2263 A 192-iridium brachytherapy source-based computed tomography scanner

Proceedings

2 orders half-life),

Boccuzzi

ASTRO

Meeting

413

At the extreme (95% edema magnitude and 29 days edema for “‘1 and 1”3Pd seed implants respectively. is more than 3 orders and 5 orders of magnitude for iZ5I and io3Pd seed implants respectively. due to the cell survival for prostate undergoing CLDRI using “‘1 or ‘03Pd seeds is increased substantially caused by surgical trauma. The increase is about 1 order and 2 orders of magnitude for lZ51 and io3Pd seed in a typical edematous prostate. At the extreme, it could be as much as 3 and 5 orders of magnitude for implants, respectively.

of magnitude the increase

Conclusion: Tumor presence of edema implants respectively I”1 and ioiPd seed

2262

of the 41st Annual

A DOSIMETRIC COMPARISON OF STEREOTACTIC RADIOSURGERY WITH A MICRO-MULTILEAF COLLIMATOR VERSUS ARCS FOR ARTERIOVENOUS MALFORMATIONS DE’,

Kim

St. Luke’s-Roosevelt

S’, Pryor

J2, Berenstein

Hospital

Center,

New

A’, Shih JA’, York,

NY, USA’;

Chiu-Tsao Beth Israel

ST’,*,

Harrison

Medical

Center,

USING TREATMENT

STATIC OF

BEAMS

LB’,’ New

York, NY, USA’

Purpose: Both linac-based and “gamma-knife” stereotactic radiosurgery (SRS) are routinely employed in th’e treatment of arteriovenous malformations (AVM). These techniques typically involve fixed circular beam apertures. The AVM’s of patients referred as candidates for radiosurgery are often highly irregular in shape, and may be located in eloquent areas of the brain. Therefore, excellent conformality of prescribed dose to the target volume and minimizing irradiation of normal brain tissue are crucial. Stereotactic treatment of AVM’s with conformally-shaped static beams using a micro-multileaf collimator (mMLC) may improve conformality and homogeneity of dose within the target, as well as dramatically improving the efficiency of treatment planning and implementation. Methods & Materials: SRS was performed on 15 patients to treat AVM targets ranging from 1.0 cm to 4.0 cm in maximum dimension, and from 0.3 cm3 to 10 cm3 in volume. Each patient was treated with a single isocenter using multiple non-coplanar conformal static beams shaped by a commercially-available BrainLAB mMLC (M3) with 3 mm leaf width at isocenter. These plans were compared to those generated using both single and multiple-isocenter arcs with circular collimators. Multi-planar isodose distributions and dose-volume histograms were calculated for each case. Plans were evaluated for conformality and homogeneity based on RTOG-defined figures of merit (PITV = volume of prescription isodose surface divided by target volume, MDPD = maximum dose divided by prescription dose). Results: Treatment plans using the mMLC with a reasonable number of static beams (6-12) achieved excellent conformality (PITV 5 1.9) and homogeneity (MDPD 5 1.3) in about 30 minutes of planning time, typically with the 80% isodose line as the minimum covering the target. Single-isocenter circular aperture arc plans were also generated quickly, and produced acceptable homogeneity. However, for irregularly-shaped targets, these plans invariably included a significant amount of normal tissue (PITV ~3.0). Considerable planning time of 2-3 hours was required to generate multiple-isocenter arc plans with 2-4 isocenters which approached the conformality of the fixed static beam plans, typically with the 50% isodose line as the minimum covering the target. Furthermore, the dose heterogeneity for multiple isocenter solutions was consistently worse than that of the conformal static beam plans (MDPD 23.0). The volume of tissue enclosed by the 50% and 30% isodose surfaces was also analyzed for each plan. Conclusion: With a large inventory of closely-spaced circular cone sizes and significant time investment for planning, QA, and treatment, stereotactic radiosurgery plans using arcs and multiple isocenters may achieve conformality equivalent to those using a commercially-available mMLC. However, the mMLC substantially improves dose homogeneity (MDPD 5 1.3 versus MDPD ~3.0), as well as reducing the amount of time required for both planning and treatment from several hours each to less than one hour each. The implementation of dynamic conformal arc therapy and intensity modulated SRS using mMLC also requires dosimetric investigation and comparison to current treatment techniques.

2263 Rathee MCTRF,

A 192-IRIDIUM S, Bemdt Winnipeg,

BRACHYTHERAPY

AG, Rickey MB,

DW,

Bews

SOURCE-BASED

COMPUTED

TOMOGRAPHY

SCANNER

JA

Canada

Purpose: Treatment planning of high-dose-rate brachytherapy requires accurate localization of the tumor and any nearby critical structures with respect to the applicators. The conventional localization technique using two orthogonal radiographs has a number of inherent limitations. We have developed a solid state g-channel detector designed for use in a novel computed tomography (CT) scanner which utilizes a 192-Ir high-dose-rate brachytherapy source to provide the photons needed to form an image instead of an x-ray tube. This low-cost scanner will allow CT imaging of the patient in the treatment position, in the brachytherapy room. Materials & Methods: We constructed an X-channel prototype detector which consists of eight CdWO, scintillating crystals optically coupled to a 16-element photodiode array. Each channel comprises a switched-capacitor integrator followed by a low-pass filter amplifier. The electronics integrate the photocurrent into a voltage signal for each of the eight measurement channels. Thus, the output voltage is proportional to the integration time and the photocurrent in the diodes. The electronic noise was minimized by using low-junction-capacitance photodiodes, the low-pass-gain-filter stage and by subtracting the voltage measured at the beginning of each integration cycle. The detector output to 192-Ir gamma-rays was measured using a narrow beam irradiation geometry, a source-to-detector distance of 71.8 cm and a source strength of 9.2 Ci. The gamma-ray fluence incident on the detector was varied by placing various thicknesses of Plexiglas in the beam path. The detector noise was assessed by measuring the standard deviation (SD) in the signal amplitude as a function of Plexiglas thickness. The 192-Ir source and the detector were collimated with 1 cm thick by 14 cm long lead blocks separated by 1.1 cm. The source-detector separation was 82.5 cm. Preliminary CT images were obtained using a first generation scanner geometry in which only a single channel of a detector

414

I. J. Radiation

assembly projections

Oncology

was used. Collimation were acquired from

Biology

l

l

Physics

Volume

similar to the noise measurements 0 degrees to 178.2 degrees.

45, Number

3 Supplement

was used. A sample

spacing

1999

of 1.40 mm was used and 99

Results: The measured detector output, a small sample of which is listed in the table, agrees well with the expected output (standard error of estimate with respect to the mean is 1.6%). The total signal-to-noise ratio (SNR) listed in the table was found by dividing the measured voltage by its SD, while the electronic component of the SNR was found by dividing the measured voltage by the electronic noise of 115 I&V. The electronic SNR is much greater than the total SNR for Plexiglas thicknesses up to 43 cm (49 cm tissue equivalent) indicating that the detector is not limited by electronic noise. An image of a non-eviscerated chicken shows the bones, heart and lungs of the chicken. An image of brachytherapy needles in a water phantom clearly shows the needles without any streak artifacts. Conclusion: We have constructed an S-channel detector which is linear over three orders of magnitude. The detector signal is greater than the electronic noise floor for up to at least 43 cm of Plexiglas (49 cm tissue equivalent). Preliminary images taken using a first generation scanner geometry have proved that this detector is well-suited for use in a 192.Ir based CT scanner.

Plexiglas

Thickness

(cm) 0 16.8 30.8 44.0

2264 Siochi

PENUMBRA INTENSITY

Measured Output (V)

Expected Output (V)

Total SNR

Electronic SNR

0.102 0.013 0.0020 0.00037

0.102 0.014 0.0023 0.00046

118 37 19

3996 536 159

10

61

CONSIDERATIONS IN FLUENCE MODULATED FIELDS

CALCULATION

AND

VERIFICATION

OF

RC

Siemens Purpose: its effects

Medical

Systems,

OCS,

Concord,

CA, USA

To develop a penumbra model that is consistent with intensity modulated on the usefulness of approximate fluence measurements for verification

radiation of IMRT

therapy (IMRT) delivery.

fields and assess

Materials and Methods: A 4 cm x 6 cm IMRT field consisting of 1 cm x 1 cm pencil beams was segmented with Siemens’ IMFAST into 9 multi-leaf collimator (MLC) shaped fields. The sequence of MLC fields was delivered at 6 MV on a Siemens KD2 linac with SIMTEC and PRIMEVIEW. Films were exposed in a solid phantom (SSD = 98.5 cm, depth = 1.5 cm) perpendicular to the beam axis and relative film dosimetry was performed. Measured data were compared with calculations that accounted for leaf transmission, head scatter (detector’s eye view of an extrafocal scattering planar source), and geometrical penumbra (edge and comer parameters model). The penumbra parameters were estimated using a least squares fit between the film data and theory. To check these parameters, a calculation using the derived parameters for a non-modulated 5 cm x 5 cm field was compared with inplane and crossplane scans at the points where penumbra can add to the fluence of neighboring pencil beams. Finally, the delivery error statistics were determined to assess the use of this measurement for delivery verification. Results: At 5 mm from the 50% point of the profile for the 5 cm x 5 cm field, the calculation yielded 12.4% while the experimental value was 12.7%, (parameters: edge = 4.5%, corner = 1.9%). The good agreement permits the use of fluence calculations to determine the accuracy of dose delivery and leaf positioning. An average error of 2% resulted when only dose errors were considered (using the best data points within a 2 mm diameter region around the center of the pencil beams), while an error of 4% resulted when dose and leaf positioning errors were considered (using the exact center of the pencil beams). When compared to the matrix of pencil beam weights, the maximum error was 40%, since the segmentation did not consider penumbra during fluence correction. Conclusion: For radiation treatment planning (RTP) systems that calculate dose using pencil beams with a penumbra model (e.g. NOMOS Corvus), approximate fluence measurements can not be compared with the matrix of relative pencil beam weights for verification. To assess the accuracy of the delivery, the measurement must be compared against the calculated fluence. For RTP systems that produce fluence maps instead of pencil beam weights, penumbra must be corrected for in the segmentation process before the delivery of the treatment can be verified with the fluence measurement technique. In both cases, scatter and leakage must be accounted for during the segmentation step.

2265 Thomas

BEAM’S EYE VIEW ANALYSIS OF LOCALIZATION FORUNRESECTALBELOCALLYADVANCEDHEADANDNECKCANCER CT’,

The University Comprehensive

Dawson

LA’,

Normolle

D’, Fiveash

JB’,

of Michigarz, Department of Radiation Cancer Center, Ann Arbor, MI, USA’

McGinn Oncology,

Purpose: To evaluate whether the use of beam’seye-view may improve target coverage in radiation treatment fields

ERRORS Cl, Marry

IN RADIATION

JM’,

Ann Arbor,

(BEV) representation for locally advanced

Eisbruch

MI,

USA’;

THERAPY

FIELDS

A’ The University

of Michigan

of the relationship of tumor to normal tissues unresectable cancer of the head and neck.

Materials and Methods: Three experienced radiation oncologists (observers) were asked to draw conventional two-dimensional radiation treatment fields for 16 patients who had been treated consecutively from 1995 to 1998 under a University of Michigan radiation therapy protocol for patients with locally advanced unresectable head and neck cancer. Observers were required to draw initial treatment fields, off-cord treatment fields and gross tumor boost fields using