Jongmin Cho

Quantitative investigation of physical factors contributing to gold nanoparticle-mediated proton dose enhancement.

Some investigators have shown tumor cell killing enhancement in vitro and tumor regression in mice associated with the loading of gold nanoparticles (GNPs) before proton treatments. Several Monte Carlo (MC) investigations have also demonstrated GNP-mediated proton dose enhancement. However, further studies need to be done to quantify the individual physical factors that contribute to the dose enhancement or cell-kill enhancement (or radiosensitization). Thus, the current study investigated the contributions of particle-induced x-ray emission (PIXE), particle-induced gamma-ray emission (PIGE), Auger and secondary electrons, and activation products towards the total dose enhancement. Specifically, GNP-mediated dose enhancement was measured using strips of radiochromic film that were inserted into vials of cylindrical GNPs, i.e. gold nanorods (GNRs), dispersed in a saline solution (0.3 mg of GNRs/g or 0.03% of GNRs by weight), as well as vials containing water only, before proton irradiation. MC simulations were also performed with the tool for particle simulation code using the film measurement setup. Additionally, a high-purity germanium detector system was used to measure the photon spectrum originating from activation products created from the interaction of protons and spherical GNPs present in a saline solution (20 mg of GNPs/g or 2% of GNPs by weight). The dose enhancement due to PIXE/PIGE recorded on the films in the GNR-loaded saline solution was less than the experimental uncertainty of the film dosimetry (<2%). MC simulations showed highly localized dose enhancement (up to a factor 17) in the immediate vicinity (<100 nm) of GNRs, compared with hypothetical water nanorods (WNRs), mostly due to GNR-originated Auger/secondary electrons; however, the average dose enhancement over the entire GNR-loaded vial was found to be minimal (0.1%). The dose enhancement due to the activation products from GNPs was minimal (<0.1%) as well. In conclusion, under the currently investigated conditions that are considered clinically relevant, PIXE, PIGE, and activation products contribute minimally to GNP/GNR-mediated proton dose enhancement, whereas Auger/secondary electrons contribute significantly but only at short distances (<100 nm) from GNPs/GNRs.

Radiosensitization of Prostate Cancers in Vitro and in Vivo to Erbium-filtered Orthovoltage X-rays Using Actively Targeted Gold Nanoparticles

Theoretical investigations suggest that gold nanoparticle (GNP)-mediated radiation dose enhancement and radiosensitization can be maximized when photons interact with gold, predominantly via photoelectric absorption. This makes ytterbium (Yb)-169, which emits photons with an average energy of 93 keV (just above the K-edge of gold), an ideal radioisotope for such purposes. This investigation tests the feasibility of tumor-specific prostate brachytherapy achievable with Yb-169 and actively targeted GNPs, using an external beam surrogate of Yb-169 created from an exotic filter material - erbium (Er) and a standard copper-filtered 250 kVp beam. The current in vitro study shows that treatment of prostate cancer cells with goserelin-conjugated gold nanorods (gGNRs) promotes gonadotropin releasing hormone receptor-mediated internalization and enhances radiosensitivity to both Er-filtered and standard 250 kVp beams, 14 and 10%, respectively. While the degree of GNP-mediated radiosensitization as seen from the in vitro study may be considered moderate, the current in vivo study shows that gGNR treatment plus Er-filtered x-ray irradiation is considerably more effective than radiation treatment alone (p < 0.0005), resulting in a striking reduction in tumor volume (50% smaller) 2 months following treatment. Overall, the current results provide strong evidence for the feasibility of tumor-specific prostate brachytherapy with Yb-169 and gGNRs.

TU-H-CAMPUS-TeP3-04: Probing the Dose Enhancement Due to a Clinically-Relevant Concentration of Gold Nanoparticles and Yb-169 Gamma Rays Using PRESAGE Dosimeters

PURPOSE To probe physical evidences of the dose enhancement due to a low/clinically-relevant concentration of gold nanoparticles (GNPs) and Yb-169 gamma rays using PRESAGE dosimeters. METHODS A PRESAGE cuvette was placed at approximately 2 mm above the plane containing three novel Yb-169 brachytherapy seeds (3.2, 3.2, and 5.3 mCi each). Two types of PRESAGE dosimeters were used - plain PRESAGEs (controls) and PRESAGEs loaded with 0.02 wt. % of GNPs (GNP-PRESAGEs). Each PRESAGE dosimeter was irradiated with different time durations (0 to 24 hours) to deliver 0, 4, 8, 16 and 24 Gy of dose. For a reference/comparison, both types of PRESAGEs were also irradiated using 250 kVp x-rays with/without Er-filter to deliver 0, 3, 10, and 30 Gy of dose. Er-filter was used to emulate Yb-169 spectrum using 250 kVp x-rays. The absorption spectra of PRESAGEs were measured using a UV spectrophotometer and used to determine the corresponding optical densities (ODs). RESULTS GNP-PRESAGEs exposed to Yb-169 sources showed ∼65% increase in ODs compared with controls. When exposed to Er-filtered and unfiltered 250 kVp x-rays, they produced smaller increases in ODs, ∼41% and ∼37%, respectively. There was a linear relationship between ODs and delivered doses with a goodness-of-fit (R2) greater than 0.99. CONCLUSION A notable increase in the ODs (∼65%) was observed for GNP-PRESAGEs irradiated by Yb-169 gamma rays. Considering the observed OD increases, it was highly likely that Yb-169 gamma rays were more effective than both Er-filtered and unfiltered 250 kVp x-rays, in terms of producing the dose enhancement. Due to several unknown factors (e.g., possible difference in the dose response of GNP-PRESAGEs vs. PRESAGEs), however, a further investigations is necessary to establish the feasibility of quantifying the exact amount of macroscopic or microscopic/local GNP-mediated dose enhancement using PRESAGE or similar volumetric dosimeters. Supported by DOD/PCRP grant W81XWH-12-1-0198 This investigation was supported by DOD/PCRP grant W81XWH-12-1-0198.

Feasibility study of using fall-off gradients of early and late PET scans for proton range verification

Purpose While positron emission tomography (PET) allows for the imaging of tissues activated by proton beams in terms of monitoring the therapy administered, most endogenous tissue elements are activated by relatively high‐energy protons. Therefore, a relatively large distance off‐set exists between the dose fall‐off and activity fall‐off. However, 16O(p,2p,2n)13N has a relatively low energy threshold which peaks around 12 MeV and also a residual proton range that is approximately 1 to 2 mm. In this phantom study, we tested the feasibility of utilizing the 13N production peak as well as the differences in activity fall‐off between early and late PET scans for proton range verification. One of the main purposes for this research was developing a proton range verification methodology that would not require Monte Carlo simulations. Methods and materials Both monoenergetic and spread‐out Bragg peak beams were delivered to two phantoms — a water‐like gel and a tissue‐like gel where the proton ranges came to be approximately 9.9 and 9.1 cm, respectively. After 1 min of postirradiation delay, the phantoms were scanned for a period of 30 min using an in‐room PET. Two separate (Early and Late) PET images were reconstructed using two different postirradiation delays and acquisition times; Early PET: 1 min delay and 3 min acquisition, Late PET: 21 min delay and 10 min acquisition. The depth gradients of the PET signals were then normalized and plotted as functions of depth. The normalized gradient of the early PET images was subtracted from that of the late PET images, to observe the 13N activity distribution in relation to depth. Monte Carlo simulations were also conducted with the same set‐up as the measurements stated previously. Results The subtracted gradients show peaks at 9.4 and 8.6 cm in water‐gel and tissue‐gel respectively for both pristine and SOBP beams. These peaks are created in connection with the sudden change of 13N signals with depth and consistently occur 2 mm upstream to where 13N signals were most abundantly created (9.6 and 8.8 cm in water‐gel and tissue‐gel, respectively). Monte Carlo simulations provided similar results as the measurements. Conclusions The subtracted PET signal gradient peaks and the proton ranges for water‐gel and tissue‐gel show distance off‐sets of 4 to 5 mm. This off‐set may potentially be used for proton range verification using only the PET measured data without Monte Carlo simulations. More studies are necessary to overcome various limitations, such as perfusion‐driven washout, for the feasibility of this technique in living patients.

Development of bimetallic (Zn@Au) nanoparticles as potential PET-imageable radiosensitizers

PURPOSE Gold nanoparticles (GNPs) are being investigated actively for various applications in cancer diagnosis and therapy. As an effort to improve the imaging of GNPs in vivo, the authors developed bimetallic hybrid Zn@Au NPs with zinc cores and gold shells, aiming to render them in vivo visibility through positron emission tomography (PET) after the proton activation of the zinc core as well as capability to induce radiosensitization through the secondary electrons produced from the gold shell when irradiated by various radiation sources. METHODS Nearly spherical zinc NPs (∼5-nm diameter) were synthesized and then coated with a ∼4.25-nm gold layer to make Zn@Au NPs (∼13.5-nm total diameter). 28.6 mg of these Zn@Au NPs was deposited (∼100 μm thick) on a thin cellulose target and placed in an aluminum target holder and subsequently irradiated with 14.15-MeV protons from a GE PETtrace cyclotron with 5-μA current for 5 min. After irradiation, the cellulose matrix with the NPs was placed in a dose calibrator to assess the induced radioactivity. The same procedure was repeated with 8-MeV protons. Gamma ray spectroscopy using an high-purity germanium detector was conducted on a very small fraction (<1 mg) of the irradiated NPs for each proton energy. In addition to experimental measurements, Monte Carlo simulations were also performed with radioactive Zn@Au NPs and solid GNPs of the same size irradiated with 160-MeV protons and 250-kVp x-rays. RESULTS The authors measured 168 μCi of activity 32 min after the end of bombardment for the 14.15-MeV proton energy sample using the (66)Ga setting on a dose calibrator; activity decreased to 2 μCi over a 24-h period. For the 8-MeV proton energy sample, PET imaging was additionally performed for 5 min after a 12-h delay. A 12-h gamma ray spectrum showed strong peaks at 511 keV (2.05 × 10(6) counts) with several other peaks of smaller magnitude for each proton energy sample. PET imaging showed strong PET signals from mostly decaying (66)Ga. The Monte Carlo results showed that radioactive Zn@Au NPs and solid GNPs provided similar characteristics in terms of their secondary electron spectra when irradiated. CONCLUSIONS The Zn@Au NPs developed in this investigation have the potential to be used as PET-imageable radiosensitizers for radiotherapy applications as well as PET tracers for molecular imaging applications.

MO-FG-BRA-02: Modulation of Clinical Orthovoltage X-Ray Spectrum Further Enhances Radiosensitization of Cancer Cells Targeted with Gold Nanoparticles

Purpose: To assess the potential to amplify radiosensitization of cancer cells targeted with gold nanoparticles by augmenting selective spectral components of X-ray beam. Methods: Human prostate cancer cells were treated for 24h with gold nanorods conjugated to goserelin acetate or pegylated, systematically washed and irradiated with 250 kVp X-rays (25mA, 0.25mm Cu- filter, 8x8cm2 field size, 50cm SSD) with or without an additional 0.25 mm Erbium (Er) filter. As demonstrated in a companion Monte Carlo study, Er-filter acted as an external target to feed Erbium K-shell X-ray fluorescence photons (∼50 keV) into the 250 kVp beam. After irradiation, we performed measurements of clonogenic viability with doses between 0 -6Gy, irreparable DNA damage assay to measure double-strand breaks via γH2AX-foci staining, and production of stable reactive oxygen species (ROS). Results: The clonogenic assay for the group treated with conjugated nanoparticles showed radiosensitization enhancement factor (REF), calculated at the 10% survival fraction aisle, of (1.62±0.07) vs. (1.23±0.04) with/without the Er-filter in the 250 kVp beam, respectively. The group treated with pegylated nanoparticles, albeit retained in modest amounts within the cells, also showed statistically significant REF (1.13±0.09) when the Erbium filter was added to the beam. No significant radiosensitization was observed for other groups. Measurements of ROS levels showed increments of (1.9±0.2) vs. (1.4±0.1) for combined treatment with targeted nanoparticles and Er-filtered beam. γH2AX-foci showed 50% increase for the same treatment combination, confirming the enhanced radiosensitization in a consistent fashion. Conclusion: Our study demonstrates the feasibility of enhancing radiosensitization of cancer cells by combining actively targeted gold nanoparticles and modulating the X-ray spectrum in the desired energy range. The established technique will not only help develop strategies to maximize nanoparticle-mediated radiosensitization but also offer a convenient way to acquire unprecedented insights into the role of photon energy for the observed radiosensitization effects. Supported by DOD/PCRP grant W81XWH-12-1-0198

WE-EF-303-01: FEATURED PRESENTATION: Hydrogel Fiducial Markers for In-Vivo Proton Therapy and Range Verifications Using PET

Purpose: Currently there are no clinically used techniques for in-vivo proton treatment/range verification. Our aim was to develop patient implantable markers that can be visualized in CT/x-ray for treatment-planning/beam-positioning, and also in PET for proton treatment/range verification. Methods: Biocompatible/biodegradable hydrogel polymers were immersed in O18-enriched water and O16 water, respectively to create O18-water hydrogels (0.5 cm3) and O16-water hydrogels (1 cm3) (both >99% water and <1% polymer). Also, 5–8 µm Zn powder was suspended in O16 water and O18-enriched water and cross-linked with hydrogel polymers to create Zn/16O-water hydrogels (30%/70% mass ratio, <1% polymer) and Zn/18O-water hydrogels (10%/90%). A block of extra-firm “wet” tofu (12.3×8.8×4.9 cm, ρ⁼1) immersed in water was injected with Zn/O16-water hydrogels (0.9 cm3 each) at four different depths using an 18 gauge needle. Similarly, Zn/18O-water hydrogels (0.9 cm3) were injected in a different tofu phantom. As a reference, both 16O-water and O18-water hydrogels in petri-dishes were irradiated in a “dry” environment. The hydrogels in the “wet” tofu phantoms and “dry” petri-dishes were CT-scanned and treatment-planned. Then, they were positioned at the proton distal dose fall-off region and irradiated (2 Gy) followed by PET/CT imaging. Results: Significantly high PET signals were observed only at O18-water hydrogels in the “dry” environment. Zn/O16-water hydrogels injected in the tofu phantom showed outstanding CT visibility but provided no noticeable PET signals. Zn/O18-water hydrogels in the “wet” tofu showed excellent CT visibility and moderate PET visibility, however, weaker PET signals than the “dry” environment possibly due to O18-water leaching out. Conclusion: The developed hydrogel markers can be used as universal fiducial markers due to their CT/PET/MRI/US visibility. Their PET visibility (possibly contributed more by activated O18-water than Zn) after proton irradiation can be utilized for proton therapy/range verification. More investigation is needed to slow down the leaching of O18-water.

MO-FG-303-08: PET-Detectable Bimetallic (Zn@Au) Nanoparticles for Radiotherapy and Molecular Imaging Applications

Purpose: A technical challenge in clinical translation of GNP-mediated radiotherapy is lack of in-vivo imaging tools for monitoring biodistribution of GNPs. While several modalities (x-ray fluorescence, photoacoustic, etc.) are investigated, we propose a potentially more effective technique based on PET imaging. We developed Zn@Au NPs whose Zn core acts as positron emitters when activated by protons, while the Au shell plays the original role for GNP-mediated radiosensitization. Methods: Spherical Zn NPs (∼7nm diameter) were synthesized and then coated with ∼7nm thick Au layer to make Zn@Au NPs (∼20nm diameter). A water slurry containing 29mg of Zn@Au NPs was deposited (<10µm thickness) on a thin cellulose target and subsequently baked to remove the water. The cellulose matrix was placed in an aluminum target holder and irradiated with 14.5MeV protons from a GE PETtrace cyclotron with 4µA for 5min. After irradiation the cellulose matrix with the NPs was placed in a dose calibrator to assay radioactivity. Gamma spectroscopy using a HPGe detector was conducted on a very small fraction (<1mg) of the irradiated NPs. Results: We measured 158µCi of activity 32min after end of bombardment (EOB) using 66Ga setting on the dose calibrator (contribution from the cellulose matrix is negligible) which decreased to 2µCi over a 24hrs period. A gamma spectrum started one hour after EOB on the small fraction and acquired for 700sec showed a strong peak at 511keV (∼40,000 counts) with several other peaks (highest peak <1200 counts) of smaller magnitude. Conclusion: Strong 511keV gamma emission from proton-activated Zn cores can potentially be utilized to image the biodistribution of Zn@Au NPs using a PET scanner. The developed Zn@Au NPs are expected to retain radiosensitizing capability similar to solid GNPs, while observable through PET imaging for human-sized objects. Moreover, bioconjugated PET-detectable GNPs would allow a new option to perform molecular imaging.

SU‐E‐T‐231: Measurements of Gold Nanoparticle‐Mediated Proton Dose Enhancement Due to Particle‐Induced X‐Ray Emission and Activation Products Using Radiochromic Films and CdTe Detector

PURPOSE There have been several reports of enhanced cell-killing and tumor regression when tumor cells and mouse tumors were loaded with gold nanoparticles (GNPs) prior to proton irradiation. While particle-induced xray emission (PIXE), Auger electrons, secondary electrons, free radicals, and biological effects have been suggested as potential mechanisms responsible for the observed GNP-mediated dose enhancement/radiosensitization, there is a lack of quantitative analysis regarding the contribution from each mechanism. Here, we report our experimental effort to quantify some of these effects. METHODS 5-cm-long cylindrical plastic vials were filled with 1.8 mL of either water or water mixed with cylindrical GNPs at the same gold concentration (0.3 mg Au/g) as used in previous animal studies. A piece of EBT2 radiochromic film (30-µm active-layer sandwiched between 80/175-µm outer-layers) was inserted along the long axis of each vial and used to measure dose enhancement due to PIXE from GNPs. Vials were placed at center-of-modulation (COM) and 3-cm up-/down-stream from COM and irradiated with 5 different doses (2-10 Gy) using 10-cm-SOBP 160-MeV protons. After irradiation, films were cleaned and read to determine the delivered dose. A vial containing spherical GNPs (20 mg Au/g) was also irradiated, and gamma-rays from activation products were measured using a cadmium-telluride (CdTe) detector. RESULTS Film measurements showed no significant dose enhancement beyond the experimental uncertainty (∼2%). There was a detectable activation product from GNPs, but it appeared to contribute to dose enhancement minimally (<0.01%). CONCLUSION Considering the composition of EBT2 film, it can be inferred that gold characteristic x-rays from PIXE and their secondary electrons make insignificant contribution to dose enhancement. The current investigation also suggests negligible dose enhancement due to activation products. Thus, previously-reported GNP-mediated proton dose enhancement/radiosensitization needs to be attributed to one or more of the other mechanisms listed earlier. Supported in part by NIH/NCI grant R01CA155446;This investigation was supported in part by NIH/NCI grant R01CA155446.

Proton beam range verification using off-site PET by imaging novel proton-activated markers

Previously we showed that when 68Zn and Cu markers are placed along the distal end of the proton beam range, they result in higher PET signals than surrounding material such as Plastic Water® and balsa wood following proton treatment. In this work we investigated the use of PET signals from activated 68Zn and Cu markers for accurate proton range verification in two pseudo-clinical settings. 68Zn (>97%) and natural Cu (63Cu, 69% ; 65Cu, 31%) markers (10 × 10 mm) of different thicknesses (0.1, 0.25, and 0.5 mm) were imbedded at 4 different distal fall-off depths in a balsa wood (simulating lung tissue) phantom. In a separate experiment, a rectangular block of raw beef (12 × 16 × 5 cm) simulating soft tissue was cut diagonally and imbedded with the same kind of 68Zn and Cu markers. Both phantoms were CT scanned for treatment planning and irradiated by a 160 MeV, 10-cm SOBP proton beam with 5 Gy and scanned for 5 hrs using an off-site PET scanner. Images were reconstructed using a 30 min interval without decay correction and using various post-irradiation delays to determine the image with the best visibility of activated markers. Treatment planned isodose lines of the phantom were overlaid on top of marker locations to correlate the isodose line level to marker activation. The best visibility of a 30 min PET scan was obtained after 1 and 2 hr delays for balsa wood and beef phantoms, respectively. Marker visibility increased with marker volume and scan time post irradiation until 1 hr (for balsa) and 2 hrs (for beef) before it decreased due to a decrease in accumulated coincidence events. 68Zn markers showed signals when located at > 50% isodose line irrespective of different post-irradiation delays. However, Cu markers show signals in balsa phantom (1 hr delay) when located at > 50% isodose line while in beef phantom (2 hrs delay) when located at > 95% isodose line. This is due to the proton energy (and depth) dependency of different progeny radioisotopes from activated Cu. Markers located beyond those isodose lines were not activated. In both cases, markers of > 25 mm3 were clearly visible. Activation of 68Zn and Cu markers and their correspondence with isodose lines suggest the possibility of using implanted markers for proton range verification.