Collimator Scatter Research Articles

SU-E-T-149: Scatter Factors Comparison of 6MV Flattened and 7MV Unflattened Beams.

To compare the Collimator scatter factor (Sc), Phantom scatter factor (Sp) and Total scatter factors (Sc,p) of 6MV flattened Beam (6MV FB) and 7MV Unflattened beams (7MV UFB). The flattening filter and primary collimator are the major sources of producing the scattered radiation. In this study, the field sizes from 5×5 cm2 to 40 × 40 cm2 compared for 6MV FB and 7MV UFB. We measured Sc,p with CC 13 chamber at the depth of 10 g/cm2 using IBA blue phantom and Sc measured with CC 13 chamber at the depth of 10 g /cm2 using columnar phantom (TG 74) for 6MV FB and 7MV UFB x-ray beams from a Siemens - ARTISTE linear accelerator. The Sp values derived from the Sc,p and Sc Values. All the values of Sc,p, Sc and Sp are normalized to 10 × 10 cm2 field size the measured values of Sc,p for 6MV FB and 7MV UFB varies from 0.9437 to 1.0651 and 0.9690 to 1.0283 respectively. The Sc values for 6MV FB and 7MV UFB varies from 0.9676 to 1.0212 and 0.9882 to 1.0075 respectively. The Sp values for 6MV FB and 7MV UFB varies from 0.9752 to 1.0429 and 0.9806 to 1.0206 respectively. Our study results shows that Sc, Sp & Sc,p for 7MV UFB for smaller fields up to 10 × 10 cm2 were higher than 6MV FB and for larger fields greater than 10 ×10 cm2 it is vice versa. A scatter factors are measured and compared for 6MV FB and 7MV UFB. A significant variation observed for Sc, Sp and Sc,p values of 6MV FB and 7MV UFB .This is may be due to flattening filter and beam quality.

A method for energy window optimization for quantitative tasks that includes the effects of model-mismatch on bias: application to Y-90 bremsstrahlung SPECT imaging

Quantitative Yttrium-90 (90Y) bremsstrahlung single photon emission computed tomography (SPECT) imaging has shown great potential to provide reliable estimates of 90Y activity distribution for targeted radionuclide therapy dosimetry applications. One factor that potentially affects the reliability of the activity estimates is the choice of the acquisition energy window. In contrast to imaging conventional gamma photon emitters where the acquisition energy windows are usually placed around photopeaks, there has been great variation in the choice of the acquisition energy window for 90Y imaging due to the continuous and broad energy distribution of the bremsstrahlung photons. In quantitative imaging of conventional gamma photon emitters, previous methods for optimizing the acquisition energy window assumed unbiased estimators and used the variance in the estimates as a figure of merit (FOM). However, for situations, such as 90Y imaging, where there are errors in the modeling of the image formation process used in the reconstruction there will be bias in the activity estimates. In 90Y bremsstrahlung imaging this will be especially important due to the high levels of scatter, multiple scatter, and collimator septal penetration and scatter. Thus variance will not be a complete measure of reliability of the estimates and thus is not a complete FOM. To address this, we first aimed to develop a new method to optimize the energy window that accounts for both the bias due to model-mismatch and the variance of the activity estimates. We applied this method to optimize the acquisition energy window for quantitative 90Y bremsstrahlung SPECT imaging in microsphere brachytherapy. Since absorbed dose is defined as the absorbed energy from the radiation per unit mass of tissues in this new method we proposed a mass-weighted root mean squared error of the volume of interest (VOI) activity estimates as the FOM. To calculate this FOM, two analytical expressions were derived for calculating the bias due to model-mismatch and the variance of the VOI activity estimates, respectively. To obtain the optimal acquisition energy window for general situations of interest in clinical 90Y microsphere imaging, we generated phantoms with multiple tumors of various sizes and various tumor-to-normal activity concentration ratios using a digital phantom that realistically simulates human anatomy, simulated 90Y microsphere imaging with a clinical SPECT system and typical imaging parameters using a previously validated Monte Carlo simulation code, and used a previously proposed method for modeling the image degrading effects in quantitative SPECT reconstruction. The obtained optimal acquisition energy window was 100–160 keV. The values of the proposed FOM were much larger than the FOM taking into account only the variance of the activity estimates, thus demonstrating in our experiment that the bias of the activity estimates due to model-mismatch was a more important factor than the variance in terms of limiting the reliability of activity estimates.

Radiotherapy and Pregnancy: Together or Alone?

DOI: 10.7247/jiumf.19.2.13 Organ malformations and mental retardation (the sensitivity is high from 2 to 8 weeks, and 8 to 25 weeks after conception, respectively) are the most serious results of fetal exposure to radiation that are observed after birth and probably arise above a threshold dose of 0.1–0.2 Gy. This threshold dose is not generally reached with curative radiotherapy during pregnancy, because most of the tumors are located sufficiently far from the fetus and that precautions have been taken to protect the unborn child against leakage radiation and collimator scatter. Generally, pregnant women with malignant diseases are advised to delay the radiotherapy until after delivery. If a pregnant patient necessitates radiotherapy the physician should inform the risk of the fetus and the benefits of the mother. Subjects like week of pregnancy, stage of the disease and radiation safety must be discussed in details and the final decision should be taken by the patient. In this review, patients who were exposed to radiation during pregnancy because of radiotherapy and their fetal exposure were discussed.

Radiotherapy pre-treatment dose validation: A second verification of monitor units (MU) with a commercial software.

Inversely planned intensity-modulated radiotherapy (IMRT) and stereotactic small field radiotherapy should be verified before treatment execution. A second verification is carried out for planned treatments in IMRT and 3D conformal radiotherapy (3D-CRT) using a monitor verification commercial dose calculation management software (DCMS). For the same reference point the ion-chamber measured doses are compared for IMRT plans. DCMS (Diamond) computes dose based on modified Clarkson integration, accounting for multi-leaf collimators (MLC) transmission and measured collimator scatter factors. DCMS was validated with treatment planning system (TPS) (Eclipse 6.5 Version, Varian, USA) separately. Treatment plans computed from TPS are exported to DCMS using DICOM interface. Doses are re-calculated at selected points for fields delivered to IMRT phantom (IBA Scanditronix Wellhofer) in high-energy linac (Clinac 2300 CD, Varian). Doses measured at central axis, for the same points using CC13 (0.13 cc) ion chamber with Dose 1 Electrometer (Scanditronix Wellhofer) are compared with calculated data on DCMS and TPS. The data of 53 IMRT patients with fields ranging from 5 to 9 are reported. The computed dose for selected monitor units (MU) by Diamond showed good agreement with planned doses by TPS. DCMS dose prediction matched well in 3D-CRT forward plans (0.8 ± 1.3%, n = 37) and in IMRT inverse plans (–0.1 ± 2.2%, n = 37). Ion chamber measurements agreed well with Eclipse planned doses (–2.1 ± 2.0%, n = 53) and re-calculated DCMS doses (–1.5 ± 2.6%, n = 37) in phantom. DCMS dose validation is in reasonable agreement with TPS. DCMS calculations corroborate well with ionometric measured doses in most of the treatment plans.

Peripheral doses in patients undergoing Cyberknife treatment for intracranial lesions. A single centre experience

BackgroundStereotactic radiosurgery/radiotherapy procedures are known to deliver a very high dose per fraction, and thus, the corresponding peripheral dose could be a limiting factor for the long term surviving patients. The aim of this clinical study was to measure the peripheral dose delivered to patients undergoing intracranial Cyberknife treatment, using the MOSFET dosimeters. The influence of the supplemental shielding, the number of monitor units and the collimator size to the peripheral dose were investigated.MethodsMOSFET dosimeters were placed in preselected anatomical regions of the patient undergoing Cyberknife treatment, namely the thyroid gland, the nipple, the umbilicus and the pubic symphysis.ResultsThe mean peripheral doses before the supplemental shielding was added to the Cyberknife unit were 51.79 cGy, 13.31 cGy and 10.07 cGy while after the shielding upgrade they were 38.40 cGy, 10.94 cGy, and 8.69 cGy, in the thyroid gland, the umbilicus and the pubic symphysis, respectively. The increase of the collimator size corresponds to an increase of the PD and becomes less significant at larger distances, indicating that at these distances the PD is predominate due to the head leakage and collimator scatter.ConclusionWeighting the effect of the number of monitor units and the collimator size can be effectively used during the optimization procedure in order to choose the most suitable treatment plan that will deliver the maximum dose to the tumor, while being compatible with the dose constraints for the surrounding organs at risk. Attention is required in defining the thyroid gland as a structure of avoidance in the treatment plan especially in patients with benign diseases.

A Simulation Study of the Effect of Phase-Shift on Dual Gated Myocardial Perfusion ECT.

We evaluated the effect of relative phase-shift of cardiac and respiratory (C&R) motions in myocardial perfusion (MP) ECT. Previously, we generated a set of realistic 3D XCAT (eXtended CArdiac Torso) phantoms that model simultaneous C&R motions for use in the study of new data acquisition methods and corrective image reconstruction techniques for improved gated MP ECT, including PET and SPECT. The respiratory motion (RM) over a respiration cycle was modeled using 24 equally-spaced time frames while the cardiac beating motion (CBM) over a cardiac cycle was divided into 48 equally-spaced time frames for each of the 24 RM phases. Almost noise-free projection datasets were generated separately from the heart, blood pool, lungs, liver, kidneys, stomach, gall bladder and remaining body at each of the 24 × 48 time points using Monte Carlo simulation techniques that include the effect of collimator detector response, photon attenuation and scatter. To demonstrate the effect of relative phase-shift, a typical 99mTc Sestamibi MP SPECT projection dataset were generated. They were then scaled and combined to model different degrees of relative C&R phase shifting and grouped into 6 respiratory-gates with 8 cardiac-gates. Each projection was reconstructed using a 3D OS-EM without and with attenuation correction using an averaged and phase-mismatched gated attenuation maps. The image artifacts of the reconstructed images were compared by visual inspection of the MP polar maps. The results showed significant changes of artifactual non-uniformity in the polar maps for off-phase of RM compared to those of CBM. The changes in the polar maps also demonstrated the effect of phase shifting accordingly. We conclude that the 4D XCAT phantom dataset with simultaneous C&R motions provides a powerful tool in the study of the effects of C&R motions with relative phase shifts, and development of C&R gating schemes and motion correction methods for improved ECT/CT imaging.

SU-E-T-100: Measurement of a Cobalt-60 Teletherapy Irradiator Spectrum Using a Compton Spectrometer

Purpose: To characterize the photon spectrum from a Theratronics T1000 cobalt-60 teletherapy unit using Compton spectrometry and compare to primary beam measurement and Monte Carlo (MC) simulation. Methods: A well-shielded high-purity germanium detector was used to measure the photon spectrum scattered at 22.5 and 70 degrees from the central axis of a 10cm×10cm beam of a cobalt-60 teletherapy unit. The central-axis photons were scattered using an aluminum rod (6mm diameter) positioned 100cm from the source, and the scattered photons were admitted to the detector through a 30cm-long collimating aperture (2mm diameter). The measured Compton-scattered spectra were corrected for detector response, determined using a model of the measurement apparatus in the MCNP5 MC code, and Compton relations (energy-angle and Klein-Nishina) to determine the central-axis spectrum at 100cm from the source. The primary photon spectrum was measured with the collimating aperture aligned to the central axis and corrected for detector response. A MC model of the cobalt-60 unit was developed in MCNP5 and used to generate the central-axis spectrum for a 10cmx10cm field. Results: The primary photon peaks from the cobalt-60 source were appropriately shifted to lower energies in the Compton-scattered spectra. The resolution of the shifted peaks degraded with increasing scattering angle. The corrected scattered spectra compared well with the primary measurement and MC simulation for peak position, but disagreed in the lower-energy continuum where source and collimator scatter are present. Differences are likely due to resolution loss with scattering and incomplete measurement correction. Conclusions: The spectrum from a cobalt-60 teletherapy unit was measured using Compton spectrometry. This spectrum was compared to direct measurement and MC simulation of the unit. Results indicate that small scattering angles should be used with this measurement technique and that further work is required to reconcile differences between measurements and simulations for source and collimator scatter.

SU‐E‐T‐861: Assessment of Collimator Jaw Optimization in Reducing Normal Tissue Irradiation with Intensity Modulated Radiation Therapy

Purpose: To evaluate normal tissue dose reduction in step‐and‐shoot intensity‐modulated radiation therapy (IMRT) on the Varian 2100 platform by tracking the multileaf collimator (MLC) apertures with the accelerator jaws. Methods: Clinical radiation treatment plans for 10 thoracic, 3 pediatric and 3 head and neck patients were converted to plans with the jaws tracking each segmentˈs MLC apertures and compared to the original plans in a commercial radiation treatment planning system (TPS). Each segment was then renormalized to account for the change in collimator scatter to obtain target coverage within 1 % of that in the original plan. Reduction in normal tissue dose was evaluated in the new plan by using the parameters V5, V10, and V20 in the cumulative dose‐volume histogram for the following structures: total lung minus GTV (gross target volume), heart, esophagus, spinal cord, liver, parotids, and brainstem. In order to validate the accuracy of our beam model, MLC transmission measurements were made and compared to those predicted by the TPS. Results: The greatest change between the original plan and new plan occurred at lower dose levels. The reduction in V20 was never more than 6.3% and was typically less than 1% for all patients. The reduction in V5 was 16.7% maximum and was typically less than 3% for all patients. The variation in normal tissue dose reduction was not predictable, and we found no clear parameters that indicated which patients would benefit most from jaw tracking. Our TPS model of MLC transmission agreed with measurements with absolute transmission differences of less than 0.1 % and thus uncertainties in the model did not contribute significantly to the uncertainty in the dose determination. Conclusion: The amount of dose reduction achieved by collimating the jaws around each MLC aperture in step and shoot IMRT is probably not clinically significant.

Testing of a treatment planning system with beam data from IAEA TECDOC 1540

Quality assurance of external-beam treatment-planning systems is recommended, and this can be partly achieved with predefined type tests. The beam data and test geometries of IAEA TECDOC 1540 have been used to test the analytical anisotropic algorithm (AAA) and pencil-beam convolution (PBC) algorithm of the Varian Eclipse treatment planning system. Beam models were created in Eclipse for 6 MV, 10 MV and 18 MV from the available beam data. Twelve test geometries were re-created in Eclipse, and the differences between Eclipse dose calculations and dose measurements were recorded. The AAA algorithm generally performed better than the PBC algorithm for the 12 tests, but both algorithms did not meet predefined tolerances for asymmetric wedge fields. An in-house monitor unit check program based on collimator and phantom scatter factors with tissue-phantom ratios was also tested and its calculations were found to agree with measurements to within 3.2% for on-axis points.

Dosimetric analysis of beam-matching procedure of two similar linear accelerators

Linear accelerators from the same vendor in a radiation therapy center are usually beam-matched following Vendor's acceptance criteria. This protocol is limited to check the difference at particular points on the ionization curve for depth dose or beam profiles. This article describes different tests done after commissioning to evaluate the level of agreement between matched beams of two ONCOR Impression plus linear accelerators from Siemens. Total scatter factors, collimator scatter factors, wedge transmission factors were measured in water for 6-MV photon. All these factors for ONCOR2 were within ±1% of those values for ONCOR1. Along with these point dose measurements we have essentially used γ-index to compare the planar dose distribution from two beam-matched accelerators. For this study a set of ready packed EDR2 films was exposed on both accelerators. The set consisted of films for percentage depth dose, beam profiles, a pyramid shape, multileaf collimator's positional and dose delivery accuracy, and a film to compare head scatter at tray level. To include treatment planning system calculations, a film kept in axial plane was exposed to 3DCRT and IMRT plans with actual gantry angles and monitor units. These films were analyzed for γ in OmniPro IMRT software using different combinations of Δdose and - Δdistances. All these films have shown good agreement for - Δdistance of 3 mm and Δdose of 3 %.

A Comparison of Out-of-Field Dose and Its Constituent Components for Intensity-Modulated Radiation Therapy Versus Conformal Radiation Therapy: Implications for Carcinogenesis

To investigate differences in scatter and leakage between 6-MV intensity-modulated radiation therapy (IMRT) and three-dimensional conformal radiation therapy (3DCRT); to describe the relative contributions of internal patient scatter, collimator scatter, and head leakage; and to discuss implications for second cancer induction. Dose was measured at increasing distances from the field edge in a water bath with a sloping wall (1) under full scatter conditions, (2) with the field edge abutting but outside the bath to prevent internal (water) scatter, and (3) with the beam aperture plugged to reflect leakage only. Internal patient scatter from IMRT is 11% lower than 3DCRT, but collimator scatter and head leakage are five and three times higher, respectively. Ultimately, total scattered dose is 80% higher with IMRT; however this difference is small in absolute terms, being 0.14% of prescribed dose. Secondary dose from 3DCRT is mostly due to internal patient scatter, which contributes 70% of the total and predominates until 25 cm from the field edge. For IMRT, however, machine scatter/leakage is the dominant source, contributing 65% of the secondary dose. Internal scatter predominates for just the first 10 cm from field edge, collimator scatter for the next 10 cm, and head leakage thereafter. Out-of-field dose is 80% higher with IMRT, but differences are tiny in absolute terms. Reductions in internal patient scatter with IMRT are outweighed by increased machine scatter and leakage, at least for small fields. Reductions from IMRT in dose to tissues within the portals and in internal scatter, which predominates close to the field edge, means that calculations based solely on dose to distant tissues may overestimate carcinogenic risks.

Assessment of Out-of-Field Doses in Radiotherapy of Brain Lesions in Children

To characterize the out-of-field doses in pediatric radiotherapy and to identify simple methods by which out-of-field dose might be minimized, with a view to reducing the risk of secondary cancers. With the aim of characterizing the peripheral doses under different treatment conditions, the dose measurements in an anthropomorphic child phantom were taken in various organs and critical structures outside the primary field using thermoluminescent dosimetry. The doses from a Varian 600C and Varian Trilogy linear accelerator, both at 6 MV, were investigated. Larger field sizes have been shown to result in greater peripheral doses close to the primary beam, with the difference becoming less significant at large distances, indicating that most of out-of-field doses result from head leakage and collimator scatter>40 cm from the primary field. The use of lead shields has been shown to reduce the absorbed dose resulting from leakage. Aligning the craniocaudal axis of the patient with the x-plane of the collimator resulted in a dose reduction of 40%, for both machines. Out-of-field doses from the Varian Trilogy were shown to be approximately 40% greater than those from the 600C linear accelerator, despite being operated at the same energy. Out-of-field doses to pediatric patients can be minimized by using simple treatment options, such as using the single-energy mode linear accelerator rather than the multimode, orienting the couch and collimator such that the patient lies along the x-plane and avoiding fields directed along the trunk of the body.

I-123 MIBG cardiac uptake measurements: limitations of collimator choice and scatter correction in the clinical context

Low uptake of metaiodobenzylguanidine (MIBG) in patients with heart failure generally indicates poor prognosis. Our objective was to determine the best method for calculating I-123 MIBG uptake. MIBG uptake as a percentage of the injected dose is presented as an alternative method for serial assessment. Patients with chronic heart failure were imaged with I-123 MIBG with both a medium-energy (ME) collimator and a low-energy high-sensitivity (LEHS) collimator. Scatter correction was used to correct the LEHS images. Heart-to-mediastinal (H/M) ratio and the percentage of myocardial uptake of MIBG were obtained. Mean H/M ratios for the ME images, LEHS images and scatter-corrected LEHS images were 2.45+/-0.61, 2.22+/-0.47 and 2.51+/-0.62, respectively. Mean H/M ratio was significantly different among all the three sets (P<0.001) of images. The average difference in H/M ratios between the ME images and LEHS images was lower when scatter correction was applied (4.95% vs. 9.79%). The error in calculating the myocardial uptake as a percentage of the injected dose was significantly lower than the error in calculating H/M ratio (0.2 vs.10.2% LEHS; 0.3 vs.16.0% ME; 0.2 vs.11.8% LEHS scatter corrected). For quantitative assessment of H/M ratio in I-123 MIBG imaging a LEHS collimator can be used in place of a ME collimator to achieve better counting statistics, but scatter correction must be used. The calculation of the myocardial uptake as a percentage of the injected dose has potential as an alternative method of measurement, particularly for serial assessment.

Evaluation of MatriXX for IMRT and VMAT dose verifications in peripheral dose regions

MatriXX is a two-dimensional ion chamber array designed for IMRT/VMAT (RapidArc, IMAT, etc.) dose verifications. Its dosimetric properties have been characterized for megavoltage beams in a number of studies; however, to the best of the authors' knowledge, there is still a lack of an investigation into its performance in the peripheral or low dose regions. In this work, the authors have carried out a systematic study on this issue. The authors compare the performance of MatriXX with a cylindrical ion chamber in solid water phantoms in the peripheral dose regions. The comparisons are performed for a number of typical irradiation conditions that involve different gantry and/or MLC motions, field sizes, and distances to the target including static gantry/open fields, static gantry/sweeping MLC gap (mimicking an IMRT delivery), dynamic gantry/oscillating sweeping MLC gap (mimicking a VMAT delivery), as well as clinical IMRT and VMAT plans. MatriXX, when used according to the manufacturer's recommendations, is found to disagree with an ion chamber in peripheral dose regions. This disagreement has been attributed to four types of MatriXX errors, namely, positive bias, over-response to scattered doses, round-off error, and angular dependence, all of which contribute to dose inaccuracies in the peripheral regions. The positive bias, which is independent of the dose level, is cumulative when MatriXX operates in the movie mode. The accumulation is proportional to the number of movie frames (snaps) when the sampling time is greater than 500 ms and is proportional to the overall movie time for a sampling time shorter than 500 ms. This behavior suggests multiple sources of the bias. MatriXX is also found to over-respond to peripheral doses by about 2.0% for the regions investigated in this work (3-15 cm from the field edge), where phantom scatter and collimator scatter dominate. Round-off error is determined to be due to insufficient precision in conversion of the raw signals to MatriXX software data for low doses. Angular dependence is defined as the dose response of MatriXX at different gantry angles. Up to 8% difference in detector response has been observed between 0 degree and 180 degrees. Possible sources of these errors are discussed and a correction method is suggested. With corrections, MatriXX shows good agreement with the ion chamber in all cases involving different gantry and/or MLC dynamics, as well as the clinical plans. For both primary and peripheral doses, MatriXX shows dose linearity down to 2 cGy with an accuracy of within 1% of the local dose. The performance of MatriXX has been systematically evaluated in the peripheral dose regions. Major sources of error associated with MatriXX are identified and a correction method is suggested. This method has been successfully tested using both experimental and clinical plans. In all cases, good agreements between MatriXX and an ion chamber are achieved after corrections. The authors conclude that with proper corrections, MatriXX can be reliably used for peripheral dose measurements within the ranges studied.

SU‐GG‐T‐305: Evaluation of Some Essential Dosimetric Parameters for Beam Matching of Similar Accelerators

Purpose: To dosimetrically evaluate beam matching procedure of two similar accelerators using gamma index. Method and Materials: The second accelerator was beam matched with first one using Vendor's acceptance test (ATP) protocol. Depth dose and inline‐crossline profiles were matched within ±1%. In order to quantify the level of agreement between the matched beams we performed a set of measurements including total scatter, collimator scatter, wedge transmission and absolute dose measurement. The ATP specifications are at particular points on the ionization curve for depth dose or profile. Gamma was used to compare planar dose distribution from EDR2 films exposed on both units. For 6MV photon we exposed a film for PDD (10×10cm), beam profile (5,10 and 20cm), a pyramid shape, to compare dosimetric and positional accuracy of MLC a pattern of strips with different MUs at different positions and a diamond shape was exposed. We also exposed a film at tray level to a segmented IMRT field to compare collimator scatter. To account for TPS calculations a film kept in axial plane was exposed to 3DCRT and IMRT plan with actual gantry angles. Absolute dose was measured simultaneously using point chamber. Results: Total scatter, collimator scatter and wedge transmission factors were within ±1%. The absolute dose for 3DCRT and IMRT plan was within ±1% of reference value and within ±3% of TPS value. EDR2 films exposed on both units were analyzed using OmniPro IMRT software. Gamma was calculated for different delta dose and delta distance. The analysis has shown that all these comparisons were in good agreement for delta dose of 2% and delta distance of 2mm. Conclusion: All these tests were done at the commissioning time. We are periodically checking few parameters like PDD, beam profile and MLC accuracy to assure the consistency in beam matching

WE-B-BRB-01: Out of Field Dose from Photon Radiotherapy: Magnitude, Evaluation, and Impact

Dose outside of the target volume is an unavoidable consequence of radiation therapy. This undesirable radiation is clearly a concern for pregnant patients as the fetus is highly radiosensitive. Similarly, out‐of‐field radiation is a concern for patients with implanted cardiac pacemakers or defibrillators as electronic failures may be induced by relatively low doses of radiation. Out‐of‐field dose has also received attention due to its potential for inducing late effects such as second malignant neoplasms,heart disease, or stroke. In recent years patient survival has increased due to more efficacious treatments resulting in more long‐term survivors. These long‐term survivors have an increased risk of developing a radiation induced late effect. Compounding this, modern radiotherapy techniques, such as intensity‐modulated radiation therapy(IMRT) may increase the out‐of‐field dose, increasing the risk per person of developing a late effect. The above concerns surrounding out‐of‐field dose are confounded by the number and complexity of modern radiotherapy delivery techniques, such as IMRT, Tomotherapy, Cyberknife, and Gamma Knife treatments. Out‐of‐field dose is not accurately reported by commercial treatment planning systems and other methods for assessing this dose are necessary. Therefore, measurements or calculations are often performed to evaluate out‐of‐field doses. As there are a variety of measurement and calculational options available, it is worthwhile exploring the strengths and limitations of each, as well as particular cautions that may be required. Monte Carlo calculations may be much more accurate than treatment planning system calculations, but require substantial infrastructure and modeling. Measurements may be a reasonable approach, but it is nevertheless possible to have substantial error in the measurement, particularly with TLD. This presentation will examine the out‐of‐field dose associated with the variety of different treatment modalities that are currently available. The sources and relative importance of head leakage, collimator scatter, and patient scatter will be compared, and parameters that impact the out‐of‐field dose will be examined. The impact of treatment energy will be discussed, particularly in the context of neutron production and the biological potency of neutrons. Finally, approaches to measure and calculate the out‐of‐field dose will be examined, and the limitations and challenges of many such approaches will be explored. Learning Objectives: 1. Understand the magnitude of out‐of‐field dose and treatment parameters that affect it 2. Understand the limitations of treatment planning system calculations 3. Understand techniques and challenges of measuring out‐of‐field doses 4. Understand the impact of these doses on patient care

Out-of-field photon dose following removal of the flattening filter from a medical accelerator

The aim of this paper is to determine the effect of removing the flattening filter from a linear accelerator on the out-of-field photon dose. A Monte Carlo model was used to simulate 6 MV and 18 MV photon beams from a Varian 2100 accelerator with the flattening filter in place and with it removed. The out-of-field photon doses and composition (head leakage, patient scatter and collimator scatter) were calculated from square open fields in a water tank as a function of distance from central axis, field size and depth. The out-of-field doses resulting from intensity-modulated radiation therapy to the prostate at 6 MV were also calculated, with and without the flattening filter, to sensitive organs in an anthropomorphic Rando phantom. Removal of the flattening filter reduced the out-of-field dose near the treatment field (<3 cm from the field edge) because of decreased collimator scatter. It increased the out-of-field dose at intermediate distances from the field edge (3–15 cm) because of increased patient scatter. At greater distances, the out-of-field dose was decreased because of reduced head leakage. For the clinical treatment examined, the out-of-field dose was generally reduced following removal of the flattening filter, particularly at large distances from the treatment field. Removal of the flattening filter may be advantageous by reducing the out-of-field dose to the patient. This could contribute to reducing the long-term risk of secondary malignancies. In general, however, the out-of-field dose depends on treatment and patient parameters, and a reduction may not always be achievable.

Impact of scatter correction on D2 receptor occupancy measurements using 123I-IBZM SPECT: Comparison to 11C-Raclopride PET

Reported values of D2 receptor occupancy (RO) achieved by antipsychotic drugs tend to be lower when measured with 123I-IBZM SPECT than with 11C-Raclopride PET. Image degrading factors such as attenuation, distance-dependent collimator response and scatter could account for this difference. While attenuation correction is routinely applied to SPECT images, the other degradations are not usually accounted for. The aim of this work was to assess the impact of scatter correction on D2 RO quantification with 123I-IBZM SPECT, and to compare the results of both corrected and un-corrected SPECT values with 11C-Raclopride PET measurements. Phantom experiments as well as within-subject human data from a previous study were used for this purpose. SPECT images were reconstructed using filtered back-projection including attenuation correction (FBPA), ordered subsets expectation maximization including attenuation and point spread function corrections (OSEMA+PSF) and ordered subsets expectation maximization including attenuation, point spread function and scatter corrections (OSEMA+PSF+SCT). PET images were reconstructed using the FBP algorithm and corrected for attenuation, scatter, random coincidences and dead time. Quantification of receptor availability was performed using the tissue ratio at pseudoequilibrium for SPECT, and the simplified reference tissue model (SRTM) for PET. Analysis was performed using both occipital cortex (occ) and cerebellum (cer) as reference regions for both modalities. When images were reconstructed using FBPA, SPECT D2 RO values were significantly lower as compared with PET leading to a D2 RO difference of −20% (CI95%: −13, −27%) (occ) and −23% (CI95%: −14, −31%) (cer). When images were reconstructed using OSEMA+PSF, SPECT D2 RO values were also lower as compared with PET leading to a D2 RO difference of −21% (CI95%: −14, −27%) (occ) and −24% (CI95%: −18, −30%) (cer). When images were reconstructed using OSEMA+PSF+SCT, the D2 RO bias was reduced to −6% (CI95%: 0, −13%) (occ) and −11% (CI95%: −4, −18%) (cer). These data suggest that the scatter correction plays a major role in explaining the differences between D2 RO measurements using 123I-IBZM SPECT and 11C-Raclopride PET.

Peripheral dose measurement in high-energy photon radiotherapy with the implementation of MOSFET

To study the peripheral dose (PD) from high-energy photon beams in radiotherapy using the metal oxide semiconductor field effect transistor (MOSFET) dose verification system. The radiation dose absorbed by the MOSFET detector was calculated taking into account the manufacturer's Correction Factor, the Calibration Factor and the threshold voltage shift. PD measurements were carried out for three different field sizes (5 cm × 5 cm, 10 cm × 10 cm and 15 cm × 15 cm) and for various depths with the source to surface distance set at 100 cm. Dose measurements were realized on the central axis and then at distances (1 to 18 cm) parallel to the edge of the field, and were expressed as the percentage PD (% PD) with respect to the maximum dose (d(max)). The accuracy of the results was evaluated with respect to a calibrated 0.3 cm(3) ionization chamber. The reproducibility was expressed in terms of standard deviation (s) and coefficient of variation. % PD is higher near the phantom surface and drops to a minimum at the depth of d(max), and then tends to become constant with depth. Internal scatter radiation is the predominant source of PD and the depth dependence is determined by the attenuation of the primary photons. Closer to the field edge, where internal scatter from the phantom dominates, the % PD increases with depth because the ratio of the scatter to primary increases with depth. A few centimeters away from the field, where collimator scatter and leakage dominate, the % PD decreases with depth, due to attenuation by the water. The % PD decreases almost exponentially with the increase of distance from the field edge. The decrease of the % PD is more than 60% and can reach up to 90% as the measurement point departs from the edge of the field. For a given distance, the % PD is significantly higher for larger field sizes, due to the increase of the scattering volume. Finally, the measured PD obtained with MOSFET is higher than that obtained with an ionization chamber with percentage differences being from 0.6% to 34.0%. However, when normalized to the central d(max) this difference is less than 1%. The MOSFET system, in the early stage of its life, has a dose measurement reproducibility of within 1.8%, 2.7%, 8.9% and 13.6% for 22.8, 11.3, 3.5 and 1.3 cGy dose assessments, respectively. In the late stage of MOSFET life the corresponding values change to 1.5%, 4.8%, 11.1% and 29.9% for 21.8, 2.9, 1.6 and 1.0 cGy, respectively. Comparative results acquired with the MOSFET and with an ionization chamber show fair agreement, supporting the suitability of this measurement for clinical in vivo dosimetry.

Ｖｉｒｔｕａｌ ｓｏｕｒｃｅ ｐｌａｎｅを用いた 矩形照射野のコリメータ散乱係数の検討

Kim introduced the geometric weighting factor concept into the field mapping method, and estimated collimator scatter factors of rectangular fields by correcting collimator exchange effects. The source plane is present at a specific position in the field mapping method, and, accordingly, the geometric weighting factor is constant. In this study, we changed the position of the source plane based on the measurement results, and we estimated the collimator factors of rectangular fields using the field mapping method. A geometric weighting factor at which the measured collimator scatter factors optimally fitted a square collimator scatter factor was calculated in each field. Collimator scatter factors can be accurately calculated recursively by changing the geometric weighting factor, resulting in altering the position of the source plane, as in this method.