Collimator Jaws Research Articles

SU‐E‐J‐144: Analysis of Intra‐Fraction Chest Wall Motion during Breath Hold Treatment for Left Sided Breast Cancer

Purpose: To quantify chest wall motion during treatment while the patient executes a breath hold (BH) maneuver for left sided breast cancer patients. Methods: Three breast cancer patients (left side) who were to receive radiation utilizing the BH technique were prospectively chosen to take part in this study. Treatment plans were generated according to standard clinical treatment planning guidelines. Images were acquired in cine mode using the EPID (Varian Medical Systems, Palo Alto, CA) on a per fraction basis for non step and shoot fields. The chest wall was defined in each image using a logarithmic edge detection algorithm. The distance of the chest wall orthogonal to the reference axis (collimator jaw parallel to chestwall) was determined in 1 cm intervals along this axis. Motion was assessed by the standard deviation of chest wall position and the maximum distance traveled, for each field, each treatment day. The root mean square (RMS) of the standard deviations was computed as well as the average and standard deviations of maximum motion for each 1 cm interval. Results: 420, 205, and 360 images were acquired respectively for patients 1, 2, and 3 for both medial and lateral tangential fields. The total RMS of the motion computed over the entire length of the chestwall was 0.7mm, 0.8mm, and 0.6mm for patients 1–3 respectively. The maximum motion over the entire chest wall for each treatment day was 1.4+1.6mm, 1.9+1.4mm, 1.1+1.3mm on average for the course of treatment. Conclusions: Chest wall motion during a breath hold treatment was analyzed for three patients and was found to be less than 2mm on average. When using the deep inspiration BH technique to displace the heart from the posterior tangential field border, our study suggests that delivery can be executed with minimal intra‐fractional motion.

Outgassing measurement of an LHC collimator and estimation for the NEG performances

The outgassing rate of the collimators in the Large Hadron Collider (LHC) at CERN has an important role for the life-time of the Non-Evaporable Getter (NEG), and an accurate analysis allows the definition of future activities, like NEG vacuum activation. For these reasons, both, total outgassing rate and gas composition of a secondary collimator have been measured in the laboratory. The outgassing rate decreases by about two orders of magnitude by after bake-out and moreover, repeated bake-out further reduced the outgassing rate. The gas transmission through the NEG coated beam pipes and the resulting pressure distributions near the collimator were also measured in a dedicated setup. It is found that the main gas component after just 2 m of NEG coated beam pipe is CH 4 due to the extreme pumping speed of NEG for the other gases. Large amount of outgassing for H 2 and carbon related molecules are released when moving the collimator jaws. It is found that the NEG is very effective even in such case with large gas load.

Measuring output factors of small fields formed by collimator jaws and multileaf collimator using plastic scintillation detectors

As the practice of using high-energy photon beams to create therapeutic radiation fields of subcentimeter dimensions (as in intensity-modulated radiotherapy or stereotactic radiosurgery) grows, so too does the need for accurate verification of beam output at these small fields in which standard practices of dose verification break down. This study investigates small-field output factors measured using a small plastic scintillation detector (PSD), as well as a 0.01 cm3 ionization chamber. Specifically, output factors were measured with both detectors using small fields that were defined by either the X-Y collimator jaws or the multileaf collimator (MLC). A PSD of 0.5 mm diameter and 2 mm length was irradiated with 6 and 18 MV linac beams. The PSD was positioned vertically at a source-to-axis distance of 100 cm, at 10 cm depth in a water phantom, and irradiated with fields ranging in size from 0.5 x 0.5 to 10 x 10 cm2. The field sizes were defined either by the collimator jaws alone or by a MLC alone. The MLC fields were constructed in two ways: with the closed leaves (i.e., those leaves that were not opened to define the square field) meeting at either the field center line or at a 4 cm offset from the center line. Scintillation light was recorded using a CCD camera and an estimation of error in the median-filtered signals was made using the bootstrapping technique. Measurements were made using a CC01 ionization chamber under conditions identical to those used for the PSD. Output factors measured by the PSD showed close agreement with those measured using the ionization chamber for field sizes of 2.0 x 2.0 cm2 and above. At smaller field sizes, the PSD obtained output factors as much as 15% higher than those found using the ionization chamber by 0.6 x 0.6 cm2 jaw-defined fields. Output factors measured with no offset of the closed MLC leaves were as much as 20% higher than those measured using a 4 cm leaf offset. The authors' results suggest that PSDs provide a useful and possibly superior alternative to existing dosimetry systems for small fields, as they are inherently less susceptible to volume-averaging and perturbation effects than larger, air-filled ionization chambers. Therefore, PSDs may provide more accurate small-field output factor determination, regardless of the collimation mechanism.

Practical quantitative measurement of graticule misalignment relative to collimator axis of rotation

We design a practical procedure for measuring translational and rotational misalignment of graticule with collimator axis of rotation and collimator jaws, respectively. The procedure's quantitative results are accurate to less than 0.2 mm (at isocenter) and do not assume alignment of radiation focal spot with collimator axis of rotation. When provided with these quantitative results, the manufacturer can custom‐adjust graticules to the purchaser's collimator head.PACS number: 87.56.Fc

SU-GG-T-129: Sparing of Neural Stem Cells during Whole-Brain Radiation Treatments

Purpose: Sparing of neural stem cells during whole‐brain radiation treatments could help retain cognitive function. However, these cells are extremely radiosensitive, with impairment of neurogenesis being observed even at 2 Gy. This study investigates the fundamental physics issues associated with sparing the neural stem cells. Method and Materials: Three factors are particularly important in determining the stem cell dose: intracranial scattering, collimator leaf leakage, and jaw positioning. A simple planning system was developed to study the relative contributions of these factors. This planning system includes a basic representation of scatter and average collimator leakage. In addition to these theoretical investigations, an IMRT plan was delivered in conjunction with blocks to further test collimator leakage. The effect of the blocks on the delivered fluence patterns was measured with a 2‐D diode array. Results: Plans covered 50% of the PTV at 30 Gy. With fixed collimator jaws and average leaf leakages of 0, 0.37, 0.5, 0.9, 1, 1.5, and 2%, half of the stem cells were kept below the doses of 2.27, 3.61, 4.00, 5.01, 5.24, 6.25, and 7.09 Gy, respectively. Moving collimator jaws to the edge of the field reduced these doses to 2.27, 2.62, 2.74, 3.05, 3.12, 3.44, and 3.73 Gy, respectively. These results are in qualitative agreement with an extrapolation of the experimental data, which indicates that collimator transmission from a machine with 0.9% leaf leakage contributes a dose of 2.4 Gy to the stem cells. Conclusion: Intracranial scattering creates a substantial dose to the stem cells; thus, it is important to minimize the other contributing factors, in particular collimator leakage. Utilization of moving jaws and state‐of‐the‐art collimators can minimize this leakage. In addition, treatment with these collimators could be combined simultaneously with blocks to further decrease this dose.Conflict of Interest: Research sponsored by Siemens corporation.

SU‐GG‐T‐327: Measurement of Small Field Output Factors Using Plastic Scintillation Detectors

Purpose: To investigate the performance of a small volume, water‐equivalent plastic scintillation detector (PSD) for measuring output factors for small IMRT fields, as compared to 0.01cm3 ionization chamber. Method and Materials: A PSD of 0.5 mm diameter and 2 mm length was irradiated with 6 and 18 MV beams from a Varian Clinac 21EX. The PSD was positioned vertically at a source‐to‐axis distance of 100 cm, at 10 cm depth in a water phantom, and irradiated with fields ranging in size from 0.5 × 0.5 cm2 to 10 × 10 cm2. The field sizes were defined either by the collimator jaws alone, or by a multi‐leaf collimator (MLC) alone. The MLC fields were constructed in two ways: with the closed leaves (i.e., those leaves that were not opened to define the square field) meeting at either the field center‐line, or at a 4 cm offset from center‐line. Scintillation light was recorded using a CCD camera. Measurements were made using a CC01 ionization chamber under conditions identical to those used for the PSD. Results: Output factors measured using the PSD were found to be in good agreement with those measured using the CC01 down to a field size of 2.0 × 2.0 cm2. The PSD measured higher output factors than the CC01 at smaller fields, e.g. by 7.8 % at 0.5 × 0.5 cm2 under 6 MV photons using MLC fields with 4 cm leaf offset. Also, output factors were found to depend on the field defining geometry. Conclusion: This study suggests that PSDs provide a useful alternative to existing dosimetry systems for small fields, as they are less susceptible to volume averaging and perturbation effects than larger, air‐filled ionization chambers. Therefore, PSDs may provide more accurate output factor determination due to their inherent properties. Supported by the NCI 1R01CA120198‐01A2.

SU-GG-T-135: Comparison of Treatment Planning Systems in Elekta Volumetric Modulated Arc Therapy (Elekta VMAT) - Prostate Cancer Study

Purpose: To compare the performance of treatment planning systems (TPS's) for prostate cancer with Elekta VMAT. Method and Materials: Five prostate cancer patients at low risk (LR) and two prostate cancer patients at high risk (HR) were randomly selected for this study. Single‐arc VMAT plans with D_95 prescription (dose to 95% of target volume) of 76Gy in 38 fractions have been created by using three commercial TPS's; Monaco v2.03beta (Elekta CMS), ERGO++ v1.7.2, (Elekta CMS), and Pinnacle v9.0 SmartArc (Philips). Effort was made to maintain the rectum dose volume histograms (DVHs) almost identical among the three plans. The resulting plans were compared in terms of total monitor unit (MU), the dose homogeneity defined by D_max/76Gy, and dose in organ at risk (OAR). All plans were delivered three times on Elekta Synergy. The dynamical parameters of dose rates, gantry angles, multi‐leaf collimator(MLC) positions and jaw positions have been recorded with 0.25 sec interval in the controller. Result: All rectum DVHs for LR were similar for doses more than 40Gy, while for HR, ERGO++ provided somewhat higher dose in rectum and also a cold spot in seminal vesicle. The averaged MU was 595, 411, 430, and the averaged dose homogeneity in target volume was 1.08, 1.07, and 1.05 for Monaco, ERGO++, and SmartArc, respectively. Delivery time was less than 3 minutes for all plans. Monaco and ERGO++ needed a longer delivery time, mainly due to the creation of move‐only segments. Observed errors in the gantry angle, MLC position, and jaw position were small and reproducible. They did not impact the dose distribution significantly. Conclusions: All TPS's used in this study provided satisfactory VMAT plans for prostate cancer. The dynamical parameters were sufficiently well controlled.

An EPID based method for efficient and precise asymmetric jaw alignment quality assurance

The aim of this work was to investigate the use of amorphous silicon electronic portal imaging devices (EPIDs) for regular quality assurance of linear accelerator asymmetric jaw junctioning. The method uses the beam central axis position on the EPID measured to subpixel accuracy found from two EPID images with 180 degrees opposing collimator angles. Individual zero jaw position ("half-beam blocked") images are then acquired and the jaw position precisely determined for each using penumbra interpolation. The accuracy of determining jaw position with the EPID method was measured by translating a block (simulating a jaw) by known distances, using a translation stage, and then measuring each translation distance with the EPID. To establish the utility of EPID based junction dose measurements, radiographic film measurements of junction dose maxima/minima as a function of jaw gap/overlap were made and compared to EPID measurements. Using the method, the long-term stability of zero jaw positioning was assessed for four linear accelerators over a 1-1.5 yr time period. The stability at nonzero gantry angles was assessed over a shorter time period. The accuracy of determining jaw translations with the method was within 0.14 mm found using the translation stage [standard deviation (SD) of 0.037 mm]. The junction doses measured with the EPID were different from film due to the nonwater equivalent EPID scattering properties and hence different penumbra profile. The doses were approximately linear with gap or overlap, and a correction factor was derived to convert EPID measured junction dose to film measured equivalent. Over a 1 yr period, the zero jaw positions at gantry zero position were highly reproducible with an average SD of 0.07 mm for the 16 collimator jaws examined. However, the average jaw positions ranged from -0.7 to 0.9 mm relative to central axis for the different jaws. The zero jaw position was also reproducible at gantry 90 degrees position with 0.1 mm SD variation with the mean jaw position offset from the gantry zero position consistently by 0.3-0.4 mm for the jaws studied. The EPID based method is efficient and yields more precise data on linear accelerator jaw positioning and reproducibility than previous methods. The results highlight that zero jaw positions are highly reproducible to a level much smaller than the displayed jaw resolution and that there is a need for better methods to calibrate the jaw positioning.

Comparison between laboratory measurements, simulations, and analytical predictions of the transverse wall impedance at low frequencies

The prediction of the transverse wall beam impedance at the first unstable betatron line (8kHz) of the CERN Large Hadron Collider (LHC) is of paramount importance for understanding and controlling the related coupled-bunch instabilities. Until now only novel analytical formulas were available at this frequency. Recently, laboratory measurements and numerical simulations were performed to cross-check the analytical predictions. The experimental results based on the measurement of the variation of a probe coil inductance in the presence of (i)sample graphite plates, (ii)stand-alone LHC collimator jaws, and (iii)a full LHC collimator assembly are presented in detail. The measurement results are compared to both analytical theories and simulations. In addition, the consequences for the understanding of the LHC impedance are discussed. © 2009 The American Physical Society.

An effective correction algorithm for off‐axis portal dosimetry errors

Portal dosimetric images acquired for IMRT pretreatment verification show dose errors of up to 15% near the detector edges as compared to dose predictions calculated by a treatment planning system for these off-axis regions. A method is proposed to account for these off-axis effects by precisely correcting the off-axis output factors, which calibrate the imager for absolute dose. Using this method, agreement between the predicted and the measured doses improves by up to 15% for fields near the detector edges, resulting in passing rate improvements of as much as 60% for gamma evaluation of 3 mm, 3% within the collimator jaws.

SU‐FF‐T‐130: Radiotherapy Treatment Plans with RapidArc for Head‐And‐Neck Cancer

Purpose: The purpose of this study is to compare RapidArc plans and conventional IMRT plans for head‐and‐neck cancer treatment involving bilateral nodes. Methods and Materials: This study included 10 locally advanced head‐and‐neck cancer patients. The planning target volume (PTV) included a 3‐mm margin onto the clinical target volume. The prescription was 50Gy to the PTV (2Gy/fraction). Right/left parotids, oral cavity, larynx, brainstem, spinal cord and mandible were contoured as critical organs. For each patient, 9‐fixed‐field IMRT and 3‐arc RapidArc plans were generated. For IMRT plans, the beams were spaced 40° apart and the collimator jaws were fixed to avoid shoulders. RapidArc plans had a total of 600° rotation, one full rotation and two anterior 120° rotations (179°–181°, 60°–300° and 300°–60°). All plans were normalized to cover 90% of PTV with 100% of the prescribed dose. Dosimetric results and treatment time were compared. Results: The homogeneity index of PTV was (1.07; 1.06) and the normal‐tissue receiving dose greater than 110% of the prescription dose was (0.11%; 0.01%) for (RapidArc; IMRT). The average median doses were (right parotid; 46.6%; 43.1%), (left parotid; 39.9%; 34.5%), (oral cavity; 50.0%; 51.7%) and (larynx; 45.7%; 41.4%) for (structure; RapidArc; IMRT). The average maximum doses were (brainstem; 45.5%; 49.9%), (spinal cord; 52.9%; 65.6%) and (mandible; 108.5%; 107.1%) for (organ; RapidArc; IMRT). The statistical analysis showed that the differences in homogeneity index and median doses to right/left parotids and spinal cords were statistically significant (p<0.05), and all others were not. The MU values were reduced by 56% in average and the total treatment time was reduced by 7 minutes when RapidArc was compared to IMRT. Conclusions: IMRT performs better for parotid glands whereas RapidArc performs better for spinal cord. IMRT and RapidArc provide comparable plans for other organs. Treatment time with RapidArc is much reduced compared to IMRT.

SU‐FF‐T‐95: Reduction of Larynx Dose in Head and Neck IMRT: A Restricted Field Approach

Purpose: To develop a Whole‐Field Head and Neck IMRT technique that allows for a considerable reduction in dose to the uninvolved larynx and post cricoid/inferior constrictor structures without sacrificing target coverage or introducing uncertainties associated with matched field techniques. Materials and Methods: Seven coplanar beams are used to treat target volumes located superior to the larynx. The inferior jaws of these beams are fixed at the superior extent of the larynx. Three additional beams, on both the left and right sides, are typically used to treat the entire superior to inferior extent of the disease, but with the medial collimator jaw edges fixed at the edge of the larynx. The three beams chosen on each side, are typically an AP and 2 steep posterior obliques approximating an AP/PA approach in the inferior portion of the target volumes. All beams are placed in a single plan and undergo optimization simultaneously (Eclipse, Varian, Palo Alto, CA). Results: Excellent target coverage is maintained while dramatically reducing dose centrally through the larynx region. Point dose measurements for a sample patient in the larynx, post cricoid and inferior constrictors are 14Gy, 8.8Gy and 8.9Gy respectively. There are also no adverse effects on parotid sparing or dose conformality in the region superior to the larynx. Conclusion: The restricted field head and neck IMRT technique allows for excellent coverage of the target volumes while providing dramatically reduced larynx and post cricoid/inferior constrictor doses when compared to using the optimization algorithm alone in the whole field IMRT technique. In addition, this technique eliminates the inhomogeneity associated with a matched field technique due to collimator position tolerances and avoids matching through disease.

Quality assurance of volumetric modulated arc therapy using Elekta Synergy

Purpose. Recently, Elekta has supplied volumetric modulated arc therapy (VMAT) in which multi-leaf collimator (MLC) shape, jaw position, collimator angle, and gantry speed vary continuously during gantry rotation. A quality assurance procedure for VMAT delivery is described. Methods and materials. A single-arc VMAT plan with 73 control points (CPs) and 5-degree gantry angle spacing for a prostate cancer patient has been created by ERGO + + treatment planning system (TPS), where MLC shapes are given by anatomic relationship between a target and organs at risk and the monitor unit for each CP is optimized based on given dose prescriptions. Actual leaf and jaw positions, gantry angles and dose rates during prostate VMAT delivery were recorded in every 0.25 seconds, and the errors between planned and actual values were evaluated. The dose re-calculation using these recorded data has been performed and compared with the original TPS plan using the gamma index. Results. Typical peak errors of gantry angles, leaf positions, and jaw positions were 3 degrees, 0.6 mm, and 1 mm, respectively. The dose distribution obtained by the TPS plan and the recalculated one agreed well under 2%-2 mm gamma index criteria. Conclusions. Quality assurance for prostate VMAT delivery has been performed with a satisfied result.

USE OF ELECTRONIC PORTAL IMAGE DEVICE IN SOME QA TESTS

Introduction: In the present study a method to investigate light and radiation field congruence utilizing an amorphoussilicon electronic portal imaging device (EPID) was developed. An alternative method for routine dynamic multi-leafcollimator (DMLC) quality assurance (QA) using EPID was also described.Materials and Methods: This study employed an amorphous silicon (aSi) EPID, the associated EPI software and adiamond-shaped template. The collimator jaws of the linac (Elekta iViewGT) were aligned such that the light field fell directly on the corners of the diamond. A radiation detection algorithm within the EPI software determined the extent of the radiation field. The light and radiation field congruence was evaluated by comparing the vertices of the diamond reference structure to the detected radiation field. In addition, the digital jaw settings were recorded and later compared to the light field detected on Kodak x-omat films and EPIs. In this study an alternative method for routine dynamic multi-leaf collimator (DMLC) quality assurance (QA) using (EPID) was described. This QA was done by producing apattern of five 0.5-cm bands 5-cm apart and performing a visual spot-check for dose peak location, dose peak values,interband distance and band gaps measured at Full Width at Half-Maximum (FWHM) of the test pattern for this QA experiment. An aSi EPID and films are used for the DMLC QA to test the practicality and efficacy of EPID vis film.Results: Light radiation field congruence tests with films and EPIs were comparable, yielding a difference of less than 1.2 mm, well within the allowed 2-mm tolerance. A disparity was observed in the magnitude of the detected light field.The X and Y dimensions of the light field measured with film differed by less than or equal to 1.6 mm from the digital collimator settings, whereas the values extracted from the EPIs differed by up to 1.8 mm. The FWHM of the multi-leaf collimator (MLC) obtained by the EPID is 6.34 mm, which is slightly higher than the 6.18 mm for the film. The lowest dose at the valley is significantly lower for the EPID than for the film. The dose peak value of the EPID is found to be97.9 with standard deviation of ±1.59 and the dose peak value of film is found to be 96.5 with standard deviation of±2.41, respectively. Film and EPID track the distance between peaks with a mean of 49.8 and 49.53 mm and standarddeviation of ±0.3 and ±1.02 mm, respectively. EPID is able to track the location of individual dose peaks for all the leafpairs consistently to a standard deviation of ±0.12 to ±0.47 mm, slightly better than that of ±1.00 to ±1.26 mm for thefilm.Conclusion: EPIs were found to be a quick and reliable alternative to film for qualitative and relative analyses. AlsoEPID is as good as the film in resolving the band pattern of the DMLC test field. Although the resolution of the EPIDis lower than that of the film, it is high enough to faithfully reproduce the band pattern without significant distortion.Since EPID images can be acquired, analyzed and stored much more conveniently than film, EPID is a good alternativeto film for routine DMLC QA.

IMRT in a Pregnant Patient: How to Reduce the Fetal Dose?

The purpose of our study was to find a solution for fetal dose reduction during head-and-neck intensity modulated radiation therapy (IMRT) of a pregnant patient. The first step was optimization of the IMRT treatment plan with as few monitor units (MUs) as possible, while maintaining an acceptable dose distribution. The peripheral dose originating from the final IMRT plan was measured at distances reaching from the most proximal to the most distal fetal position, along the accelerator's longitudinal axis, using an anthropomorphic phantom extended with water-equivalent plastic. The measured peripheral dose was divided into leakage, and internal and collimator scatter, to find the degree to which each component influences the peripheral dose to build an appropriate shield. Collimator scatter was the greatest contributor to the peripheral dose throughout the range of the growing fetus. A shield was built and placed beneath the accelerator head, extending caudally from the field edge, to function as an extra collimator jaw. This shield reduced the fetal dose by a factor of 3.5. The peripheral dose components were also measured for simple rectangular fields and also here the collimator scatter was the greatest contributor to the peripheral dose. Therefore, the shielding used for the IMRT treatment of our patient could also be used when shielding in conventional radiotherapy. It is important for a radiation therapy department to be prepared for treatment of a pregnant patient to shield the fetus efficiently.

飽和モデルによる矩形照射野のコリメータ散乱係数Ｓｃの推定

We estimated collimator scatter factor, S(c), of symmetric rectangular fields of any size by applying a two-component scatter model to measured in-air output data in width and length directions of measured rectangles. The in-air output was measured for symmetric rectangles with combined width and length sizes of 7 x 7 and 6 x 6 using 10 MV and 4 MV X-rays of Varian's Clinac 2100 C/D, respectively. The model consists of scatter components from the primary collimator and flattening filter and from the collimator jaws: the former shows a saturation curve and the latter increases linearly with enlarging field size. This model was fitted to the measured dataset firstly in the width and secondly in the length directions of rectangles; then, by compiling interpolated matrix data, the S(c) table of symmetric rectangles was constructed. In addition, using this S(c) table, values of S(c) were calculated for a few asymmetric rectangles by Day's method, and were in good agreement with measured values. Therefore, we think that our method is practical and precise for constructing the S(c) table of symmetric rectangles from measured data, and that using this table, the S(c) of any asymmetric rectangles may be calculated.

Comparision of beam data requirements for MLC commissioning on a TPS

Comparision of beam data requirements for MLC commissioning on a TPS The treatment planning system (TPS) has become a key element in the radiotherapy process with the introduction of computer tomography (CT) based 3D conformal treatment planning. Commissioning of a MLC on a TPS either for conformal radiotherapy or intensity modulated radiation therapy (IMRT) requires beam data to be generated on a linear accelerator. Most of the TPS require these beam data to be generated with routine collimator jaws. However some TPS demand the data to be provided for MLC shaped fields. This prompted us to investigate whether beam data with jaws differ than that with MLC and whether the jaw based beam data would suffice for the commissioning of a MLC on a TPS. Beam data like percentage depth dose (PDD), cross beam profiles and output factors was acquired for jaws and MLC defined square fields for 6, 10 and 23 MV photon beams. Percentage depth dose and cross beam profiles were acquired with a radiation field analyzer RFA-200, CC13-S ion chambers with active volume of 0.13 cm3 and OmniPro-Accept software from Scanditronix-Wellhofer. A Medtec-TG51 water tank with Max-4000 electrometer and 0.6 cc PTW ionization chamber and a mini phantom from Standard Imaging was utilized for output measurements for millennium-120 MLC (Varian Medical Systems) and SRS diode detector (Scanditronix-Wellhofer) of 0.6 mm diameter of active area and 0.3 mm of active volume thickness for micro-MLC (BrainLab). The difference in PDD in the build-up region for millennium MLC was ±1.0% for 6 MV photons. For 10 MV photons the PDD difference was within ±4.0%. The difference in PDD for 23 MV photons ranged from 0% to 40.0%. PDD difference from build-up depth to about 28 cm was within ±1.0%. Difference in PDD crossed ±1.0% at 30 cm depth for 6 MV photons. The difference in PDD in the build-up region for mMLC was ±8.0% for 6 MV photons. For the smallest field size studied with micro-MLC i.e. 0.6 × 0.6 cm2 difference in PDD was more than ±1.0% in the build-up region and beyond a depth of 8.0 cm. The profiles for jaws and MLC agreed within the umbra region. However in the penumbra region small differences in doses were observed. The collimator scatter factor (Sc), phantom scatter factor (Sp) and output factor values for MLC were different that those for jaws. The differences in beam characteristics could have implication for intensity modulated radiation therapy and stereotactic radiosurgery in terms of dose in the build up region, exit dose, dose to the planning target volume (PTV) and organ at risk (OAR). Impact of these dosimetric differences between jaw and MLC needs to be further studied in terms of dose volume histograms for PTV and OAR and its further impact on tumor control probability (TCP) and normal tissue complication probability (NTCP).

Dosimetric investigation of dual energy photon beams with assymmetric collimator jaws

Dosimetric investigation of dual energy photon beams with assymmetric collimator jaws Many modern linear accelerators are equipped with asymmetric collimators or jaws that can be moved independently. Asymmetric jaws have got many clinical applications in radiation therapy. In the present study, the dosimetric characteristics of asymmetric collimators from our linear accelerator with 6 and 18 MV X-rays were carried out. The field size factors (FSF) and half value layer (HVL) were measured in a water phantom using 0.6 cc Farmer chamber for symmetric and asymmetric fields for both 6 and 18 MV X-rays. Measurements of beam penumbra, percentage depth dose (PDD), cross beam profiles and calculated isodose curves were measured by RFA 300 for both asymmetric and symmetric fields. The FSF were found to agree with in 3% for symmetric and asymmetric fields. The HVL in water was found to be 15.8 cm and 14.4 cm for 6 MV photons and 26 cm and 22.9 cm for 18 MV photons at the central axis and at 20 cm off the central axis. At 30 cm depth the percentage depth dose for symmetric and asymmetric fields were found to differ as high as 6% for 6 MV and 4% for 18 MV fields. No observable difference in penumbra was noticed for symmetric and asymmetric fields of same dimensions. The constrictions of isodose curves at the edge nearer to central axis were noticed for asymmetrically placed fields. The observed differences could be due to the passage of primary beam through differential thickness of the flattening filter which alters the beam quality.

Leaf transmission reduction using moving jaws for dynamic MLC IMRT

The aim of this work is to investigate to what extent it is possible to use the secondary collimator jaws to reduce the transmitted radiation through the multileaf collimator (MLC) during an intensity modulated radiation therapy (IMRT). A method is developed and introduced where the jaws follow the open window of the MLC dynamically (dJAW method). With the aid of three academic cases (Closed MLC, Sliding-gap, and Chair) and two clinical cases (prostate and head and neck) the feasibility of the dJAW method and the influence of this method on the applied dose distributions are investigated. For this purpose the treatment planning system Eclipse and the Research-Toolbox were used as well as measurements within a solid water phantom were performed. The transmitted radiation through the closed MLC leads to an inhomogeneous dose distribution. In this case, the measured dose within a plane perpendicular to the central axis differs up to 40% (referring to the maximum dose within this plane) for 6 and 15 MV. The calculated dose with Eclipse is clearly more homogeneous. For the Sliding-gap case this difference is still up to 9%. Among other things, these differences depend on the depth of the measurement within the solid water phantom and on the application method. In the Chair case, the dose in regions where no dose is desired is locally reduced by up to 50% using the dJAW method instead of the conventional method. The dose inside the chair-shaped region decreased up to 4% if the same number of monitor units (MU) as for the conventional method was applied. The undesired dose in the volume body minus the planning target volume in the clinical cases prostate and head and neck decreased up to 1.8% and 1.5%, while the number of the applied MU increased up to 3.1% and 2.8%, respectively. The new dJAW method has the potential to enhance the optimization of the conventional IMRT to a further step.

Segmental MLC is superior to dynamic MLC for IMRT delivery

Segmental MLC is superior to dynamic MLC for IMRT delivery