Microscopic Gold Particle-Based Fiducial Markers for Proton Therapy of Prostate Cancer

A clinical study comparing polymer and gold fiducials for prostate cancer radiotherapy

Introduction

Image guidance with gold fiducials improves outcomes of prostate radiotherapy. However, gold produces artefact on CT imaging, interfering with contouring and verification. The purpose of this study was to compare polymer to standard gold fiducials using radiotherapy imaging modalities to assess the visibility and artefact.

Methods

Twenty eight patients with locally advanced prostate cancer were enrolled, half had three polymer fiducials implanted into the prostate and half underwent insertion of gold fiducials. Patients were imaged with CT, T2 weighted MRI, cone-beam CT (CBCT) and planar KV images. Fiducials were scored for visibility and assessed for CT artefact in surrounding prostate tissue. The artefact was quantified from Hounsfield number histograms and separated into percentile ranges and proportion of voxels in HU normal tissue range of a 2cm sphere surrounding the fiducial.

Results

Gold and polymer fiducials were sufficiently visible for CT and CBCT verification. The gold fiducials could be visualized well on KV planar imaging; however, the polymer markers were obscured by pelvic bones. Neither polymer nor gold fiducials could be visualized on MRI. The polymer fiducial produced less artefact than gold on CT, having less voxel spread for the HU percentile ranges and a greater proportion of voxels in the normal tissue range.

Conclusions

Polymer fiducials are a more suitable fiducial than gold for CT/CBCT in prostate cancer radiotherapy, demonstrating minimal artefact and good visibility on CT. However, they were not well seen on MRI or KV imaging and thus not suitable for co-registration or planar KV verification.

A review of artificial intelligence applications for motion tracking in radiotherapy

During radiotherapy, the organs and tumour move as a result of the dynamic nature of the body; this is known as intrafraction motion. Intrafraction motion can result in tumour underdose and healthy tissue overdose, thereby reducing the effectiveness of the treatment while increasing toxicity to the patients. There is a growing appreciation of intrafraction target motion management by the radiation oncology community. Real-time image-guided radiation therapy (IGRT) can track the target and account for the motion, improving the radiation dose to the tumour and reducing the dose to healthy tissue. Recently, artificial intelligence (AI)-based approaches have been applied to motion management and have shown great potential. In this review, four main categories of motion management using AI are summarised: marker-based tracking, markerless tracking, full anatomy monitoring and motion prediction. Marker-based and markerless tracking approaches focus on tracking the individual target throughout the treatment. Full anatomy algorithms monitor for intrafraction changes in the full anatomy within the field of view. Motion prediction algorithms can be used to account for the latencies due to the time for the system to localise, process and act.

A phantom study to contrast and compare polymer and gold fiducial markers in radiotherapy simulation imaging

To assess visibility and artifact characteristics of polymer fiducials compared to standard gold fiducials for radiotherapy CT and MRI simulation. Three gold and three polymer fiducials were inserted into a CT and MRI tissue-equivalent phantom that approximated the prostate cancer radiotherapy configuration. The phantom and fiducials were imaged on CT and MRI. Images were assessed in terms of fiducial visibility and artifact. ImageJ was employed to quantify the pixel gray-scale of each fiducial and artifact. Fiducial gray-scale histograms and profiles were generated for analysis. Objective measurements of the contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and artifact index (AI) were calculated. The CT images showed that the gold fiducials are visually brighter, with greater contrast than the polymer. The higher peak values illustrate this in the line profiles. However, they produce bright radiating and dark shadowing artifacts. This is depicted by the greater width of line profiles and the disruption of phantom area profiles. Quantitatively this results in greater percentile ranges of the histograms. Furthermore, for CT, gold had a higher CNR than polymer, relative to the phantom. However, the gold CNR and SNR were degraded by the greater artifact and thus AI. Both fiducials were visible on MRI and had similar histograms and profiles that were also reflected in comparable CNR, SNR and AI. Polymer fiducials were well visualized in a phantom on CT and MR and produce less artifact than the gold fiducials. Polymer markers could enhance the quality and accuracy of radiotherapy co-registration and planning but require clinical confirmation.

Biological impact of dosimetric perturbations of a fiducial marker and the daily number of fields in proton therapy for prostate cancer

The purpose of this study was to estimate the biological impact of dosimetric perturbations of a fiducial marker and the daily number of fields in proton therapy for prostate cancer. Using a linear-quadratic model, normalized total doses (NTDs) of points where deposited dose was reduced from the prescribed dose by dosimetric perturbation of a fiducial marker were calculated in two hypothetical prostate cancer treatment schedules: a) irradiation of both parallel-opposed lateral fields and b) irradiation of alternate field in each daily treatment. The impact of hypofractionation and sublethal damage repair between irradiation on NTD was also estimated. The NTD of two fields/day schedule becomes lower than that of one field/day schedule. The difference becomes larger as dose reduction from one of two fields becomes more enhanced. The NTD reduction from the total dose in the two fields/day schedule is largest (30% of total dose) where the dose from one beam is completely lost by a fiducial marker. In contrast, the NTD reduction from the total dose in the one field/day schedule is largest (9% of total dose) where the half dose from one beam is decreased by a fiducial marker. In addition, the NTD reduction becomes larger as the fractional dose increases in a hypofractionated regimen, and when the effect of sublethal damage repair was incorporated. These influences become significant in prostate cancer since the radiobiological sensitivity α/β of prostate cancer is lower than other cancer types and normal tissues late complication. Treating with one alternate field in a daily treatment can improve a deteriorating treatment effect by dosimetric distortion of a fiducial marker in prostate cancer treatment. However, the choice of the number of beams in a fraction must also be determined by considering the sparing of normal tissues and patient-specific status.

GEANT4 simulation of a range verification method using delayed γ spectroscopy of a 92Mo marker

In this work, we propose a novel technique for in-vivo proton therapy range verification. This technique makes use of a molybdenum hadron tumour marker, implanted at a short distance from the clinical treatment volume. Signals emitted from the marker during treatment can provide a direct measurement of the proton beam energy at the marker's position. Fusion-evaporation reactions between the proton beam and marker nucleus result in the emission of delayed characteristic γ rays, which are detected off-beam for an improved signal-to-noise ratio. In order to determine the viability of this technique and to establish an experimental setup for future work, the Monte Carlo package GEANT4 was used in combination with ROOT to simulate a treatment scenario with the new method outlined in this work. These simulations show that the intensity of delayed γ rays produced from competing reactions yields a precise measurement of the range of the proton beam relative to the marker, with sub-millimetre uncertainty.

Results from the experimental evaluation of CeBr 3 scintillators for 4 He prompt gamma spectroscopy

PURPOSE

The presence of range uncertainties hinders the exploitation of the full potential of charged particle therapy. Several range verification techniques have been proposed to mitigate this limitation. Prompt gamma spectroscopy (PGS) is among the most promising solutions for online and in vivo range verification. In this work, we present the experimental results of the detection of prompt gamma radiation, induced by 4 He beams at the Heidelberg Ion-Beam Therapy Center (HIT). The results were obtained, using a spectroscopic unit of which the design has been optimized using Monte Carlo simulations.

METHODS

The spectroscopic unit is composed by a primary cerium bromide (CeBr 3 ) crystal surrounded by a secondary bismuth germanate (BGO) crystal for anticoincidence detection (AC). The digitalization of the signals is performed with an advanced FADC/FPGA system. The 4 He beams at clinical energies and intensities are delivered to multiple targets in the experimental cave at the HIT. We analyze the production of prompt gamma on oxygen and carbon targets, as well as high Z materials such as titanium and aluminum. The quantitative analysis includes a systematic comparison of the signal-to-noise ratio (SNR) improvement for the spectral lines when introducing the AC detection. Moreover, the SNR improvement could provide a reduction of the number of events required to draw robust conclusions. We perform a statistic analysis to determine the magnitude of such an effect.

RESULTS

We present the energy spectra detected by the primary CeBr 3 and the secondary BGO. The combination of these two detectors leads to an average increase of the signal-to-noise ratio by a factor 2.1, which confirms the Monte Carlo predictions. The spectroscopic unit is capable of detecting efficiently the discrete gamma emission over the full energy spectrum. We identify and analyze 19 independent spectral lines in an energy range spacing from E γ = 0.718  MeV to E γ = 6.13  MeV. Moreover, when introducing the AC detection, the number of events required to determine robustly the intensity of the discrete lines decreases. Finally, the analysis of the low-energy reaction lines determines whether a thin metal insert is introduced in the beam direction.

CONCLUSIONS

This work provides the experimental characterization of the spectroscopy unit in development for range verification through PGS at the HIT. Excellent performances have been demonstrated over the full prompt gamma energy spectrum with 4 He beams at clinical energies and intensities.

Knowledge of endoscopic ultrasound-delivered fiducial composition and dimension necessary when planning proton beam radiotherapy

BACKGROUND AND STUDY AIMS

Little consideration has been given to selection of endoscopic ultrasound-guided fiducials for proton radiotherapy and the resulting perturbations in the therapy dose and pattern. Our aim was to assess the impact of perturbations caused by six fiducials of different composition and dimensions in a phantom gel model.

MATERIALS AND METHODS

 The phantom was submerged in a water bath and irradiated with a uniform 10 cm × 10 cm field of 119.7 MeV monoenergetic spot scanning protons delivered through a 45 mm range shifter. The proton "Bragg Peak" was evaluated.

RESULTS

 Dose perturbations manifesting as dose reductions up to 30 % were observed. A carbon composite (1 × 5 mm) and gold (0.4 × 10 mm) fiducial with backload potential rather than dedicated EUS pre-loaded gold fiducial needles had the best performance in terms of minimizing the dose perturbation.

CONCLUSIONS

 Our data demonstrate that a carbon composite fiducial has a less untoward effect on proton therapy dose distribution than dedicated EUS pre-loaded gold fiducial needles. Such information is important to consider when selecting fiducials specifically for proton therapy.

Novel real-time tumor-contouring method using deep learning to prevent mistracking in X-ray fluoroscopy

Robustness to obstacles is the most important factor necessary to achieve accurate tumor tracking without fiducial markers. Some high-density structures, such as bone, are enhanced on X-ray fluoroscopic images, which cause tumor mistracking. Tumor tracking should be performed by controlling "importance recognition": the understanding that soft-tissue is an important tracking feature and bone structure is unimportant. We propose a new real-time tumor-contouring method that uses deep learning with importance recognition control. The novelty of the proposed method is the combination of the devised random overlay method and supervised deep learning to induce the recognition of structures in tumor contouring as important or unimportant. This method can be used for tumor contouring because it uses deep learning to perform image segmentation. Our results from a simulated fluoroscopy model showed accurate tracking of a low-visibility tumor with an error of approximately 1 mm, even if enhanced bone structure acted as an obstacle. A high similarity of approximately 0.95 on the Jaccard index was observed between the segmented and ground truth tumor regions. A short processing time of 25 ms was achieved. The results of this simulated fluoroscopy model support the feasibility of robust real-time tumor contouring with fluoroscopy. Further studies using clinical fluoroscopy are highly anticipated.

Liquid fiducial marker applicability in proton therapy of locally advanced lung cancer

BACKGROUND AND PURPOSE

We investigated the clinical applicability of a novel liquid fiducial marker (LFM) for image-guided pencil beam scanned (PBS) proton therapy (PBSPT) of locally advanced lung cancer (LALC).

MATERIALS AND METHODS

The relative proton stopping power (RSP) of the LFM was calculated and measured. Dose perturbations of the LFM and three solid markers, in a phantom, were measured. PBSPT treatment planning on computer tomography scans of five patients with LALC with the LFM implanted was performed with 1-3 fields.

RESULTS

The RSP was experimentally determined to be 1.164 for the LFM. Phantom measurements revealed a maximum relative deviation in dose of 4.8% for the LFM in the spread-out Bragg Peak, compared to 12-67% for the solid markers. Using the experimentally determined RSP, the maximum proton range error introduced by the LFM is about 1mm. If the marker was displaced at PBSPT, the maximum dosimetric error was limited to 2 percentage points for 3-field plans.

CONCLUSION

The dose perturbations introduced by the LFM were considerably smaller than the solid markers investigated. The RSP of the fiducial marker should be corrected in the treatment planning system to avoid errors. The investigated LFM introduced clinically acceptable dose perturbations for image-guided PBSPT of LALC.

Two-dimensional in vivo rectal dosimetry using an endorectal balloon with unfoldable radiochromic film during prostate cancer radiotherapy

BACKGROUND AND PURPOSE

The present study aims to investigate the feasibility of two-dimensional (2D) in vivo rectal dosimetry using an endorectal balloon for the radiotherapy of prostate cancer.

MATERIALS AND METHODS

The endorectal balloon was equipped with an unfoldable radiochromic film. The film was unrolled as the balloon was inflated, and rolled as it was deflated. Its mechanical and imaging properties were tested, and the dosimetric effectiveness was evaluated in clinical photon and proton beams.

RESULTS

The size of the endorectal balloon including the film was linearly proportional to the volume of water filled in the balloon, and its position could be identified by X-ray radiography. The loss of dose information due to film cutting was within ±1mm from the cutting line. Applying linear interpolation on cut film, the gamma passing rate was more than 95% for 2%/2mm criteria. The measured dose profiles agreed with the plan within 3% and 4% for the photon and proton beams, respectively. A dose-volume histogram of the anterior rectal wall could be obtained from the measured dose distribution in the balloon, which also agreed well with the plan.

CONCLUSIONS

2D in vivo rectal dosimetry is feasible using the endorectal balloon with a radiochromic film in the radiotherapy of prostate cancer.

The physics of proton therapy

The physics of proton therapy has advanced considerably since it was proposed in 1946. Today analytical equations and numerical simulation methods are available to predict and characterize many aspects of proton therapy. This article reviews the basic aspects of the physics of proton therapy, including proton interaction mechanisms, proton transport calculations, the determination of dose from therapeutic and stray radiations, and shielding design. The article discusses underlying processes as well as selected practical experimental and theoretical methods. We conclude by briefly speculating on possible future areas of research of relevance to the physics of proton therapy.

Gold sputtered fiducial markers for combined secondary ion mass spectrometry and MALDI imaging of tissue samples

Mass spectrometry imaging (MSI) is a label free technique capable of providing simultaneous identification and localization of biomolecules. A multimodal approach is required that allows for the study of the complexity of biological tissue samples to overcome the limitations of a single MSI technique. Secondary ion mass spectrometry (SIMS) allows for high spatial resolution imaging while matrix-assisted laser desorption (MALDI) offers a significantly wider mass range. The combination of coregistered SIMS and MALDI images results in detailed and unique biomolecular information. In this Technical Note, we describe how gold sputtered/implanted fiducial markers (FM) are created and can be used to ensure a proper overlay and coregistration of the two-dimensional images provided by the two MSI modalities.

Dose perturbations by electromagnetic transponders in the proton environment

Surgically implanted electromagnetic transponders have been used in external beam radiotherapy for target localization and position monitoring in real time. The effect of transponders on proton therapy dose distributions has not been reported. A Monte Carlo implementation of the transponder geometry is validated against film measurements in a proton SOBP and subsequently used to generate dose distributions for transponders at different positions and orientations in the proton SOBP. The maximum dose deficit is extracted in each case. Dose shadows of up to 60% occur for transponders positioned very near the end of range of the Bragg peak. However, if transponders are positioned further than 5 mm from the end of range, and are not oriented parallel to the beam direction, then the dose deficit can be kept below 10%.

Determination of optimal fiducial marker across image-guided radiation therapy (IGRT) modalities: visibility and artifact analysis of gold, carbon, and polymer fiducial markers

The purpose of this study was to evaluate the visibility and artifact created by gold, carbon, and polymer fiducial markers in a simple phantom across computed tomography (CT), kilovoltage (kV), and megavoltage (MV) linear accelerator imaging and MV tomotherapy imaging. Three types of fiducial markers (gold, carbon, and polymer) were investigated for their visibility and artifacts in images acquired with various modalities and with different imaging parameters (kV, mAs, slice thickness). The imaging modalities include kV CT, 2D linac‐based kilovoltage and megavoltage X‐ray imaging systems, kV cone‐beam CT, and normal and fine tomotherapy imaging. The images were acquired on a phantom constructed using Superflab bolus in which markers of each type were inserted into the center layer. The visibility and artifacts produced by each marker were assessed qualitatively and quantitatively. All tested markers could be identified clearly on the acquired CT and linac‐based kV images; gold markers demonstrated the highest contrast. On the CT images, gold markers produced a significant artifact, while no artifacts were observed with polymer markers. Only gold markers were visible when using linac‐based MV and tomotherapy imaging. For linac‐based kV images, the contrast increased with kV and mAs values for all the markers, with the gold being the most pronounced. On CT images, the contrast increased with kV for the gold markers, while decreasing for the polymer and carbon marker. With the bolus phantom used, we found that when kV imaging‐based treatment verification equipment is available, polymer and carbon markers may be the preferred choice for target localization and patient treatment positioning verification due to less image artifacts. If MV imaging will be the sole modality for positioning verification, it may be necessary to use gold markers despite the artifacts they create on the simulation CT images.

PACS number: 87

Investigation of dose perturbations and the radiographic visibility of potential fiducials for proton radiation therapy of the prostate

Image guidance using implanted fiducial markers is commonly used to ensure accurate and reproducible target positioning in radiation therapy for prostate cancer. The ideal fiducial marker is clearly visible in kV imaging, does not perturb the therapeutic dose in the target volume and does not cause any artifacts on the CT images used for treatment planning. As yet, ideal markers that fully meet all three of these criteria have not been reported. In this study, 12 fiducial markers were evaluated for their potential clinical utility in proton radiation therapy for prostate cancer. In order to identify the good candidates, each fiducial was imaged using a CT scanner as well as a kV imaging system. Additionally, the dose perturbation caused by each fiducial was quantified using radiochromic film and a clinical proton beam. Based on the results, three fiducials were identified as good candidates for use in proton radiotherapy of prostate cancer.

Dose perturbations and image artifacts caused by carbon-coated ceramic and stainless steel fiducials used in proton therapy for prostate cancer

Image-guided radiation therapy using implanted fiducial markers is a common solution for prostate localization to improve targeting accuracy. However, fiducials that are typically used for conventional photon radiotherapy cause large dose perturbations in patients who receive proton radiotherapy. A proposed solution has been to use fiducials of lower atomic number (Z) materials to minimize this effect in tissue, but the effects of these fiducials on dose distributions have not been quantified. The objective of this study was to analyze the magnitude of the dose perturbations caused by select lower-Z fiducials (a carbon-coated zirconium dioxide fiducial and a plastic-coated stainless steel fiducial) and compare them to perturbations caused by conventional gold fiducials. Sets of phantoms were used to assess select components of the effects on dose. First, the fiducials were assessed for radiographic visibility using both conventional computed tomography (CT) and an on-board kilovoltage imaging device at our proton therapy center. CT streak artifacts from the fiducials were also measured in a separate phantom. Second, dose perturbations were measured downstream of the fiducials using radiochromic film. The magnitude of dose perturbation was characterized as a function of marker material, implantation depth and orientation with respect to the beam axis. The radiographic visibility of the markers was deemed to be acceptable for clinical use. The dose measurements showed that the perpendicularly oriented zirconium dioxide and stainless steel fiducials located near the center of modulation of the proton beam perturbed the dose by less than 10%, but that the same fiducials in a parallel orientation near the end of the range of the beam could perturb the dose by as much as 38%. This suggests that carbon-coated and stainless steel fiducials could be used in proton therapy if they are located far from the end of the range of the beam and if they are oriented perpendicular to the beam axis.

Investigation of the dosimetric impact of a Ni-Ti fiducial marker in carbon ion and proton beams

INTRODUCTION

Fiducial markers based on a removable stent are currently used in image guided radiotherapy. Here it is investigated what the possible dosimetric impact of such a marker could be, if used in proton or carbon ion treatment.

MATERIAL AND METHODS

The simulations have been done using the Monte Carlo particle transport code FLUKA with its default hadron therapy settings. A 3 cm long stent is approximated in FLUKA by stacking hollow tori. To simulate realistic clinical conditions a field 5 × 5 cm has been used, delivering a 5 cm wide spread out Bragg peak located 5 cm deep for protons and carbon ions. For protons fields mimicking active and passive beam delivery have been investigated. The stent has been arranged perpendicular, turned 45 degrees, and parallel to the beam axis.

RESULTS

The position of the 95% dose level shifts for carbon ions 7 mm in proximal direction for the marker perpendicular to the beam and 8 mm if the stent is turned 45 degree for a 1 × 1 cm dose binning on the centre beam axis. For the case where the stent was parallel to beam direction the 95% dose level shifts 26 mm. For active delivered protons, the shift of the 95% dose level is less. The shift for a perpendicular arranged marker is 6 mm, for 45 degrees turned it is 7 mm. For the case where the stent was oriented parallel to the beam, the observed shift is 21 mm. Dose inhomogeneities caused by straggling effects occur only near the distal edge of the field.

CONCLUSIONS

The results of our investigations show that the Ni-Ti marker has a non negligible impact on the dose distributions for the used radiation types. However if the treatment plan rules out narrow angles between symmetry axis of the stent and the beam direction, this may be compensated.