F. Lopez

Evaluation of Kodak EDR2 film for dose verification of intensity modulated radiation therapy delivered by a static multileaf collimator.

A new type of radiographic film, Kodak EDR2 film, was evaluated for dose verification of intensity modulated radiation therapy (IMRT) delivered by a static multileaf collimator (SMLC). A sensitometric curve of EDR2 film irradiated by a 6 MV x-ray beam was compared with that of Kodak X-OMAT V (XV) film. The effects of field size, depth and dose rate on the sensitometric curve were also studied. It is found that EDR2 film is much less sensitive than XV film. In high-energy x-ray beams, the double hit process is the dominant mechanism that renders the grains on EDR2 films developable. As a result, in the dose range that is commonly used for film dosimetry for IMRT and conventional external beam therapy, the sensitometric curves of EDR2 films cannot be approximated as a linear function, OD = c * D. Within experimental uncertainty, the film sensitivity does not depend on the dose rate (50 vs 300 MU/min) or dose per pulse (from 1.0 x 10(-4) to 4.21 x 10(-4) Gy/pulse). Field sizes and depths (up to field size of 10 x 10 cm2 and depth = 10 cm) have little effect on the sensitometric curves. Percent depth doses (PDDs) for both 6 and 23 MV x rays were measured with both EDR2 and XV films and compared with ion chamber data. Film data are within 2.5% of the ion chamber results. Dose profiles measured with EDR2 film are consistent with those measured with an ion chamber. Examples of measured IMRT isodose distributions versus calculated isodoses are presented. We have used EDR2 films for verification of all IMRT patients treated by SMLC in our clinic. In most cases, with EDR2 film, actual clinical daily fraction doses can be used for verification of composite isodose distributions of SMLC-based IMRT.

Characteristics of sensitometric curves of radiographic films.

A new type of radiographic film, EDR (extended dose range) film, has been recently become available for film dosimetry. It is particularly attractive for composite isodose verification of intensity modulated radiation therapy because of its low sensitivity relative to the more common Kodak XV film. For XV film, the relationship between optical density and dose, commonly known as the sensitometric curve, depends linearly on the dose at low densities. Unlike XV film, the sensitometric curve of EDR film irradiated by megavoltage x rays is not linearly dependent on the dose at low densities. In this work, to understand the mechanisms governing the shape of the sensitometric curves, EDR film was studied with kilovoltage x rays, 60Co gamma rays, megavoltage x rays, and electron beams. As a comparison, XV film was also studied with the same beams mentioned above. The model originally developed by Silberstein [J. Opt. Soc. Am. 35, 93-107, 1945)] is used to fit experimental data. It is found that the single hit model can be used to predict the sensitometric curve for XV films irradiated by all beams used in this work and for EDR films exposed to kilovoltage x rays. For EDR film irradiated by 60Co gamma rays, megavoltage x rays, and electron beams, the double hit model is used to fit the sensitometric curves. For doses less than 100 cGy, a systematic difference between measured densities and that predicted by the double hit model is observed. Possible causes of the observed differences are discussed. The results of this work provide a theoretical explanation of the sensitometric behavior of EDR film.

Performance characteristics of an orthovoltage x‐ray therapy machine

Performance characteristics sufficient to provide physical data base specific to the Siemens Stabilipan 2 orthovoltage x-ray therapy machine are presented. Operating conditions covering the working range of the unit from 100 to 300 kVp are selected. Beam quality, output, the central axis depth dose, relative output factors, field flatness, uniformity index, and filtration characteristics of the beams are studied. Selected results are reported.

Comparison of dosimetric characteristics of Siemens virtual and physical wedges

Dosimetric properties of Virtual Wedge (VW) and physical wedge (PW) in 6 and 23 MV photon beams from a Siemens Primus linear accelerator, including wedge factors, depth doses, dose profiles, peripheral doses and surface doses, are compared. While there is a great difference in absolute values of wedge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have a stronger depth dependence than VWF due to beam hardening in PW fields. VW dose profiles in the wedge direction, in general, match very well with PW, except in the toe area of large wedge angles with large field sizes. Dose profiles in the nonwedge direction show a significant reduction in PW fields due to off-axis beam softening and oblique filtration. PW fields have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open fields while PW fields have lower surface doses. Surface doses for both VW and PW increase with field size and slightly with wedge angle. For VW fields with wedge angles 45 degrees and less, the initial gap up to 3 cm is dosimetrically acceptable when compared to dose profiles of PW. VW fields in general use less monitor units than PW fields.

Dependence of virtual wedge factor on dose calibration and monitor units.

One of the important features of the Siemens Virtual Wedge (VW) is that the VW factor (VWF) is approximately equal to unity for all beams with a total deviation for a given wedge no greater than 0.05, as specified by Siemens. In this note we report the observed dependence of VWF on dose calibration (cGy/MU), monitor units (MU), and beam tuning for a Primus, a linear accelerator with two dose-rate ranges available for VW operation. The VWF is defined as the ratio of doses measured on the beam central axis for the wedge field to the open field; the open field dose is always measured with the nominal high dose-rate beam. When VW operates in the high dose-rate range, the VWF is independent of calibration (cGy/MU). When VW works in the low dose-rate range, the VWF varies linearly with the calibration of the low dose-rate mode. For a linear accelerator that has only one dose-rate range for VW, there is no observable dependence of VWF on the calibration. We also studied the monitor unit dependence of VWF. A discontinuity in VWF was observed at the switching point between the high and low dose-rate ranges. Working with Siemens, we have investigated causes of this discontinuity. As a result of this investigation, the discontinuity in VWF as a function monitor unit is practically removed.

Comparison of measured and calculated dose distributions around an iridium‐192 wire

The relative dose distribution around a 5.0-cm-long piece of 192Ir wire has been measured using LiF chips. Measurements were made at distances of 0.25 to 5.0 cm away from the source and distances of 0.0 to 4.0 cm along the source. In addition, measurements were also made at several distances along the axis of the source. Attention was paid to the errors associated with these measurements. A comparison was made between a commercial software program, ISODOS, an analytical solution to the Sievert integral, and the measurements. Good agreement was obtained at distances along and away from the source. Major disagreements were found at points along the source axis.

Clinical implementation of AAPM TG61 protocol for kilovoltage x-ray beam dosimetry

Historically, there have been a variety of dosimetry protocols used for kilovoltage x-ray therapy beams with a set of conversion factors and correction factors taken from different references. Corresponding to the continued installation and use of kilovoltage machines, the American Association of Physicists in Medicine (AAPM) presented a unified protocol developed by Task Group 61 (TG61). TG61 determines the absorbed dose to water with an ionization chamber calibrated in air in terms of air kerma (Nk). TG61 presents both an in-air method and an in-phantom method. In this work we only examine the TG61 in-air method. Our traditional dosimetry procedure, which is based upon NCRP Report 69 and on material found in standard medical physics texts, has been compared to the TG61. A variety of kilovoltage beam energies were examined with a set of various field sizes and source to surface distances. TG61 published updated data for the mass absorption coefficient ratios, backscatter factors, and the average energy per ion pair factor. The following conclusions have been reached: (1) Our traditional procedures and the TG61 protocol for in-air measurements are equivalent. (2) The conversion and correction factors used in TG61 are different by up to 4.5% compared to the old factors that we have used. (3) The application of the TG61 factors can result in up to 5% differences in the determination of the absorbed dose.

TH‐C‐224C‐07: EUD‐Assessed Impacts of Respiratory Motion On Breast Irradiation

Purpose:Respiration results in intrafractional motion and anatomic changes for both target and normal structures (e.g., lung,heart) during the radiation treatment for breast cancer. The purpose of this work is to quantify the dosimetric and radiobiological impacts using the concept of equivalent uniform dose (EUD). Method and Materials: Intrafractional variations were assessed based on 4DCT datasets acquired using a GE LightSpeed‐RT scanner and Varian RPM‐respiratory‐gating system. The 4DCT datasets along with the conventional 3DCT images for 10 patients were analyzed retrospectively. Each set of 4DCT consisted of 10 CT image sets at a phase between 0–90% during one respiration cycle. For each case, a 3D dosimetric plan of two tangential beams irradiating the whole breast was generated based on the 3DCT images using Xio (CMS) planning system. The parameters for this dosimetric plan (e.g., energy, beam angles, beam shape, wedge, weighting, isocenter location) were copied to each phase image set of the 4DCT to generated 3D dose distribution. DVHs for each phase image set were generated and were used for EUD calculation based on LQ model for breast tumor and Lyman model for lung.Results: 4DCT showed breast position/shape and lung position/shape/volume are changed with respiration. For example, lung volume changed up to 20% for the cases studied. These changes result in significant intrafractional variations in dose distributions/DVHs. Our calculations show that, compared to the planned EUD (based on the 3DCT), the breast EUD was lowered by an average of 5% (when including all 10 breathing phases) and up to 10% (at a particular phase). Lung EUD varied by ±3% during respiration.Conclusion: Respiratory motion in breast radiation treatment can potentially result in decreased target coverage and normal structure sparing. This effect that can be assessed using EUD, and decreased EUD may be an indicator for gated breast irradiation.

MO‐FF‐A2‐02: Respiration Induced Heart Motion and Indications of Gated Delivery for Left‐Sided Breast Irradiation

Purpose: To study dosimetric gain of respiratory gating to account for heart motion during left‐sided breast irradiation and to determine indications for gating treatment during treatment planning.Methods and materials: The 4DCT data acquired with free breathing for 13 (out of 68) left‐sided breast cancer patients, who underwent whole breast irradiation with or without regional nodal irradiation, were analyzed retrospectively. Contours of the targets, lung and heart from the planning CT, selected to be the CT at 20% phase, were populated to 0‐ and 50%‐phase CT using deformable registration. The 3D dose distributions were reconstructed in these three phases (0, 20 and 50%). The heart dislocation between the breathing phases was measured in three selected transverse CT slices for the three phases by the changes of DLAD [the distance from left ascending aorta (LAD) to a fixed line drawn on each slice], and maximal heart depth (MDH, the distance of the forefront of the heart to the line). These distances were correlated with the changes of mean heart dose (MHD) and V25.2 for heart between the breathing phases. Results: Significant respiration induced heart displacement was seen, which resulted in substantial variations in dose delivered to the heart. In particular, the heart appeared to move towards to the chest wall during respiration, DLAD changed up to 9 mm, and MDH changed 10.4 mm, 11.0 mm and 10.7mm, respectively, on the three transverse CT slices from superior to inferior. The MHD and V25.2 varied up to 38% and 39%, respectively. These variations were reduced substantially with gating. Conclusion: The respiration induced heart displacement can result in significant variation in heart dose during left‐sided breast irradiation. A large variation in the distances: MDH and/or DLAD, can be used as an indicator to trigger respiratory management, such as gating prior to the treatment delivery.

Comparison of dosimetric properties of virtual wedge and physical wedges

The authors present a direct comparison of dosimetric properties of Siemens virtual wedge (VW) and physical wedge (PW). While there is a great difference in absolute values of wedge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have stronger depth dependence than VWF because of beam hardening in PW fields. Dose profiles in the wedge direction are match very well between VW and PW except in the toe area of the 45/spl deg/ wedge with large field sizes, where PW fields have a higher peak. The authors have demonstrated that it is dosimetrically acceptable to have an initial gap up to 3 cm for VW fields when compared with PW dose profiles. While maintaining the desired wedged dose distribution, VW has an advantage of creating smaller hot spot, when compared with PW, in the toe region of 45/spl deg/ wedge fields with large field sizes. Dose profiles in the non-wedge direction shows a significant reduction in PW fields due to off-axis beam softening and oblique filtration. PW fields also have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open fields while PW fields have slightly lower surface doses.