Evaluation of performance of an accelerator-based BNCT facility for the treatment of different tumor targets

Out-of-field dosimetry using a validated PHITS model and computational phantom in clinical BNCT

BACKGROUND

The out-of-field radiation dose for boron neutron capture therapy (BNCT), which results from both neutrons and γ-rays, has not been extensively evaluated. To safely perform BNCT, the neutron and γ-ray distributions inside the treatment room and the whole-body dose should be evaluated during commissioning. Although, certain previous studies have evaluated the whole-body dose in the clinical research phase, no institution providing BNCT covered by health insurance has yet validated the neutron distribution inside the room and the whole-body dose.

PURPOSE

To validate the Monte Carlo model of the BNCT irradiation room extended for the whole-body region and evaluate organ-at-risk (OAR) doses using the validated model with a human-body phantom.

METHODS

First, thermal neutron distribution inside the entire treatment room was measured by placing Au samples on the walls of the treatment room. Second, neutron and gamma-ray dose-rate distributions inside a human-body water phantom were measured. Both lying and sitting positions were considered. Bare Au, Au covered by Cd (Au+Cd), In, Al, and thermoluminescent dosimeters were arranged at 11 points corresponding to locations of the OARs inside the phantom. After the irradiation, γ-ray peaks emitted from the samples were measured by a high-purity germanium detector. The measured counts were converted to the reaction rate per unit charge of the sample. These measurements were compared with results of simulations performed with the Particle and Heavy Ion Transport code System (PHITS). A male adult mesh-type reference computational phantom was used to evaluate OAR doses in the whole-body region. The relative biological effectiveness (RBE)-weighted doses and dose-volume histograms (DVHs) for each OAR were evaluated. The median dose (D50% ) and near-maximum dose (D2% ) were evaluated for 14 OARs in a 1-h-irradiation process. The evaluated RBE-weighted doses were converted to equivalent doses in 2 Gy fractions.

RESULTS

Experimental results within 60 cm from the irradiation center agreed with simulation results within the error bars except at ±20, 30 cm, and those over 70 cm corresponded within one digit. The experimental results of reaction rates or γ-ray dose rate for lying and sitting positions agreed well with the simulation results within the error bars at 8, 4, 11, 7 and 7, 4, 7, 6, 5, 6 out of 11 points, respectively, for Au, Au+Cd, In, Al, and TLD. Among the detectors, the discrepancies in reaction rates between experiment and simulation were most common for Au+Cd, but were observed randomly for measurement points (brain, lung, etc.). The experimental results of γ-ray dose rates were systematically lower than simulation results at abdomen and waist regions for both positions. Extending the PHITS model to the whole-body region resulted in higher doses for all OARs, especially 0.13 Gy-eq increase for D50% of the left salivary gland.

CONCLUSION

The PHITS model for clinical BNCT for the whole-body region was validated, and the OAR doses were then evaluated. Clinicians and medical physicists should know that the out-of-field radiation increases the OAR dose in the whole-body region.

Comparison of Conventional and Radiomic Features between 18F-FBPA PET/CT and PET/MR

Boron-10-containing positron emission tomography (PET) radio-tracer, 18F-FBPA, has been used to evaluate the feasibility and treatment outcomes of Boron neutron capture therapy (BNCT). The clinical use of PET/MR is increasing and reveals its benefit in certain applications. However, the PET/CT is still the most widely used modality for daily PET practice due to its high quantitative accuracy and relatively low cost. Considering the different attenuation correction maps between PET/CT and PET/MR, comparison of derived image features from these two modalities is critical to identify quantitative imaging biomarkers for diagnosis and prognosis. This study aimed to investigate the comparability of image features extracted from 18F-FBPA PET/CT and PET/MR. A total of 15 patients with malignant brain tumor who underwent 18F-FBPA examinations using both PET/CT and PET/MR on the same day were retrospectively analyzed. Overall, four conventional imaging characteristics and 449 radiomic features were calculated from PET/CT and PET/MR, respectively. A linear regression model and intraclass correlation coefficient (ICC) were estimated to evaluate the comparability of derived features between two modalities. Features were classified into strong, moderate, and weak comparability based on coefficient of determination (r2) and ICC. All of the conventional features, 81.2% of histogram, 37.5% of geometry, 51.5% of texture, and 25% of wavelet-based features, showed strong comparability between PET/CT and PET/MR. With regard to the wavelet filtering, radiomic features without filtering (61.2%) or with low-pass filtering (59.2%) along three axes produced strong comparability between the two modalities. However, only 8.2% of the features with high-pass filtering showed strong comparability. The linear regression models were provided for the features with strong and moderate consensus to interchange the quantitative features between the PET/CT and the PET/MR. All of the conventional and 71% of the radiomic (mostly histogram and texture) features were sufficiently stable and could be interchanged between 18F-FBPA PET with different hybrid modalities using the proposed equations. Our findings suggested that the image features high interchangeability may facilitate future studies in comparing PET/CT and PET/MR.

A Novel Approach to Design and Evaluate BNCT Neutron Beams Combining Physical, Radiobiological, and Dosimetric Figures of Merit

(1) Background:The quality of neutron beams for Boron Neutron Capture Therapy (BNCT) is currently defined by its physical characteristics in air. Recommendations exist to define whether a designed beam is useful for clinical treatment. This work presents a new way to evaluate neutron beams based on their clinical performance and on their safety, employing radiobiological quantities. (2) Methods: The case study is a neutron beam for deep-seated tumors from a 5 MeV proton beam coupled to a beryllium target. Physical Figures of Merit were used to design five beams; however, they did not allow a clear ranking of their quality in terms of therapeutic potential. The latter was then evaluated based on in-phantom dose distributions and on the calculation of the Uncomplicated Tumor Control Probability (UTCP). The safety of the beams was also evaluated calculating the in-patient out-of-beam dosimetry. (3) Results: All the beams ensured a UTCP comparable to the one of a clinical beam in phantom; the safety criterion allowed to choose the best candidate. When this was tested in the treatment planning of a real patient treated in Finland, the UTCP was still comparable to the one of the clinical beam. (4) Conclusions: Even when standard physical recommendations are not met, radiobiological and dosimetric criteria demonstrate to be a valid tool to select an effective and safe beam for patient treatment.

Diffuse bone marrow uptake related to granulocyte colony-stimulating factor-producing maxillary sinus carcinoma on 4-borono-2-18F-fluoro-L-phenylalanine positron emission tomography/computed tomography

Granulocyte colony-stimulating factor (G-CSF) can be produced by tumor cells and is known to promote tumor growth, thereby potentially accelerating disease progression. Squamous cell carcinoma (SCC) at maxillary sinus is aggressive growth with poor prognosis. Maxillary sinus carcinomas are rare and can be clinically silent in the early stages or manifest with the same signs and symptoms of more common illnesses, leading to their delayed diagnosis of disease. Hypermetabolic uptake of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) but not of 4-borono-2-¹⁸F-fluoro- L-phenylalanine (¹⁸F-FBPA), in the bone marrow of patients with G-CSF-producing tumors without bone marrow involvement during positron emission tomography (PET), has been reported. The present case report describes our first experience of bone marrow uptake in PET/computed tomography examination using¹⁸F-FBPA, high uptake seen in the bone marrow of a patient with a G-CSF-secreting SCC of the maxillary sinus that it relapsed following chemoradiation therapy and surgical resection of the tumor.

Investigation of the neutron spectrum measurement method for dose evaluation in boron neutron capture therapy

In boron neutron capture therapy, it is important to evaluate the dose administered to a patient's body outside the tumour area. The exposure dose is evaluated by calculation; however, the calculated value must be validated using a measured value. The dose evaluations based on the measured neutron spectrum are investigated. Multi-foil activation, combined with a LiCaAlF₆ scintillation detector and an imaging plate, is proposed as a measurement method. The proposed method can measure the neutron spectrum at various points quickly.

Beam shaping assembly design of 7Li(p,n)7Be neutron source for boron neutron capture therapy of deep-seated tumor

The development of a medical facility for boron neutron capture therapy at Budker Institute of Nuclear Physics is under way. The neutron source is based on a tandem accelerator with vacuum insulation and lithium target. The proposed accelerator is conceived to deliver a proton beam around 10 mA at 2.3 MeV proton beam. To deliver a therapeutic beam for treatment of deep-seated tumors a typical Beam Shaping Assembly (BSA) based on the source specifications has been explored. In this article, an optimized BSA based on the ⁷Li(p,n)⁷Be neutron production reaction is proposed. To evaluate the performance of the designed beam in a phantom, the parameters and the dose profiles in tissues due to the irradiation have been considered. In the simulations, we considered a proton energy of 2.3 MeV, a current of 10 mA, and boron concentrations in tumor, healthy tissues and skin of 52.5 ppm, 15 ppm and 22.5 ppm, respectively. It is found that, for a maximum punctual healthy tissue dose seated to 11 RBE-Gy, a mean dose of 56.5 RBE Gy with a minimum of 52.2 RBE Gy can be delivered to a tumor in 40 min, where the therapeutic ratio is estimated to 5.38. All of these calculations were carried out using the Monte Carlo MCNP code.

Dynamic changes in 18F-borono-L-phenylalanine uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck and malignant melanoma during boron neutron capture therapy patient selection

Background

We evaluated dynamic changes in ¹⁸F–borono-L-phenylalanine (¹⁸F–BPA) uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck (SCC) and malignant melanoma (MM) during boron neutron capture therapy (BNCT) patient selection.

Methods

Dynamic changes in the maximum standardized uptake value (SUVmax), tumor-to-normal tissue ratio (TNR), and tumor-to-blood pool ratio (TBR) for ¹⁸F–BPA were evaluated in 20 patients with SCC and 8 patients with MM.

Results

SUVmax in SCC tumors decreased significantly from 30 to 120 min. There was a non-statistically significant decrease in SUVmax for SCC tumors from 30 to 60 min and from 60 to 120 min. Patients with MM had nonsignificant SUVmax changes in ¹⁸F–BPA uptake on delayed imaging. Nonsignificant ¹⁸F–BPA TNR and TBR changes were seen in patients with SCC and MM.

Conclusions

Dynamic changes in SUVmax for ¹⁸F–BPA uptake had a washout pattern in SCC and a persistent pattern in MM. Dynamic ¹⁸F–BPA -PET studies should be performed to investigate the pharmacokinetics of ¹⁸F–BPA in humans and select appropriate candidates who may benefit from BNCT.

Relationship between the uptake of 18F-borono-L-phenylalanine and L-[methyl-11C] methionine in head and neck tumors and normal organs

BACKGROUND AND PURPOSE

The purpose of this study was to determine the distribution of 4-borono-2-¹⁸F-fluoro-phenylalanine (¹⁸F-BPA) and L-[methyl-¹¹C] methionine (¹¹C-Met) in normal organs and tumors and to evaluate the usefulness of ¹¹C-Met/PET in screening potential candidates for boron neutron capture therapy (BNCT).

MATERIAL METHODS

Seven patients who had at least one histologically confirmed head and neck tumor were included in this study. They underwent both whole-body ¹⁸F-BPA-PET/CT and ¹¹C-Met-PET/CT within a span of 6 months. Uptake was evaluated using the maximum standardized uptake value (SUVmax). Regions of interest (ROIs) were placed within the tumors and target organs of brain, thyroid, submandibular gland, lung, liver, esophagus, stomach pancreas, spleen, muscle, and bone marrow.

RESULTS

The tumor SUVmax of FBPA and ¹¹C-Met showed strong correlation (r ² = 0.72, P = 0.015). Although ¹⁸F-BPA and ¹¹C-Met showed markedly different uptake in some organs (submandibular gland, liver, heart, stomach pancreas, spleen, and bone marrow), the uptake of ¹¹C-Met was consistently higher than that of ¹⁸F-BPA in these cases.

CONCLUSION

¹¹C-Met PET/CT might be used instead of ¹⁸F-BPA PET/CT to predict the accumulation of ¹⁰B in tumors and to select candidates for BNCT. However, it would not be suitable for evaluating accumulation in some normal organs. Therefore, the ¹⁸F-BPA-PET study remains a prerequisite for BNCT. This is the first report of the correlation between ¹⁸F-BPA and ¹¹C-Met accumulation.

A 13C(d,n)-based epithermal neutron source for Boron Neutron Capture Therapy

PURPOSE

Boron Neutron Capture Therapy (BNCT) requires neutron sources suitable for in-hospital siting. Low-energy particle accelerators working in conjunction with a neutron producing reaction are the most appropriate choice for this purpose. One of the possible nuclear reactions is ¹³C(d,n)¹⁴N. The aim of this work is to evaluate the therapeutic capabilities of the neutron beam produced by this reaction, through a 30mA beam of deuterons of 1.45MeV.

METHODS

A Beam Shaping Assembly design was computationally optimized. Depth dose profiles in a Snyder head phantom were simulated with the MCNP code for a number of BSA configurations. In order to optimize the treatment capabilities, the BSA configuration was determined as the one that allows maximizing both the tumor dose and the penetration depth while keeping doses to healthy tissues under the tolerance limits.

RESULTS

Significant doses to tumor tissues were achieved up to ∼6cm in depth. Peak doses up to 57Gy-Eq can be delivered in a fractionated scheme of 2 irradiations of approximately 1h each. In a single 1h irradiation, lower but still acceptable doses to tumor are also feasible.

CONCLUSIONS

Treatment capabilities obtained here are comparable to those achieved with other accelerator-based neutron sources, making of the ¹³C(d,n)¹⁴N reaction a realistic option for producing therapeutic neutron beams through a low-energy particle accelerator.

In vivo spatial correlation between (18)F-BPA and (18)F-FDG uptakes in head and neck cancer

BACKGROUND AND PURPOSE

Borono-2-(18)F-fluoro-phenylalanine ((18)F-BPA) has been used to estimate the therapeutic effects of boron neutron capture therapy (BNCT), while (18)F-fluorodeoxyglucose ((18)F-FDG) is the most commonly used positron emission tomography (PET) radiopharmaceutical in a routine clinical use. The aim of the present study was to evaluate spatial correlation between (18)F-BPA and (18)F-FDG uptakes using a deformable image registration-based technique.

MATERIAL AND METHODS

Ten patients with head and neck cancer were recruited from January 2014 to December 2014. All patients underwent whole-body (18)F-BPA PET/computed tomography (CT) and (18)F-FDG PET/CT within a 2-week period. For each patient, (18)F-BPA PET/CT and (18)F-FDG PET/CT images were aligned based on a deformable image registration framework. The voxel-by-voxel spatial correlation of standardized uptake value (SUV) within the tumor was analyzed.

RESULTS

Our image processing framework achieved accurate and validated registration results for each PET/CT image. In 9/10 patients, the spatial distribution of SUVs between (18)F-BPA and (18)F-FDG showed a significant, positive correlation in the tumor volume.

CONCLUSIONS

Deformable image registration-based voxel-wise analysis demonstrated a spatial correlation between (18)F-BPA and (18)F-FDG uptakes in the head and neck cancer. A tumor sub-volume with a high (18)F-FDG uptake may predict high accumulation of (18)F-BPA.

Present status of Accelerator-Based BNCT

AIM

This work aims at giving an updated report of the worldwide status of Accelerator-Based BNCT (AB-BNCT).

BACKGROUND

There is a generalized perception that the availability of accelerators installed in hospitals, as neutron sources, may be crucial for the advancement of BNCT. Accordingly, in recent years a significant effort has started to develop such machines.

MATERIALS AND METHODS

A variety of possible charged-particle induced nuclear reactions and the characteristics of the resulting neutron spectra are discussed along with the worldwide activity in suitable accelerator development.

RESULTS

Endothermic (7)Li(p,n)(7)Be and (9)Be(p,n)(9)B and exothermic (9)Be(d,n)(10)B are compared. In addition to having much better thermo-mechanical properties than Li, Be as a target leads to stable products. This is a significant advantage for a hospital-based facility. (9)Be(p,n)(9)B needs at least 4-5 MeV bombarding energy to have a sufficient yield, while (9)Be(d,n)(10)B can be utilized at about 1.4 MeV, implying the smallest possible accelerator. This reaction operating with a thin target can produce a sufficiently soft spectrum to be viable for AB-BNCT. The machines considered are electrostatic single ended or tandem accelerators or radiofrequency quadrupoles plus drift tube Linacs.

CONCLUSIONS

(7)Li(p,n)(7)Be provides one of the best solutions for the production of epithermal neutron beams for deep-seated tumors. However, a Li-based target poses significant technological challenges. Hence, Be has been considered as an alternative target, both in combination with (p,n) and (d,n) reactions. (9)Be(d,n)(10)B at 1.4 MeV, with a thin target has been shown to be a realistic option for the treatment of deep-seated lesions.

Whole-body dose evaluation with an adaptive treatment planning system for boron neutron capture therapy

Dose evaluation for out-of-field organs during radiotherapy has gained interest in recent years. A team led by University of Tsukuba is currently implementing a project for advancing boron neutron capture therapy (BNCT), along with a radiation treatment planning system (RTPS). In this study, the authors used the RTPS (the 'Tsukuba-Plan') to evaluate the dose to out-of-field organs during BNCT. Computed tomography images of a whole-body phantom were imported into the RTPS, and a voxel model was constructed for the Monte Carlo calculations, which used the Particle and Heavy Ion Transport Code System. The results indicate that the thoracoabdominal organ dose during BNCT for a brain tumour and maxillary sinus tumour was 50-360 and 120-1160 mGy-Eq, respectively. These calculations required ∼29.6 h of computational time. This system can evaluate the out-of-field organ dose for BNCT irradiation during treatment planning with patient-specific irradiation conditions.

Correlation of (18)F-BPA and (18)F-FDG uptake in head and neck cancers

BACKGROUND AND PURPOSE

The aim of this study was to compare the accumulation of 4-borono-2-(18)F-fluoro-phenylalanine ((18)F-BPA) with that of (18)F-fluorodeoxyglucose ((18)F-FDG) in head and neck cancers, and to assess the usefulness of (18)F-FDG PET for screening candidates for boron neutron capture therapy (BNCT).

MATERIAL AND METHODS

Twenty patients with pathologically proven malignant tumors of the head and neck were recruited from March 2012 to January 2014. All patients underwent both whole-body (18)F-BPA PET/CT and (18)F-FDG PET/CT within 2weeks of each other. The uptakes of (18)F-BPA and (18)F-FDG at 1h after injection were evaluated using the maximum standardized uptake value (SUVmax).

RESULTS

The accumulation of (18)F-FDG was significantly correlated with that of (18)F-BPA. The SUVmax of (18)F-FDG ⩾5.0 is considered to be suggestive of high (18)F-BPA accumulation.

CONCLUSIONS

(18)F-FDG PET might be an effective screening method performed prior to (18)F-BPA for selecting patients with head and neck cancer for treatment with BNCT.

Exploring Boron Neutron Capture Therapy for non-small cell lung cancer

Boron Neutron Capture Therapy (BNCT) is a radiotherapy that combines biological targeting and high LET radiation. It consists in the enrichment of tumour with (10)B and in the successive irradiation of the target with low energy neutrons producing charged particles that mainly cause non-repairable damages to the cells. The feasibility to treat Non Small Cells Lung Cancer (NSCLC) with BNCT was explored. This paper proposes a new approach to determine treatment plans, introducing the possibility to choose the irradiation start and duration to maximize the tumour dose. A Tumour Control Probability (TCP) suited for lung BNCT as well as other high dose radiotherapy schemes was also introduced. Treatment plans were evaluated in localized and disseminated lung tumours. Semi-ideal and real energy spectra beams were employed to assess the best energy range and the performance of non-tailored neutron sources for lung tumour treatments. The optimal neutron energy is within [500 eV-3 keV], lower than the 10 keV suggested for the treatment of deep-seated tumours in the brain. TCPs higher than 0.6 and up to 0.95 are obtained for all cases. Conclusions drawn from [Suzuki et al., Int Canc Conf J 1 (4) (2012) 235-238] supporting the feasibility of BNCT for shallow lung tumours are confirmed, however discussions favouring the treatment of deeper lesions and disseminated disease are also opened. Since BNCT gives the possibility to deliver a safe and potentially effective treatment for NSCLC, it can be considered a suitable alternative for patients with few or no treatment options.