Leakage-Penumbra effect in intensity modulated radiation therapy step-and-shoot dose delivery.

AIM: To study the leakage-penumbra (LP) effect with a proposed correction method for the step-and-shoot intensity modulated radiation therapy (IMRT). METHODS: Leakage-penumbra dose profiles from 10 randomly selected prostate IMRT plans were studied. The IMRT plans were delivered by a Varian 21 EX linear accelerator equipped with a 120-leaf multileaf collimator (MLC). For each treatment plan created by the Pinnacle(3) treatment planning system, a 3-dimensional LP dose distribution generated by 5 coplanar photon beams, starting from 0(o) with equal separation of 72(o), was investigated. For each photon beam used in the step-and-shoot IMRT plans, the first beam segment was set to have the largest area in the MLC leaf-sequencing, and was equal to the planning target volume (PTV). The overshoot effect (OSE) and the segment positional errors were measured using a solid water phantom with Kodak (TL and X-OMAT V) radiographic films. Film dosimetric analysis and calibration were carried out using a film scanner (Vidar VXR-16). The LP dose profiles were determined by eliminating the OSE and segment positional errors with specific individual irradiations. RESULTS: A non-uniformly distributed leaf LP dose ranging from 3% to 5% of the beam dose was measured in clinical IMRT beams. An overdose at the gap between neighboring segments, represented as dose peaks of up to 10% of the total BP, was measured. The LP effect increased the dose to the PTV and surrounding critical tissues. In addition, the effect depends on the number of beams and segments for each beam. Segment positional error was less than the maximum tolerance of 1 mm under a dose rate of 600 monitor units per minute in the treatment plans. The OSE varying with the dose rate was observed in all photon beams, and the effect increased from 1 to 1.3 Gy per treatment of the rectal intersection. As the dosimetric impacts from the LP effect and OSE may increase the rectal post-radiation effects, a correction of LP was proposed and demonstrated for the central beam profile for one of the planned beams. CONCLUSION: We concluded that the measured dosimetric impact of the LP dose inaccuracy from photon beam segment in step-and-shoot IMRT can be corrected.

Effect of the bone heterogeneity on the dose prescription in orthovoltage radiotherapy: A Monte Carlo study.

BACKGROUND: In orthovoltage radiotherapy, since the dose prescription at the patient's surface is based on the absolute dose calibration using water phantom, deviation of delivered dose is found as the heterogeneity such as bone present under the patient's surface. AIM: This study investigated the dosimetric impact due to the bone heterogeneity on the surface dose in orthovoltage radiotherapy. MATERIALS AND METHODS: A 220 kVp photon beam with field size of 5 cm diameter, produced by a Gulmay D3225 orthovoltage X-ray machine was modeled by the BEAMnrc. Phantom containing water (thickness = 1-5 mm) on top of a bone (thickness = 1 cm) was irradiated by the 220 kVp photon beam. Percentage depth dose (PDD), surface dose and photon energy spectrum were determined using Monte Carlo simulations (the BEAMnrc code). RESULTS: PDD results showed that the maximum bone dose was about 210% higher than the surface dose in the phantoms with different thicknesses of water. Surface dose was found to be increased in the range of 2.5-3.7%, when the distance between the phantom surface and bone was increased in the range of 1-5 mm. The increase of surface dose was found not to follow the increase of water thickness, and the maximum increase of surface dose was found at the thickness of water equal to 3 mm. CONCLUSIONS: For the accepted total orthovoltage radiation treatment uncertainty of 5%, a neglected consideration of the bone heterogeneity during the dose prescription in the sites of forehead, chest wall and kneecap with soft tissue thickness = 1-5 mm would cause more than two times of the bone dose, and contribute an uncertainty of about 2.5-3.7% to the total uncertainty in the dose delivery.

Dosimetry of a small air cavity for clinical electron beams: A Monte Carlo study.

This study investigated dosimetric changes in a water phantom when a small air cavity was presented at the central axis of a clinical electron beam. We used 6-, 9-, and 16-MeV electron beams with a 10 x 10 cm(2) applicator and cutout produced by a Varian 21 EX linear accelerator. Percentage depth doses (PDDs) for different depths (0.5-7 cm), thicknesses (2-10 mm), and widths (1-5 cm) of air cavities were calculated using Monte Carlo simulations (EGSnrc code) validated by film measurements. By comparing PDDs of phantoms with and without the air cavity, it was found that when the depth or thickness of cavity was changed, the PDD curve below the cavity was shifted with a distance equal to the thickness of the cavity. However, when the width of the air cavity was changed, both the PDD curve and its slope within and below the cavity were changed. A larger width of the air cavity resulted in a shallower PDD curve within the cavity. The slope of the PDD curve below the cavity tended towards a value as the width of the air cavity was increased to 3-5 cm for the 6-, 9-, and 16-MeV electron beams. The dependence of the depth dose on the width of the air cavity is a result of the contribution of the electron side scattering in the water surrounding the cavity. The change in depth dose resulting from the presence of an air cavity can cause discrepancies between the calculated and actual dose during radiotherapy, unless the effects of the air cavity are properly characterized during treatment planning. From the dosimetry data in this study, neglecting an air cavity of 1-cm thickness in the build-up region of a 6-MeV electron beam resulted in a delivered dose 10-12% larger than the original prescription. Delivered doses 3% and 6% higher than the prescribed dose were observed when doses were prescribed at R(80) for a 16-MeV electron beam. These results were obtained by neglecting air cavities with thicknesses equal to 2 and 4 mm, respectively, at a depth of 5 cm.

Dosimetric dependence of the dimensional characteristics on a lead shield in electron radiotherapy: a Monte Carlo study.

This study investigates the dosimetric dependence of the dimension of a lead (Pb) layer for shielding using clinical electron beams with different energies. Monte Carlo simulations were used to generate phase space files of the 4, 9 and 16 MeV electron beams produced by a Varian 21 EX linear accelerator using the EGSnrc-based BEAMnrc code, and validated by measurements using films. Pb layers with different thicknesses (2, 4, 6 and 8 mm) and diameters (2.5, 3, 3.5 and 4 cm) were placed at the center of an electron field on a Solid Water phantom. Beam profiles were determined at the depth of maximum dose (d(m)) using Monte Carlo simulations. The line doses under the Pb layer at d(m) including the penumbra at the edge of the layer and relative dose at the central beam axis (CAX) were studied with varying thickness and diameter of the layer. It is found that 2 mm of Pb layer is adequate to provide 5 half value layer (HVL) attenuation for the 4 MeV electron beams, and the beam profiles at dm under the Pb layer depend on the diameter but the thickness of the Pb. However, for the 9 and 16 MeV electron beams, the relative dose at the CAX and dm depends on both the thickness and diameter of the Pb layer. For 8 mm thickness of Pb, 4 and 5 HVL attenuation of electron beams with energies of 9 and 16 MeV can be achieved at d(m), respectively. Moreover, the beam profile under the Pb layer at dm depends on (1) the penumbra region at the edge of the Pb layer, (2) the beam attenuation varying with the thickness of the Pb layer, (3) the electron side scatter contributing to the CAX under the Pb layer, and (4) the photon contamination produced by the Pb layer. A parameter called "shielding area factor", defined as the ratio of the length between two points of 50% relative doses in the beam profile at dm to the diameter of the Pb layer, is suggested to the radiation oncology staff to predict the required size and thickness of Pb for shielding a target with known dimension at d(m). The dosimetric data calculated by Monte Carlo simulations in this study is useful to select what thickness and size of Pb are suitable for the protection of critical tissue in electron radiotherapy.

Monte Carlo simulation of backscatter from lead for clinical electron beams using EGSnrc.

In electron radiotherapy of superficial lesions in the eyelid, lip, buccal mucosa, ear, and nose, backscattered electrons are produced from the lead shield used to protect the critical tissue underneath the tumor. In this study, the backscattered electrons, produced by clinical electron beams using a Varian 21 EX linear accelerator, were studied using Monte Carlo simulations. The electron backscatter factor (EBF), defined as the ratio of dose at the tissue-lead interface to the dose at the same point without the presence of backscatter, was calculated using the Monte Carlo EGSnrc-based code. The calculated EBFs were verified with measurements using metal-oxide-semiconductor field effect transistor detectors. The effect of the (1) initial electron beam energy, (2) thickness of bolus over the lead shield, (3) beam's angle of incidence, and (4) presence of an aluminum sheet used to absorb backscattered electrons, on the EBF, were studied. It is found that for lead shielding positioned at any fixed depth, the EBF decreases with an increase in initial electron beam energy (4-16 MeV). In addition, for depths within the electron practical range, Rp, and at a particular beam energy, the EBF increases with depth (or thickness of the treatment volume). When the electron beam angle increases from 0 degrees to 5 degrees, the EBF only decreases slightly (<4%) for all energies. The influence of the beam obliquity on the EBF is important when the treatment surface is not flat and perpendicular to the central beam axis. The use of an aluminum sheet to reduce backscattered electrons was also investigated. For a relatively low electron beam energy (4 MeV), a 2 mm aluminum sheet can reduce backscattering by 31%. While the electron beam energy increased, less backscattered electrons were produced and therefore removed by the same thickness of aluminum (only about 6% for 16 MeV). The Monte Carlo calculated EBFs from this study, characterized by the electron beam energy, depth of bolus above the lead shield, beam obliquity, and presence of an aluminum sheet, may provide important clinical information for radiation oncology staff when considering the effect of electron backscatter on radiotherapy using internal shielding.

Surface dosimetry for oblique tangential photon beams: a Monte Carlo simulation study.

The effect of beam obliquity on the surface relative dose profiles for the tangential photon beams was studied. The 6 and 15 MV photon beams with 4 x 4 and 10 x 10 cm2 field sizes produced by a Varian 21 EX linear accelerator were used. Phase-space models of the photon beams were created using Monte Carlo simulations based on the EGSnrc code, and were verified using film measurements. The relative dose profiles in the phantom skin, at 2 mm depth from the surface of the half-phantom geometry, or HPG, were calculated for increasing gantry angles from 270 to 280 deg clockwise. Relative dose profiles of a full phantom enclosing the whole tangential beam (full phantom geometry, or FPG) were also calculated using Monte Carlo simulation as a control for comparison. The results showed that, although the relative dose profiles in the phantom skin did not change significantly with an oblique beam using a FPG, the surface relative depth dose was increased for the HPG. In the HPG, with 6 MV photon beams and field size = 10 x 10 cm2, when the beam angle, starting from 270 deg, was increased from 1 to 3 deg, the relative depth doses in the phantom skin were increased from 68% to 79% at 10 cm depth. This increase in dose was slightly larger than the dose from 15 MV photon beams with the same field size and beam angles, where the relative depth doses in phantom skin were increased from 81% to 87% at 10 cm depth. A parameter called the percent depth dose (PDD) ratio, defined as the relative depth dose from the HPG to the relative depth dose from the FPG at a given depth along the phantom skin, was used to evaluate the effect of the phantom-air interface. It is found that the PDD ratio increased significantly when the beam angle was changed from zero to 1-3 degrees. Moreover, the PDD ratio, for a given field size, experienced a greater increase for 6 MV than for 15 MV. For the same photon beam energy, the PDD ratio increased more with a 4 x 4 cm2 field compared to 10 x 10 cm2. The results in this study will be useful for physicists and dosimetrists to predict the surface relative dose variations when using clinical tangential-like photon beams in radiation therapy.

Effect of electron beam obliquity on lateral buildup ratio: a Monte Carlo dosimetry evaluation.

The impact of the oblique electron beam on the lateral buildup ratio (LBR), used in the electron pencil beam model to predict the per cent depth dose (PDD) and dose per monitor unit (MU) for an irregular electron field, was examined using Monte Carlo simulation. The EGSnrc-based Monte Carlo code was used to model electron beams produced by a Varian 21 EX linear accelerator for different beam energies, angles of obliquity and field sizes. The Monte Carlo phase space model was verified by measurements using electron diode and radiographic film. For PDDs of oblique electron beams, it is found that the depth of maximum dose (d(m)) shifts towards the surface as the beam obliquity increases. Moreover, for increasing the beam angle of obliquity, the depth doses just beyond d(m) decrease with depth. The depth doses then increase eventually in a deeper depth close to the practical range. The LBRs and pencil beam radial spread function, calculated using PDDs with different field sizes, are found varying with electron beam energies, angles of obliquity and cutout diameters. It is found that LBR increases along the normalized depth when the beam angle of obliquity increases. This results in a decrease of the radial spread function with an increase of beam obliquity. When the size of the electron field increases, the variation of LBR with beam angle of obliquity decreases. It should be noted that when calculating dose per MU for an oblique electron beam with an irregular field misunderstanding and neglecting the effect of beam obliquity would lead to a significant deviation. A database of LBRs for oblique electron beams can be created using Monte Carlo simulation conveniently and is recommended when an oblique beam is used in electron radiotherapy.

Electron radiotherapy: a study on dosimetric uncertainty using small cutouts.

This note investigated the dosimetric uncertainties due to the positional error when centring a small cutout to the machine central beam axis (CAX) in electron radiotherapy. A group of six circular cutouts with 4 cm diameter were made with their centres shifting 0, 2, 4, 6, 8 and 10 mm from the machine CAX for the 6 x 6 cm(2) applicator. The per cent depth doses, beam profiles and output factors were measured using the 4, 9 and 16 MeV clinical electron beams produced by a Varian 21 EX linear accelerator. The 2D isodose distributions in the z-x (or cross-line) and z-y (or in-line) plane were calculated by Monte Carlo simulation using the EGSnrc system. When the cutout centre was shifted away from the machine CAX for the 4 MeV beam, the d(m), R(80) and R(90) at the machine CAX had no significant change (<0.1 mm). For higher energies of 9 and 16 MeV beams, the d(m) was reduced by 0.45 and 1.63 mm per mm, between the cutout centre and the machine CAX with off-axis shift <6 mm respectively. R(80) and R(90) were reduced by more than 0.3 mm per mm off-axis shift for both energies. The isodose coverage of the in-line axis beam profile was reduced when the cutout centre was shifted away from machine CAX. It is important for oncology staff to note such dosimetric changes in the clinical electron radiotherapy, particularly when a high energy electron beam is used for small cutout. Such positional uncertainty is unavoidable in fabricating an electron cutout in the mould room.

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment.

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.

Measurement for the MLC leaf velocity profile by considering the leaf leakage using a radiographic film.

A method to measure the velocity profile of a multi-leaf collimator (MLC) leaf along its travel range using a radiographic film is reported by considering the intra-leaf leakage. A specific dynamic MLC field with leaves travelling from the field edge to the isocentre line was designed. The field was used to expose a radiographic film, which was then scanned, and the dose profile along the horizontal leaf axis was measured. The velocity at a sampling point on the film can be calculated by considering the horizontal distance between the sampling point and the isocentre line, dose at the sampling point, dose rate of the linear accelerator, the total leaf travel time from the field edge to isocentre line and the pre-measured dose rate of leaf leakage. With the leaf velocities and velocity profiles for all MLC leaves measured routinely, a comprehensive and simple QA for the MLC can be set up to test the consistency of the leaf velocity performance which is essential to the IMRT delivery using a sliding window technique.

Peripheral dose outside applicators in electron beams.

The peripheral dose outside the applicators in electron beams was studied using a Varian 21 EX linear accelerator. To measure the peripheral dose profiles and point doses for the applicator, a solid water phantom was used with calibrated Kodak TL films. Peak dose spot was observed in the 4 MeV beam outside the applicator. The peripheral dose peak was very small in the 6 MeV beam and was ignorable at higher energies. Using the 10 x 10 cm(2) cutout and applicator, the dose peak for the 4 MeV beam was about 12 cm away from the field central beam axis (CAX) and the peripheral dose profiles did not change with depths measured at 0.2, 0.5 and 1 cm. The peripheral doses and profiles were further measured by varying the angle of obliquity, cutout and applicator size for the 4 MeV beam. The local peak dose was increased with about 3% per degree angle of obliquity, and was about 1% of the prescribed dose (angle of obliquity equals zero) at 1 cm depth in the phantom using the 10 x 10 cm(2) cutout and applicator. The peak dose position was also shifted 7 mm towards the CAX when the angle of obliquity was increased from 0 to 15 degrees.

IMRT: improvement in treatment planning efficiency using NTCP calculation independent of the dose-volume-histogram.

The normal tissue complication probability (NTCP) is a predictor of radiobiological effect for organs at risk (OAR). The calculation of the NTCP is based on the dose-volume-histogram (DVH) which is generated by the treatment planning system after calculation of the 3D dose distribution. Including the NTCP in the objective function for intensity modulated radiation therapy (IMRT) plan optimization would make the planning more effective in reducing the postradiation effects. However, doing so would lengthen the total planning time. The purpose of this work is to establish a method for NTCP determination, independent of a DVH calculation, as a quality assurance check and also as a mean of improving the treatment planning efficiency. In the study, the CTs of ten randomly selected prostate patients were used. IMRT optimization was performed with a PINNACLE3 V 6.2b planning system, using planning target volume (PTV) with margins in the range of 2 to 10 mm. The DVH control points of the PTV and OAR were adapted from the prescriptions of Radiation Therapy Oncology Group protocol P-0126 for an escalated prescribed dose of 82 Gy. This paper presents a new model for the determination of the rectal NTCP (R(NTCP)). The method uses a special function, named GVN (from Gy, Volume, NTCP), which describes the R(NTCP) if 1 cm3 of the volume of intersection of the PTV and rectum (R(int)) is irradiated uniformly by a dose of 1 Gy. The function was "geometrically" normalized using a prostate-prostate ratio (PPR) of the patients' prostates. A correction of the R(NTCP) for different prescribed doses, ranging from 70 to 82 Gy, was employed in our model. The argument of the normalized function is the R(int), and parameters are the prescribed dose, prostate volume, PTV margin, and PPR. The R(NTCPs) of another group of patients were calculated by the new method and the resulting difference was < +/- 5% in comparison to the NTCP calculated by the PINNACLE3 software where Kutcher's dose-response model for NTCP calculation is adopted.

A graphical user interface for an electron monitor unit calculator using a sector-integration algorithm and exponential curve-fitting method.

A new electron monitor unit (MU) calculator program called "eMUc" was developed to provide a convenient electron MU calculation platform for the physics and radiotherapy staff in electron radiotherapy. The program was written using the Microsoft Visual Basic.net framework and has a user-friendly front-end window with the following features: (1) Apart from using the well-known polynomial curvefitting method for the interpolation and extrapolation of relative output factors (ROFs), an exponential curve-fitting method was used to obtain better results. (2) A new algorithm was used to acquire the radius in each angular segment in the irregular electron field during the sector integration. (3) A comprehensive graphical user interface running on the Microsoft Windows operating system was used. (4) Importing irregular electron cutout field images to the calculator program was simplified by using only a commercial optical scanner. (5) Interlocks were provided when the input patient treatment parameters could not be handled by the calculator database accurately. (6) A patient treatment record could be printed out as an electronic file or hard copy and transferred to the patient database. The data acquisition mainly required ROF measurements using various circular cutouts for all the available electron energies and applicators for our Varian 21 EX linear accelerator. To verify and implement the calculator, the measured results using our specific designed irregular and clinical cutouts were compared to those predicted by the calculator. Both agreed well with an error of +/-2%.

Dosimetry limitations and a dose correction methodology for step-and-shoot IMRT.

For the step-and-shoot intensity-modulated radiation therapy (IMRT) technique, the combination of high dose rate, multiple beam segments and low dose per segment can lead to significant differences between the planned dose and the dose delivered to the patient. In this technique, a dose delivery inaccuracy known as the 'overshoot' effect is caused by the dose servo control system. This typically occurs in the first and last beam segments and causes an over- and underdose, respectively. Some dose positional error in the segment sequence is also possible there. Commercial ionization chambers (RK-type) and radiographic Kodak films were used for the measurements. The reported results were obtained using the Pinnacle(3)-V6.2 treatment planning system and a Varian Clinac 21 EX linear accelerator equipped with a 120-leaf Millennium MLC. The dose inaccuracy measurements were based on the comparison of the dose and profiles for reference fields and fields irradiated with the step-and-shoot technique. For our linear accelerators, an 'overshoot' effect ranging from 0.1 to 0.6 MU was found, corresponding to a dose rate from 100 to 600 MU min(-1), respectively. For segments with off-axis distances from 0 to 5.5 cm with >3.5 MU per segment and all dose rates, a MLC leaf-position error of <1 mm was measured. For segments with an off-axis distance of 9.5 cm, a positional error >2 mm was measured for 600 MU min(-1) and 1 MU per segment. The purpose of this study was to find a correction method for segmental dose errors caused by the 'overshoot' effect when small monitor unit and high dose rate are used. To better represent the fluctuation of the segment doses in the beam, a dose ratio between reference and step-and-shoot irradiated fields was defined. A method for the correction of segment dose inaccuracies and a quality assurance programme for the 'overshoot' effect were developed. The ordering of the biggest segment shape in the segment sequence was studied for ten randomly selected prostate patients planned for IMRT. The results of this work can be used to improve the agreement between the planned and delivered doses for IMRT.

SWIMRT: a graphical user interface using sliding window algorithm to construct a fluence map machine file.

A custom-made computer program, SWIMRT, to construct "multileaf collimator (MLC) machine" file for intensity-modulated radiotherapy (IMRT) fluence maps was developed using MATLAB and the sliding window algorithm. The user can either import a fluence map with a graphical file format created by an external treatment-planning system such as Pinnacle3 or create his or her own fluence map using the matrix editor in the program. Through comprehensive calibrations of the dose and the dimension of the imported fluence field, the user can use associated image-processing tools such as field resizing and edge trimming to modify the imported map. When the processed fluence map is suitable, a "MLC machine" file is generated for our Varian 21 EX linear accelerator with a 120-leaf Millennium MLC. This machine file is transferred to the MLC console of the LINAC to control the continuous motions of the leaves during beam irradiation. An IMRT field is then irradiated with the 2D intensity profiles, and the irradiated profiles are compared to the imported or modified fluence map. This program was verified and tested using film dosimetry to address the following uncertainties: (1) the mechanical limitation due to the leaf width and maximum traveling speed, and (2) the dosimetric limitation due to the leaf leakage/transmission and penumbra effect. Because the fluence map can be edited, resized, and processed according to the requirement of a study, SWIMRT is essential in studying and investigating the IMRT technique using the sliding window algorithm. Using this program, future work on the algorithm may include redistributing the time space between segmental fields to enhance the fluence resolution, and readjusting the timing of each leaf during delivery to avoid small fields. Possible clinical utilities and examples for SWIMRT are given in this paper.

Intensity modulated radiation therapy with irregular multileaf collimated field: a dosimetric study on the penumbra region with different leaf stepping patterns.

Using a Varian 21 EX linear accelerator with a multileaf collimator (MLC) of 120 leaves, the penumbra regions of beam profiles within an irregular multileaf collimated fields were studied. MLC fields with different leaf stepping angles from 21.8 degrees to 68.2 degrees were used. Beam profiles in different directions: (1) along the cross-line and in-line axis, (2) along the leaf stepping edges of the field, and (3) parallel to the stepping edges but in the middle of the field, were measured and calculated using Kodak XV radiographic film and Pinnacle3 treatment planning system version 7.4f. These beam profiles were measured and calculated at source to axis distance= 100 cm with 5 cm of solid water slab on top. On the one hand, for both cross-line and in-line beam profiles, the penumbra widths of 20%-80% did not vary with the leaf stepping angles and were about 0.4 cm. On the other hand, the penumbra widths of 10%-90% of the above two profiles varied with the stepping angles and had maximum widths of about 1.9 cm (cross-line) and 1.65 cm (in-line) for stepping angles of 38.7 degrees and 51.3 degrees , respectively. For profiles crossing the "rippled" stepping edges of the field, the penumbra widths (10%-90%) at the regions between two opposite leaves (i.e., profile end at the Y1/Y2 jaw position) decreased with the stepping angles. At the penumbra regions between two leaf edges with the tongue-and-groove structure of the same bank (i.e., profile end at the X1/X2 jaw position), the penumbra widths increased with the stepping angles. When the penumbra widths were measured between two opposite leaf edges and at corners between two leaves, the widths first decreased with the stepping angles and then increased beyond the minimum width point at stepping angle of 45 degrees. The penumbra width (10%-90%) measured at the leaf edge was larger than that at the corner. For the beam profiles calculated using Pinnacle3, it is found that the results agreed well with the measurements along the cross-line and in-line axis, while there was a deviation for the profiles along the leaf stepping edge of the field compared to the film measurements. The measured results in this study can help us to understand the dosimetric effect of the leaf stepping (due to finite leaf width), tongue-and-groove and rounded leaf end structure in the penumbra region of an irregular MLC field. A more dedicated penumbra model can be developed for the treatment planning system.

Monte Carlo simulations of dose near a nonradioactive gold seed.

The relative doses and hot/cold spot positions around a non-radioactive gold seed, irradiated by a 6 or 18 MV photon beam in water, were calculated using Monte Carlo simulation. Phase space files of 6 and 18 MV photon beams with a field size of 1 x 1 cm2 were generated by a Varian 21 EX linear accelerator using the EGSnrc and BEAMnrc code. The seed (1.2 x 1.2 x 3.2 mm3) was positioned at the isocenter in a water phantom (20 x 20 x 20 cm2) with source-to-axis distance = 100 cm. For the single beam geometry, the relative doses (normalized to the dose at 5 mm distance above the isocenter) at the upstream seed surface were calculated to be 1.64 and 1.56 for the 6 and 18 MV beams respectively when the central beam axis (CAX) is parallel to the width of the seed. These doses were slightly higher than those (1.58 and 1.52 for 6 and 18 MV beams respectively) calculated when the CAX is perpendicular to the width of the seed. Compared to the relative dose profiles with the same beam geometry without the seed in the water phantom, the presence of the seed affects the dose distribution at about 3 mm distance beyond both the upstream and downstream seed surface. For a pair of opposing beams with equal and unequal beam weight, the hot and cold spots of both opposing beams were mixed. For a 360 degree photon arc around the longitudinal axis of the seed, the relative dose profile along the width of the seed was similar to that of the opposing beam pair, except the former geometry has a larger dose gradient near the seed surface. In this study, selected results from our simulation were compared to previous measurements using film dosimetry.

Dose measurements near a non-radioactive gold seed using radiographic film.

The dose distribution near a non-radioactive gold seed under a 6 MV photon beam was measured using radiographic film, water equivalent bolus and solid water slabs. This type of small seed is typically used as a marker in target positional verification using a portal imager for conformal prostate treatment such as intensity modulated radiation therapy. A stack of three films was placed on top of the seed located on a soft bolus. Solid water slabs were then placed on top of the film. The films were exposed using a small 1x1 cm2 field. Then, using a similar experimental set-up and exposure, another stack of three films was placed under the seed, which was then covered by the soft bolus and solid water slabs. The cross-plane axial beam profiles at different depths, depending on the thickness of the film package, were measured. From the group of beam profiles above and below the seed, the dose distribution along a selected vertical line within the profiles was easily plotted. Compared to the dose with no seed at the isocentre and 5 cm of solid water, there was about a 21% increase in dose at 0.35 mm above the seed. On the other hand, there was about a 22% decrease in dose at the same distance below the seed. The dosimetry of the calibrated film was verified with a MOSFET detector. The change in dose due to the seed by varying the incident beam angles was also measured for this note.