Field Size Factor Research Articles

A procedure for calculation of monitor units for passively scattered proton radiotherapy beams

The purpose of this study is to validate a monitor unit (MU) calculation procedure for passively scattered proton therapy beams. The output dose per MU (d/MU) of a therapeutic radiation beam is traditionally calibrated under specific reference conditions. These conditions include beam energy, field size, suitable depth in water or water equivalent phantom in a low dose gradient region with known relative depth dose, and source to point of calibration distance. Treatment field settings usually differ from these reference conditions leading to a different d/MU that needs to be determined for delivering the prescribed dose. For passively scattered proton beams, the proton specific parameters, which need to be defined, are related to the energy, lateral scatterers, range modulating wheel, spread out Bragg peak (SOBP) width, thickness of any range shifter, the depth dose value relative to the normalization point in the SOBP, and scatter both from the range compensator and inhomogeneity in the patient. Following the custom for photons or electrons, a set of proton dosimetry factors, representing the changes in the d/MU relative to a reference condition, can be defined as the relative output factor (ROF), SOBP factor (SOBPF), range shifter factor (RSF), SOBP off-center factor (SOBPOCF), off-center ratio (OCR), inverse square factor (ISF), field size factor (FSF), and compensator and patient scatter factor (CPSF). The ROF, SOBPF, and RSF are the major contributors to the d/MU and were measured using an ion chamber in water tank during the clinical commissioning of each beam to create a dosimetry beam data table to be used for calculating the monitor units. The following simple formula is found to provide an independent method to determine the d/MU at the point of interest (POI) in the patient, namely, (d/MU) = ROF SOBPF. RSF SOBPOCF.OCR.FSF.ISF.CPSF. The monitor units for delivering the intended dose (D) to the POI can be obtained from MU = D / (d/MU). The accuracy and robustness of the above formula were validated by calculating the d/MU in water for many different combinations of beam parameters and comparing it with the corresponding measured d/MU by an ion chamber in a water or water/plastic phantom. This procedure has been in use for MU calculation for patient treatment fields at our facility since May 2006. The differences in the calculated and measured values of the d/MU for 623 distinct fields used for patient treatment during the period of May 2006 to February 2007 are within 2% for 99% of these fields. The authors conclude that an intuitive formula similar to the one used for monitor unit calculation of therapeutic photon beams can be used to compute the monitor units of passively scattered proton therapy beams.

Standing Waves and Traveling Waves Distinguish Two Circuits in Visual Cortex

The visual cortex represents stimuli through the activity of neuronal populations. We measured the evolution of this activity in space and time by imaging voltage-sensitive dyes in cat area V1. Contrast-reversing stimuli elicit responses that oscillate at twice the stimulus frequency, indicating that signals originate mostly in complex cells. These responses stand clear of the noise, whose amplitude decreases as 1/frequency, and yield high-resolution maps of orientation preference and retinotopy. We first show how these maps are combined to yield the responses to focal, oriented stimuli. We then study the evolution of the oscillating activity in space and time. In the orientation domain, it is a standing wave. In the spatial domain, it is a traveling wave propagating at 0.2-0.5 m/s. These different dynamics indicate a fundamental distinction in the circuits underlying selectivity for position and orientation, two key stimulus attributes.

TU‐D‐AUD‐07: The Design and Performance of a Simplified Independent Tomotherapy Treatment Planning System

Purpose: A simplified helical‐tomotherapy independent planning system (SHIPS) is developed for independent verification of the TomoTherapy Hi‐ART (TomoTherapy Inc. Madison, WI) treatment scheme and to provide an alternative method to produce helical tomotherapy plans. The SHIPS software is designed to be simple, so the program can run a desktop PC, straightforward, so the delivery scheme is intuitive, and streamlined, to speed the treatment planning process. Treatment plans created with SHIPS may be used in cases where the Hi‐ART's inverse planner may be unnecessarily complex. We present the design of SHIPS and compare its results against forty Hi‐ART treatment plans. Several SHIPS derived tomotherapy plans are presented. Methods and Materials:SHIPS produces a treatment delivery scheme based on patient geometry and contours, prescription, delivery settings (e.g. pitch, field size, and modulation factor), and measured beam data. SHIPS calculates a leaf‐sequencing pattern (a treatment sinogram), gantry rotation period (GRP), couch speed (CS), couch travel length (CTL), monitor units (MU), and time of treatment (T). Treatment plans used to compare against SHIPS consist mostly of prostate patients with some head and neck, pelvis, lung, and phantom cases. SHIPS is used to create example prostate, breast, and brain treatments. Results: SHIPS can produce conformal tomotherapy plans for imbedded (i.e. prostate), distal/convex (i.e. breast), and large/shallow (i.e. brain) targets in less than 30 seconds. When compared against Hi‐ART treatment schemes, the average deviation of the calculated CTL, T, CS, and MU are, respectively, 0.08%, 0.05%, 0.37%, and 0.74%. Conclusion: SHIPS bypasses the Hi‐ART's optimizer producing independently derived plans, quickly. SHIPS can be used for quality assurance of the entire treatment planning process. Treatment parameters generated from SHIPS agree within 1% of the Hi‐ART values. SHIPS challenges the degree of which the Hi‐ART's complex inverse‐planning is necessary.

Daily monitoring of linear accelerator beam parameters using an amorphous silicon EPID

An amorphous silicon EPID has been investigated to test its suitability as a daily check device for linac output and to provide daily monitoring of beam profile parameters such as flatness, symmetry, field size and wedge factor. Open and wedged 6 and 8 MV photon beams were collected on a daily basis for a period of just over a year and analysed in software to determine daily values of these parameters. Daily output results gave agreement between EPID measured dose and ion chamber measurements with a standard deviation of 0.65%. Step changes in flatness, symmetry and field size were readily detected by the EPID and could be correlated with adjustments made on service days and QC sessions. The results could also be used to assess the long term beam stability. Recalibration of the EPID required new baseline values of the parameters to be set. Wedge factors measured at one collimator angle proved stable but sensitive to changes in beam steering. The EPID proved to be a useful daily check device for linac output which can simultaneously be used for daily monitoring of beam profiles and field sizes.

How Much the Eye Tells the Brain

In the classic "What the frog's eye tells the frog's brain," Lettvin and colleagues showed that different types of retinal ganglion cell send specific kinds of information. For example, one type responds best to a dark, convex form moving centripetally (a fly). Here we consider a complementary question: how much information does the retina send and how is it apportioned among different cell types? Recording from guinea pig retina on a multi-electrode array and presenting various types of motion in natural scenes, we measured information rates for seven types of ganglion cell. Mean rates varied across cell types (6-13 bits . s(-1)) more than across stimuli. Sluggish cells transmitted information at lower rates than brisk cells, but because of trade-offs between noise and temporal correlation, all types had the same coding efficiency. Calculating the proportions of each cell type from receptive field size and coverage factor, we conclude (assuming independence) that the approximately 10(5) ganglion cells transmit on the order of 875,000 bits . s(-1). Because sluggish cells are equally efficient but more numerous, they account for most of the information. With approximately 10(6) ganglion cells, the human retina would transmit data at roughly the rate of an Ethernet connection.

SU-FF-T-221: Evaluation of An Electron Monte Carlo Dose Calculation Algorithm for Electron Beam Radiotherapy

Purpose: To systematically evaluate the electron Monte Carlo (eMC) dose calculation algorithm of the Eclipse treatment planning system and to identify factors that affect its calculation accuracy. Method and Materials: Percent depth dose (PDD) and profiles for each electron energy/applicator/depth combination were measured for four Varian Clinac 21EX Linacs. Cutout factors for various cutout sizes down to 3×3 cm2 were also measured. Statistical comparison between eMC calculation and measurement was performed for parameters such as shift in depth of PDD curves, field size, penumbra width, average dose difference within 80% of field size, beam symmetry, Bremsstrahlung dose, and cutout factor. Analysis of variance was used to identify factors that have significant influence on the observed discrepancies. Results: The agreement between measured and calculated 50% PDD depth was within ±2 mm for nearly all machine/energy/applicator combinations except the 6×6 6 MeV beams on three machines where maximum difference was 3.8 mm. The calculated field width was larger than that of measured in most of the cases but within 3%. The differences between calculated and measured penumbra were also within 3%. The calculated Bremsstrahlung doses were smaller than measurements, with a few exceptions. Analysis of variance identified machine as a significant variable for the observed discrepancies in PDD, field width, and penumbra, suggesting possible variations among linacs or the quality of measured data. Calculation grid size was found to have significant effect on calculation accuracy. Using grid size of 5 mm resulted in approximate 10% discrepancies. Reducing grid size yielded significant improvement on the calculated cutout factor: from 5% discrepancy to less than 3% when grid size of 1 mm was used. Conclusion: The overall agreement between the eMC calculation and measurements was acceptable. Calculation grid size of 2.5 mm or smaller is recommended for all field sizes for eMC algorithm.

External radiation therapy boost to the vaginal vault: feasibility of intracavitary dosimetry using a commercial diode system

Purpose: An overall check of the whole dosimetry procedure by intracavitary in vivo dosimetry, using n-type silicon diode dosimeter, was performed during 6-MV x-ray irradiation of the vaginal vault. The dose delivered to the isocenter by all treatment fields was evaluated. Methods and Materials: The diode dosimeter was calibrated against an ion chamber and tissue maximum ratio, field size factor, SSD factor, and temperature dependence studies were performed. Diode system accuracy, linearity, and reproducibility were also tested. Patients’ dose data were collected and comparision was made with respect to treatment-planning dose calculations. Ten patients with cervical cancer and endometrial cancer were treated with surgery and irradiation. During the boost to the vaginal vault, a diode was inserted by an intravaginal device and the vaginal vault was the isocenter of the four fields. The field size generally was not larger than 10 × 10 cm 2. Results: Diode-measured “tissue maximum ratio” agreed to within 1% with those measured with an ion chamber in field from 7 × 7 to 10 × 10 cm 2. The diode also exhibited a temperature dependence of 0.1%°C −1. For 10 patients treated with a 6-MV beam, the agreement with treatment-planning dose calculations was shown to be better than ±4%. Conclusion: The good accuracy and reproducibility of the diode system shows that determination of the dose at isocenter, for patients treated in the pelvic region, can be performed with n-type diodes accurately. On the other hand, in the vaginal vault boost, external-beam radiotherapy is delivered accurately and in vivo dosimetry is really not indicated.

Technique charts for Kodak’s new film-screen systems for portal localization

In July 1996, Kodak released new film-screen systems with enhanced contrast (EC) for portal localization with megavoltage therapeutic beams. This study presents the generation of general-purpose technique charts for Kodak’s new film-screen combinations: the Enhanced-Contrast localization (EC-L) film in EC-L cassette and the EC-L film in fast ECL (fECL) cassette for use with cobalt-60, 6 MV, 10 MV, and 18 MV beams. These technique charts were based on the assumption that a film with an optical density (OD) of 1.8 provides the best viewing density. The doses to produce such as OD, D exp, were obtained from the H & D curves and were 1.5, 1.6, 1.7, and 1.8 cGy for cobalt-60, 6 MV, 10 MV, and 18 MV beams, respectively, with the EC-L-film+EC-L-cassette combination. The corresponding values were 1.3, 1.3, 1.3, and 1.4 cGy, respectively, for the above four beams with the EC-L-film+fEC-L-cassette combination. The dose to the film is assumed to be proportional to the calibrated dose rate ( D 0), field size factor (FSF), inverse square factor relative to 100.0 cm (INV), and the transmission factor through the patient, which is equal to e −uT, where u is the broad beam attenuation coefficient and T is the patient thickness. With the above assumptions, the exposure time or monitor unit, t, is then calculated from the following equation: t = D exp/(D 0∗FSF∗INV∗e −uT). For an average port size of 15 × 15 cm, the attenuation coefficients were obtained from the fitting of TAR (cobalt-60) or TMR (6 MV, 10 MV, and 18 MV) as a function of depth from 10 to 30 cm and were 0.0564 cm −1, 0.03714 cm −1, and 0.02271 cm −1 for cobalt-60, 6 MV, 10 MV, and 18 MV beams, respectively. The FSF were explicitly obtained from the clinical physics data books and were 1.028, 1.032, 1.036, and 1.053 for cobalt-60, 6 MV, 10 MV, and 18 MV, respectively. For cobalt-60 beam, the D 0 was assumed to be 100.0 cGy/min. For the 6 MV, 10 MV, and 18 MV beams, the D 0 in cGy per monitor unit is 1.030, 1.051, and 1.061, respectively. Technique charts were then generated as a function of patient thickness from 10 to 45 cm for filming distance from 110 to 140 cm for all four beams. These technique charts can be easily customized to portal localization practices in a radiation therapy department.

Verification of a simple point dose calculation method for a dual energy linear accelerator with asymmetric jaws

Certain fundamental dosimetrical parameters involving the applications of asymmetric jaws were investigated. The nominal accelerating potentials (NAPs) were found to decrease from 5.1 to 4.2 and from 18.0 to 13.4 for the 6 and 18 MV beams, respectively, as the off-axis distance (OAD) increases from 0.0 to 15.0 cm. The relative beam intensity increases from 1.00 to 1.07 at OAD of 15.0 cm for the 6 MV beam, and to 1.02 at OAD of 7.0 cm for the 18 MV beam. The percentage depth doses (PDDs) for half-blocked fields of 4 × 4 cm, 10 × 10 cm and 20 × 20 cm were found to deviate from those of corresponding symmetric fields by less than 2% down to the depth of 35.0 cm. The field size factor (FSF) for the asymmetric field from 4 × 4 cm to 20 × 20 cm deviates less than 1.0% from those of the corresponding symmetric fields. The equivalent square concept was found to be applicable to asymmetric fields within 1% error if the jaw exchange effect is taken into consideration. The measured point doses for half-blocked fields of 4 × 4 cm, 10 × 10 cm and 20 × 20 cm for both 6 and 18 MV were within 3% of the calculated dose based on a published dose calculation method which employs symmetric field beam parameters, such as field size factor (FSF), percentage depth dose (PDD), and off-axis correction factors (OAFs). The efficacy of this point dose calculation method is discussed.

Patient dosimetry quality assurance program with a commercial diode system

Purpose : To evaluate a commercial silicone diode dosimeter for a patient dosimetry quality assurance program. Methods and Materials : The diode dosimeter was calibrated against an ion chamber and percentage depth dose, linearity, anisotropy, virtual source position, and field size factor studies were performed. Correction factors for lack of full scatter medium in the diode entrance and exit dose measurements were acquired. Dosimetry equations were proposed for calculation of dose delivered at isocenter. Diode dose accuracy and reproducibility were tested on phantom and on four patients. A patient dosimetry quality assurance program based on diode measured dose was instituted and patient dose data were collected. Results : Diode measured percentage depth dose and field factors agreed to within 3% with those measured with an ion chamber. The diode exhibited less than 1.7% angular dose anisotropy and less than 0.5% nonlinearity up to 4 Gy. Diode dose measurements in phantom showed that the calculated doses differed from the prescribed dose by less than 1.5%; the diode exhibited a daily dose reproducibility of better than 0.2%. On four selected patients, the measured dose reproducibility was 1.5%; the average calculated doses were all within ± 7% of the prescribed doses. For 33 of 40 patients treated with a 6 MV beam, measured doses were within ± 7% of the prescribed doses. For 58 of 63 patients treated with an 18 MV beam, measured doses were within ± 7% of the prescribed doses. For 11 out of 12 patients, a second repeat measurements yielded doses within ± 7% of the prescribed doses. Conclusions : The proposed diode-based patient dosimetry quality assurance program with dose tolerance at ± 7% is simple and feasible. It is capable of detecting certain serious treatment errors such as incorrect daily dose greater than 7%, incorrect wedge use, incorrect photon energy and patient setup errors involving some incorrect source-to-surface-distance vs. source-to-axis-distance treatments.

PROJECTION OF THE RETINA ON TO STRIATE AND PRESTRIATE CORTEX IN THE SQUIRREL MONKEY, SAIMIRI SCIUREUS.

PROJECTION OF THE RETINA ON TO STRIATE AND PRESTRIATE CORTEX IN THE SQUIRREL MONKEY, SAIMIRI SCIUREUS.