Prompt gamma ray detection and imaging for boron neutron capture therapy using CdTe detector and novel detector shield - Monte Carlo study.

BACKGROUND: For boron neutron capture therapy (BNCT), the improvements in patient dosimetry will require information about the spatial variation of 10 B concentration in the tumor and critical organs. A non-invasive approach, based on the detection of prompt gamma (PG) rays from the BNC reaction, may be well-suited to obtain such information. The detectability of the BNC PG rays has been shown experimentally utilizing energy-resolving cadmium telluride (CdTe) detectors. However, the feasibility of this approach under the clinically relevant conditions of BNCT is currently unknown. PURPOSE: The present work aimed to investigate the aforementioned feasibility by performing Monte Carlo (MC) simulations under the phantom irradiation geometry relevant to accelerator-based BNCT (a-BNCT). Especially, this investigation focused on demonstrating the enhanced detection of the BNC PG rays using a novel neutron shield for CdTe detectors. Upon demonstrating the efficacy of the proposed detector shield, the BNC PG ray-based quantitative imaging of clinically relevant concentrations of 10 B was also demonstrated. METHODS: The Geant4 MC simulation toolkit was used to model the phantom irradiation by an epithermal neutron beam as well as the detection of the BNC PG rays from the phantom by CdTe detectors with and without the proposed gadolinium (Gd)-based detector shield. It was also used to model the BNC PG ray-based quantitative imaging of 10 B concentrations under a-BNCT scenarios. Each model included a 20 cm-diameter/24 cm-height cylindrical PMMA phantom containing 10 B inserts at various concentrations. Arrays of CdTe crystals of 5 × 5 × 1 mm3 each (up to 120 in the case of a ring detector) were modeled for acquiring the BNC PG ray signals and quantitative imaging. RESULTS: According to the MC simulations, thermalized neutrons from the phantom were found to reach the CdTe detector and captured by Cd and Te, resulting in the gamma ray background noise that directly interfered with the BNC PG ray signal. The proposed Gd-based detector shield was found to be highly effective in shielding thermal neutrons from the phantom, thereby reducing the unwanted gamma ray background noise. Owing to this shield, the detection of as low as seven parts-per-million (ppm) of 10 B within the phantom of clinically relevant size was possible using 20 billion incident neutron histories. Furthermore, quantitative imaging of 10 B distributed at low concentration (down to 50 ppm) within the phantom was demonstrated using computed tomography (CT) simulations with 16 billion incident neutron histories per angular projection. The 10 B detection limit (7.5 ppm) was also estimated using the reconstructed CT image. Both 10 B detection limits determined from this investigation are deemed clinically relevant for BNCT. CONCLUSIONS: The proposed Gd-based detector shield played an essential role for achieving the currently reported 10 B detection limits. Overall, the present MC simulation work demonstrated highly sensitive BNC PG ray detection and imaging under a-BNCT scenarios using CdTe detectors coupled with a novel detector shield.

Effect of boron compounds on the biological effectiveness of proton therapy.

PURPOSE: We assessed whether adding sodium borocaptate (BSH) or 4-borono-l-phenylalanine (BPA) to cells irradiated with proton beams influenced the biological effectiveness of those beams against prostate cancer cells to investigate if the alpha particles generated through proton-boron nuclear reactions would be sufficient to enhance the biological effectiveness of the proton beams. METHODS: We measured clonogenic survival in DU145 cells treated with 80.4-ppm BSH or 86.9-ppm BPA, or their respective vehicles, after irradiation with 6-MV X-rays, 1.2-keV/µm (low linear energy transfer [LET]) protons, or 9.9-keV/µm (high-LET) protons. We also measured γH2AX and 53BP1 foci in treated cells at 1 and 24 h after irradiation with the same conditions. RESULTS: We found that BSH radiosensitized DU145 cells across all radiation types. However, no difference was found in relative radiosensitization, characterized by the sensitization enhancement ratio or the relative biological effectiveness, for vehicle- versus BSH-treated cells. No differences were found in numbers of γH2AX or 53BP1 foci or γH2AX/53BP1 colocalized foci for vehicle- versus BSH-treated cells across radiation types. BPA did not radiosensitize DU145 cells nor induced any significant differences when comparing vehicle- versus BPA-treated cells for clonogenic cell survival or γH2AX and 53BP1 foci or γH2AX/53BP1 colocalized foci. CONCLUSIONS: Treatment with 11 B, at concentrations of 80.4 ppm from BSH or 86.9 ppm from BPA, had no effect on the biological effectiveness of proton beams in DU145 prostate cancer cells. Our results agree with published theoretical calculations indicating that the contribution of alpha particles from such reactions to the total absorbed dose and biological effectiveness is negligible. We also found that BSH radiosensitized DU145 cells to X-rays, low-LET protons, and high-LET protons but that the radiosensitization was not related to DNA damage.

A new MLC segmentation algorithm/software for step-and-shoot IMRT delivery.

We present a new MLC segmentation algorithm/software for step-and-shoot IMRT delivery. Our aim in this work is to shorten the treatment time by minimizing the number of segments. Our new segmentation algorithm, called SLS (an abbreviation for static leaf sequencing), is based on graph algorithmic techniques in computer science. It takes advantage of the geometry of intensity maps. In our SLS approach, intensity maps are viewed as three-dimensional (3-D) "mountains" made of unit-sized "cubes." Such a 3-D "mountain" is first partitioned into special-structured submountains using a new mixed partitioning scheme. Then the optimal leaf sequences for each submountain are computed by either a shortest-path algorithm or a maximum-flow algorithm based on graph models. The computations of SLS take only a few minutes. Our comparison studies of SLS with CORVUS (both the 4.0 and 5.0 versions) and with the Xia and Verhey segmentation methods on Elekta Linac systems showed substantial improvements. For instance, for a pancreatic case, SLS used only one-fifth of the number of segments required by CORVUS 4.0 to create the same intensity maps, and the SLS sequences took only 25 min to deliver on an Elekta SL 20 Linac system in contrast to the 72 min for the CORVUS 4.0 sequences (a three-fold improvement). To verify the accuracy of our new leaf sequences, we conducted film and ion-chamber measurements on phantom. The results showed that both the intensity distributions as well as dose distributions of the SLS delivery match well with those of CORVUS delivery. SLS can also be extended to other types of Linac systems.