PURPOSE In this study, an experiment was devised to establish the dependency of the impact of pulse forming network (PFN) and injection current (IC) parameters on output and energy variations of helical tomotherapy (HT) on the radiation beam output and energy. METHODS HT has unique radiation beam characteristics due to the absence of a flattening filter. As with conventional linear accelerators, the machine output and energy should be within a + or - 2% tolerance according to published studies. However, because a dose servo is not utilized in a HT unit, these parameters may drift out of the + or - 2% tolerance due to various reasons such as high machine temperatures. With this in mind, physicists and engineers must adjust certain machine parameters to reset the output and energy to within the tolerance of the commissioned baseline. Two parameters commonly adjusted are: PFN voltage (V(PFN)) and IC voltage (V(IC)). RESULTS Results showed that the HT unit possesses different working zones defined by the V(PFN) and V(IC) parameter settings. The working zones were classified into 5 zones: 1) low dose rate zone - radiation dose rate much lower than nominal dose rate and machine cannot run due to low dose rate fault; 2) normal dose rate zone - dose rate is within tolerance of nominal dose rate and machine can run without dose rate fault; 3) dose rate failure during treatment zone - dose rate within the tolerance of the nominal dose rate however machine interrupts during treatment due to dose rate fault; 4) high dose rate zone - dose rate is higher than nominal dose rate and machine cannot run due to high dose rate fault; and 5) inoperable dose rate zone - dose rate is much higher than the nominal dose rate and machine cannot run. CONCLUSION The results of this study may provide a quick guide for physicists to adjust their HT unit V(PFN) and V(IC) values in order to reset the radiation beam output and energy back to within the tolerance of the commissioned baseline.

Purpose: The objective of this study is to perform a plan evaluation using Californium-252 neutron brachytherapy for the treatment of malignant gliomas and compare the neutron brachytherapy planning with photon IMRT planning. Materials/Methods: After a phase I trial of neutron brachytherapy (Cf-252 implant) for the treatment of malignant gliomas, there is renewed interest to evaluate the effectiveness of radiation therapy using neutron brachytherapy compared with photon IMRT. Here we performed a dosimetric comparison of two treatment plans based on the same patient using equivalent prescribed dose. Isodose lines and dose volume histogram of brain tumor and adjacent critical structures were used for plan evaluation. Dose of neutron brachytherapy was calculated using CT-converted Monte Carlo model and simulated by Monte Carlo code MCNPX. The tissue component of Monte Carlo model was adapted from the sectioned images of human cadavers of the Visible Human Project of NLM. Dose of IMRT plan was calculated using Pinnacle3 TPS. A relative biological effectiveness of 6 was used to determine the neutron equivalent dose (ncGy) for central nervous system (CNS) tissues. An equivalent dose of 6000 cGy was prescribed for both the IMRT plan and the neutron brachytherapy plan. Results: The targets were well covered by the 95% isodose line in both IMRT and neutron brachytherapy plans. Comparing the IMRT and Cf-252 neutron brachytherapy, the mean dose was 6102 cGy and 6708 ncGy for target, 403 cGy and 177 ncGy for brainstem, 1022 cGy and 311 cGy for chiasm, 311 cGy and 275 ncGy for pituitary, and 705 cGy and 420 ncGy for brain, respectively. Conclusions: Cf-252 Brachytherapy provided conformal dose distribution to the brain tumor and reduced the dose to the surrounding critical organs compared to IMRT. The implanted Cf-252 source provides high dose to brain tumor and reduces the radiation exposure of normal brain.

Purpose: The purpose of this work is to study the Bragg peak shifts and degradation caused by density and boundary changes in proton beam dose calculation Method and Material: Proton beam delivery provides promising dose characteristics as radiation dose can conform tightly to tumor while sparing surrounding healthy tissues. Proton particles deposit energy in a narrow range around the Bragg peak and as such the dose calculation is more challenging for that the Bragg peak is sensitive to tissue density, tissue composition and organ boundaries along the proton track path. We simulated a few scenarios to study the proton Bragg peak shift due to density and Bragg peak degradation due to change and boundary changes. The calculation of the three dimension dose matrix was performed using a 2 × 2 × 1 mm3 voxels in the depth peak dose range in water phantom after some rough simulation for the dose peak estimation. Results: Bragg peak shift at the iso‐center slice were found to follow a linear relationship with the density of heterogeneity insert based on our simulations with density ranging [0.4 2.0] g/cm∧3 which we studied. Bragg peak degradation and proton dose changed significantly due to low density and small beams size. Proton dose degraded when high energy proton beam irradiated to low density material. Proton dose degraded also when small beam with beam radius at several mm range. Conclusion: Proton dose calculation depends on many factors as Bragg peak is sensitive to tissue density and composition. Besides that, there exist several scenarios causing Bragg peak shift due to density change, causing Bragg peak degradation due to low density and small proton beam. The Monte Carlo simulation is a very accurate solution to provide precise dose distribution in inhomogeneous structures by simulating transport and energy deposition.

The purpose of this study was to commission a first-of-its-kind dual-layer micro multileaf collimator (mMLC) system by using Monte Carlo dose calculations. The mMLC is attached on a Varian 600C linac. Having a lower and an upper layer of MLC leaves, this mMLC allows for field shaping in two orthogonal directions. The commissioning of the system was performed in two steps: without and with the mMLC attached on the linac. The treatment head without and with the mMLC was modeled in the BEAMnrc Monte Carlo (MC) code. The scoring planes for the phase space files were specified below the linac's secondary collimators (jaws) and above and below the mMLC. With the mMLC attached to the linac the field size was defined by the jaws as 10 x 10 cm(2), which is also the maximum possible field size that can be shaped by the mMLC. For the commissioning of the linac, several fields of various sizes were simulated and compared against ionization chamber measurements in a water phantom. Output factors for several field sizes, as well as percent depth dose curves and dose profiles for rectangular and irregular shape fields, were calculated and compared against measurements in water. Agreement between measured and calculated data was better than 1% and less than 1.0 mm in the penumbra region for open fields. With the mMLC attached, the agreement between measurements and MC calculations is within 1.0% or 1.0 mm in the penumbra region.

Purpose: We aim to provide accurate proton dose calculations for ocular tumors and adjacent critical organs using intensity modulated proton therapy (IMPT) using a human anatomy‐based Monte Carlo model. Dose is simulated using Monte Carlo code MCNPX and compared to standard photon IMRT planning using Pinnacle3® TPS. Method and Materials: The human anatomy model was adapted from the Visible Human Project from the National Library in Medicine. Sectioned images were assigned physical properties. Two independent trials delivering 90% prescription dose to 100% tumor volume were developed using IMRT and IMPT, respectively. Dose profiles for each transverse, sagittal and coronal view of the model were provided for evaluation. Both treatment plans were optimized to deliver maximum dose to the tumor and minimize dose elsewhere. The dose volume histograms for the PTV (tumor), eye, lens, optic nerve, lacrimal gland, brain, chiasm, and pituitary gland were compared between IMRT and IMPT, respectively. Results: IMPT delivered superior isodose coverage to all tissues. Comparing IMRT and IMPT, the mean dose was 4499 cGy and 4750 cGy‐Eq (PTV), 2334 cGy and 1700 cGy‐Eq (eye), 2705 cGy and 1330 cGy‐Eq (lens), 156 cGy and 181 cGy‐Eq (optic nerve), 142 cGy and 23 cGy‐Eq (lacrimal gland), 21 cGy and 0.0 cGy‐Eq (brain), 31 cGy and 0.0 cGy‐Eq (chiasm), and 43 cGy and 0.00 cGy‐Eq (pituitary gland), respectively. The PTV was well covered by 90% isodose to 100% of the tumor volume with an average % prescription dose of 99.9% and 105.6 % for IMRT and IMPT, respectively. Conclusions: IMPT provided conformal dose to the ocular tumor and significantly spared dose to critical organs compared to IMRT. The human‐anatomy dose model performs very well in dose calculation; however, further validation using additional human anatomy‐based models and more specified proton source configuration is needed for optimization purposes.

Purpose: To quantity evaluate the impact of pulse forming network (PFN) and injection current (IC) parameters on the output and energy variations of a helical TomoTherapy (HT) unit. Method and Materials: Tomotherapy quality assurance (TQA™) platform, especially the step wedge phantom and step wedge static module, was used for the whole measurement. PFN and IC voltage values were coarsely sampled from 3.0 to 5.0 V in 0.2 V increments, and finely sampled ( and ) in 0.02 V increments. Results: Five working zones were found for different combination of PFN and IC voltage values: low dose rate zone, normal dose rate zone, dose rate failure during treatment zone, high dose rate zone, inoperable dose rate zone. It was noted that a 1.0% increase in VIC yields an average 1.4% increase in the average dose rate. Additionally, a 0.02 V increase in VPFN yields an average 1.0% increase in the average dose rate. A 1.0% increase in the VIC value yielded an average 0.3% decrease in the energy ratio. Furthermore, changes in the energy ratio were more dependent on VIC than VPFN based on the fact that only a 0.5% variation in energy was noted when varying the VPFN from 4.00 to 4.10 V while a 2.0% change was noted when varying the VIC from 3.5 to 3.7 V. Conclusions: In this study, several working zones based on the VPFN and VIC parameter setting were found to exist for a HT unit. Inside the normal dose rate zone, the output and energy vary linearly with VIC and VPFN parameter values. The results of this study may provide a quick guide for physicists to adjust their HT unit VPFN and VIC values in order to reset the radiation beam output and energy back to within the tolerance of the commissioned baseline.

Purpose: In this study, we performed dependency study of proton dose on tissue composition using Monte Carlo models of Hounsfield number conversion and cadaver‐based anatomical data Method and Materials: Monte Carlo methods provide the most accurate radiation dose calculation technology as it take into account detailed materials properties, such as materials composition, mass density and interaction cross section. Monte Carlo simulation calculates the energy deposit per mass of each small volume (voxel) after a patient is presented by a large number of voxels. Two methods of building patient‐specified Monte Carlo models have been used in this study: one is to convert patient's CT Hounsfield numbers to materials; the other way is assign anatomical detailed materials using cadavers' segments. Dose distribution and dose volume histogram were compared based on the Monte Carlo models. Results: The dose distribution at the iso‐center slice, the 95% did not cover conformally to the ROI for Hounsfield MC model with shifting 2∼3 mm superior to the ROI. Dose volume history for planning tumor volume (PTV), Brain, Pituitary and Chiasm were used for evaluating the effect of tissue composition. The mean doses difference for PTV was 2.1% for the cadaver‐based MC and Hounsfield conversion MC model. The mean dose difference for Brain, Pituitary and Chiasm was less than 1.0%. Conclusion: Proton radiation dose was calculated and closely compared using two Monte Carlo models: one from CT Hounsfield number conversion and the other from human anatomically detailed Cadaver segments. It is found that the effect of different tissue composition on proton radiation dose calculation is complex involving organs at risk. Our method using cadaver‐based Monte Carlo model for proton dose calculation was shown to be suitable for benchmarking other Monte Carlo dose calculation methods and for providing tissue heterogeneity correction due to the effect of tissue composition.

Purpose: To evaluate the performance characteristics of a first-of-its-kind dual layer micro multi-leaf collimator (DmMLC) and compare it dosimetrically with standard, single layer MLCs for use in intensity modulated radiation therapy (IMRT). Method and Materials: The DmMLC performance was studied using a cross shaped field generated by both a single layer MLC and the DmMLC. The DmMLC by Initia Medical Technologies was mounted on a Varian 600C linac unit. Film measurements were obtained using a 6 MV x-ray beam and EDR2 film at a depth of 5 cm in solid water at 100 cm source to film distance. Film analysis was performed using the RIT V5 software. The leaf-end transmission of the single bank and dual bank of mMLC was measured. The maximum and average leaf-end transmissions for the cross pattern were compared for both modes of operation of the mMLC and for the standard Varian MLC. Results: The leaf-end transmission for the single layer of the DmMLC was at its maximum 22.4% with an average value of 15.4% The transmission was reduced to a maximum of 2.4% and an average of 2.1% when both layers of the DmMLC leaves were used. Dual layer MLC provided more conformal field edge as compared to the standard single layer MLC with approximately ten fold less transmission at the leaf end. Conclusion: The results of this study indicate that the DmMLC provides more precise field shaping at field edges than the standard MLC. The DmMLC reduces the leaf-end transmission to about 2.1%. The dual layer MLC offers more accurate IMRT delivery to the planned target volume and spares the underlined surrounding health tissue from increased transmission and leakage dose. Conflict of Interest: The work was partially supported by a research grant from Initia RT.

Purpose: Developing a method of an HPGe detector precise γ efficiency calibration which is very important for accurate radiation detection during cancer radiotherapy practices. Method and Materials: radioactive nucleus produced and separated with Momentum Achromat Recoil Spectrometer (MARS) at the K500 superconducting cyclotron of Texas A&M University has positron decays followed by γ transitions up to 8 MeV from excited states, which is used for a β-γ coincidence measurement with a 1-mm-thick BC404 plastic scintillator, an HPGe detector and a fast tape-transport system to calibrate the HPGe detector. Results: By carefully considering the effects of summing, positron annihilation, internal conversion, and β detector efficiency when analyzing spectrum, we got the efficiency for γ-ray 7070 keV at 49 mm distance away from the source sample , which was 0.192(6)%. The Monte Carlo (MC) simulation with CYLTRAN code gave a value of 0.189%, which was in agreement with our measurements. The precise efficiency calibration curve of the HPGe detector up to 7070 KeV at 49 mm distance away from the source sample was obtained. By using the same procedure, we got the efficiency for the 7070 keV γ-ray at 151 mm distance away from the source sample , which was 0.0385(8)%. MC simulation value was 0.0399%, which differed from measurement by 4(2)%. This discrepancy led us to assign an uncertainty of 4% to our efficiencies at 151 mm up to 7070 KeV. The Monte Carlo calculations also reproduced the intensity of observed single-and double-escape peaks, providing that the effects of positron annihilation-in-flight were incorporated. Conclusion: A new method was established. The precise calibration curves obtained from this work are useful for accurate radiation detection and improving quality control to quality assurance (QA) for intensity-modulated radiation therapy (IMRT). Research sponsored by Department of Energy and Robert Welch Foundation.

Purpose: To reconstruct a 4D dose distribution from the planned dose based on one phase of a 4D-CT image and compare with results from 4D dose reconstructed from multiple phases and phantom measurements. Method and Materials: A treatment plan was developed for a set of CT images from a single phase of a respiratory motion phantom using ADAC pinnacle TPS. The single phase reconstructed dynamic dose distribution to a coronal pane was obtained for several tumor trajectories by applying a linear transformation on the optimized dose from pinnacle. This was then compared to the dynamic dose distribution obtained by considering a weighted sum of dose distributions from plans based on eight phases of the tumor cycle. Measurements using radiochromic films were made following plan delivery on a LINAC to validate our results. Results: Using gamma index analysis, the number of pixels exceeding a gamma index of 1 was 7% and 0% for the measured versus single phase, and the single versus multi-phase reconstructed dynamic dose respectively. This was based on a dose difference tolerance of 5% and DTA tolerance of 5mm. The effects of phase dependent weighting versus equal weighting were in general negligible on the dose distribution for the trajectories studied. Using dose difference tolerance of 3% and a DTA tolerance of 4mm, the comparison yielded 0.5%, 0.72% and 1.72% as the percentage of pixels exceeding the gamma index of 1 for sinusoidal tumor motion amplitudes of 1cm, 1.5cm and 2cm respectively. Conclusion: There was close agreement between the two methods of reconstruction and with measurements. Recreating the dynamic dose distribution from the static dose distribution from a single CT image set can be an efficient way of accurately accounting for relative motion in a static or dynamic dose delivery and can be useful in pre-treatment verification analysis.

The need for high accuracy dose calculation has long been an issue since the complexity of human body. In this work, we aim to build an accurate dose calculation algorithm based on human anatomy-based model and simulated by Monte Carlo code MCNPX. The Anatomy-based Monte Carlo dose for a prostate tumor was compared to a widely used TPS (Pinnacle3, 8.0d). The human anatomy-based model was adapted from the sectioned images of human cadavers of the Visible Human Project from the National Library in Medicine. The sectioned images were digitalized to voxel-based volume arrays for generating computational models that represent the human anatomy. The human anatomy-based model in this work was built with 4mm x 4mm x 4mm voxel resolution with total over 6 million voxels with each voxel assigned physical properties based on the component molecular make up of the tissue and includes density and isotopic composition. The Monte Carlo code we used in this work is MCNPX (V2.5.0), developed by Los Alamos National Laboratory exclusively benchmarked against measurements for over sixty years history. Monte Carlo code simulated particles transport and energy deposition in each voxel of the human anatomy model. Two trials were developed using anterior-posterior (AP)-PA and left-right (LR)-RL four beams treatment planning based on the Pinnacle3 and HAMD respectively. Isodose lines and dose profiles for each CT slices in the transverse, sagittal and coronal view of the VHP human model were provided by both of the two dose algorithm. Dose volume histogram and the mean dose in bladder, rectum and prostate tumor were compared between Pinnacle3 and HAMD. HAMD provided more responses and conformal dose to the heterogeneity regions in the pelvis. It was much easier to tell hot spots in the bone areas and cold spots in the air cavity areas from the isodose lines by HAMD. Pinnacle3 provides smoother isodose lines and profiles where the beam went through such heterogeneous regions. For the four-beam planning, the mean dose for the bladder is 2232.9 and 2148.9 cGy based on Pinnacle3 and HAMD respectively, with the mean dose difference at 3.8%. The mean dose for the prostate is 3882.0 and 3177.0 cGy based on Pinnacle3 and HAMD with difference at 22.3%. The mean dose for the rectum is 2684.5 and 1797.4.0 cGy based on Pinnacle3 and HAMD respectively with the mean dose difference at 33.0%. Human-anatomy based Monte Carlo dose potentially provides a more accurate and more realistic dose distribution in human body. The significant difference in the prostate and rectum is dose between the two modalities needs to be investigated further. If verified, HAMD can offer basic benchmarking data for clinically simplified CT-based dose calculation.

Purpose: To examine the effect of the source size to the neutron energy spectrum and dose contribution in a tissue equivalent material, with and without boron neutron capture enhancement. Method and Materials: Version 2.5.0 of the MCNPX computer code (Pelowitz, 2005) was used in this study to calculate the neutron energy spectra and dose distribution with and without 10B loading for 252Cf sources of various geometries. A spherical phantom geometry with a centrally positioned point-source was first implemented in order to verify our simulation code by reproducing existing data in the literature. The neutron energy flux was calculated for various loadings and various distances from the source. Cylindrical sources of different sizes were simulated in a 30×30×30 cm3 water phantom and the neutron energy flux and energy deposition in the medium was calculated on cylindrical surfaces enclosing each source at various distances from each source surface. The boron enhancement of 30 ppm was also studied in this case. The neutron energy spectrum was modeled as an isotropic Watt distribution and all calculated spectra were normalized assuming the same amount of distributed uniformly inside the source volume. Results: Calculated neutron energy spectra for the point-source geometry showed very good agreement with existing literature, verifying our simulation model. For the cylindrical sources, preliminary results showed increased fast neutron contribution from the compact source, as compared to the conventional one, especially at small distances from the source surface. Conclusion: Smaller size sources can be more beneficial for brachytherapy treatments, not only due to localized dose distribution, but also due to higher dose contribution from fast neutrons of high RBE at small distances from the source.

Purpose: The aim of this study is to provide a pre‐clinical evaluation of four‐dimensional tracking radiation therapy to lung tumor using a prototype tracking system. The evaluation was based on films dosimetric analysis, time delay measurements and treatment planning DVH analysis by using two types of dynamic phantoms. Method and Materials: The key component of the 4DTRT system was a prototype of TrackBeam. It consists of an image processing tools and a first‐of‐its‐kind dual‐layer micro MLC. DmMLC has two layers of orthogonal leaves which provide advantages in speed and conformality when forming beam aperture for tracking. The TrackBeam was mounted to a Varian Linac and connected to a workstation which process the online MV fluence and controls each leaf's motion. A Quasar dynamic phantom was used for radiographic film irradiation with 4DTRT and also 3DCRT. The phantom has a Gafchromic film insert and a gold marker in the insert. It can move in Sinusoid mode as well as real patient respiratory cycle. Another tissue‐equivalent thorax dynamic phantom was used for DVH analysis after a phantom‐based 3DCRT planning and a 4DTRT planning developed respectively. Results: The synchronization of marker motion and the DmMLC leaf motion was achieved within less than 0.05 seconds. The films analysis indicated that total 29.91% over the tolerance of 5% and 5.09% of over the tolerance of 5% when the 3DCRT and 4DTRT films compared to a static film. The DVH comparisons indicate 4DTRT reduces significant dose to the Ring from 75% (80% volume) to 60% (50% volume). 4DTRT also reduces considerable amount dose to the total lung from 33% (30% volume) to 22% (22%). Conclusion: The 4D tracking using MLC provides a feasible solution delivering conformal dose to lung tumor and spare the surrounding tissue. Conflict of Interest: The work was partially supported by Initia‐RT medical device.

Purpose: In this work, we aim to build an accurate dose calculation algorithm based on human anatomy‐based model using Monte Carlo simulation for providing basic and benchmarking data for therapeutic protons. Method and Materials: Both phantom‐based and human anatomy‐based models were used. The human anatomy model was developed from VHP® at National Library in Medicine. The human anatomy‐based model was built with 4 mm × 4 mm × 4mm voxel resolution with total over 6 million voxels for describing the whole body. Each voxel was assigned physical properties, including density and isotopic composition. MCNPX was used to simulate the transport and energy deposit to each voxel. An in‐house dosimety software package, Human Anatomy‐based Monte Carlo Dose (HAMD) was developed to analysis the huge dose dataset based on Monte Carlo simulation and the three dimensional dose matrix was super positioned to the CT image correspondingly. Results: The Monte Carlo simulation provided very close agreement to the two widely used proton range‐energy tables with average depth peak difference less than 0.70% and −0.37% to ICRU Report 49 and Janni DNDT respectively from 40 MeV to 250 MeV energy range. HAMD performed well in proton treatment dose calculation. HAMD offers very friendly and familiar interface for physicians to conveniently review a treatment plan. The isodose lines in transverse, sagittal and coronal views provided very conformal coverage to the contours in lung. Conclusion: The simulated proton range‐energy table has been accurately benchmarked compared to measurements. The in‐house developed dose algorithm HAMD performs very well in dose calculation both in phantom‐based and human anatomy‐based heterogeneity. The HAMD needs further validation by using additional human anatomy‐based models and specified beam source configuration. The long‐term and board objective is to provide an extreme accurate dose calculation based on human model for benchmarking clinic treatment planning systems.