Monte Carlo modeling and optimization of contrast-enhanced radiotherapy of brain tumors

Nanomaterial-Based Molecular Imaging in Cancer: Advances in Simulation and AI Integration

Nanomaterials represent an innovation in cancer imaging by offering enhanced contrast, improved targeting capabilities, and multifunctional imaging modalities. Recent advancements in material engineering have enabled the development of nanoparticles tailored for various imaging techniques, including magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), and ultrasound (US). These nanoscale agents improve sensitivity and specificity, enabling early cancer detection and precise tumor characterization. Monte Carlo (MC) simulations play a pivotal role in optimizing nanomaterial-based imaging by modeling their interactions with biological tissues, predicting contrast enhancement, and refining dosimetry for radiation-based imaging techniques. These computational methods provide valuable insights into nanoparticle behavior, aiding in the design of more effective imaging agents. Moreover, artificial intelligence (AI) and machine learning (ML) approaches are transforming cancer imaging by enhancing image reconstruction, automating segmentation, and improving diagnostic accuracy. AI-driven models can also optimize MC-based simulations by accelerating data analysis and refining nanoparticle design through predictive modeling. This review explores the latest advancements in nanomaterial-based cancer imaging, highlighting the synergy between nanotechnology, MC simulations, and AI-driven innovations. By integrating these interdisciplinary approaches, future cancer imaging technologies can achieve unprecedented precision, paving the way for more effective diagnostics and personalized treatment strategies.

Radio-enhancement effects by radiolabeled nanoparticles

In cancer radiation therapy, dose enhancement by nanoparticles has to date been investigated only for external beam radiotherapy (EBRT). Here, we report on an in silico study of nanoparticle-enhanced radiation damage in the context of internal radionuclide therapy. We demonstrate the proof-of-principle that clinically relevant radiotherapeutic isotopes (i.e. 213Bi, 223Ra, 90Y, 177Lu, 67Cu, 64Cu and 89Zr) labeled to clinically relevant superparamagnetic iron oxide nanoparticles results in enhanced radiation damage effects localized to sub-micron scales. We find that radiation dose can be enhanced by up to 20%, vastly outperforming nanoparticle dose enhancement in conventional EBRT. Our results demonstrate that in addition to the favorable spectral characteristics of the isotopes and their proximity to the nanoparticles, clustering of the nanoparticles results in a nonlinear collective effect that amplifies nanoscale radiation damage effects by electron-mediated inter-nanoparticle interactions. In this way, optimal radio-enhancement is achieved when the inter-nanoparticle distance is less than the mean range of the secondary electrons. For the radioisotopes studied here, this corresponds to inter-nanoparticle distances <50 nm, with the strongest effects within 20 nm. The results of this study suggest that radiolabeled nanoparticles offer a novel and potentially highly effective platform for developing next-generation theranostic strategies for cancer medicine.

Influence of concentration, nanoparticle size, beam energy, and material on dose enhancement in radiation therapy

The purpose of this study was to analyse the effects of the type, concentration, and nanoparticle diameter of dose enhancement materials on the dose enhancement of low- and high-energy megavoltage (MV) X-rays acquired from a medical linear accelerator using Monte Carlo simulation. Monte Carlo simulation was performed with the Monte Carlo N-Particle Transport (MCNPX) code, using the energy spectrum of the linear accelerator and a mathematical Snyder head phantom. A 5-cm-diameter virtual tumour was defined in the centre of the phantom. Gold, gadolinium, iodine and iron oxide were used as dose enhancement materials. Varying concentrations (7, 18 and 30 mg/g) of nanoparticles of different diameters (25, 50, 75, 100 and 125 nm) were applied, and the dose enhancement was comparatively evaluated for 4, 6, 10 and 15 MV X-rays, and a 60Co source. Higher dose enhancement factors (DEFs) were observed when the incident energy was low. Moreover, the dose enhancement effects were greatest with gold nanoparticles, followed by gadolinium, iodine, and iron oxide nanoparticles; the DEFs were 1.011-1.047 (gold), 1.005-1.030 (gadolinium), 1.002-1.028 (iodine) and 1.002-1.014 (iron oxide). The dose enhancement effects increased with increasing nanoparticle diameter and concentration. However, the concentration of the material had a greater impact than the diameter of the nanoparticles. As the concentration and diameter of nanoparticles increased, the DEF also increased. The 4 and 6 MV X-rays demonstrated higher dose enhancement compared with the 10 and 15 MV X-rays.

Feasibility of robotic stereotactic body radiotherapy of lung tumors with kilovoltage x-ray beams

PURPOSE

Robotic Stereotactic body radiation therapy (SBRT) for lung tumors is treatment modality that, for cases of inoperable lung tumors, has shown excellent treatment outcomes. The typical photon energy when delivering this type of treatments is 6 MeV. In this work, using Monte Carlo simulation and realistic patient models we evaluate the characteristics of the absorbed dose distributions that result when x-ray beams of peak spectral energy of 220 keV are used to irradiate lung tumors assuming a robotic SBRT delivery mode.

METHODS

Both male and female patient models, based on voxelized phantoms, are used in our study. Two types of tumors are considered: centrally and peripherally located lung tumors. The Monte Carlo code PENELOPE was used to calculate absorbed dose distributions for each of the beams used in the treatments which were assumed circular with diameter ranging from 1 cm-3 cm. An optimization algorithm was then applied to determine the appropriate beam weight necessary to accomplish the treatment objectives. The feasibility of our proposed approach is determined based on the guidelines set by Radiation Therapy Oncology Group (RTOG) 0813 for central tumors and RTOG 0915 for peripheral tumors.

RESULTS

While the dose to the skin and bony structures is higher for the kilovoltage treatment, they are within the safe limits established by both RTOG 0813 and 0915. Conversely, the maximum dose to distant structures, such as the heart wall and esophagus, are up to 10 Gy higher for some of the megavoltage treatments but, again, within the limits recommended by the aforementioned clinical protocols.

CONCLUSIONS

We have shown that robotic SBRT of lung tumors using kilovoltage beams is feasible and that, therefore, it may represent an attractive alternative to the use of more expensive megavoltage linear accelerators, particularly in developing countries, where the high cost of such equipment poses an increasing economic burden.

Stereotactic breast irradiation with kilovoltage x-ray beams

The purpose of this work is to determine, using Monte Carlo simulation and a realistic patient model, the characteristics of the resultant absorbed dose distributions when breast tumors are irradiated using small-field stereotactic body radiation therapy (SBRT) with kilovoltage x-ray beams instead of the standard megavoltage energies currently in use. The Rensselaer Polytechnic Institute (RPI) female phantom was used to model a pair of small-field SBRT breast treatments: in one treatment the tumor at depth and another one with the tumor located close to the breast surface. Each treatment consisted of 300 circular beams aimed at the tumor from a plurality of positions. The PENELOPE Monte Carlo code was used to determine the absorbed dose distribution for each beam and subsequently an optimization algorithm determined each beam weight according to a set of prescription goals. Both kilo- and megavoltage beam treatments were modeled, the latter to be used as a reference. Cumulative dose-volume histograms for eleven structures were used to compare the kilovoltage and reference treatments. Integral dose values are also reported. Absorbed dose distributions for the target volumes as well as the organs at risk were within the parameters reported in a clinical trial for both treatments. While for the ipsilateral healthy breast tissue the megavoltage treatment does offer an advantage in terms of less volume irradiated to intermediate doses, for the contralateral structures, breast and lung, the low penetration ability of the kilovoltage treatment results in a lower maximum dose. Skin dose is higher for the kilovoltage treatment but still well within the tolerance limits reported in the clinical trial.

Robotic stereotactic radioablation of breast tumors: Influence of beam size on the absorbed dose distributions

Robotic stereotactic radioablation (RSR) therapy for breast tumors has been shown to be an effective treatment strategy when applied concomitantly with chemotherapy, with the purpose of reducing the tumor volume thus making it more amenable for breast conserving surgery. In this paper we used Monte Carlo simulation within a realistic patient model to determine the influence that the variation in beam collimation radius has on the resultant absorbed dose distributions for this type of treatment. Separate optimized plans were obtained for treatments using 300 circular beams with radii of 0.5 cm, 0.75 cm, 1.0 cm and 1.5 cm. Cumulative dose volume histograms were obtained for the gross, clinical and planning target volumes as well as for eight organs and structures at risk. Target coverage improves as the collimator size is increased, at the expense of increasing the volume of healthy tissue receiving mid-level absorbed doses. Interestingly, it is found that the maximum dose imparted to the skin is highly dependent on collimator size, while the dosimetry of other structures, such as both the ipsilateral and contralateral lung tissue are basically unaffected by a change in beam size.

Nanoparticles for Radiation Therapy Enhancement: the Key Parameters

This review focuses on the radiosensitization strategies that use high-Z nanoparticles. It does not establish an exhaustive list of the works in this field but rather propose constructive criticisms pointing out critical factors that could improve the nano-radiation therapy. Whereas most reviews show the chemists and/or biologists points of view, the present analysis is also seen through the prism of the medical physicist. In particular, we described and evaluated the influence of X-rays energy spectra using a numerical analysis. We observed a lack of standardization in preclinical studies that could partially explain the low number of translation to clinical applications for this innovative therapeutic strategy. Pointing out the critical parameters of high-Z nanoparticles radiosensitization, this review is expected to contribute to a larger preclinical and clinical development.

Does administering iodine in radiological procedures increase patient doses?

PURPOSE

The authors investigated the changes in the pattern of energy deposition in tissue equivalent phantoms following the introduction of iodinated contrast media.

METHODS

The phantom consisted of a small "contrast sphere," filled with water or iodinated contrast, located at the center of a 28 cm diameter water sphere. Monte Carlo simulations were performed using mcnp5 codes, validated by simulating irradiations with analytical solutions. Monoenergetic x-rays ranging from 35 to 150 keV were used to simulate exposures to spheres containing contrast agent with iodine concentrations ranging from 1 to 100 mg/ml. Relative values of energy imparted to the contrast sphere, as well as to the whole phantom, were calculated. Changes in patterns of energy deposition around the contrast sphere were also investigated.

RESULTS

Small contrast spheres can increase local absorbed dose by a factor of 13, but the corresponding increase in total energy absorbed was negligible (<1%). The highest localized dose increases were found to occur at incident photon energies of about 60 keV. For a concentration of about 10 mg/ml, typical of clinical practice, localized absorbed doses were generally increased by about a factor of two. At this concentration of 10 mg/ml, the maximum increase in total energy deposition in the phantom was only 6%. These simulations demonstrated that increases in contrast sphere doses were offset by corresponding dose reductions at distal and posterior locations.

CONCLUSIONS

Adding iodine can result in values of localized absorbed dose increasing by more than an order of magnitude, but the total energy deposition is generally very modest (i.e., <10%). Their data show that adding iodine primarily changes the pattern of energy deposition in the irradiated region, rather than increasing patient doses per se.

Calculation of fluence and absorbed dose in head tissues due to different photon energies

Calculations of fluence and absorbed dose in head tissues due to different photon energies were carried out using the MCNPX code, to simulate two models of a patient's head: one spherical and another more realistic ellipsoidal. Both head models had concentric shells to describe the scalp skin, the cranium and the brain. The tumor was located at the center of the head and it was a 1 cm-radius sphere. The MCNPX code was run for different energies. Results showed that the fluence decreases as the photons pass through the different head tissues. It can be observed that, although the fluence into the tumor is different for both head models, absorbed dose is the same.

Feasibility of kilovoltage x-ray energy modulation by gaseous media and its application in contrast-enhanced radiotherapy

PURPOSE

To present a method to modulate the energy contents of a kilovoltage x-ray beam that makes use of a gas as the modulating medium. The method is capable of producing arbitrary x-ray spectra by varying the pressure of the modulating gas and the peak kilovoltage (kVp) of the x-ray beams whose energy is being modulated.

METHODS

An aluminum chamber was machined with a 0.5 cm wall thickness, designed to withstand pressures of more than 80 atm. A pressure sensor and electrovalves were used to monitor and regulate the gas pressure. Argon was used as the modulating gas. A CdTe spectrometer was used to measure x-ray spectra for different combinations of kVp and gas pressure, thus obtaining a set of basis x-ray functions. An arbitrary x-ray spectrum can then be formed by the linear combination of such basis functions. In order to show one possible application of the modulation method, a contrast-enhanced radiotherapy prostate treatment was optimized with respect to the x-ray beam energy, without restrictions on the possible shape of the resultant x-ray spectra.

RESULTS

The x-ray spectra basis functions obtained display a smooth and gradual variation of their average energy as a function of the gas pressure for a given kVp, sometimes in the order of 1 or 2 keV. This gradual variation would be difficult to obtain with a conventional aluminum or copper filters, as the change in thickness necessary to reproduce the data presented would be in the order of micrometers, making necessary the use of a large number of such filters. Using the modulation method presented here, the authors were able to reconstruct the optimized x-ray spectra from the measured basis functions, for different optimization objectives.

CONCLUSIONS

A method has been developed that allows for the controlled modulation of the energy contents of kilovoltage x-ray spectra. The method has been shown to be able to reproduce spectra of arbitrary shape, such as those obtained from the optimization of contrast-enhanced radiotherapy. The method may have other applications as well, such as in the precise matching of diagnostic x-ray catalog spectra.

Microdosimetry of X-ray-irradiated gold nanoparticles

The use of contrast agents, particularly those made of high atomic number elements like gold nanoparticles, to enhance the X-ray absorption properties of tissue has recently gained attention in the context of radiotherapy treatments. Because these contrast agents alter the secondary electron field in the irradiated medium by adding an Auger electron component, it is necessary to determine the change in the microdosimetric spectra brought about by the incorporation of such agents. Using Monte Carlo simulation, it is shown that the linear energy transfer and the beam quality factor in the vicinity of a gold nanoparticle irradiated with kilovoltage X-ray beams increase substantially when compared with irradiation without the gold nanoparticles present.

Monte Carlo modeling of converging small-field contrast-enhanced radiotherapy of prostate

Radiation therapy using a kilovoltage X-ray source to irradiate a target previously loaded with a radiological contrast agent, contrast-enhanced radiotherapy (CERT), has been shown both theoretically and in a preliminary experimental study to represent a potential alternative to high-energy treatments. It has also been shown, however, to produce an integral dose that can be up to twice that resulting from a conventional megavoltage treatment. In this work, using a realistic patient model and Monte Carlo simulation, a CERT prostate treatment plan is designed that makes use of a plurality of small circular beams aimed at the target in such a way as to minimize the radiological trajectory to the target volume. Gold nanoparticles are assumed to be the contrast agent. Two cases are examined, one with a concentration level in the target of 10 mg-Au per gram of tissue and the second with a concentration of 3 mg-Au per gram of tissue in the target. A background concentration of 1 mg of contrast agent per gram of tissue was assumed everywhere else in both cases. The Cimmino feasibility algorithm was then used to find each beam weight in order to obtain the prescribed target dose, set at 72 Gy to 100% of the tumor volume. It is shown that the approach using the small circular fields, a radiosurgery treatment, produces treatment plans with excellent absorbed dose distributions while at the same time it reduces by up to 60% the non-tumor integral dose imparted to the irradiated subject. A brief discussion on the technology necessary to clinically implement this treatment modality is also presented.