Field Proton Research Articles

SU-E-T-170: Evaluation of Rotational Errors in Proton Therapy Planning of Lung Cancer

Purpose: To investigate the impact of rotational (roll, yaw, and pitch) errors in proton therapy planning of lung cancer. Methods: A lung cancer case treated at our center was used in this retrospective study. The original plan was generated using two proton fields (posterior-anterior and left-lateral) with XiO treatment planning system (TPS) and delivered using uniform scanning proton therapy system. First, the computed tomography (CT) set of original lung treatment plan was re-sampled for rotational (roll, yaw, and pitch) angles ranged from −5° to +5°, with an increment of 2.5°. Second, 12 new proton plans were generated in XiO using the 12 re-sampled CT datasets. The same beam conditions, isocenter, and devices were used in new treatment plans as in the original plan. All 12 new proton plans were compared with original plan for planning target volume (PTV) coverage and maximum dose to spinal cord (cord Dmax). Results: PTV coverage was reduced in all 12 new proton plans when compared to that of original plan. Specifically, PTV coverage was reduced by 0.03% to 1.22% for roll, by 0.05% to 1.14% for yaw, and by 0.10% to 3.22% for pitch errors. In comparison to original plan, the cord Dmax in new proton plans was reduced by 8.21% to 25.81% for +2.5° to +5° pitch, by 5.28% to 20.71% for +2.5° to +5° yaw, and by 5.28% to 14.47% for −2.5° to −5° roll. In contrast, cord Dmax was increased by 3.80% to 3.86% for −2.5° to −5° pitch, by 0.63% to 3.25% for −2.5° to −5° yaw, and by 3.75% to 4.54% for +2.5° to +5° roll. Conclusion: PTV coverage was reduced by up to 3.22% for rotational error of 5°. The cord Dmax could increase or decrease depending on the direction of rotational error, beam angles, and the location of lung tumor.

SU-E-T-561: Development of Depth Dose Measurement Technique Using the Multilayer Ionization Chamber for Spot Scanning Method

Purpose: To develop a measurement technique which suppresses the difference between profiles obtained with a multilayer ionization chamber (MLIC) and with a water phantom. Methods: The developed technique multiplies the raw MLIC data by a correction factor that depends on the initial beam range and water equivalent depth. The correction factor is derived based on a Bragg curve calculation formula considering range straggling and fluence loss caused by nuclear reactions. Furthermore, the correction factor is adjusted based on several integrated depth doses measured with a water phantom and the MLIC. The measured depth dose profiles along the central axis of the proton field with a nominal field size of 10 by 10 cm were compared between the MLIC using the new technique and the water phantom. The spread out Bragg peak was 20 cm for fields with a range of 30.6 cm and 6.9 cm. Raw MLIC data were obtained with each energy layer, and integrated after multiplying by the correction factor. The measurements were performed by a spot scanning nozzle at Nagoya Proton Therapy Center, Japan. Results: The profile measured with the MLIC using the new technique is consistent with that of the water phantom. Moreover, 97% of the points passed the 1% dose /1mm distance agreement criterion of the gamma index. Conclusion: We have demonstrated that the new technique suppresses the difference between profiles obtained with the MLIC and with the water phantom. It was concluded that this technique is useful for depth dose measurement in proton spot scanning method.

MO‐B‐18C‐01: Proton Therapy II: Proton Stereotactic Radiotherapy

Proton stereotactic radiotherapy shares fundamental principles with general proton therapy physics, specifically range uncertainties and broad beam measurement techniques. Significant differences emerge when treating with smaller field sizes that suffer lateral disequilibrium and when fractions are reduced. This session will explore the history and scope of proton stereotactic radiotherapy in clinical practice. Uncertainties and treatment planning methods specific to stereotactic treatments will be discussed. The session will include an overview of the physical properties of small proton fields and resulting needs for accurate measurements and modeling of dose distributions for radiosurgery treatment planning.Learning Objectives: Understand the clinical rationale for proton radiosurgery. Understand the similarities and differences from general proton therapy. Understand the similarities and differences from photon stereotactic radiosurgery. Understand the basic physics and clinical physics methods for measuring and commissioning a radiosurgery program.

TH-C-19A-07: Output Factor Dependence On Range and Modulation for a New Proton Therapy System

Purpose: Proton treatment planning systems are not able to accurately predict output factors and do not calculate monitor units (MU) for proton fields. Output factors (cGy/MU) for patient-specific fields are usually measured in phantoms or modeled empirically. The purpose of this study is to predict the output factors (OFs) for a given proton (R90) and modulation width (Mod) for the first Mevion S250 proton therapy system. Methods: Using water phantoms and a calibrated ionization chamber-electrometer, over 100 OFs were measured for various R90 and Mod combinations for 24 different options. OFs were measured at the center of the Mod, which coincided with the isocenter. The measured OFs were fitted using an analytic model developed by Kooy (Phys.Med.Biol. 50, 2005) for each option and a derived universal empirical-based polynomial as a function of R90 and Mod for all options. Options are devised for ranges of R90 and Mod. The predicted OFs from both models were compared to measurements. Results: Using the empirical-based model, the values could be predicted to within 3% for at least 90% of measurements and within 5% for 98% of the measurements. Using the analytic model to fit each option with the same effective source position, the predictionmore » is much more accurate. The maximal uncertainty between measured and predicted is within 2% and the averaged root-mean-square is 1.5%. Conclusion: Although the measured data was not exhaustive, both models predicted OFs within acceptable uncertainty. Both models are currently used for a sanity check of our continual patient field OF measurements. As we acquire more patient-field OFs, the model will be refined with an ultimate goal of eliminating the time-consuming patient-specific OF measurements.« less

SU-E-T-621: Analysis of Robustness of Proton Pencil Beam Scanning Technique for Delivery of Craniospinal Irradiation

Purpose: To investigate the robustness and safety of craniospinal irradiation (CSI) planned with a proton pencil beam scanning (PBS) technique which overcomes the complexity of the planning associated with feathering match lines. Methods: Six CSI patients were planned with gradient-dose matching using PBS technique. Uniform dose coverage to the entire target volumes was achieved with averaged junction lengths of 6.9±0.3 cm. Robustness of the plans was evaluated by shifting the isocenter of each treatment field by ±3 mm in longitudinal direction and compared with the original non-shifted plan with metrics of conformity number (CN) and homogeneity index (HI). An anthropomorphic phantom study using film measurements was also carried out on a plan with 5 cm junction length. Results: For a given junction length, the dose errors were directly proportional to the setup errors. Setup errors of 3 mm from each field caused on average 3.5% lower CN and 2.1% higher HI. Minimal D95% to PTV and D98% to CTV were reduced by 2.2%±1.5% and 2.8%±1.7% respectively. A drop of maximal 6.8%±5.5% on the minimal dose to the cribriform plate was also observed. When the junction length was 5cm or longer, these 3mm setup errors from each field resulted in up to 12% dose errors. Consistent results were reached between film measurements and planned dose profiles in the junction area. Due to near-zero exit doses beyond the target volume, sparing of anterior organs such as heart, liver, lung and kidney were observed. Conclusions: Longitudinal setup errors directly reduce the dosimetric accuracy of the CSI treatment with matched proton fields. The reported technique creates a slow dose gradient in the junction area, which makes the treatment more robust and safe to longitudinal setup errors compared to conventional feathering methods.

Dose distribution outside the target volume for 170-MeV proton beam.

Dose delivered outside the proton field during radiotherapy can potentially lead to secondary cancer development. Measurements with a 170-MeV proton beam were performed with passive detectors (track etched detectors and thermoluminescence dosemeters) in three different depths along the Bragg curve. The measurement showed an uneven decrease of the dose outside of the beam field with local enhancements. The major contribution to the delivered dose is due to high-energy protons with linear energy transfer (LET) up to 10 keV µm(-1). However, both measurement and preliminary Monte Carlo calculation also confirmed the presence of particles with higher LET.

Solar transients disturbing the terrestrial magnetic environment at higher latitudes

Geomagnetic field variations during five major Solar Energetic Particle (SEP) events of solar cycle 23 have been investigated in the present study. The SEP events of 01 oct 2001, 04 Nov 2001, 21 Apr 2002 and 14 May 2005 have been selected to study the geomagnetic field variations at two high-latitude stations, Thule and Resolute Bay of the northern polar cap. We have used the GOES protn flux in seven different energy channels. All the proton events were associated with geoeffective or Earth directed CMEs that caused intense geomagnetic storms in response to geospace. We have taken high-latitude indices, AE and PC, under consideration and found fairly good correlation of thees with the ground magnetic field records during the five proton events. The departure of H component during the events were calculated from the quietest day of the month for each event. The correspondence of spectral index, inferred from event integrated spectra, with ground magnetic signatures along with Dst and PC indices have been brought out. From the correlation analysis we found very strong correlation to exist between the geomagnetic field variations and high latitude indices AE and PC. To find the association of geomagnetic storm intensity with proton and geomagnetic field variations along with the Dst and AE index. We found a strong correlation (0.88) to exist between the spectral indices and magnetic field deviations and also between spectral indices and AE and PC.

An algorithm to assess the need for clinical Monte Carlo dose calculation for small proton therapy fields based on quantification of tissue heterogeneity

In proton therapy, complex density heterogeneities within the beam path constitute a challenge to dose calculation algorithms. This might question the reliability of dose distributions predicted by treatment planning systems based on analytical dose calculation. For cases in which substantial dose errors are expected, resorting to Monte Carlo dose calculations might be essential to ensure a successful treatment outcome and therefore the benefit is worth a presumably long computation time. The aim of this study was to define an indicator for the accuracy of dose delivery based on analytical dose calculations in treatment planning systems for small proton therapy fields to identify those patients for which Monte Carlo dose calculation is warranted. Fourteen patients treated at our facility with small passively scattered proton beams (apertures diameters below 7 cm) were selected. Plans were generated in the XiO treatment planning system in combination with a pencil beam algorithm developed at the Massachusetts General Hospital and compared to Monte Carlo dose calculations. Differences in the dose to the 50% of the gross tumor volume (D50, GTV) were assessed in a field-by-field basis. A simple and fast methodology was developed to quantify the inhomogeneity of the tissue traversed by a single small proton beam using a heterogeneity index (HI)-a concept presented by Plugfelder et al. [Med. Phys. 34, 1506-1513 (2007)] for scanned proton beams. Finally, the potential correlation between the error made by the pencil beam based treatment planning algorithm for each field and the level of tissue heterogeneity traversed by the proton beam given by the HI was evaluated. Discrepancies up to 5.4% were found in D50 for single fields, although dose differences were within clinical tolerance levels (<3%) when combining all of the fields involved in the treatment. The discrepancies found for each field exhibited a strong correlation to their associated HI-values (Spearman's ρ=0.8, p<0.0001); the higher the level of tissue inhomogeneities for a particular field, the larger the error by the analytical algorithm. With the established correlation a threshold for HI can be set by choosing a tolerance level of 2-3%-commonly accepted in radiotherapy. The HI is a good indicator for the accuracy of proton field delivery in terms of GTV prescription dose coverage when small fields are delivered. Each HI-value was obtained from the CT image in less than 3 min on a computer with 2 GHz CPU allowing implementation of this methodology in clinical routine. For HI-values exceeding the threshold, either a change in beam direction (if feasible) or a recalculation of the dose with Monte Carlo would be highly recommended.

Dosimetric accuracy of proton therapy for chordoma patients with titanium implants

To investigate dosimetric errors in proton therapy treatment planning due to titanium implants, and to determine how these affect postoperative passively scattered proton therapy for chordoma patients with orthopedic hardware. The presence of titanium hardware near the tumor may affect the dosimetric accuracy of proton therapy. Artifacts in the computed tomography (CT) scan can cause errors in the proton stopping powers used for dose calculation, which are derived from CT numbers. Also, clinical dose calculation algorithms may not accurately simulate proton beam transport through the implants, which have very different properties as compared to human tissue. The authors first evaluated the impact of these two main issues. Dose errors introduced by metal artifacts were studied using phantoms with and without titanium inserts, and patient scans on which a metal artifact reduction method was applied. Pencil-beam dose calculations were compared to models of nuclear interactions in titanium and Monte Carlo simulations. Then, to assess the overall impact on treatment plans for chordoma, the authors compared the original clinical treatment plans to recalculated dose distributions employing both metal artifact reduction and Monte Carlo methods. Dose recalculations of clinical proton fields showed that metal artifacts cause range errors up to 6 mm distal to regions affected by CT artifacts. Monte Carlo simulations revealed dose differences >10% in the high-dose area, and range differences up to 10 mm. Since these errors are mostly local in nature, the large number of fields limits the impact on target coverage in the chordoma treatment plans to a small decrease of dose homogeneity. In the presence of titanium implants, CT metal artifacts and the approximations of pencil-beam dose calculations cause considerable errors in proton dose calculation. The spatial distribution of the errors however limits the overall impact on passively scattered proton therapy for chordoma.

SU‐E‐T‐180: Empirical Determination of Output Factors for Proton Therapy Fields Using a Uniform Scanning Proton Beam and 10cm Snout

Purpose: he output factor of a proton field is affected by energy, width of spread‐out Bragg Peak (SOBP), source to measurement point distance, shape and size of the aperture, and the thickness of the compensator. It is generally measured in a water phantom to determine MU needed for each treatment field. This is time consuming and labor intensive. Previous studies employed empirical fits to measured data and then applied correction factors to account for various parameters. In this study, we have developed an empirical model to determine the output factors for proton fields with a 10cm snout using a cubic equation. Methods: Measured output factors with (OFclosed) and without (OFopen) a compensator for 693 fields delivered with the uniform scanning beam were analyzed. The measurements were made for various ranges, SOBP widths, air gaps, aperture shapes and sizes using a 10cm snout. 3D empirical equations for predicting OFclosed and OFopen were determined by fitting the data to several 3D curves and evaluating each fit. Results: The proposed model uses a simplified cubic fit. The distribution of closed measured OF vs predicted OF for this model has a mean of 1.000 and 0.013 the standard deviation and agrees within 3% of measured data 88.9% of all fields. The current model being used has a mean of 0.994 and a standard deviation of 0.009 and agrees within 3% of measured data 98.2% of all fields. Conclusion: We have shown that a simplified cubic fitting of the measured data allows for calculation of output factor with a single equation of the form OF=f(Inverse Square Law Correction, SOBP, Range). This empirical equation may be used to double check the OFclosed obtained with the separate model that is currently used. The model will significantly reduce the QA time required for measurements

SU-E-CAMPUS-T-06: Dose Perturbations by Electromagnetic Transponders in the Proton Environment

Purpose: Surgically implanted electromagnetic transponders have been used in external beamradiotherapy for target localization and position monitoring in real time. The effect of transponderson proton therapy dose distributions has not been reported. This study determines the level of thedose deficit that results from the presence of Calypso Beacon™ transponders in the path of thetreatment beam. Methods: A Monte Carlo implementation of the transponder geometry is validated against measurements using Gafchromic EBT2 film in a proton field and subsequently used to generate dosedistributions for transponders at different positions and orientations in the proton spread-out Braggpeak. The maximum dose deficit is extracted in each case. Results: Dose shadows of up to 60% occur for transponders positioned very near the end ofrange of the Bragg peak. However, if transponders are positioned further than 5 mm from the endof range, and are not oriented parallel to the beam direction, then the dose deficit can be keptbelow 10%. Conclusion: Beacon electromagnetic transponders could be used for patient setup and motionmanagement for proton therapy provided some guidelines regarding their placement and orien-tation with respect to the beam can be met. In cases of difficult transponder positions, optionsto minimise the shadow include careful plan design with multiple beam angles to distribute theshadow over a larger volume, or possibly increasing the dose in the expected shadow region tooffset the deficit.

SU-E-T-73: Investigation of 3D Dosimetry for Proton Therapy Using PRESAGE

Purpose: This investigation studies the feasibility of PRESAGE, a 3D polyurethane dosimeter, for relative dosimetry measurements of clinically relevant proton treatments using an anthropomorphic head phantom developed by the Radiological Physics Center (RPC). Performance of a low-LET dependent PRESAGE was evaluated by comparison to the traditionally used radiochromic film, EBT2, and thermoluminescent dosimeters (TLDs). Methods: The head phantom consists of a human skeleton with synthetic isocyanate rubber having similar tissue-equivalent radiological properties to high-energy protons cast around it to mimic natural heterogeneous anatomical structures. The phantom has a cutout that admits either a film and TLD insert or a PRESAGE dosimeter. A treatment plan was developed and delivered twice: once with the film/TLD insert and again with the PRESAGE dosimeter. The 3D dose distribution from the PRESAGE was read out using the Duke medium-field-of-view optical-CT scanner (DMOS). The measured dose distributions were compared to the treatment plan and the distance to agreement (DTA) within the high dose regions was calculated in the sagittal and coronal planes as measured by the EBT2 film. Results: PRESAGE dose profiles showed agreement within 3mm and 4mm in the coronal and sagittal planes respectively. Film data showed agreement to 2mm in both planes. A 2D gamma comparison between the PRESAGE and planning data gave 95% agreement with 5%/3mm and 5%/4mm in those respective fields. Conclusion: PRESAGE dosimeters show great potential for measuring dose distributions from proton fields within heterogeneous volumes, with the opportunity to measure volumetric dose data. Data within the high dose regions of the treatment showed comparable DTAs from two dosimetry systems. With film measurements confirming PRESAGE readings, dose data in low dose regions in clinically sensitive regions can now be better determined. Additional work will include organs at risk (OAR) considerations allowing for detailed DVH comparisons, as well as 3D gamma analysis. NIH grant 5R01CA100835

TU-A-108-02: A 4D Monte Carlo Study Quantifying Changes in the Interplay Effect as a Function of Treatment Delivery Parameters in Proton Beam Scanning for Lung

Purpose: To assess the impact of delivery parameters of a pencil beam scanning proton therapy system on the dosimetric interplay effect during lung treatments. Methods: 4D Monte Carlo simulations were performed using TOPAS for 5 lung cancer patients with varying tumor sizes (50.4–167.1cc) and superior‐inferior (SI) motion amplitudes (2.9–30.1mm). Each patient was planned based on a 4DCT scan. The complete time structure of the proton fields was incorporated into the simulation framework by including the time required to deposit dose, scan the beam laterally, change the beam energy and the time required for magnet settling. Changes in the 4D dose distribution resulting from altering the spot size, energy switch time, magnet settling time, initial breathing phase, spot spacing, scanning direction, scanning speed, beam current and breathing period were determined using deformable image registration. The metrics used for analysis were mean dose, minimum dose, maximum dose, dose homogeneity, equivalent uniform dose (EUD), lung V 5, V 20 and mean lung dose. Results: In general, longer delivery times led to less interplay and the interplay effect could not be predicted using motion amplitude alone. Larger spot sizes (σ ∼9–16mm) gave a EUD of 99.0% + 4.4% (1 standard deviation) of the prescription dose in 1 fraction compared to 86.1% + 13.1% for smaller spots (σ ∼2–4mm). Smaller spot sizes gave EUD values as low as 65.3% in 1 fraction. Target dose homogeneity improved with reduced spot spacing. The interplay effect can vary significantly with initial breathing phase. No definitive benefit was observed when scanning predominantly parallel or perpendicular to the SI axis. Conventional fractionation reduced interplay, giving EUD values of at least 84.7% and 100.0% for the small and larger spot sizes respectively. Conclusion: The interplay effect is highly patient specific and varies based on the motion amplitude, tumor location and various delivery parameters. This project was supported by the National Cancer Institute Grant No. R01 CA111590

TH-A-137-03: Application of the Dose Magnifying Glass to Proton Radiosurgery

Purpose: To evaluate the performance of a pixelated‐Silicon detector for proton radiosurgery applications and determine a methodology for response correction as a function of proton energy/LET. Methods: Proton radiosurgery requires metrology apparatus that have a high spatial resolution and a stable (or well characterized) response to LET. To meet this need the dose magnifying glass or DMG was developed at the University of Wollongong. This device is a pixelated silicon strip detector comprising an array of 128 phosphor implanted n+ strips on a p‐type silicon wafer. A 100μm pitch device was tested with proton radiation to evaluate its performance. The DMG was mounted in a water tank and irradiated with various discrete energies to develop correction factors as a function of proton energy. Depth dose and lateral profiles of various proton radiosurgery beams were then collected for comparison with standard metrology techniques. Results: The DMG performed well providing realtime depth dose and lateral profile information. The device did over‐respond to proton dose especially below 100MeV resulting in over‐estimation in the spread‐out Bragg peak region of almost 30%. This over‐response could be accounted for using a power law correction, the parameters of which were established for a range of discrete proton energies down to 20MeV. Using the central channel of the DMG, accurate depth dose profiles proton fields to 1mm diameter were obtained without errors associated with partial volume sampling, while the full array could be used to provide real time profile information of fields below 1.5cm diameter without the need for detector scanning. Conclusion: The DMG is a potentially useful device for proton therapy, in particular in providing realtime data for small fields associated with SRS. Further work is required to better characterize the performance of the device as a function of depth/LET before it can be used clinically.

TU‐A‐108‐08: Comparison of Full Intensity Modulated and Single Field Uniform Dose Proton Beam Scanning Lung Treatments

Purpose: To determine differences in motion interplay effects between single‐field uniform dose (SFUD) and intensity modulated proton therapy (IMPT) treatments for lung patients. Methods: SFUD and IMPT treatment plans were generated for 10 lung cancer patients. The spot spacing was set to 0.7σ for all treatment plans and all plans used 2 fields delivered to the planning target volume (PTV). Treatment plans required that at least 95% of the PTV and 99% of the ICTV received the prescription dose. The ICTV was defined as the IGTV+8mm and covered the CTV in all 10 phases of the 4DCT. SFUD plans require each field homogeneously, within a given tolerance, cover the PTV, whilst IMPT has no such constraint. Monte Carlo simulations were performed using the detailed timing structure of the proton fields and the patient 4DCT for 2.5Gy(RBE) and 12Gy(RBE) fractions using 2 different spot sizes (σ ∼8–17mm and ∼2–4mm). Simulations were performed on the different phases of the patient 4DCT. The 4D dose distribution was generated via deformable image registration, using the 50% phase as the reference. Degradation in the dose distributions due to motion and interplay effects was quantified via equivalent uniform dose (EUD), V95 and dose homogeneity. Results: All values are quoted as percentages of the dose per fraction. For 2.5Gy(RBE) fractions, the average EUD loss due to interplay is 3.7+/−3.6% and 4.4+/−3.3% for IMPT and SFUD respectively for the larger spot size. For the smaller spot size, the average EUD loss for IMPT and SFUD treatments is 12.2+/−10.5% and 15.1+/−12.2% respectively. The average EUD loss for 12Gy(RBE) fractions is within 1% of the 2.5Gy(RBE) values. No significant differences between the SFUD and IMPT plans were observed for V95 and target dose homogeneity. Conclusion: No significant difference in motion effects was observed between SFUD and IMPT treatments. This project was supported by the National Cancer Institute Grant No. R01 CA111590

MO‐F‐108‐08: A Revolving Collimator for Proton Stereotactic Radiosurgery (pSRS)

Purpose: Stereotactic radiosurgery (SRS) is a non‐invasive treatment modality appropriate for a wide array of tumors. Particularly in patients with benign tumors receiving proton SRS (pSRS), the generation of secondary neutrons from brass apertures may be associated with higher risk of secondary radiation‐induced tumors. A specially designed revolving titanium alloy collimator system with multiple sized collimator openings in circular and elliptical shapes is proposed and investigated in this study. Methods: Simulations of the passage of a proton beam through the proposed collimator system was performed using the FLUKA Monte Carlo general purpose code with treatment nozzle model implemented. The set of materials potentially applicable for the collimator fabrication, i.e. brass, stainless steel (SS), tungsten alloy (WA) and titanium alloy (TiA) (Ti‐6A1‐4V Grade 5) were simulated. The neutron fluence, dose equivalent, activation of the collimator material, the cooling times and beam shaping properties are analyzed. Results: Utilization of TiA results in less neutron dose equivalent at the patient compared to brass, WA or SS. The neutron flux spectra for TiA is an order of magnitude lower than those for the other materials in the energy interval with the maximum neutron weighting factor. The activation of TiA is lower and the cooling time 10 times shorter compared to WA. Dose shaping with the pSRS collimator is similar to a brass aperture. Dose enhancement at shallow depths for small proton fields limits the practical collimator opening size. Conclusion: The specially designed revolving pSRS titanium alloy collimator proposed in this study may reduce secondary neutron dose to the patient compared to other collimator materials investigated. This may reduce the risk of secondary tumors and malignancies and reduce late effects to the patient. The pSRS collimator allows the establishment of a pSRS system and methodology without the need for a dedicated pSRS treatment vault.

SU‐E‐T‐451: Patient and Site‐Specific Assessment of the Value of Routine Monte Carlo Dose Calculation in Proton Therapy

Purpose: To assess the clinical impact of Monte Carlo (MC) dose calculations in proton therapy by analyzing uncertainties of analytical algorithms when predicting dose to target and critical structures and beam ranges in patient geometries. Methods: We used TOPAS to simulate double scattering proton treatments, which were compared to analytical treatment planning dose calculations (TPC). We investigated: 1)beam ranges, dose distributions, does‐rate profiles and output factors using various dedicated experiments.range differences caused by the patient heterogeneity considering 48 fields and 4 treatment sites. We calculated distal 2)dose surfaces composed of beam ranges and assessed average range difference (dR) and root‐mean‐square deviation (RMSD).3)dose delivery accuracy using a DVH based analysis considering patient treatment plans from 3 sites. 4)the accuracy of dose delivery for small fields (diameter below 7cm) based on patient heterogeneity for 38 head fields. We assess differences in D50. Results: 1)MC and measurements agree within statistical uncertainties.2)For 3 of 4 patient sites the RMSD between TPC and TOPAS was &lt;2mm. Head treatments indicate significant RMSD of up to 6mm. 3)TPC DVHs showed underdosage for head cases by 2% compared to TOPAS, discrepancies in critical volumes can reach up to 50%, prostate patients show significant penumbra differences, liver patients show good agreement between TOPAS and TPC. The site‐specific findings can be explained by beam range and geometrical complexities.4)For small proton fields, discrepancies of up to 5.4% were found in D50 for single fields. A clear correlation between the accuracy and patient heterogeneity was established. Conclusion: This study highlights treatment sites and scenarios where MC simulations are recommended for proton therapy, i.e. small fields (due to scattering disequilibrium) and head treatments with bone/air/tissue interfaces affecting the ability of analytical algorithms to predict the correct beam range. Absolute dose differences where typically within 2% for large fields.

Compensation method for respiratory motion in proton treatment planning for mobile liver cancer

We evaluated the dosimetric effect of a respiration motion, and sought an effective planning strategy to compensate the motion using four‐dimensional computed tomography (4D CT) dataset of seven selected liver patients. For each patient, we constructed four different proton plans based on: (1) average (AVG) CT, (2) maximum‐intensity projection (MIP) CT, (3) AVG CT with density override of tumor volume (OVR), and (4) AVG CT with field‐specific proton margin which was determined by the range difference between AVG and MIP plans (mAVG). The overall effectiveness of each planning strategy was evaluated by calculating the cumulative dose distribution over an entire breathing cycle. We observed clear differences between AV G and MIP CT‐based plans, with significant underdosages at expiratory and inspiratory phases, respectively. Only the mAVG planning strategy was fully successful as the field‐specific proton margin applied in the planning strategy complemented both the limitations of AVG and MIP CT‐based strategies. These results demonstrated that respiration motion induced significant changes in dose distribution of 3D proton plans for mobile liver cancer and the changes can be effectively compensated by applying field‐specific proton margin to each proton field.PACS numbers: 87.55.D; 87.53.Bn; 87.53.Jw; 87.55.dk

Technique for sparing previously irradiated critical normal structures in salvage proton craniospinal irradiation

BackgroundCranial reirradiation is clinically appropriate in some cases but cumulative radiation dose to critical normal structures remains a practical concern. The authors developed a simple technique in 3D conformal proton craniospinal irradiation (CSI) to block organs at risk (OAR) while minimizing underdosing of adjacent target brain tissue.MethodsTwo clinical cases illustrate the use of proton therapy to provide salvage CSI when a previously irradiated OAR required sparing from additional radiation dose. The prior radiation plan was coregistered to the treatment planning CT to create a planning organ at risk volume (PRV) around the OAR. Right and left lateral cranial whole brain proton apertures were created with a small block over the PRV. Then right and left lateral “inverse apertures” were generated, creating an aperture opening in the shape of the area previously blocked and blocking the area previously open. The inverse aperture opening was made one millimeter smaller than the original block to minimize the risk of dose overlap. The inverse apertures were used to irradiate the target volume lateral to the PRV, selecting a proton beam range to abut the 50% isodose line against either lateral edge of the PRV. Together, the 4 cranial proton fields created a region of complete dose avoidance around the OAR. Comparative photon treatment plans were generated with opposed lateral X-ray fields with custom blocks and coplanar intensity modulated radiation therapy optimized to avoid the PRV. Cumulative dose volume histograms were evaluated.ResultsTreatment plans were developed and successfully implemented to provide sparing of previously irradiated critical normal structures while treating target brain lateral to these structures. The absence of dose overlapping during irradiation through the inverse apertures was confirmed by film. Compared to the lateral X-ray and IMRT treatment plans, the proton CSI technique improved coverage of target brain tissue while providing the least additional radiation dose to the previously irradiated OAR.ConclusionsProton craniospinal irradiation can be adapted to provide complete sparing of previously irradiated OARs. This technique may extend the option of reirradiation to patients otherwise deemed ineligible for further radiotherapy due to prior dose to critical normal structures.

Predicting the Neutron and Proton Masses Based on Baryons which Are Yang-Mills Magnetic Monopoles and Koide Mass Triplets

We show how the Koide relationships and associated triplet mass matrices can be generalized to derive the observed sum of the free neutron and proton rest masses in terms of the up and down current quark masses and the Fermi vev to six parts in 10,000. This sum can then be solved for the separate neutron and proton masses using the neutron minus proton mass difference derived by the author in a recent, separate paper. The oppositely-signed charges of the up and down quarks are responsible for the appearance of a complex phase exp(iδ) and real rotation angle θ which leads on an independent basis to mass and mixing matrices similar to that of Cabibbo, Kobayashi and Maskawa (CKM). These can then be used to specify the neutron and proton mass relationships to unlimited accuracy using θ as a nucleon fitting angle deduced from empirical data. This fitting angle is then shown to be related to an invariant of the CKM mixing angles within experimental errors. Also developed is a master mass and mixing matrix which may help to interconnect all baryon and quark masses and mixing angles. The Koide generalizations developed here enable these neutron and proton mass relationships to be given a Lagrangian formulation based on neutron and proton field strength tensors that contain vacuum-amplified and current quark wavefunctions and masses. In the course of development, we also uncover new Koide relationships for the neutrinos, the up quarks, and the down quarks.