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P S Nizin

Publications and source records attributed to P S Nizin.

14 recordsLinked to original sources
PubMed2001

Independent dose calculations for the PEACOCK System.

An independent dose calculation method has been developed to validate intensity-modulated radiation therapy (IMRT) plans from the NOMOS PEACOCK System. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle and each arc position. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The independent calculation uses the pencil beam data viz tissue maximum ratio (TMR) and beam profiles for a single 1 x 0.8-cm beamlet formed by the NOMOS multileaf intensity-modulating collimator (MIMiC) leaf. The pencil beam data were measured for the 6-MV photon beam from Siemens PRIMUS linear accelerator using film dosimetry. The dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for its beam intensity. Isodose distributions are generated using the independent dose calculations and compared to the CORVUS plans. Isodose distributions show good agreement with the CORVUS plans for a number of clinical cases. The independent dose calculation algorithm is described in this paper.

Algorithms↗
PubMed2001

Basic concepts of CORVUS dose model.

Basic concepts of the dose model utilized in the CORVUS treatment planning system are reviewed. Following the Peacock delivery tool (MIMiC) by NOMOS Corporation, CORVUS "delivers" radiation to a patient by means of narrow x-ray beams (pencil beams), which are subject to lateral electronic disequilibrium. Dose data for such beams are difficult to obtain experimentally. Therefore, the CORVUS dose model uses analytically calculated (rather than experimentally measured) narrow-beam dose data. The model is based on the idea that physical parameters necessary to calculate absorbed dose in narrow x-ray beams can be derived from measured broad-beam dose data. Calculation of central-axis and off-axis absorbed dose in narrow beams as well as a method of generating beam profiles are described.

Humans↗
PubMed2001

Independent dose calculations for the corvus MLC IMRT.

Two independent dose calculation methods have been explored to validate MLC-based IMRT plans from the NOMOS CORVUS system. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The 2 independent calculation methods are based on a pencil beam kernel convolution and a Clarkson-type differential scatter summation, respectively. The pencil beam data for a 1 x 1-cm beam, as formed by the multileaf collimator, were measured for the 6-MV photon beam from a Siemens PRIMUS linear accelerator using film dosimetry. In the pencil beam method, the dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for beam intensity. The scatter summation method used the conversion of measured depth dose data into scatter maximum ratios. In this method, the differential scatter from each pencil beam is corrected for the beam intensity. Isodose distributions were generated using the independent dose calculations and compared to the CORVUS plans. Although isodose distributions from both methods show good agreement with the CORVUS plan, our implementation of the differential scatter summation approach seems more favorable. The 2 independent dose calculation algorithms are described in this paper.

Radiotherapy Dosage↗
PubMed2000

Dosimetric study of the narrow beams of 60Co teletherapy unit for stereotactic radiosurgery.

This study explores the possibility of using a telecobalt unit for radiosurgery. A dosimetric study was performed for the narrow beam of Cobalt 60 (60Co) unit with circular radiation fields in diameters of 11, 17, 20, 27, 32, 35, 40, and 44 mm. Percentage depth dose and off-axis ratio were measured with ion chamber and radiographic film. The tissue air ratio values derived from measurements agreed well with the calculated values for all cone sizes and depths, ranging from the depth of maximum ionization of 24 cm in water. A quantitative evaluation of treatment plans with 60Co and 6-MV photon beams was carried out. The penumbra of the narrow beam of 60Co was larger than that of the 6-MV beam by 1.3 mm on average. This difference in penumbra can be attributed to the large source size of 60Co units. The feasibility of using narrow-beam 60Co for stereotactic radiosurgery/radiotherapy is discussed.

Cobalt Radioisotopes↗
PubMed1999

Phenomenological dose model for therapeutic photon beams: basic concepts and definitions.

A model for central-axis absorbed dose in therapeutic photon beams is developed. An expression for absorbed dose in a unit density material, including that in the regions of longitudinal and lateral electronic disequilibrium, is derived. The model is based on the concept of primary and scatter. Primary and scatter dose components are approximated using two identical analytical functions. Monte Carlo simulated dose data for 60Co gamma rays and 15 MV x rays in water are used to test the model. The accuracy of the model is demonstrated.

Biophysical Phenomena↗
PubMed1998

Tissue-air ratios for narrow 60Co gamma-ray beams.

This study introduces a table of tissue-air ratios (TAR) for narrow 60Co gamma-ray beams. The table is consistent with recently published TAR data for broad 60Co gamma-ray beams [Table 4.1, Br. J. Radiol. Suppl. 25 (1996)]. Narrow-beam TARs are derived analytically from broad-beam data of Table 4.1 and are tabulated for circular fields ranging from 0.2 to 2.2 cm in radius--an approximate equivalent of a 0.4 cm x 0.4 cm to 4 cm x 4 cm square-field range. The extent of depth is from 0.5 to 30 cm in water.

Air↗
PubMed1998

On absorbed dose in narrow 60Co gamma-ray beams and dosimetry of the gamma knife.

Using separate analytical functions describing primary dose, P0(dm,r), collimator scatter, Sc(r), and phantom scatter, TAR(d,r), an expression for absorbed dose in narrow 60Co gamma-ray beams is developed and each function is quantified: D(d,r) = P0(dm,r) Sc(r) TAR(d,r). The absorbed dose is calculated in beams as narrow as 0.2 cm in radius. Analytical and experimental results are compared using measured dose data for the Gamma Knife. Close agreement with experimental data is observed.

Cobalt Radioisotopes↗
PubMed1997

An approximation of central-axis absorbed dose in narrow photon beams.

In narrow photon beams of therapeutic energy range, the absorbed dose derived from experimental measurements is subject to a significant error. The error stems from high dose gradients characteristic to small radiation fields and from finite probe dimensions. In this study, a simple model for the narrow-beam absorbed dose is described. It is shown that broad-beam dose data are sufficient to predict a narrow-beam dose. The dose is calculated as a sum of primary and scatter components given in the form of respective analytical functions. For both functions, numerical coefficients are determined in broad-beam geometry. The model is evaluated by comparing calculated dose values with the Monte Carlo simulated narrow-beam dose data for 6 and 15 MV x rays.

Biophysical Phenomena↗
PubMed1996

The elements of tissue-air ratio and systematic error.

The definition of tissue-air ratio (TAR) is based on the concept of primary dose. To determine TAR, both in-phantom and in-air ionization measurements are utilized. To convert ionization in the phantom into dose and that in air into primary dose, correction factors must be applied to chamber readings in both geometries. Due to difficulties in selecting proper correction factors, TAR is subject to systematic error. The error comes from two sources of uncertainty: (1) Primary dose cannot be measured. Therefore approximate methods, such as in-air ionization measurements, are used. (2) Detectors of ionization are of finite dimensions and they are inhomogeneous. In this study, analytical expression for a systematic error is derived. Because in this derivation systematic error is an accumulative error, it is no longer necessary to convert ionization, both in air and in phantom, into a dose when calculating TARs. A method of determining systematic error is described. This method is based on the ability to produce accurate zero-field data in photon beams by means of a linear extrapolation technique. Using 60Co gamma radiation in water as an example, it is shown how to generate TAR data free of systematic error. A possibility of determining TARs for therapeutic x rays is discussed.

Air Ionization↗
PubMedDate not supplied

Determination of primary dose in 60Co gamma beam using a small attenuator.

A measurement technique previously proposed for determining dose from primary radiation has been tested using 60Co gamma rays. It is shown that the dose from primary radiation is reliably determined for field sizes of 10 X 10 and 20 X 20 cm2 at depths of 0.5, 5, and 10 cm in water. With further development this technique may be useful for verifying dose from primary radiation that may be calculated using a variety of methods.

Aluminum↗
PubMedDate not supplied

Geometrical aspects of scatter-to-primary ratio and primary dose.

Using an approximation of Bjarngard and Petti [Phys. Med. Biol. 33, 21-32 (1988)] that the scatter-to-primary ratio SPR(r,d) is a linear function of the geometrical parameter z = rd/(r + d) (where r is the radius of photon beam and d is depth in phantom), a method was developed to determine primary dose directly from depth-dose data, measured within the range of fields with lateral electronic equilibrium. This method was evaluated using tabulated TAR data for Co-60 gamma radiation in water. The results for primary dose agree within approximately 1% of those from other studies. It was shown that this method is applicable within the energy range of therapeutic x rays. Primary dose was determined in the case of 6- and 18-MV x rays using proposed method.

Cobalt Radioisotopes↗
PubMedDate not supplied

Primary dose in photon beams with lateral electron disequilibrium.

It is shown that in narrow monoenergetic photon beams under conditions of lateral electron disequilibrium, primary absorbed dose P(r) is a simple function of beam radius r: P(r) = P lambda.(1 - e(-gamma.r)), where P lambda is the primary dose in broad beams for which complete lateral electron equilibrium exists, and gamma depends on photon energy and absorbing medium. This formula was evaluated using Monte-Carlo-generated data for the primary dose in water from monoenergetic photons in the energy range from 2 to 8 MeV. The primary dose was studied in beams of radii 0.006 cm less than r less than 5.0 cm and within the depth interval 0.5 cm less than or equal to d less than or equal to 24 cm. It was concluded that the saturation equation above provides an accurate description of the primary dose from monoenergetic photon beams, as judged by comparison with Monte Carlo results.

Humans↗
PubMedDate not supplied

Electronic equilibrium and primary dose in collimated photon beams.

Electronic equilibrium conditions are studied in a homogeneous medium irradiated by monoenergetic photons with Compton scattering as a predominant process. Based on the concept of straight charged particle tracks, a geometrical model for spatial distribution of Compton electrons is developed in the limit of primary photon interactions. The model is applied to examine conditions of electronic equilibrium in collimated photon beams and to define equilibrium phase diagrams which establish correlation between various degrees of electronic equilibrium and primary dose. The diagrams predict that in a single direction (longitudinal or lateral) partial electronic equilibrium can be observed in radiation fields of dimensions smaller than the maximum range of secondary electrons. Associated macroscopic effects appear as a variation of the primary dose build-up rate with beam radius and depth in phantom. These effects are observed in the case of both primary and total absorbed dose as judged by the Monte Carlo generated data in waterlike material (1-8 MeV photons).

Electrons↗