Source-to-surface Distance Research Articles

Neutron spectra and dose equivalents calculated in tissue for high‐energy radiation therapy

Neutrons are by-products of high-energy radiation therapy and a source of dose to normal tissues. Thus, the presence of neutrons increases a patient's risk of radiation-induced secondary cancer. Although neutrons have been thoroughly studied in air, little research has been focused on neutrons at depths in the patient where radiosensitive structures may exist, resulting in wide variations in neutron dose equivalents between studies. In this study, we characterized properties of neutrons produced during high-energy radiation therapy as a function of their depth in tissue and for different field sizes and different source-to-surface distances (SSD). We used a previously developed Monte Carlo model of an accelerator operated at 18 MV to calculate the neutron fluences, energy spectra, quality factors, and dose equivalents in air and in tissue at depths ranging from 0.1 to 25 cm. In conjunction with the sharply decreasing dose equivalent with increased depth in tissue, the authors found that the neutron energy spectrum changed drastically as a function of depth in tissue. The neutron fluence decreased gradually as the depth increased, while the average neutron energy decreased sharply with increasing depth until a depth of approximately 7.5 cm in tissue, after which it remained nearly constant. There was minimal variation in the quality factor as a function of depth. At a given depth in tissue, the neutron dose equivalent increased slightly with increasing field size and decreasing SSD; however, the percentage depth-dose equivalent curve remained constant outside the primary photon field. Because the neutron dose equivalent, fluence, and energy spectrum changed substantially with depth in tissue, we concluded that when the neutron dose equivalent is being determined at a depth within a patient, the spectrum and quality factor used should be appropriate for depth rather than for in-air conditions. Alternately, an appropriate percent depth-dose equivalent curve should be used to correct the dose equivalent at the patient surface.

Skin dose estimation for various beam modifiers and source-to-surface distances for 6MV photons

The purpose of this study was to learn the skin dose estimation for various beam modifiers at various source-to-surface distances (SSDs) for a 6 MV photon. Surface and buildup region doses were measured with an acrylic slab phantom and Markus 0.055 cc parallel plate (PP) ionization chamber. Measurements were carried out for open fields, motorized wedge fields, acrylic block tray fields ranging from 3 × 3 cm2 to 30 × 30 cm2. Twenty-five percent of the field was blocked with a cerrobend block and a Multileaf collimator (MLC). The effect of the blocks on the skin dose was measured for a 20 × 20 cm2 field size, at 80 cm, 100 cm and 120 cm SSD. During the use of isocentric treatments, whereby the tumor is positioned at 100 cm from the source, depending on the depth of the tumor and size of the patient, the SSD can vary from 80 cm to 100 cm. To achieve a larger field size, the SSD can also be extended up to 120 cm at times. The skin dose increased as field size increased. The skin dose for the open 10 ×10 cm2 field was 15.5%, 14.8% and 15.5% at 80 cm, 100 cm and 120 cm SSDs, respectively. The skin dose due to a motorized 60° wedge for the 10 × 10 cm2 field was 9.9%, 9.5%, and 9.5% at 80 cm, 100 cm and 120 cm SSDs. The skin dose due to acrylic block tray, of thickness 1.0 cm for a 10 × 10 cm2 field was 27.0%, 17.2% and 16.1% at 80, 100 and 120 cm SSD respectively. Due to the use of an acrylic block tray, the surface dose was increased for all field sizes at the above three SSDs and the percentage skin dose was more dominant at the lower SSD and larger field size. The skin dose for a 30 × 30 cm2 field size at 80 cm SSD was 38.3% and it was 70.4% for the open and acrylic block tray fields, respectively. The skin doses for motorized wedge fields were lower than for open fields. The effect of SSDs on the surface dose for motorized 60° wedge fields was not significant for a small field size (difference was less than 1% up to a 15 × 15 cm2 field size), but for a larger field (field size more than 15 × 15 cm2), the difference in a percentage skin dose was significant. The skin dose for the open field was more than that for the MLC blocked field and lower than that for the acrylic blocked tray field. The block was 25% of the 20 × 20 cm2 open field. Skin doses were increased as the SSD decreased and were dominant for larger field sizes. The surface dose was weakly dependent on the MLC block.

Electron beam characteristics at extended source to surface distance for a Clinac-DHX linear accelerator

Electron beam characteristics at extended source to surface distance for a Clinac-DHX linear accelerator A uniform dose to the target site is required with a knowledge of delivered dose, central axis depth dose and beam flatness for successful electron treatment at an extended source to surface distance (SSD). In an extended SSD treatment under dosage of the lateral tissue may occur due to reduced beam flatness. To study the changes in beam characteristics, the depth dose curves, beam flatness and isodose distributions were measured at different SSDs from 100 to 120 cm for clinically used field sizes from (4×4) to (25×25) cm2 and beam energies ranging from 6 MeV to 20 MeV. Our results suggest that the change in depth dose is minimal except in the buildup region for most energy. In general surface dose is decreased as the SSD increased moderately. It was observed that the loss in beam flatness is significant for smaller fields, higher isodose lines, and lower energies. The penumbra enlarged and the uniformity index reduced with increasing SSD.

Influence of source parameters on large-field electron beam profiles calculated using Monte Carlo methods

The purpose of this paper was to study the source model for a Monte Carlo simulation of electron beams from a medical linear accelerator. In a prior study, a non-divergent Gaussian source with a full-width at half-maximum (FWHM) of 0.15 cm was successful in predicting relative dose distributions for electron beams with applicators. However, for large fields with the applicator removed, discrepancies were found between measured and calculated profiles, particularly in the shoulder region. In this work, the source was changed to a divergent Gaussian spatial distribution and the FWHM parameter was varied to produce better agreement with measured data. The influence of the FWHM source parameter on profiles was observed at multiple locations in the simulation geometry including in-air fluence profiles at a 95 cm source-to-surface distance (SSD), percent depth dose profiles and off-axis profiles (OARs) in a water phantom for two SSDs, 80 and 100 cm. For a 6 MeV 40 × 40 cm2 OAR profile, discrepancies in the shoulder region were reduced from 15% to 4% using a FWHM value of 0.45 cm. The optimal FWHM values for the other energies were 0.45 cm for 9 MeV, 0.22 for 12 MeV, 0.25 for 16 MeV and 0.2 cm for 20 MeV. Although this range of values was larger than measured focal spot sizes reported by other researchers, using the increased FWHM values improved the fit at most locations in the simulation geometry, giving confidence that the model could be used with a variety of SSDs and field sizes.

A modified formula for defining tissue phantom ratio of photon beams

Tissue phantom ratio (TPR), for square fields of various dimensions has been determined at varying depths in water. The dose in water has been measured at a fixed source-to-surface distance (SSD) of 100 cm and reference depth of 5 cm for 6 MV photon beam of Siemens Linear Accelerator Primus 11 in German Cancer Research Center (DKFZ), Heidelberg, Germany. A modified formula has been developed to calculate the TPR value for isocentric treatment. The present article describes the conversion of the measured data values into a comprehensive and consistent data set by the modified formula, that gives the TPR from Percentage Depth Dose (PDD) with depth as a function of field sizes from 10 mm x 10 mm upto 300 mm x 300 mm) and depth (from 0 mm to 300 mm).

Electron beam therapy at extended source‐to‐surface distance: a Monte Carlo investigation

Electron‐beam therapy is used to treat superficial tumors at a standard 100 cm source‐to‐surface distance (SSD). However, certain clinical situations require the use of an extended SSD. In the present study, Monte Carlo methods were used to investigate clinical electron beams, at standard and non‐standard SSDs, from a Siemens Oncor Avant Garde (Siemens Healthcare, Erlangen, Germany) linear accelerator (LINAC). The LINAC treatment head was modeled in BEAMnrc for electron fields 5 cm in diameter and 10×10 cm, 15×15 cm, and 20×20 cm; for 6 MeV, 9 MeV, and 12 MeV; and for 100 cm, 110 cm, and 120 cm SSD. The DOSXYZnrc code was used to calculate extended SSD factors and dose contributions from various parts of the treatment head.The main effects of extended SSD on water phantom dose distributions were verified by Monte Carlo methods. Monte Carlo–calculated and measured extended SSD factors showed an average difference of ±1.8%. For the field 5 cm in diameter, the relative output at extended SSD declined more rapidly than it did for the larger fields. An investigation of output contributions showed this decline was mainly a result of a rapid loss of scatter dose reaching the dmax point from the lower scrapers of the electron applicator. The field 5 cm in diameter showed a reduction in dose contributions; the larger fields generally showed an increased contribution from the scrapers with increase in SSD. Angular distributions of applicator‐scattered electrons have shown a large number of acute‐angle electron tracks contributing to the output for larger field sizes, explaining the shallow output reduction.PACS numbers: 87.53.Wz, 87.53.Vb, 87.53.Hv

SU‐GG‐T‐534: Investigation of Electron Beam Collimated by Motorized Electron Multi Leaf Collimator (eMLC) Designed for Fixed and Modulated Electron Beam Therapy

Purpose: Investigate the electron beam characteristics after been collimated by a motorized eMLC and study its feasibility for modulated electron radiation therapy. In addition we aimed at accurately simulating the eMLC and verify dose calculations for the inverse treatment planning system for MERT. Method and Materials: A motorized eMLC was developed for beam delivery for fixed beam delivery and MERT. The eMLC leafs are positioned at 82 cm source collimator distance. Film and ion chamber measurement were conducted in this study to investigate the characteristics of electron beam collimated by the eMLC. Measurement was done at 89 cm source surface distance allowing for 5cm gap between the eMLC and the surface of the phantom. Leakage from the eMLC was measured. Abutments of fields formed by eMLC were investigated. Comparison is also done between this new generation of motorized eMLC and our first generation of manually driven eMLC. Monte Carlo simulation was performed to accurately model the eMLC. Results: Profiles from the eMLC shows very close penumbra to that from the applicator for all energies. Film measurement showed that one leaf is still distinguished in a cross profile. In addition the leaf width is shown to be appropriate for defining a beamlet. Leakage from 16MeV beam through the eMLC was found to be 1.8%. The abutment fields showed enhancement at junction point which is blurred with depth. Good agreement was achieved between Monte Carlo simulations and measurements for profiles collimated by both electron applicators/cutouts and by the eMLC for all electron energies. Conclusion: The electron beam characteristics from the motorized eMLC were examined. Measurement done for this motorized eMLC showed promising results and a potential for modulated electron therapy. Monte Carlo code with the source represented by phase space files is capable of accurately modeling electron beam delivery with the eMLC.

SU-GG-T-204: Estimation of Organ Doses in Total Body Irradiation

Purpose: To estimate the lung and other critical organ doses of the patients in total body irradiation before bone marrow transplant which is a general protocol for curing leukemia. Method and Materials: Total body irradiation was performed using the photo beam produced by the linear accelerator in the conditions of 415 cm source‐to‐surface distance (SSD), 40×40 cm2field size and 15 MV beam energy in parallel opposed irradiation in this work. A Rando phantom, a diode detector and thermoluminescent dosimeter(TLD) chips were selected to perform the dose investigation. The diode detector has small volume, and can provide real‐time reading and high accuracy. TLD chips were placed inside the Rando phantom to get the dose distributions along the longitude of the phantom and the critical organs suggested by the ICRP‐60 report. In the meantime, a diode detector and TLD chips were also placed at the entrance surface point to evaluate the dose conversion factors for the critical organs to the entrance surface dose (ESD).Results: The doses of upper mediastinum lung in the Rando phantom are about 20 % higher than the prescribed doses at the center in field. The doses of thyroids, gonad glands and eyes are lower than prescribed doses. The organ‐dose conversion factors for the critical organs to the entrance surface dose were evaluated. Conclusion: The increased doses of lung should be considered in clinical practice. The organ‐dose conversion factors established in this work can be used to estimate the critical organ doses, by means of measuring only the entrance surface dose of the patient in clinical irradiation. The diode detector is accurate and convenient to be used in the evaluation of real‐time ESD of the patient in the clinical total body irradiation.

SU‐GG‐T‐352: Dosimetric Validation of a Commercial Monte‐Carlo Based IMRT Planning

Purpose: Recently, the inverse planning planning system Monaco (CMS Inc) version 1.0.0 was installed in our department. The Monaco system incorporates a two‐stage IMRT optimization procedure and a version of the XVMC Monte Carlo dose calculation algorithm. In the first stage the fluence distribution of the IMRT beams are optimized using hard (‘biological’) constraints for the organs at risk. In the second stage, the beams are segmented and their weights optimized while still satisfy the hard constraints. In this study we investigated the dosimetric accuracy of the XVMC dose engine implemented in MONACO. Method and Materials: For an ELEKTA(r) linac (6, 10 and 18 MV) depth dose curves, dose profiles and output factors were measured in a waterphantom using source‐to‐surface distances (SSD) between 80 and 100 cm. Symmetric fields ranging from 2×2 to 30×30 cm and off‐axis fields of 2×2 3×3 cm and 2×10 cm were used. In addition, GAFCHROMIC(r) film measurements were performed in anthropomorphic phantoms to validate the dose accuracy in inhomogeneous media. Furthermore, the dosimetric accuracy of IMRT fields for prostate and head‐and‐neck cancer treatments was evaluated. All fields were simulated in Monaco using a grid size of 2 mm and variances of 0.5 and 1%. Results: The agreement between measured and calculated dose distributions was generally within 2%. Only in the build‐up region larger dose differences were observed, especially for the highest photon energy. No impact of the SSD on the dose accuracy was observed. Calculation times for a 10×10 cm field and a 0.5 or 1% variance were 75 and 23 minutes, respectively. Conclusion: A very good agreement was observed between Monaco Monte Carlo dose calculations and measurements, allowing clinical introduction.

SU‐GG‐T‐306: Evaluation of Two‐Dimensional Diode Array Dosimeter for An Application of Exit Dosimetry

Purpose: To characterize and evaluate the performance of MapCHECK™ two‐dimensional diode array dosimeter for accurate measurement of exit fluence at an extended source‐to‐surface distance (SSD) for the application of in vivo dosimetry based on dose reconstruction. Method and Materials: Dose profile measurements of open field for varying field sizes (3×3cm2, 5×5cm2, 7×7cm2 and 10×10cm2), air gaps (30cm, 40cm, 50cm and 60cm) and thicknesses of primary beam attenuator (10cm and 20cm of solid water) using MapCHECK™, CC04 ionization chamber and GafChromic® EBT radiochromic film were carried out. For clinical measurements, the 2D dosimetry measurements for five IMRT boost fields were carried out at an air gap of 50 cm with MapCHECK™, and compared with GafChromic® EBT radiochromic film, which served as the gold standard. Results: With increasing air gap, the difference between MapCHECK™ system and CC04 increased slightly for both primary beam attenuator thicknesses. For air gaps of 0cm, 30cm, 40cm, 50cm and 60cm, the overall agreement between MapCHECK™ and CC04 was ±1.25%, ±1%, ±2%, ±2.5% and ±4, respectively. The overall agreement averaged over all air gaps between MapCHECK™ system and CC04 was within ±2% while agreement between GafChromic® EBT radiochromic film and CC04 was ±3.5%. For the clinical IMRT fields and using the gamma test, the comparison between MapCHECK™ and GafChromic EBT radiochromic film yielded a mean passing rate of 91.9%, with a minimum and maximum passing rate of 87.6% and 97.2% respectively. Conclusion: MapCHECK™ yielded to have an accurate dose distribution (profile and 2D dose distribution) at an extended SSD (> 130cm). It can be considered a suitable candidate for exit dosimetry.

SU‐GG‐J‐67: Detection of Gold Fluorescence X‐Rays for the In‐Vivo Quantification of Gold Concentration During Gold Nanoparticle‐Aided Radiation Therapy (GNRT)

Purpose: To determine the concentration of gold (Au) nanoparticles presented within the tumor during in vivo demonstration of tumor dose enhancement by detecting gold fluorescence x‐rays. Method and Materials: Gold nanoparticle solution of 1% by weight in saline was prepared using commercially available gold nanoparticles with 1.9 nm core diameter. A cylindrical sample container (2.5 cm in diameter and 5.0 cm in height) was irradiated with 110 kVp and 200 mA x‐rays from conventional radiotherapy simulator for 40 seconds with 1 cm × 1 cm x‐ray beam at 85 cm source‐to‐surface distance (SSD) and spectrum was obtained using either Si‐PIN or CdTe photodiode detector using a container‐to‐detector geometry to minimize unwanted photons entering the detector. Results: The spectrum collected using the Si detector successfully captured the Au Lα and Lβ fluorescence lines at 9.7 and 11.4 keV, respectively, well above the background. Au K lines could not be measured with Si‐PIN photodiode due to Si‐PIN detection inefficiency above 60 keV. To capture the Au K lines, CdTe photodiode detector was employed which has high detection efficiency for energy range of Au K lines. The spectrum collected using CdTe detector clearly showed the Au Kα2, Kα1, and Kβ lines corresponding to energies of 67.0, 68.8, and 77.9 keV, respectively. Conclusion: The magnitude of obtained Kα fluorescence signal (i.e. greater than 50% above all background scattering) is very encouraging considering the tested gold concentration. With more sophisticated geometry to minimize scattered photons reaching the detector, Au fluorescence output from K lines can be used to measure even lower concentrations of gold nanoparticles. Fluorescence output as a function of Au concentration can be developed to accurately measure gold nanoparticle concentration.

Monte-Carlo-based perturbation and beam quality correction factors for thimble ionization chambers in high-energy photon beams

This paper presents a detailed investigation into the calculation of perturbation and beam quality correction factors for ionization chambers in high-energy photon beams with the use of Monte Carlo simulations. For a model of the NE2571 Farmer-type chamber, all separate perturbation factors as found in the current dosimetry protocols were calculated in a fixed order and compared to the currently available data. Furthermore, the NE2571 Farmer-type and a model of the PTW31010 thimble chamber were used to calculate the beam quality correction factor kQ. The calculations of kQ showed good agreement with the published values in the current dosimetry protocols AAPM TG-51 and IAEA TRS-398 and a large set of published measurements. Still, some of the single calculated perturbation factors deviate from the commonly used ones; especially prepl deviates more than 0.5%. The influence of various sources of uncertainties in the simulations is investigated for the NE2571 model. The influence of constructive details of the chamber stem shows a negligible dependence on calculated values. A comparison between a full linear accelerator source and a simple collimated point source with linear accelerator photon spectra yields comparable results. As expected, the calculation of the overall beam quality correction factor is sensitive to the mean ionization energy of graphite used. The measurement setup (source–surface distance versus source–axis distance) had no influence on the calculated values.

Comparison between the lateral penumbra of a collimated double-scattered beam and uncollimated scanning beam in proton radiotherapy

Intensity modulated proton radiotherapy (IMPT) can reduce the dose to critical structures by optimizing the distribution and intensity of individual pencil beams. The IMPT can be delivered by dynamically scanning a pencil beam with variable intensity and energy across the tumor target volume. The lateral penumbra of an uncollimated pencil beam is compromised, however, by the scattering in air between the vacuum window and the patient, and by the initial beam size. In this study, we compare the transversal penumbra of a pencil beam to the one of a collimated Gaussian broad divergent beam, such as the one produced by the double scattering system, for different range compensator thicknesses, collimator-to-surface distances (CSD), proton range and pencil beam sizes (σ0). The effect of vacuum and helium in the nozzle on the pencil beam lateral profile further downstream is also investigated. The lateral spatial intensity distribution for the collimated Gaussian broad divergent proton beam is modeled using the generalized Fermi–Eyges theory. The model is validated with measurements of the lateral profile in water at different depths for two different ranges (7.7 cm and 22.1 cm, respectively). Nearly 2500 treatment fields are analyzed to establish typical clinical beam configurations, such as the range compensator thicknesses, CSD and range, which we use to predict the 80%–20% lateral penumbra. The penumbra of the collimated broad divergent beam is calculated for fixed source-to-surface distance (SSD) of 220 cm and source size of 2.5 cm (σ). The results show that the model predicts the penumbra at different water depths with accuracy better than 0.2 mm. At depths larger than 7.6 cm (minimum range of the clinical fields analyzed), the accuracy is better than 3%. The treatment fields feature the following average configuration: the range compensator thickness of 6.5 ± 2.8 cm (max 19.4 cm), CSD 11.9 ± 3.8 cm (max 29.4 cm) and range of 16.0 ± 6.1 cm. The penumbra of a pencil beam at shallow depth is in general larger (i.e., worse) than the penumbra of a collimated beam, but better at larger depths. The depth at which the two penumbras are identical exhibits only a small dependence on the proton range, but is strongly affected by the collimator-to-surface distance. For CSD 10 cm, range compensator thickness 6 cm, SSD 220 cm and source size 2.5 cm, this depth is 11.5 cm for a 5 mm pencil beam, and 9.1 cm for a 3 mm pencil beam. For most of the clinical sites considered, assuming the beam configurations of this study, the pencil beam penumbra is larger (i.e., worse). By moving the vacuum window downstream or by replacing air with helium in the gantry nozzle, the dosimetrical benefit of scanning would be drastically improved, especially for small σ0 (5 mm or less).

Experimental verification and clinical implementation of a commercial Monte Carlo electron beam dose calculation algorithm

This study describes the modeling and the experimental verification and clinical implementation of the alpha release of Pinnacle3 Monte Carlo (MC) electron beam dose calculation algorithm for patient-specific treatment planning. The MC electron beam modeling was performed for beam energies ranging from 6 to 18 MeV from a Siemens (Primus) linear accelerator using standard-shaped electron applicators and 100 cm source-to-surface distance (SSD). The agreement between MC calculations and measurements was, on average, within 2% and 2 mm for all applicator sizes. However, differences of the order of 3%-4% were noted in the off-axis dose profiles for the largest applicator modeled and for all energies. Output factors were calculated for standard electron cones and square cutouts inserted in the 10 x 10 cm2 applicator for different SSDs and were found to be within 4% of measured data. Experimental verification of the MC electron beam model was carried out using an ionization chamber and film in solid-water slab and anthropomorphic phantoms containing bone and lung materials. Agreement between calculated and measured dose distributions was within +/-3%. Clinical comparison was performed in four patient treatment plans with lesions in highly irregular anatomies, such as the ear, face, and breast, where custom-designed bolus and field shaping blocks were used in the patient treatments. For comparison purposes, treatment planning was also performed using the conventional pencil beam (PB) algorithm with the Pinnacle3 treatment planning system. Differences between MC and PB dose calculations for the patient treatment plans were significant, particularly in anatomies where the target was in close proximity to low density tissues, such as lung and air cavities. Concerning monitor unit calculations, the largest differences obtained between MC and PB algorithms were between 4.0% and 5.0% for two patients treated with oblique beams and involving highly irregular surfaces, i.e., breast and cheek. Clinical results are reported for overall uncertainty values (averaged over voxels with doses >50% dosemax) ranging from 2% to 0.3% and calculations were performed using cubic voxels with side 0.3 cm. Timing values ranged from 2 min to 24.5 h, depending on the field size, beam energy, number, and thickness of computed tomography slices used to define the patient's anatomy for the overall uncertainty values mentioned above.

Supine Craniospinal Irradiation Using Intrafractional Junction Shifts and Field-in-Field Dose Shaping: Early Experience at Methodist Hospital

To describe our preliminary experience with supine craniospinal irradiation. The advantages of the supine position for craniospinal irradiation include patient comfort, easier access to maintain an airway for anesthesia, and reduced variability of the head tilt in the face mask. The cranial fields were treated with near lateral fields and a table angle to match their divergence to the superior edge of the spinal field. The collimator was rotated to match the divergence from the superior spinal field. The spinal fields were treated using a source to surface distance (SSD) technique with the couch top at 100 cm. When a second spinal field was required, the table and collimator were rotated 90 degrees to allow for the use of the multileaf collimator and so the gantry could be rotated to match the divergence of the superior spinal field. The multileaf collimator was used for daily dynamic featherings and field-in-field dose control. With a median follow-up of 20.2 months, five documented failures and no cases of radiation myelitis occurred in 23 consecutive patients. No failures occurred in the junctions of the spine-spine or brain-spine fields. Two failures occurred in the primary site alone, two in the spinal axis alone, and one primary site failure plus distant metastasis. The median time to recurrence was 17 months. The results of our study have shown that supine approach for delivering craniospinal irradiation is not associated with increased relapses at the field junctions. To date, no cases of radiation myelitis have developed.

Correction factors for low perturbation in vivo diodes used in the determination of entrance doses in high energy photon beams

Low perturbation diodes, with thin buildup caps, can be used to reduce perturbations to the delivered dose. The literature states that additional correction factors are required for low perturbation diodes, however, there are few reported studies into their use. This report measured the dose perturbations and correction factors for diodes with varying buildup cap thicknesses. Scanditronix EDP15, EDD5, and EDD2 diodes were investigated. Dose perturbations and correction factors for field size, source-surface distance (SSD), obliquity, and wedge were measured in megavoltage photon beams. EDP15 produces a 6% dose perturbation. EDD5 produces a perturbation between 1% and 2%. EDD2 perturbation is negligible. The variation of correction factors for the full buildup EDP15 diode is small and consistent with the literature. The low perturbation diode EDD2 has large correction factors. The field size correction factor varies from 1.38 to 0.87 for 10 MV. The SSD correction factor varies from 0.92 to 1.09 for 10 MV. At the maximum angle measured, the obliquity correction factor is 0.73 for 10 MV. Intermediate results were observed for the EDD5 diode. It is expected that it will be very difficult to achieve accurate in vivo dosimetry using the EDD2 diode. The EDD5 diode may represent a reasonable compromise between EDD2 and the full buildup EDP15. The EDD5 dose perturbation is small and the correction factors are not as large as for EDD2, so accurate in vivo dosimetry may be possible as long as the obliquity is below 45 degrees.

Linear attenuation coefficients for compensator based imrt

With rapid technological improvements in computer driven 3-D radiotherapy treatment planning systems (RTPS) the use of compensating filters for intensity modulated radiation therapy (IMRT) will dramatically increase the ease of treatment. The procedure for commissioning .decimal (Sanford, Florida) compensators involved the measurement of the effective linear attenuation coefficients for aluminium and brass. Field sizes to be measured vary from small square field size of 5 cm to the larger square field size of 25 cm with additional measurements at each 5 cm2 increments. The energies commissioned where 6 MV and 18 MV photons. The depth of measurements varied from 5 cm to 10 cm within phantom material and the source surface distance varied from 100 cm to 90 cm. The beam quality was measured by obtaining percentage depth dose (PDD) curves for the various field sizes with and without the compensating material. Results of the series of measurements showed no significant differences in the effective linear attenuation coefficients with respect to chamber depth and source surface distance with constant energy and field size. The main factor that was shown to influence the effective linear attenuation coefficient was field size variation. A correlation was shown between the effective linear attenuation coefficient and field size, up to a field size of 15 cm x 15 cm. Our results showed that for optimal patient treatments using IMRT compensating filters, there is a need for establishing two field size dependent linear attenuation coefficients.

Clinical application of GAFCHROMIC ® EBT film for in vivo dose measurements of total body irradiation radiotherapy

The GAFCHROMIC ® EBT film model is a fairly new film product designed for absorbed dose measurements of high-energy photon beams. In vivo dosimetry for total body irradiation (TBI) remains a challenging task due to the extended source-to-surface distance (SSD), low dose rates, and the use of beam spoilers. EBT film samples were used for dose measurements on an anthropomorphic phantom using a TBI setup. Additionally, in vivo measurements were obtained for two TBI patients. Phantom results verified the suitability of the EBT film for TBI treatment in terms of accuracy, reproducibility, and dose linearity. Doses measured were compared to conventional dosimeter measurements using thermoluminescent dosimeters (TLDs), resulting in an agreement of 4.1% and 6.7% for the phantom and patient measurements, respectively. Results obtained from the phantom and patients confirm that GAFCHROMIC ® EBT films are a suitable alternative to TLDs as an in vivo dosimeter in TBI radiotherapy.

SU-FF-T-393: Surface Dose Determinantion for Modulated Electron Radiotherapy

Purpose: To determine the skindose from photonmultileaf collimator (pMLC) deliveredelectron beams for modulated electron therapy (MERT) and examine its dependence on source to surface distance (SSD), energy, obliquity, and field size.Method and Materials:Measurements of surface dose for various fields at six source SSDs were taken using a parallel plate ion chamber for electron energies of 6, 9, 12, 16, and 20 MeV. These measurements were supplemented by measurements of surface dose using ultra‐thin TLDs at the same SSDs used for the parallel plate measurements. The behavior of the parallel plate dose data was analyzed with respect to its dependence on energy for each field size as well as dependence of dose on field size for each energy for all different SSDs studied. The TLD data were examined for dependence of dose on SSD, and the dose to the surface was examined as a fraction of the maximum dosedelivered.Results: The dosesmeasured using the parallel plate chamber and the TLDs both indicated that the dose to the skin decreases as the SSD increases. TLD and parallel plate chamber measurements indicate the surface dose increases with increasing electron beam energy, field size and beam obliquity Conclusions: Patient skindose from electron beamsdelivered through a pMLC shows a strong dependence on field size, energy, and SSD. Lower doses were measured as field size and energy decreased and as SSD increased. Knowledge of the variation in surface dose with these factors is necessary during treatment planning for the appropriate choice of electron beam energy in modulated electron radiotherapy.

SU-FF-T-143: Determination of Electron Output Factors Using An Empirical, Mathematical Model

Purpose: To empirically develop a mathematical method to determine electron output factors with varying energies, source to surface distances (SSD), and irregular cutout shapes used in electron beam therapy. Method and Materials: Circular Cerrobend cutouts of diameters ranging from 2 cm to cone size, for various cone sizes, were used to measure electron output factors for a Varian 21EX linear accelerator. Measurements were made in a 30 × 30 × 20 cm3 solid water phantom with a 0.3 cc PTW ion-chamber and a Standard Imaging electrometer. Electrons with energies of 6, 9, 12, 16 and 20 MeV, with SSDs of 100, 105, 110 and 115 cm were measured at the position of maximum dose (Dmax) to determine output factor. The virtual source method was used to model extended SSD conditions. All measured output data from these circular cutouts were fitted with the same linear regression function using Wolfram's Mathematica software to predict output factors. The effective radius of a clinical cutout is then matched with the circular cutout data to determine an output factor. The average of twelve measured equiangular radii determines the effective radius for each irregularly shaped clinical cutout. An additional empirical mathematical formula was used to incorporate extreme minimum width and aspect ratio combinations. To validate this empirical mathematical approach, we measured output factors for 100 clinical cutouts of various shapes and sizes, and compared these with predictions. Results: It is found that the measured output factor differs from prediction by more than 3% in two out of 100 cutouts. In most cases this difference, however, is under 2%. Conclusion: The empirical method developed in the present study successfully determines electron output factors for various energy, SSD, cone, and irregular shape combinations. This method is a viable alternative to traditional methods of electron output factor determination.