In-vitro investigation of out-of-field cell survival following the delivery of conformal, intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans

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.

Cellular Damage in the Target and Out-Of-Field Peripheral Organs during VMAT SBRT Prostate Radiotherapy: An In Vitro Phantom-Based Study

Simple Summary

New developments show that patients with prostate cancer can benefit from radiotherapy delivered with a hypo-fractionated regimen. The aim of our study was to investigate the effect of hypo-fractionated stereotactic body radiation therapy (SBRT) of prostate cancer on out-of-field organs. We used a humanoid phantom to irradiate prostate cells in conditions similar to patient therapy, using SBRT planning. Our results show that radiation doses in the location of the intestine and lung resulted in significantly higher radiation doses than the further locations. We observed a high radiotoxic effect in the cells irradiated in the prostate, and a small increase in DNA damage and cell killing in the intestine location. Gene expression analysis revealed significant enrichment of the biological processes related to the radiation response in the prostate. In the lung and thyroid, the enrichment of several gene groups was revealed, however the processes were not clearly related to the response to radiation. Our study provides extensive data on out-of-field safety of prostate SBRT.

Abstract

Hypo-fractionated stereotactic body radiation therapy (SBRT) is an effective treatment for prostate cancer (PCa). Although many studies have investigated the effects of SBRT on the prostate and adjacent organs, little is known about the effects further out-of-field. The aim of this study was to investigate, both in vitro and in a quasi-humanoid phantom, the biological effects (using a dose-scaling approach) of radiation in the out-of-field peripheral organs delivered by 6 MV volumetric modulated arc therapy (VMAT) SBRT in a prostate cancer model. Healthy prostate cells were irradiated in a phantom at locations corresponding to the prostate, intestine, lung, thyroid, and brain. Seven 10 Gy fractions of VMAT SBRT were delivered to the target in a single session without intermission (scaled-up method). Radiochromic films were used to measure the doses. The radiobiological response was assessed by measuring DNA breaks, the cell survival fraction, and differences in gene expression profile. Our results showed a strong, multiparametric radiobiological response of the cells in the prostate. Outside of the radiation field, the highest doses were observed in the intestine and lung. A small increase (not statistically significant) in DNA damage and cell death was observed in the intestines. Several gene groups (cell cycle, DNA replication) were depleted in the lung and thyroid (DNA replication, endocytosis), but further analysis revealed no changes in the relevant biological processes. This study provides extensive evidence of the types and extent of radiobiological responses during VMAT SBRT in a prostate cancer model. Additional research is needed to determine whether the radiobiological effects observed in the peripheral organs are validated in a clinical context.

Can dose convolution modelling explain bath and shower effects in rat spinal cord?

Objective. ‘Bath and shower’ effects were first seen in proton irradiations of rat spinal cord, where a low dose ‘bath’ reduced the smaller field ‘shower’ dose needed for limb paralysis giving the appearance of sensitisation of the cord or disproportionate response. This was difficult to reconcile with existing tissue complication models. The purpose of this investigation is to explore a different approach using a dose convolution algorithm to model the 50% isoeffect endpoint. Approach. Bath and shower dose distributions were convolved with Gaussian functions with widths specified by the σ parameter. The hypothesis was that the maximum value from the convolved distributions was constant for isoeffect across the modelled scenarios. A simpler field length dependent relative biological effectiveness (FLRBE) approach was also used for a subset of the data which gave results independent of σ. Main results. The maximum values from the convolved distributions were constant within ±17% across the bath and shower experiments for σ = 3.5 mm, whereas the maximum dose varied by a factor of four. The FLRBE results were also within ±14% confirming the validity of the dose convolution approach. Significance. A simple approach using dose convolution modelling of the 50% isotoxicity gave compelling consistency with the full range of bath and shower results, while the FLRBE approach confirmed the results for the symmetric field data. Convolution modelling and the effect of time interval were consistent with a signalling factor diffusion mechanism such as the ‘bystander effect’. The results suggest biological effectiveness is reduced for very small field sizes, requiring a higher isoeffect dose. By implication, the bath dose does not sensitise the cord to the shower dose; when biological effectiveness is accounted for, a small increase in the bath dose requires a significantly larger reduction in shower dose for isoeffect.

Development of a quasi-humanoid phantom to perform dosimetric and radiobiological measurements for out-of-field doses from external beam radiation therapy

Our understanding of low dose, out‐of‐field radiation and their radiobiological effects are limited, in part due to the rapid technological advances in external beam radiotherapy, especially for non‐coplanar and dynamic techniques. Reliable comparisons of out‐of‐field doses produced by advanced radiotherapy techniques are difficult due to the limitations of commercially available phantoms. There is a clear need for a functional phantom to accurately measure the dosimetric and radiobiological characteristics of out‐of‐field doses, which would in turn allow clinicians and medical physicists to optimize treatment parameters. We designed, manufactured, and tested the performance of a quasi‐humanoid (Q‐H) adult phantom. To test the physics parameters, we used computed tomography (CT) scans of assembled Q‐H phantom. Static open field and dynamic techniques were measured both in‐ and out‐of‐field with ionization chambers and radiochromic films for two configurations (full solid and with water‐filled containers). In the areas simulating soft tissues, lung, and bones, median Hounsfield units and densities were, respectively: 129.8, ‐738.7, 920.8 HU and 1.110, 0.215, 1.669 g/cm³. Comparison of the measured to treatment planning systems (TPS) in‐field dose values for the sample volumetric arc therapy (VMAT) (6 MV flattening filter‐free (FFF)) plan, 96.4% of analyzed points passed the gamma evaluation criteria (L2%/2 mm, threshold (TH) 10%) and less than 1.50% for point dose verification. In the two phantom configurations: full poly(methyl) methacrylate (PMMA) and with water container, the off‐axis median doses for open field, relative to the central axis of the beam (CAX) were similar, respectively: 0.900% versus 0.907% (15 cm distance to CAX); 0.096% versus 0.120% (35 cm); 0.018% versus 0.018% (52 cm); 0.009% versus 0.008% (74 cm). For VMAT 6 MV FFF, doses relative the CAX were, respectively: 0.667% (15 cm), 0.062% (35 cm), 0.019% (52 cm), 0.016% (74 cm). The Q‐H phantom meets the International Commission on Radiation Units and Measurements (ICRU) and American Association of Physicists in Medicine (AAPM) recommended phantom criteria, providing medical physicists with a reliable, comprehensive system to perform dose calculation and measurements and to assess the impact on radiobiological response and on the risk of secondary tumor induction.

Investigating spatial fractionation and radiation induced bystander effects: a mathematical modelling approach

Radiation induced bystander effects (RIBEs) have been shown to cause death in cells receiving little or no physical dose. In standard radiotherapy, where uniform fields are delivered and all cells are directly exposed to radiation, this phenomenon can be neglected. However, the role of RIBEs may become more influential when heterogeneous fields are considered. Mathematical modelling can be used to determine how these heterogeneous fields might influence cell survival, but most established techniques account only for the direct effects of radiation. To gain a full appreciation of how non-uniform fields impact cell survival, it is also necessary to consider the indirect effects of radiation. In this work, we utilise a mathematical model that accounts for both the direct effects of radiation on cells and RIBEs. This model is used to investigate how spatially fractionated radiotherapy plans impact cell survivalin vitro. These predictions were compared to survival in normal and cancerous cells following exposure to spatially fractionated plans using a clinical linac. The model is also used to explore how spatially fractionated radiotherapy will impact tumour controlin vivo. Results suggest that spatially fractionated plans are associated with higher equivalent uniform doses than conventional uniform plans at clinically relevant doses. The model predicted only small changes changes in normal tissue complication probability, compared to the larger protection seen clinically. This contradicts a central paradigm of radiotherapy where uniform fields are assumed to maximise cell kill and may be important for future radiotherapy optimisation.

Novel treatment planning approaches to enhance the therapeutic ratio: targeting the molecular mechanisms of radiation therapy

Radiation acts not only through cell death but has also angiogenic, immunomodulatory and bystander effects. The realization of its systemic implications has led to extensive research on the combination of radiotherapy with systemic treatments, including immunotherapy and antiangiogenic agents. Parameters such as dose, fractionation and sequencing of treatments are key determinants of the outcome. However, recent high-quality research indicates that these are not the only radiation therapy parameters that influence its systemic effect. To effectively integrate systemic agents with radiation therapy, these new aspects of radiation therapy planning will have to be taken into consideration in future clinical trials. Our aim is to review these new treatment planning parameters that can influence the balance between contradicting effects of radiation therapy so as to enhance the therapeutic ratio.

The Impact of Hypoxia on Out-of-Field Cell Survival after Exposure to Modulated Radiation Fields

Advanced radiotherapy techniques such as intensity modulated radiation therapy achieve highly conformal dose distributions within target tumor volumes through the sequential delivery of multiple spatially and temporally modulated radiation fields and have been shown to influence radiobiological response. The goals of this study were to determine the effect of hypoxia on the cell survival responses of different cell models (H460, DU145, A549, MDA231 and FADU) to modulated fields and to characterize the time dependency of signaling under oxic conditions, following reoxygenation and after prolonged hypoxia. Hypoxia was induced by incubating cells at 95% nitrogen and 5% carbon dioxide for 4 h prior to irradiation. The out-of-field response in MDA231 cells was oxygen dependent and therefore selected for co-culture studies to determine the signaling kinetics at different time intervals after irradiation under oxic and hypoxic conditions. Under both oxic and hypoxic conditions, significant increases in cell survival were observed in-field with significant decreases in survival observed out-of-field (P < 0.05), which were dependent on intercellular communication. The in-field response of MDA231 cells showed no significant time dependency up to 24 h postirradiation, while out-of-field survival decreased significantly during the first 6 h postirradiation (P < 0.05). While in-field responses were oxygen dependent, out-of-field effects were observed to be independent of oxygen, with similar or greater cell killing under hypoxic conditions. This study provides further understanding of intercellular signaling under hypoxic conditions and highlights the need for further refinement of established radiobiological models for future applications in advanced radiotherapies.

Neighbor effect: penumbra-dose exposed neighbor cells contribute to the enhanced survival of high-dose targeted cells

PURPOSE

In the last decade, new types of 'bystander effect' have been suggested by multiple research groups and have been challenged by others. In this study, we explored a new type of bystander effect, which has been defined in previous studies as the enhancement of the survival of high-dose targeted cells due to the penumbra-dose exposed neighbor cells. Intensity-modulated radiation therapy, which is the most widely used treatment modality, generates local regions of gradient doses between targeted and shielded cells throughout the treatment volume; therefore, we were urged to ascertain whether the new type of effect is real and to suggest a revised treatment planning.

MATERIALS AND METHODS

Cellular responses under non-uniform beam fields were observed in rat gliosarcoma cells, rat diencephalon cells, and mouse endothelial cells. The cells were irradiated with 200 kVp X-rays in two types: (1) all the cells in the flask were exposed to the X-ray beam (whole-beam exposure) and (2) half of the cells in the flask were exposed to the beam while the other half, or neighbor cells, were shielded from the beam (half-beam exposure). Target cells were exposed to 1, 2, 4, 6, 8, and 10 Gy, and the penumbra dose was approximately 10%-20% of the target dose.

RESULTS

Target cells survived high-dose (> 6 Gy) radiation exposures better under half-beam exposure with the low penumbra-dose exposed neighbor cells around than under whole-beam exposure. The survival of the targeted cells from half-beam exposure was reduced when the radiation self-conditioned medium was replaced with a fresh one immediately after irradiation. Survival was further reduced when the targeted cells were harvested immediately after irradiation and incubated in new dishes with fresh culture media until the colony was counted.

CONCLUSION

We have collected data of good statistics by several post-irradiation treatments of targeted cells to ascertain that the new type of bystander effect is real. The low penumbra-dose exposed neighbor cells benefited the survival of the high-dose targeted cells.

A meta-analysis of the abscopal effect in preclinical models: Is the biologically effective dose a relevant physical trigger?

BACKGROUND

Preclinical in vivo studies using small animals are considered crucial in translational cancer research and clinical implementation of novel treatments. This is of paramount relevance in radiobiology, especially for any technological developments permitted to deliver high doses in single or oligo-fractionated regimens, such as stereotactic ablative radiotherapy (SABR). In this context, clinical success in cancer treatment needs to be guaranteed, sparing normal tissue and preventing the potential spread of disease or local recurrence. In this work we introduce a new dose-response relationship based on relevant publications concerning preclinical models with regard to delivered dose, fractionation schedule and occurrence of biological effects on non-irradiated tissue, abscopal effects.

METHODS

We reviewed relevant publications on murine models and the abscopal effect in radiation cancer research following PRISMA methodology. In particular, through a log-likelihood method, we evaluated whether the occurrence of abscopal effects may be related to the biologically effective dose (BED). To this aim, studies accomplished with different tumor histotypes were considered in our analysis including breast, colon, lung, fibrosarcoma, pancreas, melanoma and head and neck cancer. For all the tumors, the α / β ratio was assumed to be 10 Gy, as generally adopted for neoplastic cells.

RESULTS

Our results support the hypothesis that the occurrence rate of abscopal effects in preclinical models increases with BED. In particular, the probability of revealing abscopal effects is 50% when a BED of 60 Gy is generated.

CONCLUSION

Our study provides evidence that SABR treatments associated with high BEDs could be considered an effective strategy in triggering the abscopal effect, thus shedding light on the promising outcomes revealed in clinical practice.

Treatment of breast cancer with simultaneous integrated boost in hybrid plan technique : Influence of flattening filter-free beams

OBJECTIVE

The present study compares in silico treatment plans using hybrid plan technique during hypofractionated radiation of mammary carcinoma with simultaneous integrated boost (SIB). The influence of 6 MV photon radiation in flattening filter free (FFF) mode against the clinical standard flattening filter (FF) mode is to be examined.

PATIENTS AND METHODS

RT planning took place with FF and FFF radiation plans for 10 left-sided breast cancer patients. Hybrid plans were realised with two tangential IMRT fields and one VMAT field. The dose prescription was in line with the guidelines in the ARO-2010-01 study. The dosimetric verification took place with a manufacturer-independent measurement system.

RESULTS

Required dose prescriptions for the planning target volumes (PTV) were achieved for both groups. The average dose values of the ipsi- and contralateral lung and the heart did not differ significantly. The overall average incidental dose to the left anterior descending artery (LAD) of 8.24 ± 3.9 Gy in the FFF group and 9.05 ± 3.7 Gy in the FF group (p < 0.05) were found. The dosimetric verifications corresponded to the clinical requirements. FFF-based RT plans reduced the average treatment time by 17 s/fraction.

CONCLUSION

In comparison to the FF-based hybrid plan technique the FFF mode allows further reduction of the average LAD dose for comparable target volume coverage without adverse low-dose exposure of contralateral structures. The combination of hybrid plan technique and 6 MV photon radiation in the FFF mode is suitable for use with hypofractionated dose schemes. The increased dose rate allows a substantial reduction of treatment time and thus beneficial application of the deep inspiration breath hold technique.

Radioluminescence characterization of in situ x-ray nanodosimeters: Potential real-time monitors and modulators of external beam radiation therapy

Europium-doped yttrium oxide (Y₂O₃:Eu) has garnered considerable interest recently for its use as a highly efficient, red phosphor in a variety of lighting applications that include fluorescent lamps, plasma, and field emission display panels, light emitting diodes (LEDs), and lasers. In the present work, we describe the development of Y₂O₃:Eu nanoparticles for a very different application: in situ, in vivo x-ray dosimetry. Spectroscopic analyses of these nanoparticles during x-ray irradiation reveal surprisingly bright and stable radioluminescence at near-infrared wavelengths, with markedly linear response to changes in x-ray flux and energy. Monte Carlo modeling of incident flux and broadband, wide-field imaging of mouse phantoms bearing both Y₂O₃:Eu nanoparticles and calibrated LEDs of similar spectral emission demonstrated significant transmission of radioluminescence, in agreement with spectroscopic studies; with approximately 15 visible photons being generated for every x-ray photon incident. Unlike the dosimeters currently employed in clinical practice, these nanodosimeters can sample both dose and dose rate rapidly enough as to provide real-time feedback for x-ray based external beam radiotherapy (EBRT). The technique's use of remote sensing and absence of supporting structures enable perturbation-free dosing of the targeted region and complete sampling from any direction. With the conjugation of pathology-targeting ligands onto their surfaces, these nanodosimeters offer a potential paradigm shift in the real-time monitoring and modulation of delivered dose in the EBRT of cancer in situ.

Use dose bricks concept to implement nasopharyngeal carcinoma treatment planning

Purpose. A “dose bricks” concept has been used to implement nasopharyngeal carcinoma treatment plan; this method specializes particularly in the case with bell shape nasopharyngeal carcinoma case. Materials and Methods. Five noncoplanar fields were used to accomplish the dose bricks technique treatment plan. These five fields include (a) right superior anterior oblique (RSAO), (b) left superior anterior oblique (LSAO), (c) right anterior oblique (RAO), (d) left anterior oblique (LAO), and (e) superior inferior vertex (SIV). Nondivergence collimator central axis planes were used to create different abutting field edge while normal organs were blocked by multileaf collimators in this technique. Results. The resulting 92% isodose curves encompassed the CTV, while maximum dose was about 115%. Approximately 50% volume of parotid glands obtained 10–15% of total dose and 50% volume of brain obtained less than 20% of total dose. Spinal cord receives only 5% from the scatter dose. Conclusions. Compared with IMRT, the expenditure of planning time and costing, “dose bricks” may after all be accepted as an optional implementation in nasopharyngeal carcinoma conformal treatment plan; furthermore, this method also fits the need of other nonhead and neck lesions if organ sparing and noncoplanar technique can be executed.

High dose bystander effects in spatially fractionated radiation therapy

Traditional radiotherapy of bulky tumors has certain limitations. Spatially fractionated radiation therapy (GRID) and intensity modulated radiotherapy (IMRT) are examples of advanced modulated beam therapies that help in significant reductions in normal tissue damage. GRID refers to the delivery of a single high dose of radiation to a large treatment area that is divided into several smaller fields, while IMRT allows improved dose conformity to the tumor target compared to conventional three-dimensional conformal radiotherapy. In this review, we consider spatially fractionated radiotherapy approaches focusing on GRID and IMRT, and present complementary evidence from different studies which support the role of radiation induced signaling effects in the overall radiobiological rationale for these treatments.