Effect of Proton Therapy on Tumor Cell Killing and Immune Microenvironment for Hepatocellular Carcinoma

Commissioning of a novel gantry-less proton therapy system

Purpose

The focus of this article is to describe the configuration, testing, and commissioning of a novel gantry-less synchrotron-based proton therapy (PT) facility.

Materials and methods

The described PT system delivers protons with a water equivalent range between 4 and 38 cm in 1800 energy layers. The fixed beam delivery permits a maximum field size of 28 × 30 cm2. The patient positioning and imaging system includes a six-degree-of-freedom robotic arm, a convertible patient chair, a vertical 4DCT, and an orthogonal 2D X-ray imaging system.

Results

The spot positioning reproducibility was consistent within ±1 mm. The width (σ) of the beam profile at the isocenter was energy dependent and ranged from 2.8 mm to 7.7 mm. Absolute dose reproducibility was measured and deviations were found to be <0.62% for all possible beam scenarios. The built-in dose monitoring system was successfully tested for its ability to generate interlocks under specific conditions (beam spot deviation ≥2 mm, individual spot dose ≥10% or ≥0.25 Gy, spot energy deviation ≥0.5 MeV). The robot positioning exhibited a consistent reproducibility within ±1 mm. All tested scenarios achieved laser-free initial 3D/3D image-guided positioning within ±5 mm. Subsequent 2D/3D positioning showed an accuracy of ±1 mm. A single 2D/3D image registration event corrected positions in all cases. Results of gamma analysis (3%, 3 mm) demonstrated pass rates greater than 95% for head and neck, thorax, abdomen treatment plans.

Conclusions

We report on the performance of a novel single-room gantry-less PT system comprised of a compact synchrotron and an adjustable (from nearly horizontal to almost vertical) patient positioning system. The commissioning results show high accuracy and reproducibility of the main proton beam parameters and the patient positioning system. The new PT facility started patient treatments in March 2023, which were the first in Israel and the Middle Eastern region.

Potential Benefits of Combining Proton or Carbon Ion Therapy with DNA Damage Repair Inhibitors

The use of charged particle radiotherapy is currently increasing, but combination therapy with DNA repair inhibitors remains to be exploited in the clinic. The high-linear energy transfer (LET) radiation delivered by charged particles causes clustered DNA damage, which is particularly effective in destroying cancer cells. Whether the DNA damage response to this type of damage is different from that elicited in response to low-LET radiation, and if and how it can be targeted to increase treatment efficacy, is not fully understood. Although several preclinical studies have reported radiosensitizing effects when proton or carbon ion irradiation is combined with inhibitors of, e.g., PARP, ATR, ATM, or DNA-PKcs, further exploration is required to determine the most effective treatments. Here, we examine what is known about repair pathway choice in response to high- versus low-LET irradiation, and we discuss the effects of inhibitors of these pathways when combined with protons and carbon ions. Additionally, we explore the potential effects of DNA repair inhibitors on antitumor immune signaling upon proton and carbon ion irradiation. Due to the reduced effect on healthy tissue and better immune preservation, particle therapy may be particularly well suited for combination with DNA repair inhibitors.

Carbon Ion Irradiation Activates Anti-Cancer Immunity

Carbon ion beams have the unique property of higher linear energy transfer, which causes clustered damage of DNA, impacting the cell repair system. This sometimes triggers apoptosis and the release in the cytoplasm of damaged DNA, leading to type I interferon (IFN) secretion via the activation of the cyclic GMP-AMP synthase-stimulator of interferon genes pathway. Dendritic cells phagocytize dead cancer cells and damaged DNA derived from injured cancer cells, which together activate dendritic cells to present cancer-derived antigens to antigen-specific T cells in the lymph nodes. Thus, carbon ion radiation therapy (CIRT) activates anti-cancer immunity. However, cancer is protected by the tumor microenvironment (TME), which consists of pro-cancerous immune cells, such as regulatory T cells, myeloid-derived suppressor cells, and tumor-associated macrophages. The TME is too robust to be destroyed by the CIRT-mediated anti-cancer immunity. Various modalities targeting regulatory T cells, myeloid-derived suppressor cells, and tumor-associated macrophages have been developed. Preclinical studies have shown that CIRT-mediated anti-cancer immunity exerts its effects in the presence of these modalities. In this review article, we provide an overview of CIRT-mediated anti-cancer immunity, with a particular focus on recently identified means of targeting the TME.

Using immunotherapy to enhance the response of a C3H mammary carcinoma to proton radiation

BACKGROUND

The benefit of combining immunotherapy with photon irradiation has been shown pre-clinically and clinically. This current pre-clinical study was designed to investigate the anti-tumour action of combining immunotherapy with protons.

MATERIALS AND METHODS

Male CDF1 mice, with a C3H mammary carcinoma inoculated on the right rear foot, were locally irradiated with single radiation doses when tumours reached 200mm3. Radiation was delivered with an 83-107MeV pencil scanning proton beam in the centre of a 3 cm spread out Bragg peak. Following irradiation (day 0), mice were injected intraperitoneal with anti-CTLA-4, anti-PD-1, or anti-PD-L1 (10 mg/kg) twice weekly for two weeks. Endpoints were tumour growth time (TGT3; time to reach 3 times treatment volume) or local tumour control (percent of mice showing tumour control at 90 days). A Student's T-test (tumour growth) or Chi-squared test (tumour control) were used for statistical analysis; significance levels of p < 0.05.

RESULTS

Untreated tumours had a mean (± 1 S.E.) TGT3 of 4.6 days (± 0.4). None of the checkpoint inhibitors changed this TGT3. A linear increase in TGT3 was seen with increasing radiation doses (5-20 Gy), reaching 17.2 days (± 0.7) with 20 Gy. Anti-CTLA-4 had no effect on radiation doses up to 15 Gy, but significantly enhanced 20 Gy; the TGT3 being 23.0 days (± 1.3). Higher radiation doses (35-60 Gy) were investigated using a tumour control assay. Logit analysis of the dose response curve, resulted in a TCD50 value (radiation dose causing 50% tumour control; with 95% confidence intervals) of 48 Gy (44-53) for radiation only. This significantly decreased to 43 Gy (38-49) when mice were treated with anti-CTLA-4. Neither anti-PD-1 nor anti-PD-L1 significantly affected tumour control.

CONCLUSION

Checkpoint inhibitors enhanced the response of this C3H mammary carcinoma to proton irradiation. However, this enhancement depended on the checkpoint inhibitor and radiation dose.

Radiation-induced skin and heart toxicity in patients with breast cancer treated with adjuvant proton radiotherapy: a comparison with photon radiotherapy

This study aimed to investigate the dose parameters and incidence of radiotherapy (RT)-associated toxicity in patients with left breast cancer (LBC) treated with proton-RT, compared with photon-RT. We collected data from 111 patients with LBC who received adjuvant RT in our department between August 2021 and March 2023. Among these patients, 24 underwent proton-RT and 87 underwent photon-RT. In addition to the dosimetric analysis for organs at risk (OARs), we measured NT-proBNP levels before and after RT. Our data showed that proton-RT improved dose conformity and reduced doses to the heart and lungs and was associated with a lower rate of increased NT-proBNP than did photon-RT. Regarding skin toxicity, the Dmax for 1 c.c. and 10 c.c. and the average dose to the skin-OAR had predictive roles in the risk of developing radiation-induced dermatitis. Although pencil beam proton-RT with skin optimization had a dose similar to that of skin-OAR compared with photon-RT, proton-RT still had a higher rate of radiation dermatitis (29%) than did photon RT (11%). Using mice 16 days after irradiation, we demonstrated that proton-RT induced a greater increase in interleukin 6 and transforming growth factor-β1 levels than did photon-RT. Furthermore, topical steroid ointment reduced the inflammatory response and severity of dermatitis induced by RT. In conclusion, we suggest that proton-RT with skin optimization spares high doses to OARs with acceptable skin toxicity. Furthermore, prophylactic topical steroid treatment may decrease radiation dermatitis by alleviating proton-induced inflammatory responses in vivo.

Ultra-high dose-rate proton FLASH improves tumor control

BACKGROUND AND PURPOSE

Proton radiotherapy (PRT) offers potential benefits over other radiation modalities, including photon and electron radiotherapy. Increasing the rate at which proton radiation is delivered may provide a therapeutic advantage. Here, we compared the efficacy of conventional proton therapy (CONVpr) to ultrahigh dose-rate proton therapy, FLASHpr, in a mouse model of non-small cell lung cancers (NSCLC).

MATERIALS AND METHODS

Mice bearing orthotopic lung tumors received thoracic radiation therapy using CONVpr (<0.05 Gy/s) and FLASHpr (>60 Gy/s) dose rates.

RESULTS

Compared to CONVpr, FLASHpr was more effective in reducing tumor burden and decreasing tumor cell proliferation. Furthermore, FLASHpr was more efficient in increasing the infiltration of cytotoxic CD8+ T-lymphocytes inside the tumor while simultaneously reducing the percentage of immunosuppressive regulatory T-cells (Tregs) among T-lymphocytes. Also, compared to CONVpr, FLASHpr was more effective in decreasing pro-tumorigenic M2-like macrophages in lung tumors, while increasing infiltration of anti-tumor M1-like macrophages. Finally, FLASHpr treatment reduced expression of checkpoint inhibitors in lung tumors, indicating reduced immune tolerance.

CONCLUSIONS

Our results suggest that FLASH dose-rate proton delivery modulates the immune system to improve tumor control and might thus be a promising new alternative to conventional dose rates for NSCLC treatment.