation exposure of plain skull radiography, without compromising required image quality.

Lens dose in routine head CT: comparison of different optimization methods with anthropomorphic phantoms.

OBJECTIVE: The purpose of this study was to study different optimization methods for reducing eye lens dose in head CT. MATERIALS AND METHODS: Two anthropomorphic phantoms were scanned with a routine head CT protocol for evaluation of the brain that included bismuth shielding, gantry tilting, organ-based tube current modulation, or combinations of these techniques. Highsensitivity metal oxide semiconductor field effect transistor dosimeters were used to measure local equivalent doses in the head region. The relative changes in image noise and contrast were determined by ROI analysis. RESULTS: The mean absorbed lens doses varied from 4.9 to 19.7 mGy and from 10.8 to 16.9 mGy in the two phantoms. The most efficient method for reducing lens dose was gantry tilting, which left the lenses outside the primary radiation beam, resulting in an approximately 75% decrease in lens dose. Image noise decreased, especially in the anterior part of the brain. The use of organ-based tube current modulation resulted in an approximately 30% decrease in lens dose. However, image noise increased as much as 30% in the posterior and central parts of the brain. With bismuth shields, it was possible to reduce lens dose as much as 25%. CONCLUSION: Our results indicate that gantry tilt, when possible, is an effective method for reducing exposure of the eye lenses in CT of the brain without compromising image quality. Measurements in two different phantoms showed how patient geometry affects the optimization. When lenses can only partially be cropped outside the primary beam, organ-based tube current modulation or bismuth shields can be useful in lens dose reduction.

Effect of patient centering on patient dose and image noise in chest CT.

OBJECTIVE: The objective of our study was to evaluate the effect of vertical centering on dose and image noise in chest MDCT of different-sized patients using anthropomorphic phantoms and retrospectively studying examinations of clinical patients. MATERIALS AND METHODS: Three different anthropomorphic phantoms were scanned using different vertical centering (offset ± 6 cm) and were assessed with radiation dose-monitoring software. The effect of vertical positioning on the radiation dose was studied using the volume CT dose index, dose-length product, and size-specific dose estimates for different-sized phantoms. Image noise was determined from CT number histograms. Vertical positioning for chest CT examinations of 112 patients ranging from neonates to adults were retrospectively assessed. RESULTS: Radiation doses were highest when using the posteroanterior scout image for automatic exposure control (AEC) and when phantoms were set in the lowest table position, and radiation doses were lowest when phantoms were set in the uppermost table position. For the adult phantom, relative doses increased by 38% in the lowest table position and decreased by 23% in the highest table position. Similarly, doses for pediatric 5-year-old and newborn phantoms were 21% and 12% higher in the lowest table position and 12% and 8% lower in the highest table position, respectively. The effect decreased when a lateral scout image was used for AEC. The relative noise was lowest when the phantoms were properly centered and increased with vertical offset. In clinical patients, we observed offset with a median value varying from 25 to 35 mm below the isocenter. CONCLUSION: Regardless of patient size, most patients in this study were positioned too low, which negatively affected both patient dose and image noise. Miscentering was more pronounced in smaller pediatric patients.

Boron neutron capture therapy (BNCT) in Finland: technological and physical prospects after 20 years of experiences.

Boron Neutron Capture Therapy (BNCT) is a binary radiotherapy method developed to treat patients with certain malignant tumours. To date, over 300 treatments have been carried out at the Finnish BNCT facility in various on-going and past clinical trials. In this technical review, we discuss our research work in the field of medical physics to form the groundwork for the Finnish BNCT patient treatments, as well as the possibilities to further develop and optimize the method in the future. Accordingly, the following aspects are described: neutron sources, beam dosimetry, treatment planning, boron imaging and determination, and finally the possibilities to detect the efficacy and effects of BNCT on patients.