Variation of 4 MV X-ray dose rate strongly impacts biological response both in vitro and in vivo

Dose Rate Effects from the 1950s through to the Era of FLASH

Numerous dose rate effects have been described over the past 6-7 decades in the radiation biology and radiation oncology literature depending on the dose rate range being discussed. This review focuses on the impact and understanding of altering dose rates in the context of radiation therapy, but does not discuss dose rate effects as relevant to radiation protection. The review starts with a short historic review of early studies on dose rate effects, considers mechanisms thought to underlie dose rate dependencies, then discusses some current issues in clinical findings with altered dose rates, the importance of dose rate in brachytherapy, and the current timely topic of the use of very high dose rates, so-called FLASH radiotherapy. The discussion includes dose rate effects in vitro in cultured cells, in in vivo experimental systems and in the clinic, including both tumors and normal tissues. Gaps in understanding dose rate effects are identified, as are opportunities for improving clinical use of dose rate modulation.

Possible association of dose rate and the development of late visual toxicity for patients with intracranial tumours treated with pencil beam scanned proton therapy

BACKGROUND AND PURPOSE

Rare but severe toxicities of the optic apparatus have been observed after treatment of intracranial tumours with proton therapy. Some adverse events have occurred at unusually low dose levels and are thus difficult to understand considering dose metrics only. When transitioning from double scattering to pencil beam scanning, little consideration was given to increased dose rates observed with the latter delivery paradigm. We explored if dose rate related metrics could provide additional predicting factors for the development of late visual toxicities.

MATERIALS AND METHODS

Radiation-induced intracranial visual pathway lesions were delineated on MRI for all index cases. Voxel-wise maximum dose rate (MDR) was calculated for 2 patients with observed optic nerve toxicities (CTCAE grade 3 and 4), and 6 similar control cases. Additionally, linear energy transfer (LET) related dose enhancing metrics were investigated.

RESULTS

For the index cases, which developed toxicities at low dose levels (mean, 50 GyRBE), some dose was delivered at higher instantaneous dose rates. While optic structures of non-toxicity cases were exposed to dose rates of up to 1 to 3.2 GyRBE/s, the pre-chiasmatic optic nerves of the 2 toxicity cases were exposed to dose rates above 3.7 GyRBE/s. LET-related metrics were not substantially different between the index and non-toxicity cases.

CONCLUSIONS

Our observations reveal large variations in instantaneous dose rates experienced by different volumes within our patient cohort, even when considering the same indications and beam arrangement. High dose rate regions are spatially overlapping with the radiation induced toxicity areas in the follow up images. At this point, it is not feasible to establish causality between exposure to high dose rates and the development of late optic apparatus toxicities due to the low incidence of injury.

Voxel-wise dose rate calculation in clinical pencil beam scanning proton therapy

Objective. Clinical outcomes after proton therapy have shown some variability that is not fully understood. Different approaches have been suggested to explain the biological outcome, but none has yet provided a comprehensive and satisfactory rationale for observed toxicities. The relatively recent transition from passive scattering (PS) to pencil beam scanning (PBS) treatments has significantly increased the voxel-wise dose rate in proton therapy. In addition, the dose rate distribution is no longer uniform along the cross section of the target but rather highly heterogeneous, following the spot placement. We suggest investigating dose rate as potential contributor to a more complex proton RBE model.Approach. Due to the time structure of the PBS beam delivery the instantaneous dose rate is highly variable voxel by voxel. Several possible parameters to represent voxel-wise dose rate for a given clinical PBS treatment plan are detailed. These quantities were implemented in the scripting environment of our treatment planning system, and computations experimentally verified. Sample applications to treated patient plans are shown.Main results. Computed dose rates we experimentally confirmed. Dose rate maps vary depending on which method is used to represent them. Mainly, the underlying time and dose intervals chosen determine the topography of the resultant distributions. The maximum dose rates experienced by any target voxel in a given PBS treatment plan in our system range from ∼100 to ∼450 Gy(RBE)/min, a factor of 10-100 increase compared to PS. These dose rate distributions are very heterogeneous, with distinct hot spots.Significance. Voxel-wise dose rates for current clinical PBS treatment plans vary greatly from clinically established practice with PS. The exploration of different dose rate measures to evaluate potential correlations with observed clinical outcomes is suggested, potentially adding a missing component in the understanding of proton relative biological effectiveness (RBE).

Detection of radiation-induced senescence by the Debacq-Chainiaux protocol: Improvements and upgrade in the detection of positive events

Senescent cells are blocked in the cell cycle but remain metabolically active. These cells, once engaged in the senescence process, fail to initiate DNA replication. Due to the shortening of telomeres, replicative senescence can be triggered by a DNA damage response. Moreover, cells can also be induced to senesce by DNA damage in response to elevated reactive oxygen species (ROS), activation of oncogenes, cell-cell fusion or after ionizing radiation. There are multiple experimental ways to detect senescent cells directly or indirectly. Senescence-associated cellular traits (SA β-Gal activity, increase in cell volume and lysosome content, appearance of γ-H2AX foci, increase of ROS and oxidative damage adducts, etc.) can be identified by numerous methods of detection (flow cytometry, confocal imaging, in situ staining, etc.). Here, we improved an existing flow cytometry protocol and further developed a new one specifically tailored to ionizing radiation-induced endothelial senescence. Thus, we have upgraded the Debacq-Chainiaux protocol and added improvements in this protocol (i) to better detect positive events (ii) to offer a compatibility to simultaneously analyze various intracellular molecules including phosphorylated signaling proteins and cytokines, whether related or not to senescence processes.

Circulating microvesicles correlate with radiation proctitis complication after radiotherapy

In a large retrospective study, we assessed the putative use of circulating microvesicles (MVs), as innovative biomarkers of radiation toxicity in a cohort of 208 patients with prostate adenocarcinoma overexposed to radiation. The level of platelet (P)-, monocyte (M)- and endothelial (E)-derived MVs were assessed by flow cytometry. Rectal bleeding toxicity scores were collected at the time of blood sampling and during the routine follow-up and were tested for association with MVs using a multivariate logistic regression. MVs dosimetric correlation was investigated using dose volume histograms information available for a subset of 36 patients. The number of PMVs was significantly increased in patients with highest toxicity grades compared to lower grades. Risk prediction analysis revealed that increased numbers of PMVs, and an increased amount of MMVs relative to EMVs, were associated with worst rectal bleeding grade compared to the time of blood sampling. Moreover, a significant correlation was found between PMV and MMV numbers, with the range of doses up to the median exposure (40 Gy) of bladder/rectum and anterior rectal wall, respectively. MVs could be considered as new biomarkers to improve the identification of patients with high toxicity grade and may be instrumental for the prognosis of radiation therapy complications.

Deep models of integrated multiscale molecular data decipher the endothelial cell response to ionizing radiation

The vascular endothelium is a hot spot in the response to radiation therapy for both tumors and normal tissues. To improve patient outcomes, interpretable systemic hypotheses are needed to help radiobiologists and radiation oncologists propose endothelial targets that could protect normal tissues from the adverse effects of radiation therapy and/or enhance its antitumor potential. To this end, we captured the kinetics of multi-omics layers-i.e. miRNome, targeted transcriptome, proteome, and metabolome-in irradiated primary human endothelial cells cultured in vitro. We then designed a strategy of deep learning as in convolutional graph networks that facilitates unsupervised high-level feature extraction of important omics data to learn how ionizing radiation-induced endothelial dysfunction may evolve over time. Last, we present experimental data showing that some of the features identified using our approach are involved in the alteration of angiogenesis by ionizing radiation.

Variation of 4 MV X-ray dose rate in fractionated irradiation strongly impacts biological endothelial cell response in vitro

PURPOSE

Even though X-ray beams are widely used in medical diagnosis or radiotherapy, the comparisons of their dose rates are scarce. We have recently demonstrated in vitro (clonogenic assay, cell viability, cell cycle, senescence) and in vivo (weight follow-up of animals and bordering epithelium staining of lesion), that for a single dose of irradiation, the relative biological effectiveness (RBE) deviates from 1 (up to twofold greater severe damage at the highest dose rate depending on the assay) when increasing the dose rate of high energy X-ray beams.

MATERIAL AND METHODS

To further investigate the impact of the dose rate on RBE, in this study, we performed in vitro fractionated irradiations by using the same two dose rates (0.63 and 2.5 Gy.min^-1) of high-energy X-rays (both at 4 MV) on normal endothelial cells (HUVECs). We investigated the viability/mortality, characterized radiation-induced senescence by using flow cytometry and measured gene analysis deregulations on custom arrays.

RESULTS

The overall results enlighten that, in fractionated irradiations when varying the dose rate of high-energy X-rays, the RBE of photons deviates from 1 (up to 2.86 for viability/mortality experiments performed 21 days postirradiation).

CONCLUSION

These results strengthen the interest of multiparametric analysis approaches in providing an accurate evaluation of the outcomes of irradiated cells in support of clonogenic assays, especially when such assays are not feasible.

Review of Experimental Studies to Improve Radiotherapy Response in Bladder Cancer: Comments and Perspectives

Bladder cancer is among the top ten most common cancer types in the world. Around 25% of all cases are muscle-invasive bladder cancer, for which the gold standard treatment in the absence of metastasis is the cystectomy. In recent years, trimodality treatment associating maximal transurethral resection and radiotherapy combined with concurrent chemotherapy is increasingly used as an organ-preserving alternative. However, the use of this treatment is still limited by the lack of biomarkers predicting tumour response and by a lack of targeted radiosensitising drugs that can improve the therapeutic index, especially by limiting side effects such as bladder fibrosis. In order to improve the bladder-preserving treatment, experimental studies addressing these main issues ought to be considered (both in vitro and in vivo studies). Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for systematic reviews, we conducted a literature search in PubMed on experimental studies investigating how to improve bladder cancer radiotherapy with different radiosensitising agents using a comprehensive search string. We made comments on experimental model selection, experimental design and results, formulating the gaps of knowledge still existing: such as the lack of reliable predictive biomarkers of tumour response to chemoradiation according to the molecular tumour subtype and lack of efficient radiosensitising agents specifically targeting bladder tumour cells. We provided guidance to improve forthcoming studies, such as taking into account molecular characteristics of the preclinical models and highlighted the value of using patient-derived xenografts as well as syngeneic models. Finally, this review could be a useful tool to set up new radiation-based combined treatments with an improved therapeutic index that is needed for bladder preservation.

Biological effectiveness of very high gamma dose rate and its implication for radiological protection

Many experimental studies are carried out to compare biological effectiveness of high dose rate (HDR) with that of low dose rate (LDR). The rational for this is the uncertainty regarding the value of the dose rate effectiveness factor (DREF) used in radiological protection. While a LDR is defined as 0.1 mGy/min or lower, anything above that is seen as HDR. In cell and animal experiments, a dose rate around 1 Gy/min is usually used as representative for HDR. However, atomic bomb survivors, the reference cohort for radiological protection, were exposed to tens of Gy/min. The important question is whether gamma radiation delivered at very high dose rate (VHDR-several Gy/min) is more effective in inducing DNA damage than that delivered at HDR. The aim of this investigation was to compare the biological effectiveness of gamma radiation delivered at VHDR (8.25 Gy/min) with that of HDR (0.38 Gy/min or 0.79 Gy/min). Experiments were carried out with human peripheral mononuclear cells (PBMC) and the human osteosarcoma cell line U2OS. Endpoints related to DNA damage response were analysed. The results show that in PBMC, VHDR is more effective than HDR in inducing gene expression and micronuclei. In U2OS cells, the repair of 53BP1 foci was delayed after VHDR indicating a higher level of damage complexity, but no VHDR effect was observed at the level of micronuclei and clonogenic cell survival. We suggest that the DREF value may be underestimated when the biological effectiveness of HDR and LDR is compared.