Models for the bystander effect in gradient radiation fields: Range and signalling type

Automated target placement for VMAT lattice radiation therapy: enhancing efficiency and consistency

Objective. An algorithm was developed for automated positioning of lattice points within volumetric modulated arc lattice radiation therapy (VMAT LRT) planning. These points are strategically placed within the gross tumor volume (GTV) to receive high doses, adhering to specific separation rules from adjacent organs at risk (OARs). The study goals included enhancing planning safety, consistency, and efficiency while emulating human performance.Approach. A Monte Carlo-based algorithm was designed to optimize the number and arrangement of lattice points within the GTV while considering placement constraints and objectives. These constraints encompassed minimum spacing between points, distance from OARs, and longitudinal separation along thez-axis. Additionally, the algorithm included an objective to permit, at the user's discretion, solutions with more centrally placed lattice points within the GTV. To validate its effectiveness, the automated approach was compared with manually planned treatments for 24 previous patients. Prior to clinical implementation, a failure mode and effects analysis (FMEA) was conducted to identify potential shortcomings.Main results.The automated program successfully met all placement constraints with an average execution time (over 24 plans) of 0.29 ±0.07 min per lattice point. The average lattice point density (# points per 100 c.c. of GTV) was similar for automated (0.725) compared to manual placement (0.704). The dosimetric differences between the automated and manual plans were minimal, with statistically significant differences in certain metrics like minimum dose (1.9% versus 1.4%), D5% (52.8% versus 49.4%), D95% (7.1% versus 6.2%), and Body-GTV V30% (20.7 c.c. versus 19.7 c.c.).Significance.This study underscores the feasibility of employing a straightforward Monte Carlo-based algorithm to automate the creation of spherical target structures for VMAT LRT planning. The automated method yields similar dose metrics, enhances inter-planner consistency for larger targets, and requires fewer resources and less time compared to manual placement. This approach holds promise for standardizing treatment planning in prospective patient trials and facilitating its adoption across centers seeking to implement VMAT LRT techniques.

Radio-immune response modelling for spatially fractionated radiotherapy

Objective.Radiation-induced cell death is a complex process influenced by physical, chemical and biological phenomena. Although consensus on the nature and the mechanism of the bystander effect were not yet made, the immune process presumably plays an important role in many aspects of the radiotherapy including the bystander effect. A mathematical model of immune response during and after radiation therapy is presented.Approach.Immune response of host body and immune suppression of tumor cells are modelled with four compartments in this study; viable tumor cells, T cell lymphocytes, immune triggering cells, and doomed cells. The growth of tumor was analyzed in two distinctive modes of tumor status (immune limited and immune escape) and its bifurcation condition.Main results.Tumors in the immune limited mode can grow only up to a finite size, named as terminal tumor volume analytically calculated from the model. The dynamics of the tumor growth in the immune escape mode is much more complex than the tumors in the immune limited mode especially when the status of tumor is close to the bifurcation condition. Radiation can kill tumor cells not only by radiation damage but also by boosting immune reaction.Significance.The model demonstrated that the highly heterogeneous dose distribution in spatially fractionated radiotherapy (SFRT) can make a drastic difference in tumor cell killing compared to the homogeneous dose distribution. SFRT cannot only enhance but also moderate the cell killing depending on the immune response triggered by many factors such as dose prescription parameters, tumor volume at the time of treatment and tumor characteristics. The model was applied to the lifted data of 67NR tumors on mice and a sarcoma patient treated multiple times over 1200 days for the treatment of tumor recurrence as a demonstration.

2D mapping of radiation dose and clonogenic survival for accurate assessment ofin vitroX-ray GRID irradiation effects

Spatially fractionated radiation therapy (SFRT or GRID) is an approach to deliver high local radiation doses in an 'on-off' pattern. To better appraise the radiobiological effects from GRID, a framework to link local radiation dose to clonogenic survival needs to be developed. A549 lung cancer cells were irradiated in T25 cm²flasks using 220 kV x-rays with an open field or through a tungsten GRID collimator with periodical 5 mm openings and 10 mm blockings. Delivered nominal doses were 2, 5, and 10 Gy. A novel approach for image segmentation was used to locate the centroid of surviving colonies in scanned images of the cell flasks. Gafchromic^TMfilm dosimetry (GFD) and FLUKA Monte Carlo (MC) simulations were employed to map the dose at each surviving colony centroid. Fitting the linear-quadratic (LQ) function to clonogenic survival data for open field irradiation, the expected survival level at a given dose level was calculated. The expected survival levels were then mapped together with the observed levels in the GRID-irradiated flasks. GFD and FLUKA MC gave similar dose distributions, with a mean peak-to-valley dose ratio of about 5. LQ-parameters for open field irradiation gaveα=0.24±0.02Gy^-1andβ=0.019±0.002Gy^-2. The mean relative percentage deviation between observed and predicted survival in the (peak; valley) dose regions was (4.6; 3.1) %, (26.6; -1.0) %, and (129.8; -2.3) % for 2, 5 and 10 Gy, respectively. In conclusion, a framework for mapping of surviving colonies following GRID irradiation together with predicted survival levels from homogeneous irradiation was presented. For the given cell line, our findings indicate that GRID irradiation causes reduced survival in the peak regions compared to an open field configuration.

Radiation responses of cancer and normal cells to split dose fractions with uniform and grid fields: increasing the therapeutic ratio

PURPOSE

Radiation treatment of cancer is usually delivered in a prescribed sequence of dose fractions within which the dependence of dose on time is determined by the treatment plan. New techniques, such as stereotactic body radiation therapy (SBRT) and image guided radiation therapy (IGRT) have been introduced with the motivation of improving therapeutic outcomes, with the consequence that the time dependence of the dose within a fraction is modified. Here, we test whether an increased toxicity to cancer cells arises when a radiation treatment fraction is delivered in two equal parts, allowing time for the expression of factors, for example, RONS and cytokines, in response to the first dose which may sensitize cells to the second dose. A medium time delay between 15 and 60 minutes is proposed to allow factors to be expressed before repair takes place. A grid field is used to enhance diffusion of the factors.

MATERIALS AND METHODS

The cell lines used in the study were two prostate cancers (LNCaP and DU 145), a normal prostate (PNT1A), a non-small cell lung cancer (NCI-H460), and a glioma (Hs 683). Uniform or spatially modulated grid fields, delivering the same mean dose, were used. The results for the clonogenic survival fractions were grouped into a 'short' delay (under 10 minutes) and a 'medium' delay (between 15 and 60 minutes).

RESULTS

The medium delay with a grid field yielded a significant increase in toxicity for the four cancer cell lines. The medium delay with a uniform field gave a significant increase in toxicity for the two prostate cancer cell lines. A highly significant increase was found in the therapeutic ratio, defined as the ratio of the survival of prostate normal to prostate cancer cells.

CONCLUSIONS

The findings show that the intra-fractional dose schedule with medium time delay offers an opportunity to increase the toxicity of radiation to cancer cells, relative to a single radiation delivery. For all cancer cell lines, a grid field gives a greater toxic effect than a uniform field. The split dose treatment offers an increase in cancer toxicity while preserving normal cells, improving the outcomes of a treatment.

Impact of radiobiological models on the calculation of the therapeutic parameters of Grid therapy for breast cancer

Therapeutic advantages of Grid therapy have been demonstrated in several theoretical studies using the standard linear-quadratic (LQ) model. However, the suitability of the LQ model when describing cell killing at highly modulated radiation fields has been questioned. In this study, we have applied an extended LQ model to recalculate therapeutic parameters of Grid therapy. This study shows that incorporating the bystander effects in the radiobiological models would significantly change the theoretical predictions and conclusion of Grid therapy, especially at high dose gradient fields.

REVIEW OF QUANTITATIVE MECHANISTIC MODELS OF RADIATION-INDUCED NON-TARGETED EFFECTS (NTE)

Quantitative mechanistic modeling of the biological effects of ionizing radiation has a long rich history. Initially, it was dominated by target theory, which quantifies damage caused by traversal of cellular targets like DNA by ionizing tracks. The discovery that mutagenesis, death and/or altered behavior sometimes occur in cells that were not themselves traversed by any radiation tracks but merely interacted with traversed cells was initially seen as surprising. As more evidence of such 'non-targeted' or 'bystander' effects accumulated, the importance of their contribution to radiation-induced damage became more recognized. Understanding and modeling these processes is important for quantifying and predicting radiation-induced health risks. Here we review the variety of mechanistic mathematical models of nontargeted effects that emerged over the past 2-3 decades. This review is not intended to be exhaustive, but focuses on the main assumptions and approaches shared or distinct between models, and on identifying areas for future research.

Imaging prior to radiotherapy impacts in-vitro survival

Background and purpose

Cone Beam Computed Tomography (CBCT) is routinely used in radiotherapy to identify the position of the target volume. The aim of this study was to determine whether the CBCT dose, when followed by the treatment, influences the therapeutic outcomes as determined by in-vitro clonogenic cell survival in a radiobiological experiment.

Materials and methods

Human cell lines, four cancer and one normal, were exposed to a 6 MV photon beam, produced by a linear accelerator. For half of each sample, a prior imaging dose was delivered using the on-board CBCT. A sample size of n = 103 was used to achieve statistical power.

Results

The experimental group of cell lines exposed to CBCT imaging prior to treatment exhibited a reduction in mean cancer cell survival of ~17 times (p = 0.02) greater than predicted from the average dose response and equivalent to more than 5% of the therapeutic dose, compared to 11 times greater than predicted for normal cells (n.s.).

Conclusion

The greater than predicted reduction in survival resulting from the additional CBCT dose is consistent with radiation-induced bystander effects.

VMAT Grid Therapy: A Widely Applicable Planning Approach

PURPOSE

To describe a novel and practical volumetric modulated arc therapy (VMAT) planning approach for grid therapy.

METHODS AND MATERIALS

Dose is prescribed to 1.5-cm diameter spherical contours placed throughout the gross tumor volume (GTV). Placement of spheres is variable, but they must maintain at least a 3-cm (center to center) separation, and the edge of any sphere must be at least 1 cm from any organ at risk (OAR). Three concentric ring structures are used during optimization to confine the highest doses to the center of the spheres and maximize dose sparing between them. The end result is alternating regions of high and low dose throughout the GTV and minimal dose to OARs. High-intensity flattening filter-free (FFF) modes are used to efficiently deliver the plans, and entire treatments typically take only 15 to 20 minutes.

RESULTS

The approach is illustrated with 2 examples treated at our institution. Patient #1 had a 1703-cm³ mediastinal mass and was prescribed 20 Gray (Gy) to 24 spherical regions within the GTV. Patient #2 had a 3680-cm³ abdominal tumor and was prescribed 18 Gy to 32 spherical regions within the GTV. Both patients received additional consolidative radiation approximately 1 week after the initial VMAT grid treatment. Each patient experienced marked reduction in tumor size and symptomatic relief without treatment-related complications.

CONCLUSIONS

We have described in detail a planning approach for VMAT grid therapy treatments that can typically be delivered in a clinically practical time span. The VMAT approach is especially useful for tumors that are surrounded by sensitive critical structures. As many centers offer VMAT treatments, the approach is widely accessible and can be readily implemented once appropriate patient selection and delivery processes are established.

Coupling of cell fate selection model enhances DNA damage response and may underlie BE phenomenon

Double-strand break-induced (DSB) cells send signal that induces DSBs in neighbour cells, resulting in the interaction among cells sharing the same medium. Since p53 network gives oscillatory response to DSBs, such interaction among cells could be modelled as an excitatory coupling of p53 network oscillators. This study proposes a plausible coupling model of three-mode two-dimensional oscillators, which models the p53-mediated cell fate selection in globally coupled DSB-induced cells. The coupled model consists of ATM and Wip1 proteins as variables. The coupling mechanism is realised through ATM variable via a mean-field modelling the bystander signal in the intercellular medium. Investigation of the model reveals that the coupling generates more sensitive DNA damage response by affecting cell fate selection. Additionally, the authors search for the cause-effect relationship between coupled p53 network oscillators and bystander effect (BE) endpoints. For this, they search for the possible values of uncertain parameters that may replicate BE experiments' results. At certain parametric regions, there is a correlation between the outcomes of cell fate and endpoints of BE, suggesting that the intercellular coupling of p53 network may manifest itself as the form of observed BEs.

Changes of microbial cell survival, metabolic activity, efflux capacity, and quorum sensing ability of Aggregatibacter actinomycetemcomitans due to antimicrobial photodynamic therapy-induced bystander effects

BACKGROUND

The bystander effects, whereby naive (bystander) microbial cells near microbial cells directly exposed to certain treatment show responses that would not have happened in the absence of the directly targeted microbial cells, is recently documented in the field of microbiology. In this article, we discuss that substantial bystander responses are also observed after antimicrobial photodynamic therapy (aPDT) using curcumin (Cur).

MATERIALS AND METHODS

Bystander effects induced by whole bacterial cell suspension (WBCS^T), cell-free supernatants fluid (CFSF^T), and bacterial cell pellet (BCP^T) obtained from A. actinomycetemcomitans culture treated with Cur-aPDT on cell survival, quorum sensing (QS) ability, metabolic activity and efflux capacity of A. actinomycetemcomitans were determined using microbial viability assay, Escherichia coli-based bioassay, XTT reduction method, and ethidium bromide (EtBr) accumulation assay, respectively.

RESULTS

A. actinomycetemcomitans cell survival reduced by 82.7% (P = 0.001) and 76.2% (P = 0.01) after exposure to WBCS^T and CFSF^T, respectively. The A. actinomycetemcomitans population increased by 5.5% (P = 0.7) after exposure to BCP^T. Bacterial metabolic activity decreased by 42.6% (P = 0.02), 35.3% (P = 0.03), and 9.4% (P = 0.5) after exposure to WBCS^T, CFSF^T, and BCP^T, respectively. A. actinomycetemcomitans exposed to WBCS^T, CFSF^T, and BCP^T showed a reduction of 83.2% (P = 0.001), 77.2% (P = 0.01) and 21.9% (P = 0.09) in the QS mediator compared to the WBCS^U, CFSF^U, and BCP^U of untreated A. actinomycetemcomitans, respectively. No significant change of the EtBr accumulation was observed in the three preparations of the Cur-aPDT-treated culture (i.e. WBCS^T, CFSF^T, and BCP^T) compared to their respective controls.

CONCLUSIONS

The results of the current study revealed that Cur-aPDT could significantly reduce microbial cell survival, cell metabolic activity, efflux capacity, and QS ability through the bystander effects. As a result, the bystander effects of Cur-aPDT along with the direct effect of Cur-aPDT can enhance the efficiency of aPDT as an adjunct therapeutic strategy for treatment of local infections.

Spatially fractionated proton minibeams

Extraordinary normal tissue response to highly spatially fractionated X-ray beams has been explored for over 25 years. More recently, alternative radiation sources have been developed and utilized with the aim to evoke comparable effects. These include protons, which lend themselves well for this endeavour due to their physical depth dose characteristics as well as corresponding variable biological effectiveness. This paper addresses the motivation for using protons to generate spatially fractionated beams and reviews the technological implementations and experimental results to date. This includes simulation and feasibility studies, collimation and beam characteristics, dosimetry and biological considerations as well as the results of in vivo and in vitro studies. Experimental results are emerging indicating an extraordinary normal tissue sparing effect analogous to what has been observed for synchrotron generated X-ray microbeams. The potential for translational research and feasibility of spatially modulated proton beams in clinical settings is discussed.