Small Field Sizes Research Articles

Measurement, modelling and reporting of small field output factors requires systematic approaches for accurate dosimetry

Small field dosimetry is difficult, yet consistent data is necessary for advanced radiotherapy techniques. However, the reported data in the literature contains relatively large inconsistencies and differences. To help to resolve these and to improve the approaches and associated uncertainties requires careful standardised measurements, validated Monte-Carlo (MC)-based correction factors, possible new thinking on detectors and clear reporting of data, correlating to actual/delivered/dosimetric field size to enable comparison between linacs. For very small field sizes, greater attention is required to the experimental measurement and reporting methods, requiring clear guidelines for what is a ‘very small field size’ and for when such more detailed methods are needed.

A dosimetric study of small photon fields using polymer gel and Gafchromic EBT films

The use of small field sizes is increasingly becoming important in radiotherapy particularly since the introduction of stereotactic radiosurgery and intensity-modulated radiation therapy techniques. The reliable measurement of delivered dose from such fields with conventional dosimeters, such as ionization chambers, is a challenging task. In this work, methacrylic and ascorbic acid in gelatin initiated by copper polymer gel dosimeters are employed to measure dose in 3 dimensions. Field sizes of 5 × 5mm2, 10 × 10mm2, 20 × 20mm2, and 30 × 30mm2 are investigated for a 6-MV x-rays. The results show an agreement with Gafchromic film, with some variation in measured doses near the edge of the fields, where the film data decrease more rapidly than the other methods. Dose penumbra widths obtained with gel dosimeters and Gafchormic film were generally in agreement with each other. The results of this work indicate that polymer gel dosimetry could be invaluable for the quantification of the 3-dimensional dose distribution in small field size.

A Monte Carlo approach to lung dose calculation in small fields used in intensity modulated radiation therapy and stereotactic body radiation therapy.

In the current study, the effect of electronic disequilibrium on lung dose with small photon beams was verified. The central axis absorbed dose in lung phantom was calculated by Monte Carlo (MC) method. The 6 and 18 MV photon beams of Varian Clinac 2100EX were simulated using MCNPX MC Code (Los Alamos national lab, USA). The MC model was used to calculate the depth doses water and low density water resembling the soft-tissue and lung, respectively. Four small field sizes including 0.5 cm2×0.5 cm2, 1 cm2×1 cm2, 2 cm2×2 cm2, and 3 cm2×3 cm2 were used in this study. Percentage of dose reduction in lung region relative to homogenous phantom for 6 MV photon beam were 44.6%, 39%, 13%, and 7% for 0.5 cm2×0.5 cm2, 1 cm2×1 cm2, 2 cm2×2 cm2, and 3 cm2×3 cm2 fields, respectively. For 18 MV photon beam, the results were found to be 82%, 69%, 46%, and 25.8% for the same field sizes, respectively. The solid tumor dose inside lung was reduced considerably between 17% and 35% for 18 MV beam, while there was only 9% dose reduction for tumor dose for 0.5 and 1 cm field sizes. Our study showed that the dose reduction with small fields in the lung was very enormous. Thus, inaccurate prediction of absorbed dose inside lung and also lung soft-tissue interfaces with small photon beams may lead to critical consequences for treatment outcome.

Dosimetric distribution of the surroundings of different dental crowns and implants during LINAC photon irradiation

In radiotherapy of the head and neck, metal dentures or implants will increase the risk of complications such as mucositis and osteoradionecrosis. The aim of this study is to explore the back scatter effect of commercially available dental metal alloys on the mucosa and bone under 6MV LINAC irradiation. The Monte Carlo method has been employed to calculate the dose distribution in the heterogeneous media of the designed oral phantom based on the oral cavity geometry.Backscatter dose increases up to a maximum of 53%, and is primarily dependent on the physical density and electron density of the metal crown alloy. Ceramic metal crowns have been quantified to increase backscatter dose up to 10% on mucosa. Ceramic serves as an inherent shield of mucosa. The backscatter dose will be greater for a small field size if the tumor is located at a deeper region. Titanium implants will increase the backscatter dose by 13% to bone but will not affect the mucosa. QC-20 (polystyrene resin) is recommended as a shield material (3mm) to eliminate the backscatter dose on mucosa due to the high density metals.

Characterization of an Innovative p-type Epitaxial Diode for Dosimetry in Modern External Beam Radiotherapy

Due to the ever-increasing complexity of treatment modalities in radiation therapy, there has been a greater need for detectors to perform quality assurance to ensure patients are treated correctly and safely. Modern radiation therapy techniques involve small field sizes, high dose gradients, and varying intensity of energy and rate. The ideal dosimeter for this treatment should display high spatial resolution, high linearity, accuracy, and radiation hardness. Silicon detectors have been widely used for radiotherapy measurements and have many attractive qualities as a dosimeter; weaknesses of silicon detectors are, however, decreases in sensitivity with accumulated dose. The Centre for Medical Radiation Physics has developed a new technology with an unusual charge collection efficiency variation with accumulated dose which stabilizes the response of the detector within ±5% after 120 kGy photon irradiation. The sensor has been also characterized by irradiation by an 18 MV medical LINAC with sensitivity to a photoneutron-induced damage of less than 0.5%/100 Gy. The radiation damage mechanism has been validated by TCAD simulations which confirmed the mechanism behind the CCE increase as a function of the accumulated dose.

Monte Carlo dosimetry for the Papillon P50 contact radiotherapy and IORT device

Contact radiotherapy uses small field sizes and very short SSDs to deliver low-energy X-rays very close to the position of a tumour. We present a Monte Carlo simulation of the Papillon contact radiotherapy machine, and in particular the backscatter factor, to investigate whether the factors given in current Codes of practice for kV dosimetry remain valid under these conditions.

Radiotherapy electron beams collimated by small tubular applicators: characterization by silicon and diamond diodes

High-energy electron beams generated by linear accelerators, typically in the range 6 to 20 MeV, are used in small field sizes for radiotherapy of localized superficial tumors. Unshielded silicon diodes (Si-D) are commonly considered suitable detectors for relative dose measurements in small electron fields due to their high spatial resolution. Recently, a novel synthetic single crystal diamond diode (SCDD) showed suitable properties for standard electron beams and small photon beams dosimetry. The aim of the present study is twofold: to characterize 6 to 15 MeV small electron beams shaped by using commercial tubular applicators with 2, 3, 4 and 5 cm diameter and to assess the dosimetric performance under such irradiation conditions of the novel SCDD dosimeter by comparison with commercially available dosimeters, namely a Si-D and a plane–parallel ionization chamber. Percentage depth dose curves, beam profiles and output factors (OFs) were measured. A good agreement among the dosimeters was observed in all of the performed measurements. As for the tubular applicators, two main effects were evidenced: (i) OFs larger than unity were measured for a number of field sizes and energies, with values up to about 1.3, that is an output 30% greater than that obtained at the 10 × 10 cm2 reference field; (ii) for each diameter of the tubular applicator a noticeable increase of the OF values was observed with increasing beam energy, up to about 100% in the case of the smaller applicator. This OF behavior is remarkably different from what typically observed for small blocked fields having the same size and energy as those used in this study. OFs for tubular applicators depend considerably on the field size, so interpolation is unadvisable to predict the linear accelerator output for such applicators whereas reliable high-resolution detectors, as the silicon and diamond diodes used in this work allow OF measurements with uncertainties of about 1%.

Effect of Field Size and Length of Plantar Spur on Treatment Outcome in Radiation Therapy of Plantar Fasciitis: The Bigger the Better?

Radiation therapy is well established in the treatment of painful plantar fasciitis or heel spur. A retrospective analysis was conducted to investigate the effect of field definition on treatment outcome and to determine the impact of factors potentially involved. A review of treatment data of 250 patients (285 heels) with a mean follow-up time of 11 months showed that complete symptom remission occurred in 38%, partial remission in 32%, and no change in 19% (11% were lost to follow-up). Variables such as radiologic evidence of plantar spurs, their length, radiation dose, field size, age, sex, and onset of pain before administration of radiation therapy were investigated in univariate and multivariate regression analyses. Treatment response depended upon age >53 years, length of heel spur ≤6.5 mm (or no radiologic evidence of a heel spur), and onset of pain <12 months before radiation therapy. Patients with these clinical prerequisites stood a 93% chance of clinical response. Without these prerequisites, only 49% showed any impact. No influence of field size on treatment outcome became evident. Patients with short plantar heel spurs benefit from radiation therapy equally well as patients without any radiologic evidence. Moreover, smaller field sizes have the same positive effect as commonly used large field definitions covering the entire calcaneal bone. This leads to a recommendation of a considerable reduction of field size in future clinical practice.

Single Fraction Stereotactic Body Radiation Therapy (SBRT) for Spine Metastasis: A Dosimetric Comparison of Multiple Treatment Delivery Systems

98.9%. The mismatch became more pronounced with smaller field size. The Gamma passing rate decreased to 38% from 52% when the DMLC field size changed from 5 x 5 cm to 3 x 3 cm. These gantry/collimator rotation induced discrepancies in DMLC delivery could be significantly reduced if additional angular dependent “shift” (averaged w1.7 mm, a resultant of 2D shifts) was applied to the dose distribution. When this was done, the Gamma passing rate increased to 93% from 56%. The value of these “shifts” was independent of field size. Overall, there were no observed differences in Gamma passing rates using different dose rates or different prescribed dose gradients. Conclusions: We found that smaller DMLC fields have larger variations in dose distribution from collimator rotations. The passing rate improved by applying a set of constant shifts. However, the clinical significance of these variations needs to be further evaluated. Author Disclosure: J. Li: None. C. Burman: None. K. Schupak: None. M. Chan: None.

Comparison Between IMRT and 3D CRT in Laryngeal Cancer Patients Treated on RTOG 0522

and Dmax values remained relatively unchanged (2.7 1.6% and -0.2 3% respectively) regardless of target size. When MC plans were renormalized (MC Optimized), overall target dose was found to increase compared to PB plans. On average, MC calculated doses for PTV Dmean, Dmax and D5 were higher than PB plans by 6.4 6.5%, 10.7% 8%, and 9% 8%, respectively. Based on results for CI, R50, and D2 cm, MC Optimized plans also showed increased dose to surrounding critical structures. Due to small field size, this led to violation of RTOG constraints for optic apparatus in 3 of 9 patients. Conclusions: Traditionally, PB plans have been used in treatment of NP tumors. Although this may be acceptable for large tumors, our study highlights the need for accurate dose planning for smaller tumors (<4 cm) surrounded by air. If PB calculations are used, such targets may be significantly under-dosed. As MC based planning is clinically adopted, higher doses to PTV and OARs will result. This may require reassignment of prescription dose and dose limits on critical structures based on MC calculations. Author Disclosure: A. Sethi: None. F. Alite: None. P. Cecilio: None. I. Rusu: None. E. Melian: None. B. Emami: None.

A synthetic diamond diode in volumetric modulated arc therapy dosimetry

The aim of this work is to investigate the behavior of a single crystal diamond diode (SCDD) for volumetric modulated arc therapy (VMAT) dose verifications. This delivery technique is one of the most severe test of a dosimeter performance due to the modulation of the dose rate achieved by simultaneously changing the velocity of the gantry and the position of the collimator leaves. The performed measurements with VMAT photon beams can therefore contribute to an overall global validation of the device to be used in dose distribution verifications. The SCDD response to 6 MVRX has been tested and compared with reference ionization chambers and treatment planning system (TPS) calculations in different experiments: (a) measurements of output factors for small field sizes (square fields of side ranging between 8 mm and 104 mm) by SCDD and A1SL ionization chamber; (b) angular dependence evaluation of the entire experimental set-up by SCDD, A1SL, and Farmer ionization chambers; and (c) acquisition of dose profiles for a VMAT treatment of a pulmonary disease in latero-lateral and gantry-target directions by SCDD and A1SL ionization chamber. The output factors measured by SCDD favorably compare with the ones obtained by A1SL, whose response is affected by the lack of charged particle equilibrium and by averaging effect when small fields are involved. From the experiment on angular dependence, a good agreement is observed among the diamond diode, the ion chambers, and the TPS. In VMAT profiles, the absorbed doses measured by SCDD and A1SL compare well with the TPS calculated ones. An overall better agreement is observed in the case of the diamond dosimeter, which is also showing a better accuracy in terms of distance to agreement in the high gradient regions. Synthetic diamond diodes, whose performance were previously studied for conformal and IMRT radiotherapy techniques, were found to be suitable detectors also for dosimetric measurements in volumetric arc therapy treatments.

Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife® and linear accelerators equipped with microMLC and circular cones

The use of small photon fields is now an established practice in stereotactic radiosurgery and radiotherapy. However, due to a lack of lateral electron equilibrium and high dose gradients, it is difficult to accurately measure the dosimetric quantities required for the commissioning of such systems. Moreover, there is still no metrological dosimetric reference for this kind of beam today. In this context, the first objective of this work was to determine and to compare small fields output factors (OF) measured with different types of active detectors and passive dosimeters for three types of facilities: a CyberKnife(®) system, a dedicated medical linear accelerator (Novalis) equipped with m3 microMLC and circular cones, and an adaptive medical linear accelerator (Clinac 2100) equipped with an additional m3 microMLC. The second one was to determine the kQclin,Qmsr (fclin,fmsr) correction factors introduced in a recently proposed small field dosimetry formalism for different active detectors. Small field sizes were defined either by microMLC down to 6 × 6 mm(2) or by circular cones down to 4 mm in diameter. OF measurements were performed with several commercially available active detectors dedicated to measurements in small fields (high resolution diodes: IBA SFD, Sun Nuclear EDGE, PTW 60016, PTW 60017; ionizing chambers: PTW 31014 PinPoint chamber, PTW 31018 microLion liquid chamber, and PTW 60003 natural diamond). Two types of passive dosimeters were used: LiF microcubes and EBT2 radiochromic films. Significant differences between the results obtained by several dosimetric systems were observed, particularly for the smallest field size for which the difference in the measured OF reaches more than 20%. For passive dosimeters, an excellent agreement was observed (better than 2%) between EBT2 and LiF microcubes for all OF measurements. Moreover, it has been shown that these passive dosimeters do not require correction factors and can then be used as reference dosimeters. Correction factors for the active detectors have then been determined from the mean experimental OF measured by the passive dosimeters. Four sets of correction factors needed to apply the new small field dosimetry formalism are provided for several active detectors. A protocol for small photon beams OF determination based on passive dosimeters measurements has been recently proposed to French radiotherapy treatment centers.

Monte Carlo-based diode design for correction-less small field dosimetry

Due to their small collecting volume, diodes are commonly used in small field dosimetry. However, the relative sensitivity of a diode increases with decreasing small field size. Conversely, small air gaps have been shown to cause a significant decrease in the sensitivity of a detector as the field size is decreased. Therefore, this study uses Monte Carlo simulations to look at introducing air upstream to diodes such that they measure with a constant sensitivity across all field sizes in small field dosimetry. Varying thicknesses of air were introduced onto the upstream end of two commercial diodes (PTW 60016 photon diode and PTW 60017 electron diode), as well as a theoretical unenclosed silicon chip using field sizes as small as 5 mm × 5 mm. The metric used in this study represents the ratio of the dose to a point of water to the dose to the diode active volume, for a particular field size and location. The optimal thickness of air required to provide a constant sensitivity across all small field sizes was found by plotting as a function of introduced air gap size for various field sizes, and finding the intersection point of these plots. That is, the point at which was constant for all field sizes was found. The optimal thickness of air was calculated to be 3.3, 1.15 and 0.10 mm for the photon diode, electron diode and unenclosed silicon chip, respectively. The variation in these results was due to the different design of each detector. When calculated with the new diode design incorporating the upstream air gap, was equal to unity to within statistical uncertainty (0.5%) for all three diodes. Cross-axis profile measurements were also improved with the new detector design. The upstream air gap could be implanted on the commercial diodes via a cap consisting of the air cavity surrounded by water equivalent material. The results for the unclosed silicon chip show that an ideal small field dosimetry diode could be created by using a silicon chip with a small amount of air above it.

SU-E-T-382: Investigation of the Characteristics of the OCTAVIUS Detector 1000 SRS for Verification of the Small Field Size IMRT and SRS Beams

Purpose: The aim for the study was to investigate characteristics of the OCTAVIUS Detector 1000 SRS for verification of the small field size IMRT and SRS beams. Methods: Several different treatments sites have been chosen. High modulation steep gradient small field size beams were delivered in two different setup settings. In the first setup virtual water phantom has been placed on the detector and irradiated. In the same setup EDR film has been irradiated. Measured data has been analyzed. In the second setup detector has been placed in the PTW Octavius phantom and irradiated. In the same setup EDR film and PTW 729 have been irradiated. Measured data has been analyzed. Results: In the first setup detector has been irradiated with virtual water phantom and measured Result showed excellent agreement with the film measurements. Along the compared profiles, absolute dose measured was in ±1 % of the film measurements in the high dose region and ±3 % in low dose region. Measured results showed also very good agreement in region of high gradients. Second set of measurements showed excellent agreement with the film measurements in high dose region with absolute dose difference within ±1% and ±3% in low dose region. Agreement with PTW 729 was in ±3 % in high and low dose region along the compared profiles. Region of high gradient showed better agreement with the film than with PTW 729 due to the inferior detector resolution. Conclusion: Broad range of measurements performed in the investigation showed that OCTAVIUS Detector 1000 SRS is a dosimetrically accurate and sensitive device and it can be used for clinical verification of the highly modulated small field size beams used in the IMRT and SRS treatments.

SU‐E‐T‐204: Preliminary Study of the Comparison of Patient‐Specific QAs Using the Diode and Ionization Chamber Based Dosimetry Systems

Purpose: Patient QA comparison by using the ArcCHECK diode cylindrical and PTW 2D ionization chamber array based dosimetry systems for in‐homogenous and small field size tumor. Methods: Four head and neck (HN), 4 lung and 3 brain tumor patients were selected to study the behavior of in‐homogeneity on QA and, 3 SRS and SRT patients were selected to investigate small field size effect. The detectors used for this study are the PTW Seven29 2D ion‐chamber planar array (PTW, Freiburg, Germany) and the ArcCHECK cylindrical array (Sun‐Nuclear, FL). All the patient QA plans were generated using the Eclipse (version‐11) TPS; all the treatment plans were delivered on the TrueBeam‐STX Accelerator and measured by both the systems. A gamma‐index criterion of minimum 3% (3 mm) is used to evaluate the QA results. The Local‐Dose and the Van‐Dyk criteria are used to calculate the gamma, respectively. Results: The QA results show that gamma‐index scores are higher than 94% (3mm, 3%) for both the systems for the brain and lung patients; whereas for the HN patients, the gamma‐index score is 86% and 98% for the PTW and ArcCHECK, respectively. For the small field size, the gamma‐index scores (3%, 3 mm) are higher than 98% for the ArcCHECK; whereas the gamma‐index score is lower for the PTW. Conclusion: The tumor inhomgeneity does not affect the QA results since both systems give similar QA results for the brain and lung patients. However, ArcCHECK is better as compared to the PTW for the HN patients; this may be due to the relatively higher sensitivity of the diode and could pick up the lower signal for low dose area. For the SRS and SRT QAs, there will be volume averaging effect because of the large sensitive volume; hence, the PTW system may not be a good choice of detectors for these QAs.

SU‐E‐T‐218: Octavius 4D: Commissioning and Clinical Implementation of a New Device for VMAT Verifications

Purpose: To commission a new device for pre‐treatment patient specific verification of VMAT plans, which is capable of reconstructing 3D dose with high resolution. Methods: Octavius 4D system (PTW) consists of an ion chamber array embedded in a cylindrical phantom which, assisted by an inclinometer, rotates synchronously with the gantry. For VMAT plans, it measures planar dose distributions as a function of gantry angle in order to compute the resulting 3D dose distribution. There are two options for the ion chamber array: Octavius 729 (general purpose) and Octavius 1000 SRS (small field size, high resolution). We have used three linacs: two Varian Clinac iX and one TrueBeam with MLC HD 120, and the TPS has been Eclipse (v.10.0). The commissioning measurements consisted of the following tests: (1) homogeneity in response; (2) linearity with dose; (3) reproducibility; (4) leakage current, pre‐and post‐irradiation; (5) verification of typical VMAT and SBRT plans, comparing TPS‐calculated versus measured dose using 3D local gamma index (3%, 3 mm criteria). Results: We found the following results for Octavius 729 (and, in brackets, for Octavius 1000 SRS): (1) Homogeneity between chambers was 1.9% (0.8%). (2) Excellent linearity for both cases, with r2 value better than 0.999. (3) Reproducibility was 0.52% (0.08%). (4) Leakage increases with previously measured dose and shows its largest variation for large field sizes, fluctuating between −0.33 and +0.33 cGy/min (between −0.006 and +0.020 cGy/min). (5) Verifications of typical clinical plans, performed with the adequate array, showed a mean pass ratio of 89.3% (98.7%). Conclusion: Octavius 4D system together with Octavius 729 is an adequate tool for VMAT patient specific QA. With Octavius 1000 SRS, this capability is extended to those treatments requiring better resolution, as is the case for SBRT.

MO‐D‐105‐06: Dose‐Area Product as a Method for Small Field Geometric QA

Purpose: To investigate the use of dose‐area product (DAP) as a highresolution tool for QA of small field sizes as an alternative to more complex alternatives such as film or water tank scanning. Method: An 81 mm diameter PTW Bragg Peak parallel plate chamber was used in combination with an Iris variable circular aperture collimator fitted to a CyberKnife System and a vendor‐supplied alignment jig. The Iris collimator is designed to deliver field diameters between 5 mm and 60 mm, with a tolerance of ±0.2 mm. First the sensitivity of the chamber reading to influence parameters (SSD, angle misalignment, lateral displacement, output variation) was investigated. Next the dependency of DAP on field diameter was investigated. Finally the sensitivity of the method to detect small field size changes consistent with the device specification was evaluated. Comparisons were made with film and diode water scans using four CyberKnife systems and three chambers. Results: Expected variations in chamber position/orientation within the jig resulted in measurement changes &lt;0.2%. Measurement variations due to LINAC output changes can be removed by normalising measurements to a fixed collimator reading obtained at the same time. The variation in chamber reading with field size followed a quadratic dependency over the 5mm ‐ 60mm range. The change in chamber reading associated with a 0.2 mm field diameter change ranged from 10% at 5 mm nominal to 0.5% at 60 mm, and was reasonably consistent across tests performed on four units. Measurements were more consistent than routine film tests, and were stable over two years. Conclusion: DAP provides a practical, efficient, robust, and sensitive method for routine QA of field aperture changes as small as ±0.2mm across the field size range 5 mm – 60 mm. The method may also have applications for QA of small fields formed by MLC.

WE-F-105-04: Evaluation of the Eclipse Electron Monte Carlo Algorithm Based On Patient Specific Dosimetry Measurement

Purpose: To evaluate the Eclipse electron Monte Carlo (eMC) dose calculation algorithm using patient specific dosimetry measurements, and to investigate possible patient specific QA guideline. Methods: Electron treatments were planned for 58 patients on Eclipse (Version 10.0) using the eMC algorithm for two Varian TrueBeam linear accelerators (Linac‐A and Linac‐B). The dose calculations were based on patient anatomy (patient plan), and a solid‐water phantom (QA plan). The calculation results were compared with physics measurement using a solid‐water phantom and a pinpoint ion chamber. Results: The monitor units as calculated on the QA plans were (2.1±2.0)% higher than the measurement on both accelerators, excluding outliers with small fields (3 cm or less) and extended SSD (above 105 cm). The difference of QA plan and measurement was consistent with the calculation uncertainty of the eMC algorithm (2%). The monitor units as calculated on the patient plans were (8.0±5.0)% (Linac‐A) and (5.5±5.0)% (Linac‐B) higher than the corresponding measurement. However, we observed that the eMC algorithm under‐estimated the dose for one patient treated on nose, by as much as 28.5% for the patient plan and 12.9% for the QA plan. For this patient, a calculation based on an output look‐up table was within 2% of the measurement. Conclusion: The measurements indicate that the eMC algorithm predicts dose reasonably well for QA plans using the same homogeneous phantom. However, for CT‐based patient plans, the eMC algorithm generates a larger number of monitor units versus the QA plan. It may reflect the impact of patient surface curvature and tissue inhomogeneity to the eMC algorithm and further investigation is needed. Caution should be taken for eMC patient plan with small field size and extended SSD. A lookup table of output factors based on measurement can be generated as a quick reference for electron dose calculation.

SU-E-T-558: Photon Fluence Model for Distributed Radiation Sources Using the Convolution Method

Purpose: Sophisticated dose calculation algorithms based on principles of convolution theory are commonly used in commercial treatment planning systems for calculating photon dose from point‐like radiation sources. However, not much has been studied regarding the modeling of distributed sources such as a Cobalt‐60 (Co‐60) source of 2 cm diameter. In this work, we present a convolution based photon fluence model that can be used for dose calculations for any finite size source. Methods: The photon fluence model proposed in this work is based on two key functions: the source distribution function and the aperture function. The source distribution function, which models primary and scatter radiation independently, was determined using a one‐time Monte Carlo (MC) simulation. Since the focus of this work is on Co‐60 delivery, the MC modeling was based on a Theratronics T780 Co‐60 unit (Best Theratronics). The validity of the MC model was verified with the ion chamber measurements made in a water phantom. The aperture function is a rectangular window function representing the field size. The fluence model was evaluated by comparing the calculated photon fluence to the MC simulated fluence. Results: The comparisons between the photon fluence calculated using the proposed model and the MC simulation fluences show good agreement, better than 2% in the in‐field region of all large and small fields. In the tail and sharp dose gradient regions, this agreement is better than 5% for the large field sizes and 1.5% for the small field sizes. The primary and scatter fluence output factors are within 2% of those obtained with the MC simulations. Conclusion: The results of our investigations indicate that the proposed photon fluence model can accurately determine fluence for large and small radiation beams from finite size sources. This model has the potential of playing an integral role in Co‐60 based IMRT treatment planning.

SU-E-T-59: 3-D Dosimetric Assessment for a Proton Pencil Beam by Optical CT Scanning of Plastic Radiochromic Dosimeters

Purpose: Proton pencil beam scanning can provide precise and efficient treatment delivery for both conventional and intensity modulated proton therapy (IMPT) techniques. It is challenging to measure the 3-D dose distribution of a pencil beam (1 – 1.5 cm diameter) because of its small field size as well as the complexity of dose changes in various directions. This study is the first investigation in characterizing the dosimetry of a single proton pencil beam using PRESAGE radiochromic dosimeters and an optical CT scanner. Methods: In this study, cylindrical PRESAGE dosimeters and an optical CT scanner were used to implement dose distribution measurements for proton pencil beams at Massachusetts General Hospital. Two different dosimeter formulations, with different physical density and atomic number, were used for measurements for proton pencil beams, with energies between 93 MeV and 110 MeV. Optical density dose response was studied by irradiating the dosimeters to various known doses (up to 9 Gy). PDD distributions were studied for proton pencil beams, as well as unmodulated and modulated scattered beams.Results: PRESAGE has a linear dose response from 0 up to 8 Gy for all proton energies studied. The physical density of PRESAGE was used to scale the depth of PRESAGE measurement to depth of water. For pencil beams, normalized at surface, the dose at the Bragg peak is underestimated by 15–20% due to LET effects. The spread of a proton pencil beam as a function of depth can be observed. Measured PDDs were compared with that in water for both modulated (SOBP) and unmodulated scattered beams. Some LET effects are observed. Conclusion: The findings from this study suggest that it is feasible to use PRESAGE dosimeter for proton pencil beam dosimetry study. Future studies will be focused on the responses of various PRESAGE formulations in proton pencil beams.