Dose Profile Modulation of Proton Minibeam for Clinical Application.

The feasibility of proton minibeam radiation therapy (pMBRT) using a multislit collimator (MSC) and a scattering device was evaluated for clinical use at a clinical proton therapy facility. We fabricated, through Monte Carlo (MC) simulations, not only an MSC with a high peak-to-valley dose ratio (PVDR) at the entrance of the proton beam, to prevent radiation toxicity, but also a scattering device to modulate the PVDR in depth. The slit width and center-to-center distance of the diverging MSC were 2.5 mm and 5.0 mm at the large end, respectively, and its thickness and available field size were 100 mm and 76 × 77.5 mm2, respectively. Spatially fractionated dose distributions were measured at various depths using radiochromic EBT3 films and also tested on bacterial cells. MC simulation showed that the thicker the MSC, the higher the PVDR at the phantom surface. Dosimetric evaluations showed that lateral dose profiles varied according to the scatterer's thickness, and the depths satisfying PVDR = 1.1 moved toward the surface as their thickness increased. The response of the bacterial cells to the proton minibeams' depth was also established, in a manner similar to the dosimetric pattern. Conclusively, these results strongly suggest that pMBRT can be implemented in clinical centers by using MSC and scatterers.

3D star shot analysis using MAGAT gel dosimeter for integrated imaging and radiation isocenter verification of MR-Linac system.

PURPOSE: This study aims to investigate a star shot analysis using a three-dimensional (3D) gel dosimeter for the imaging and radiation isocenter verification of a magnetic resonance linear accelerator (MR-Linac). METHODS: A mixture of methacrylic acid, gelatin, and tetrakis (hydroxymethyl) phosphonium chloride, called MAGAT gel, was fabricated. One MAGAT gel for each Linac and MR-Linac was irradiated under six gantry angles. A 6 MV photon beam of Linac and a 6 MV flattening filter free beam of MR-Linac were delivered to two MAGAT gels and EBT3 films. MR images were acquired by MR-Linac with a clinical sequence (i.e., TrueFISP). The 3D star shot analysis for seven consecutive slices of the MR images with TrueFISP was performed. The 2D star shot analysis for the central plane of the gel was compared to the results from the EBT3 films. The radius of isocircle (ICr ) and the distance between the center of the circle and the center marked on the image (ICd ) were evaluated. RESULTS: For MR-Linac with MAGAT gel measurements, ICd at the central plane was 0.46 mm for TrueFISP. Compared to EBT3 film measurements, the differences in ICd and ICr for both Linac and MR-Linac were within 0.11 and 0.13 mm, respectively. For the 3D analysis, seven consecutive slices of TrueFISP images were analyzed and the maximum radii of isocircles (ICr_max ) were 0.18 mm for Linac and 0.73 mm for MR-Linac. The tilting angles of radiation axis were 0.31° for Linac and 0.10° for MR-Linac. CONCLUSION: The accuracy of 3D star shot analysis using MAGAT gel was comparable to that of EBT3 film, having a capability for integrated analysis for imaging isocenter and radiation isocenter. 3D star shot analysis using MAGAT gel can provide 3D information of radiation isocenter, suggesting a quantitative extent of gantry-tilting.

Plan optimization with L0-norm and group sparsity constraints for a new rotational, intensity-modulated brachytherapy for cervical cancer.

The aim of this work is to build a framework that comprehends inverse planning procedure and plan optimization algorithm tailored to a novel directional beam intensity-modulated brachytherapy (IMBT) of cervical cancer using a rotatable, single-channel radiation shield. Inverse planning is required for finding optimal beam emitting direction, source dwell position and dwell time, which begin with creating a kernel matrix for each structure based on Monte-Carlo simulated dose distribution in the rotatable shield. For efficient beam delivery and less transit dose, the number of source dwell positions and angles needs to be minimized. It can be solved by L0-norm regularization for fewest possible dwell points, and by group sparsity constraint in L2,p-norm (0≤p<1) besides L0-norm for fewest active applicator rotating angles. The dose distributions from our proposed algorithms were compared to those of conventional tandem-based intracavitary brachytherapy (ICR) plans for six cervical cancer patients. The algorithmic performance was evaluated in delivery efficiency and plan quality relative to the unconstrained algorithm. The proposed framework yielded substantially enhanced plan quality over the conventional ICR plans. The L0-norm and (group sparsity+L0-norm) constrained algorithms reduced the number of source dwell points by 60 and 70% and saved 5 and 8 rotational angles on average (7 and 11 angles for highly modulated cases), relative to the unconstrained algorithm, respectively. Though both algorithms reduced the optimal source dwell positions and angles, the group sparsity constrained optimization with L0-norm was more effective than the L0-norm constraint only, mainly because of considering physical constraints of the new IMBT applicator. With much fewer dwell points compared to the unconstrained, the proposed algorithms led to statistically similar plan quality in dose volume histograms and iso-dose lines. It also demonstrated that the plan optimized by rotating the applicator resulted in much better plan quality than that of conventional applicator-based plans.

Two-dimensional in vivo rectal dosimetry during high-dose-rate brachytherapy for cervical cancer: a phantom study.

BACKGROUND: The aim of the present study was to verify the dosimetric accuracy of two-dimensional (2D) in vivo rectal dosimetry using an endorectal balloon (ERB) with unfoldable EBT3 films for high-dose-rate (HDR) brachytherapy for cervical cancer. The clinical applicability of the technique was discussed. MATERIAL AND METHODS: ERB inflation makes the EBT3 films unrolled, whereas its deflation makes them rolled. Patient-specific quality assurance (pQA) tests were performed in 20 patient plans using an Ir-192 remote afterloading system and a water-filled cervical phantom with the ERB. The dose distributions measured in ERBs were compared with those of the treatment plans. RESULTS: The absolute dose profiles measured by the ERBs were in good agreement with those of treatment plans. The global gamma passing rates were 96-100% and 91-100% over 20 pQAs under the criteria of 3%/3 mm and 3%/2 mm, respectively, with a 30% low-dose threshold. Dose-volume histograms of the rectal wall were obtained from the measured dose distributions and showed small volume differences less than 2% on average from the patients' plans over the entire dose interval. The positioning error of the applicator set was detectable with high sensitivity of 12% dose area variation per mm. Additionally, the clinical applicability of the ERB was evaluated in volunteers, and none of them felt any pain when the ERB was inserted or removed. CONCLUSIONS: The 2D in vivo rectal dosimetry using the ERB with EBT3 films was effective and might be clinically applicable for HDR brachytherapy for cervical and prostate cancers to monitor treatment accuracy and consistency as well as to predict rectal toxicity.

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.

A new method and device of aligning patient setup lasers in radiation therapy.

The aim of this study is to develop a new method to align the patient setup lasers in a radiation therapy treatment room and examine its validity and efficiency. The new laser alignment method is realized by a device composed of both a metallic base plate and a few acrylic transparent plates. Except one, every plate has either a crosshair line (CHL) or a single vertical line that is used for alignment. Two holders for radiochromic film insertion are prepared in the device to find a radiation isocenter. The right laser positions can be found optically by matching the shadows of all the CHLs in the gantry head and the device. The reproducibility, accuracy, and efficiency of laser alignment and the dependency on the position error of the light source were evaluated by comparing the means and the standard deviations of the measured laser positions. After the optical alignment of the lasers, the radiation isocenter was found by the gantry and collimator star shots, and then the lasers were translated parallel to the isocenter. In the laser position reproducibility test, the mean and standard deviation on the wall of treatment room were 32.3 ± 0.93 mm for the new method whereas they were 33.4 ± 1.49 mm for the conventional method. The mean alignment accuracy was 1.4 mm for the new method, and 2.1 mm for the conventional method on the walls. In the test of the dependency on the light source position error, the mean laser position was shifted just by a similar amount of the shift of the light source in the new method, but it was greatly magnified in the conventional method. In this study, a new laser alignment method was devised and evaluated successfully. The new method provided more accurate, more reproducible, and faster alignment of the lasers than the conventional method.

Combination effects of tissue heterogeneity and geometric targeting error in stereotactic body radiotherapy for lung cancer using CyberKnife.

We have investigated the combined effect of tissue heterogeneity and its variation associated with geometric error in stereotactic body radiotherapy (SBRT) for lung cancer. The treatment plans for eight lung cancer patients were calculated using effective path length (EPL) correction and Monte Carlo (MC) algorithms, with both having the same beam configuration for each patient. These two kinds of plans for individual patients were then subsequently recalculated with adding systematic and random geometric errors. In the ordinary treatment plans calculated with no geometric offset, the EPL calculations, compared with the MC calculations, largely overestimated the doses to PTV by ~ 21%, whereas the overestimation were markedly lower in GTV by ~ 12% due to relatively higher density of GTV than of PTV. When recalculating the plans for individual patients with assigning the systematic and random geometric errors, no significant changes in the relative dose distribution, except for overall shift, were observed in the EPL calculations, whereas largely altered in the MC calculations with a consistent increase in dose to GTV. Considering the better accuracy of MC than EPL algorithms, the present results demonstrated the strong coupling of tissue heterogeneity and geometric error, thereby emphasizing the essential need for simultaneous correction for tissue heterogeneity and geometric targeting error in SBRT of lung cancer.

Dose response of commercially available optically stimulated luminescent detector, Al2O3:C for megavoltage photons and electrons.

This study examined the dose response of an optically stimulated luminescence dosemeter (OSLD) to megavoltage photon and electron beams. A nanoDot™ dosemeter was used to measure the dose response of the OSLD. Photons of 6-15 MV and electrons of 9-20 MeV were delivered by a Varian 21iX machine (Varian Medical System, Inc. Milpitas, CA, USA). The energy dependency was <1 %. For the 6-MV photons, the dose was linear until 200 cGy. The superficial dose measurements revealed photon irradiation to have an angular dependency. The nanoDot™ dosemeter has potential use as an in vivo dosimetric tool that is independent of the energy, has dose linearity and a rapid response compared with normal in vivo dosimetric tools, such as thermoluminescence detectors. However, the OSLD must be treated very carefully due to the high angular dependency of the photon beam.

Estimation of the secondary cancer risk induced by diagnostic imaging radiation during proton therapy.

We have estimated the secondary cancer risk (SCR) introduced by image-guided procedures during proton therapy. The physical dose from imaging radiation and the corresponding organ equivalent dose were calculated for the case of a lumbar spine patient. The maximum physical dose delivered to the patient during the imaging procedure was estimated to be ~0.35% of the prescribed dose of 46 Gy. However, this small imaging dose substantially raised the radiation-induced SCR by ~8%. In addition, the clinical benefit (improved accuracy during the procedure) and costs (extra SCR) associated with image-guided procedures were quantitatively modelled by systematically investigating the changes in SCR as a function of the prescribed dose, treatment target volume and imaging field size. The results showed that the SCR varied sensitively with the volume receiving the imaging and the therapeutic radiation, whereas the SCR depended to a lesser extent on the magnitude of the applied therapeutic radiation. These results showed that the additional SCR introduced by imaging radiation could be efficiently reduced by minimizing the imaging field size during image-guided procedures.

The effect of a contrast agent on proton beam range in radiotherapy planning using computed tomography for patients with locoregionally advanced lung cancer.

PURPOSE: We evaluated the effect of a contrast agent (CA) on proton beam range in a treatment planning system (TPS) for patients with locoregionally advanced lung cancer. METHODS AND MATERIALS: Two sets of computed tomography (CT) images (with and without CA) were obtained from 20 patients with lung cancer. Because the increase in Hounsfield unit (∆HU) value of the heart and great vessels due to the effect of CA is most prominent among thoracic structures, to evaluate the effect of CA on proton beam range in the TPS, we compared the calculated distal ranges in the plan with CA-enhanced CT with those with corrected CT, in which the HU values of the heart and great vessels in the CA-enhanced CT were replaced by average HU values obtained from the unenhanced CT. RESULTS: The mean ∆HU value and the longest length of the heart and great vessels within the proton beam path in the field that passed through these structures were 189 ± 29 HU (range, 110-250 HU) and 7.1 ± 1.1 cm (range, 2.6-11.2 cm), respectively. The mean distal range error in the TPS because of the presence of CA was 1.0 ± 0.7 cm (range, 0.2-2.6 cm). CONCLUSION: If CA-enhanced CT images are used for radiotherapy planning using a proton beam for the treatment of lung cancer, our results suggest that the HU values of the heart and great vessels should be replaced by the average HU values of soft tissue to avoid discrepancies between planned and delivered doses.

Proton range uncertainty due to bone cement injected into the vertebra in radiation therapy planning.

We wanted to evaluate the influence of bone cement on the proton range and to derive a conversion factor predicting the range shift by correcting distorted computed tomography (CT) data as a reference to determine whether the correction is needed. Two CT datasets were obtained with and without a bone cement disk placed in a water phantom. Treatment planning was performed on a set of uncorrected CT images with the bone cement disk, and the verification plan was applied to the same set of CT images with an effective CT number for the bone cement disk. The effective CT number was determined by measuring the actual proton range with the bone cement disk. The effects of CT number, thicknesses, and position of bone cement on the proton range were evaluated in the treatment planning system (TPS) to draw a conversion factor predicting the range shift by correcting the CT number of bone cement. The effective CT number of bone cement was 260 Hounsfield units (HU). The calculated proton range for native CT data was significantly shorter than the measured proton range. However, the calculated range for the corrected CT data with the effective CT number coincided exactly with the measured range. The conversion factor was 209.6 [HU · cm/mm] for bone cement and predicted the range shift by approximately correcting the CT number. We found that the heterogeneity of bone cement could cause incorrect proton ranges in treatment plans using CT images. With an effective CT number of bone cement derived from the proton range and relative stopping power, a more actual proton range could be calculated in the TPS. The conversion factor could predict the necessity for CT data correction with sufficient accuracy.

Determination of the space-charge field amplitude in polymeric photorefractive polymers.

The space-charge field built in a polymeric photorefractive polymer was calculated by a simple method based on the oriented gas model. When anisotropic chromophores in a photorefractive polymer were exposed to an external field, they oriented preferentially to exhibit a birefringence. Then, under illumination of two coherent beams and an external field, they reoriented to form a photorefractive grating. During the formation of the grating, the chromophores were reoriented by the space-charge field as well as by the external applied field. The birefringence induced in the material by an external electric field was determined by measuring the transmittance of the sample which is placed between crossed polarizers, where birefringence depicts the orientation of the chromophores. By measuring the diffraction efficiency with a modified degenerate four-wave mixing setup, the index amplitude of the grating was determined. Finally, the space-charge field was determined by comparing the diffraction efficiency with the birefringence with respect to the applied electric field. In our study, the space-charge field was about 20% of the external applied field, which coincided with previous results obtained from our laboratory.