Dosimetry and characterization of a 25-MeV proton beam line for preclinical radiobiology research

Proposing a Clinical Model for RBE Based on Proton Track-End Counts

PURPOSE

In proton therapy, the clinical application of linear energy transfer (LET) optimization remains contentious, in part because of challenges associated with the definition and calculation of LET and its exact relationship with relative biological effectiveness (RBE) because of large variation in experimental in vitro data. This has raised interest in other metrics with favorable properties for biological optimization, such as the number of proton track ends in a voxel. In this work, we propose a novel model for clinical calculations of RBE, based on proton track end counts.

METHODS AND MATERIALS

We developed an effective dose concept to translate between the total proton track-end count per unit mass in a voxel and a proton RBE value. Dose, track end, and dose-averaged LET (LETd) distributions were simulated using Monte Carlo models for a series of water phantoms, in vitro radiobiological studies, and patient treatment plans. We evaluated the correlation between track ends and regions of elevated biological effectiveness in comparison to LETd-based models of RBE.

RESULTS

Track ends were found to correlate with biological effects in in vitro experiments with an accuracy comparable to LETd. In patient simulations, our track end model identified the same biological hotspots as predicted by LETd-based radiobiological models of proton RBE.

CONCLUSIONS

These results suggest that, for clinical optimization and evaluation, an RBE model based on proton track end counts may match LETd-based models in terms of information provided while also offering superior statistical properties.

Impact of proton therapy on antitumor immune response

Radiotherapy delivered using photons induces an immune response that leads to modulation of the tumor microenvironment. Clinical studies are ongoing to evaluate immune checkpoint inhibitors in association with photon radiotherapy. At present, there is no publication on the radio-induced immune response after proton therapy. Balb/c mice bearing subcutaneous CT26 colon tumors were irradiated by a single fraction of 16.4 Gy using a proton beam extracted from a TR24 cyclotron. RNA sequencing analysis was assessed at 3 days post-treatment. Proton therapy immune response was monitored by flow cytometry using several panels (lymphoid, myeloid cells, lymphoid cytokines) at 7 and 14 days post-irradiation. RNA-Seq functional profiling identified a large number of GO categories linked to “immune response” and “interferon signaling”. Immunomonitoring evaluation showed induced tumor infiltration by immune cells. This is the first study showing the effect of proton therapy on immune response. These interesting results provide a sound basis to assess the efficacy of a combination of proton therapy and immune checkpoint inhibitors.

Comparison of the [18F]-FDG and [18F]-FLT PET Tracers in the Evaluation of the Preclinical Proton Therapy Response in Hepatocellular Carcinoma

PURPOSE

The main objective of the present study was to compare the 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]-FDG) and 3'-[¹⁸F]fluoro-3'-deoxythymidine ([¹⁸F]-FLT) PET imaging biomarkers for the longitudinal follow-up of small animal proton therapy studies in the context of hepatocellular carcinoma (HCC).

PROCEDURES

SK-HEP-1 cells were injected into NMRI nude mice to mimic human HCC. The behavior of [¹⁸F]-FDG and [¹⁸F]-FLT tumor uptake was evaluated after proton therapy procedures. The proton single-fraction doses were 5, 10, and 20 Gy, with a dose rate of 10 Gy/min. The experimental protocol consisted of 8 groups of 10 mice, each group experiencing a particular dose/radiotracer condition. A reference PET exam was performed on each mouse the day before the irradiation procedure, followed by PET exams every 3 days up to 16 days after irradiation.

RESULTS

[¹⁸F]-FDG uptake showed a linear dose-dependent increase in the first days after treatment (37%, p < 0.05), while [¹⁸F]-FLT uptake decreased in a dose-dependent manner (e.g., 21% for 5 Gy compared to 10 Gy, p = 1.1e-2). At the later time point, [¹⁸F]-FDG normalized activity showed an 85% decrease (p < 0.01) for both 10 and 20 Gy doses and no variation for 5 Gy. Conversely, a significant 61% (p = 0.002) increase was observed for [¹⁸F]-FLT normalized activity at 5 Gy and no variation for higher doses.

CONCLUSION

We showed that the use of the [¹⁸F]-FDG and [¹⁸F]-FLT radiolabeled molecules can provide useful and complementary information for longitudinal follow-up of small animal proton therapy studies in the context of HCC. [¹⁸F]-FDG PET imaging enables a treatment monitoring several days/weeks postirradiation. On the other hand, [¹⁸F]-FLT could represent a good candidate to monitor the treatment few days postirradiation, in the context of hypo-fractioned and close irradiation planning. This opens new perspectives in terms of treatment efficacy verification depending on the irradiation scheme.

Radiosensitizing Pancreatic Cancer with PARP Inhibitor and Gemcitabine: An In Vivo and a Whole-Transcriptome Analysis after Proton or Photon Irradiation

Simple Summary

Pancreatic ductal adenocarcinoma is a devastating disease. Using modern technique of radiotherapy, such as proton therapy, may simultaneously enhance dose to the tumor and decrease dose to surrounding organ, thus limiting toxicity. Moreover, associating drugs to radiotherapy also increases its effectiveness on tumor. The aim of our study was to show the benefit of proton therapy compared to standard radiotherapy with photon, and the benefit of associating different drugs with those particles in vivo. Thus, our results displayed a higher effectiveness of associating proton therapy, gemcitabine and olaparib. Finally, we pointed out that treatment induced significant transcriptomic alterations.

Abstract

Over the past few years, studies have focused on the development of targeted radiosensitizers such as poly(ADP-ribose) polymerase inhibitors. We performed an in vivo study and a whole-transcriptome analysis to determine whether PARP inhibition enhanced gemcitabine-based chemoradiosensitization of pancreatic cancer xenografts, combined with either proton or photon irradiation. NMRI mice bearing MIA PaCa-2 xenografts were treated with olaparib and/or gemcitabine and irradiated with 10 Gy photon or proton. First, a significant growth inhibition was obtained after 10 Gy proton irradiation compared to 10 Gy photon irradiation (p = 0.046). Moreover, the combination of olaparib, gemcitabine and proton therapy significantly sensitized tumor xenografts, compared to gemcitabine (p = 0.05), olaparib (p = 0.034) or proton therapy (p < 0.0001) alone or to the association of olaparib, gemcitabine and radiotherapy (p = 0.024). Simultaneously, whole RNA sequencing profiling showed differentially expressed genes implicated in categories such as DNA repair, type I interferon signaling and cell cycle. Moreover, a large amount of lncRNA was dysregulated after proton therapy, gemcitabine and olaparib. This is the first study showing that addition of olaparib to gemcitabine-based chemoradiotherapy improved significantly local control in vivo, especially after proton therapy. RNA sequencing profiling analysis presented dynamic alteration of transcriptome after chemoradiation and identified a classifier of gemcitabine response.

Design and verification of an external radiobiological beam port on a 16.5 MeV GE PETtrace proton cyclotron

PURPOSE

Protons and heavy ions are considered to be ideal particles for use in external beam radiotherapy due to the superior properties of the dose distribution. While a photon (x-ray) beam delivers considerable dose to healthy tissues around the tumor, a proton beam that is delivered with sufficient energies has: a low entrance dose (the dose in front of the tumor); a high-dose region within the tumor, known as the Bragg peak; and, no exit dose beyond the tumor. Proton therapy is the next major step in advancing radiotherapy treatment. The purpose of this project was to adapt an existing radioisotope production cyclotron, a General Electric (GE) PETtrace, to enable radiobiological studies using proton beams. During routine use the PETtrace delivers 16.5 MeV protons to target with beam currents in the range of 10-100 µA resulting in dose rates in the order of kGy/s. To achieve the aim of the project the dose rate had to be reduced to the Gy/min range, without attenuating the proton energy below 5 MeV. This paper covers the design, construction and validation of the beam port.

METHODS

Monte Carlo simulations were performed, using GEANT4, SRIM and PACE4 to design the beam port and optimize its components. Once the beam port was fabricated, validation experiments were performed using EBT3 and HD-V2 Gafchromic™ films, and a Keithley 6485 picoampere meter.

RESULTS AND CONCLUSION

The external beam port was successfully modeled, designed and fabricated. By using a 0.25 mm thick gold foil and a brass pin-hole collimator the beam was spread from a narrow full beam diameter of 10 mm to a wide beam with a 5% flatness area in the center of the beam that had a diameter of ~20 mm. In using this system the dose rate was reduced from kGy/s to ~30 Gy/min.