D. Findley

Clinical implementation of a Monte Carlo treatment planning system

The purpose of this study was to implement the Monte Carlo method for clinical radiotherapy dose calculations. We used the EGS4/BEAM code to obtain the phase-space data for 6-20 MeV electron beams and 4, 6, and 15 MV photon beams for Varian Clinac 1800, 2100C, and 2300CD accelerators. A multiple-source model was used to reconstruct the phase-space data for both electron and photon beams, which retained the accuracy of the Monte Carlo beam data. The multiple-source model reduced the phase-space data storage requirement by a factor of 1000 and the accelerator simulation time by a factor of 10 or more. Agreement within 2% was achieved between the Monte Carlo calculations and measurements of the dose distributions in homogeneous and heterogeneous phantoms for various field sizes, source-surface distances, and beam modulations. The Monte Carlo calculated electron output factors were within 2% of the measured values for various treatment fields while the heterogeneity correction factors for various lung and bone phantoms were within 1% for photon beams and within 2% for electron beams. The EGS4/DOSXYZ Monte Carlo code was used for phantom and patient dose calculations. The results were compared to the dose distributions produced by a conventional treatment planning system and an intensity-modulated radiotherapy inverse-planning system. Significant differences (>5% in dose and >5 mm shift in isodose lines) were found between Monte Carlo calculations and the analytical calculations implemented in the commercial systems. Treatment sites showing the largest dose differences were for head and neck, lung, and breast cases.

Physical characteristics of a miniature multileaf collimator

A preliminary study of the physical characteristics of a miniature multileaf collimator (mMLC) used with 4 MV x rays is reported. The mMLC attached to the accessory mount of a class C or D Varian linear accelerator (Varian Oncology Systems, Palo Alto, CA) with a source to aperture distance of 65 cm. The field penumbra using the small leaves was found to be consistent with the anticipated field penumbra using photon jaws at the same source to aperture distance as the mMLC. The percentage depth dose values of square fields were found to be consistent with the fields collimated with the upper and lower jaws. Output factors for the very small fields were found to vary rapidly. Circular fields could be produced with depth dose characteristics similar to those produced using conical tertiary collimators, commonly used for radiosurgery, but with a broader penumbra.

Monte Carlo calculations of electron beam output factors for a medical linear accelerator

The purpose of this study was to investigate the application of the Monte Carlo technique to the calculation and analysis of output factors for electron beams used in radiotherapy. The code EGS4/BEAM was used to obtain phase-space files for 6, 12 and 20 MeV clinical electron beams from a scattering-foil linac (Varian Clinac 2100C) for a clinically representative range of applicator and square or rectangular insert combinations. The source-to-surface distance used was 100 cm. The field sizes ranged from 1 x 1 cm2 to 20 x 20 cm2. These phase-space files were analysed to study the intrinsic beam characteristics and used as source input for relative dose and output factor computations in homogeneous water phantoms using the code EGS4/DOSXYZ. The calculated relative central-axis depth-dose and transverse dose profiles at various depths of clinical interest agreed with the corresponding measured dose profiles to within 2% of the maximum dose. Calculated output factors for the fields studied agreed with measured output factors to about 2%. This demonstrated that for the Varian Clinac 2100C linear accelerator, electron beam dose calculations in homogeneous water phantoms can be performed accurately at the 2% level using Monte Carlo simulations.

Evaluating the dose to the contralateral breast when using a dynamic wedge versus a regular wedge

The incidence of secondary cancers in the contralateral breast after primary breast irradiation is several times higher than the incidence of first time breast cancer. Studies have shown that the scatter radiation to the contralateral breast may play a large part in the induction of secondary breast cancers. Factors that may contribute to the contralateral breast dose may include the use of blocks, the orientation of the field, and wedges. Reports have shown that the use of regular wedges, particularly for the medial tangential field, gives a significantly higher dose to the contralateral breast compared to an open field. This paper compares the peripheral dose outside the field using a regular wedge, a dynamic wedge, and an open field technique. The data collected consisted of measurements taken with patients, solid water and a Rando phantom using a Varian 2300CD linear accelerator. Ion chambers, thermoluminescent dosimeters (TLD), diodes, and films were the primary means for collecting the data. The measurements show that the peripheral dose outside the field using a dynamic wedge is close to that of open fields, and significantly lower than that of regular wedges. This information indicates that when using a medial wedge, a dynamic wedge should be used.

Application of a video-optical beam imaging system for quality assurance of medical accelerators

Method validation techniques were developed and experiments were carried out using a beam imaging system (BIS, Wellhöfer Dosimetrie, Schwarzenbruck, Germany) for routine quality assurance of medical accelerators. The routine quality assurance tasks include x-ray beam flatness and symmetry check, light/radiation field congruence test, beam energy constancy for electrons and mechanical checks for couch and collimator rotations. Comparisons were made between the BIS application and conventional quality assurance methods that use radiographic films or detector arrays. In this work, we have demonstrated efficiency and accuracy of the BIS to perform some of the routine quality assurance tasks for medical linear accelerators.

Lens dose in IMRT treatment of the head and neck

Abstract Purpose: Intensity modulated radiotherapy (IMRT) is used to treat lesions in the head and neck where conventional delivery techniques can not adequately treat the tumor. Dose to the lens is a concern in IMRT treatments because very often the planner chooses unconventional beam angles in the planning process, where beams either enter or exit through the eye and/or lens. Additionally, there is typically a factor of 3 increase in monitor units delivered in IMRT treatments and the dose due to MLC transmission can be appreciable. Direct in-vivo measurement of the lens dose during IMRT treatments is impossible and dose calculation accuracy using conventional algorithms is limited in superficial tissues like the lens. The objective of this work is to investigate and determine the dose to the lens during step and shoot IMRT treatments. Materials and Methods: At our institution, the Corvus pencil-beam based inverse treatment planning system (Nomos Corp, Sewickley, PA) was used to develop step and shoot IMRT treatment plans for tumor sites including the nasopharynx, maxillary and ethmoid sinus, and parotid. Primary tumor prescription doses ranged from 55-64 Gy and were treated on a Varian 2100C linac with 4 MV X-rays. To date, 8 patients treated with IMRT have been studied. The lens was contoured and used for optimization in 4 cases with the dose limit set at 2-10 Gy. Mean dose to the lens was obtained from the values reported by the inverse planning system. Results from the inverse planning system were also calculated using Monte Carlo simulation of the IMRT treatment starting from the actual leaf-sequence files and using the CT data used for the inverse treatment plan. Both the Corvus and Monte Carlo systems were commissioned for the 2100C 4 MV beam. The calculated lens dose was also compared to a measurement using n-type silicon diodes (Sun Nuclear Corp, Melbourne, FL) during a single fraction of the IMRT treatment. The diode was placed over the mid-pupil of both the left and right eyes and taken as an approximation of the single fraction dose to the lens. Results: The lens dose due to MLC transmission and internal scatter was generally 1-2% of the prescription dose. For those cases where the target was near the optic apparatus, the mean dose to the lens ranged from 7.2% to 66.2% of the prescription dose which depends in part on the dose limitations set by the planner. This corresponds to 4.8 Gy to 42.7 Gy over the complete course of 25 - 30 fractions. The average difference in mean dose to the target between the inverse planning system and Monte Carlo verification results was 0.9% ± 2.5% (±1 σ). For the mean dose to the lens predicted by the two systems, the results agreed to within 3.2% ± 8.4% of the prescription dose. Similarly, the lens dose as measured by the diodes was within 1.2% ± 1.8% of the Monte Carlo results except for two cases. Some diode measurements were as much as 50% the prescription dose but these did not agree with either the inverse planning system or Monte Carlo verification results. For one case in which the Monte Carlo and inverse planning system results differed by 19.5% of the prescription dose, the Monte Carlo verification results agreed with the diode measurements. Conclusion: The lens dose can amount to an appreciable fraction of the treatment dose in head and neck IMRT treatments. Treatment plans should be developed that take this into consideration. The dose delivered to the lens depends more strongly on the dose-volume limitations set during optimization than on the number of beams or individual beam orientations since the dose due to MLC transmission and internal scatter is small. The Corvus pencil-beam based inverse planning results agree with Monte Carlo verification results and therefore would appear to be reliable dose estimates during IMRT planning and optimization. Diode measurements can be a useful verification of the lens dose predicted by the treatment planning system but they can not be relied upon in all situations. Future work is planned to develop a Monte Carlo based diode calibration to reduce this uncertainty.

TH‐D‐M100E‐01: An Electronic Compensator for Total Body Irradiation

Purpose: Dose equalization along the long axis of the patient for total body irradiation requires the use of a compensator. At our institution the compensator consists of multiple layers of lead strips and is based on measurements along the patients mid‐sagittal plane. In this study, we examine the feasibility and limitations of replacing the lead compensator with a one‐dimensional electronic compensator using fluence management tools available in our treatment planning system. Method and Materials: The patient compensator was based on body thickness, SSD and off‐axis distance of 12 mid‐sagittal patient specific points. A phantom was modeled using these measurements from a previously treated patient. A fluence map of the transmission values for the compensator used at treatment was created. Dose calculated on the phantom was compared with patient surface dose measurements. Physical limits and software limitations of this method were evaluated. Results: The maximum patient height accommodated by our treatment room and setup is 225 cm. Using a fluence map with the largest transmission factor gradient is not an issue, nor is the use of any reasonable field width that would be seen in a clinical setting. Calculated dose to the phantom using the electronic compensator was found to be on 3.4% lower (range: +2.6% to −7.8%) than diode dose measurements taken on the patient skin surface time of treatment.Conclusion: It is feasible for most patients that an electronic compensator be used. Discrepancies between calculated and measured dose can reasonably be accounted for. Further study is planned to measure the dose using an anthropomorphic phantom and also to automate the construction of patient‐specific virtual phantoms and patient‐specific optimal fluence.

Distance learning by the world wide web for medical dosimetrists

The critical shortage of medical dosimetrists will require innovative methods to rapidly and adequately educate new personnel rapidly using limited resources. Distance learning through a world wide web site offers the opportunity for resources to be pooled to create a comprehensive teaching tool that can be used at multiple sites by many mentors. Web based distance learning has the capability to provide uniform training to every student at each site. Commercially available software provides authoring tools that can be used by mentors to create didactic lectures through the web. Examination tools in the software allow control of the pool of questions used in examinations and the automation of performance scoring. A prototype web site has been constructed for training medical dosimetrists. The example demonstrates the training techniques that can be implemented and the feasibility of a national dosimetrist education program.