James C L Chow

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 18MV photon beam in water, were calculated using Monte Carlo simulation. Phase space files of 6 and 18MV photon beams with a field size of 1×1cm2 were generated by a Varian 21 EX linear accelerator using the EGSnrc and BEAMnrc code. The seed (1.2×1.2×3.2mm3) was positioned at the isocenter in a water phantom (20×20×20cm2) with source-to-axis distance=100cm. For the single beam geometry, the relative doses (normalized to the dose at 5mm distance above the isocenter) at the upstream seed surface were calculated to be 1.64 and 1.56 for the 6 and 18MV 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 18MV 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 3mm 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 360degree 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.

SWIMRT: A graphical user interface using the 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. PACS numbers: 87.53.Kn, 87.53.St, 87.53.Uv

A carbon nanotube-based radiation sensor

Dosimetric measurements and monitoring play an essential role in radiotherapy. Because of their sensitivity and relatively flat energy response ionization chambers remain the most important dosimeters. However, ionization chambers usually have large physical dimensions and require high bias voltages to achieve acceptable ionization collection efficiency. Such disadvantages limit their applications for in vivo dose measurements. The availability of novel materials such as carbon nanotubes (CNTs) has created the potential to miniaturize traditional ionization chambers and lower the bias voltages. This paper describes a new CNT-based radiation sensor. In the first stage, characteristics of the sensor were examined with two stainless steel electrodes. The sensor displayed excellent linear responses to exposure and showed accurate responses to oblique incident beam measurements. These experimental results showed that the prototype sensor is suitable for studying the ionization collection efficiency of CNTs. In the second stage, square- and irregular-shaped CNTs electrodes were designed. Saturation characteristics of the sensor with the CNTs electrodes were measured. Experimental results and ongoing work are presented and discussed in this paper.

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.

Experimental verification of the application of lateral buildup ratio on the 4-MeV electron beam.

The lateral buildup ratio (LBR) used to estimate the depth dose distribution of electron beams for an irregular cutout field was obtained for a 4‐MeV energy beam from a Varian 21 EX linear accelerator. The depth‐dose curves for a group of circular cutout fields starting from a 2‐cm diameter were measured. Electron diodes were used in a large water tank to measure the LBR values for 6, 9, 12, and 16 MeV electron beam energies and a 10×10cm2 applicator. The results agreed with the published data. When the same equipment, setup, and technique were used to determine the LBR values for the 4‐MeV energy beam, the values were only reasonable, being within the clinical treatment range (i.e., LBR <1) for the smallest 6×6cm2 applicator. The calculated LBR values were clinically unacceptable for the circular cutout fields with a diameter larger than 2 cm with the 10×10cm2 applicator. The difficulty in the LBR measurement may be due to the significant contribution of scattered electrons from the beam defining system. This study also focused on how well the sigma values for the 4‐MeV beam can predict depth‐dose curves for other field sizes and whether the values are applicator‐dependent. PACS numbers: 87.53.Fs; 87.53.Hv; 87.66.Jj

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.

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.

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 curve‐fitting 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%. PACS number(s): 87.53.Fs; 87.53.Hv; 87.66.‐a

Sci‐AM1 Sat ‐ 06: Improved absorbed dose calculations incorporating internal organ motion

The goal of radiation therapy is to deliver a highly conformal dose to a prescribed target volume and to spare surrounding healthy tissue as much as possible. Present commercial dose planning systems assume that patients anatomy is static over the course of treatment. During treatmentdelivery, however, dosimetric uncertainties arising from patient repositioning and internal organ motion are unavoidable practically. The purpose of this study is to evaluate the effect of prostate motion on the physical dose distribution by PTV ¯ 7 model based on the Pinnacle treatment planning system. Prostate motion, within the PTV, was represented by a weighted average of seven individually shifted PTVs ( PTV ¯ 7 ). As already well known, internal organ motion always leads to blurred contour surfaces. The dose coverage of PTV and critical organs is less as indicated by dose at “edge” of contours and also by decreased DVH particularly at high dose region. The averaged decrease of TCP between the static planning and PTV ¯ 7 model is 2.9%, and the rectum is spared if motion is equally weighted and symmetric. The PTV ¯ 7 configurations yield a better estimate of the actual dose in the rectal wall with decreasing NTCP. The effects of different shifting weight to TCP and NTCP in L‐R, A‐P and S‐I directions were also quantitatively analyzed. The calculation of the cumulative dose incorporating internal organ motion plays an important role in pursuing adaptive radiation therapy and dose escalation for IMRT with the goal of decreasing the dosedelivered to the normal critical structures.