Analysis of mechanical sources of patient alignment errors in radiation therapy

Certain radiation treatments, such as conformal and intensity modulated treatments, involve isocentric treatment fields delivered using multiple angles or continuous angulation of the gantry, collimator and table. At our institution, treatments involving three angles (gantry, collimator, and table) can, if uncorrected, exhibit misalignments of 2 mm or more on premarked field centers and borders on the patient surface during the initial setup on a linear accelerator (linac), even though the linac operates within allowable mechanical tolerances. This paper is an analysis of three principal mechanical sources of patient alignment errors observed on linacs: (i) errors in table and gantry angle, (ii) displacement of gantry rotational axis during gantry rotation, and (iii) displacement between collimator and table rotational axes. On patient surfaces, these small, systematic mechanical errors can each be expected to produce misalignments of up to 1.5 mm, increasing to over 2 mm with nearly horizontal fields delivered at nonzero table angles onto highly oblique patient surfaces. For the underlying target volumes, the mechanical errors can, in combination, be expected to produce target volume misalignments of up to 1 mm on newly installed linacs and 3 mm on older linacs. Thus, 1 mm appears to be a mechanical limit on the positional precision of radiation treatments.     

Benchmarking beam alignment for a clinical helical tomotherapy device

A clinical helical tomotherapy treatment machine has been installed at the University of Wisconsin Comprehensive Cancer Center. Beam alignment has been finalized and accepted by UW staff. Helical tomotherapy will soon be clinically available to other sites. Clinical physicists who expect to work with this machine will need to be familiar with its unique dosimetric characteristics, and those related to the geometrical beam configuration and its verification are described here. A series of alignment tests and the results are presented. Helical tomotherapy utilizes an array of post-patient xenon-filled megavoltage radiation detectors. These detectors have proved capable of performing some alignment verification tests. That is particularly advantageous because those tests can then be automated and easily performed on an ongoing basis.     

Radiation-induced cardiomyopathy as a function of radiation beam gating to the cardiac cycle

Portions of the heart are often unavoidably included in the primary treatment volume during thoracic radiotherapy, and radiation-induced heart disease has been observed as a treatment-related complication. Such complications have been observed in humans following radiation therapy for Hodgkin's disease and treatment of the left breast for carcinoma. Recent attempts have been made to prevent re-stenosis following angioplasty procedures using external beam irradiation. These attempts were not successful, however, due to the large volume of heart included in the treatment field and subsequent cardiac morbidity. We suggest a mechanism for sparing the heart from radiation damage by synchronizing the radiation beam with the cardiac cycle and delivering radiation only when the heart is in a relatively hypoxic state. We present data from a rat model testing this hypothesis and show that radiation damage to the heart can be altered by synchronizing the radiation beam with the cardiac cycle. This technique may be useful in reducing radiation damage to the heart secondary to treatment for diseases such as Hodgkin's disease and breast cancer.     

Estimated spectrum of a 4-MV therapeutic beam

We have made a comparative investigation of the estimated spectra obtained by the Laplace transform analysis of the transmitted exposure data measured in an absorption study of a 4-MV x-ray beam. Four transform pair models currently used with this method have been evaluated. It has been determined that the Archer-Wagner model provides a valid representation of the measured transmission data and yields an estimated spectrum which most closely resembles a Monte Carlo spectrum calculated for a 4-MV therapeutic x-ray beam available from a typical medical accelerator.     

A microdosimetric characterization of a cyclotron-produced therapeutic neutron beam

The therapeutic neutron beam of the Cyclotron Corporation's CP-42 negative-ion cyclotron is generated by protons of 42 MeV bombarding a thin beryllium target. Microdosimetric measurements were made for this neutron beam in a full-scatter water phantom at nine positions inside and outside the useful beam. The lineal energy distribution and the variations of dose-mean, frequency-mean, and saturated lineal energy are compared for these positions. The dose fraction due to gamma rays is also calculated at each of these positions, based upon previously published techniques. A theoretical relative biologic effectiveness, based upon the dual radiation action model of Kellerer and Rossi [Curr. Top. Radiat. Res. 8, 85 (1972)] is also shown for the positions of measurement.     

Mesothelioma relative to asbestos, radiation, and methylcholanthrene

The carcinogenicity of chrysotile asbestos fibers (Canadian and Rhodesian) for the mesothelium of pleura and peritoneum of NEDH rats was explored by injection of 2 mg of asbestos fibers suspended in saline intratracheally, intrapleurally, or intraperitoneally, with or without ancillary radiation treatment (1,000 rad to the whole body of parabiont rats or 2,000 rad to the right thorax of single rats), or alternatively, by injection of asbestos plus 1 mg of 3-methylcholanthrene. A highly significant incidence of mesothelioma (3.8%) was noted in 159 rats treated with asbestos alone, as compared with 0.1% in 1,417 control rats. Additional treatment with radiation or 3-methylcholanthrene increased this incidence to 11.8% and 25.5%, respectively, the latter increase alone being significant at the .01 level of probability.     

Verification of IMRT dose distributions using a water beam imaging system.

A water beam imaging system (WBIS) has been developed and used to verify dose distributions for intensity modulated radiotherapy using dynamic multileaf collimator. This system consisted of a water container, a scintillator screen, a charge-coupled device camera, and a portable personal computer. The scintillation image was captured by the camera. The pixel value in this image indicated the dose value in the scintillation screen. Images of radiation fields of known spatial distributions were used to calibrate the device. The verification was performed by comparing the image acquired from the measurement with a dose distribution from the IMRT plan. Because of light scattering in the scintillator screen, the image was blurred. A correction for this was developed by recognizing that the blur function could be fitted to a multiple Gaussian. The blur function was computed using the measured image of a 10 cm x 10 cm x-ray beam and the result of the dose distribution calculated using the Monte Carlo method. Based on the blur function derived using this method, an iterative reconstruction algorithm was applied to recover the dose distribution for an IMRT plan from the measured WBIS image. The reconstructed dose distribution was compared with Monte Carlo simulation result. Reasonable agreement was obtained from the comparison. The proposed approach makes it possible to carry out a real-time comparison of the dose distribution in a transverse plane between the measurement and the reference when we do an IMRT dose verification.     

Evaluation of a pencil beam algorithm for therapeutic carbon ion beam in presence of bolus

Hot- and cold-dose spots at a shallow depth in a target are formed by carbon ions passing through the bolus with sharp gradients. These spots are caused by sidescatter disequilibrium due to various multiple scattering effects in the different bolus thicknesses. When the dose calculation method by the broad beam algorithm (BBA) is used for treatment planning, these spots cannot be predicted, because the BBA neglects the multiple scattering effects in materials (rms error of 3.9%). On the other hand, since the dose calculation method by the pencil beam algorithm (PBA) takes into account the scattering effects, the results calculated by the PBA agreed better than the BBA with the measured hot- and cold-dose spots, having a rms error of 1.9%. Thus, dose calculation by the PBA improves the accuracy of dose prediction at the shallow depth. However, since dose distributions at deeper positions are affected by many light fragment particles generated by fragment reactions, the results calculated by the PBA disagree with the experimental ones. It is necessary that even the PBA accurately models behavior of fragment particles.     

Verification of segmented beam delivery using a commercial electronic portal imaging device.

In modern radiotherapy, three-dimensional conformal dose distributions are achieved through the delivery of beam ports having precalculated planar distributions of photon beam intensity. Although sophisticated means to calculate and deliver these spatially modulated beams have been developed, means to verify their actual delivery are relatively cumbersome, making equipment and treatment quality assurance difficult to enforce. An electronic portal imaging device of the scanning liquid ionization chamber type yields images which, once calibrated from a previously determined calibration curve, provide highly precise planar maps of the incident dose rate. For verification of an intensity-modulated beam delivered in the segmented approach with a multileaf collimator, a portal image is acquired for each subfield of the leaf sequence. Subsequent to their calibration, the images are multiplied by their respective associated monitor unit settings, and summed to produce a planar dose distribution at the measurement depth in phantom. The excellent agreement of our portal imager measurements with calculations of our treatment planning system and measurements with a one-dimensional beam profiler attests to the usefulness of this method for the planar verification of intensity-modulated fields produced in the segmented approach on a computerized linear accelerator equipped with a multileaf collimator.     

Organ/patient geometric variation in external beam radiotherapy and its effects

Treatment variation in positioning of the organ/patient with respect to the radiation beams causes a temporal dose variation in critical normal tissues adjacent to the treatment target. This temporal variation induces uncertainties in understanding the normal tissue dose response, thereby limiting reliable treatment evaluation and optimization. The aim of this study is to model and analyze the temporal variation of organ dose distribution, and its effect on the biological effective dose. The study mainly focuses on the temporal dose variation caused by intertreatment organ motion/ deformation and daily setup error. Sensitivity of the biological effective dose to organ/patient geometric variation, dose distribution, and treatment fractionation will be investigated. Significant deviation of the biological effective dose could be expected in a critical normal structure, even if the cumulative dose deviation in this structure is negligible. Patients with similar geometric variation characteristics can experience significantly different biological effective dose, and the differences are sensitive to the dose distribution and the total number of treatment fractions.     

5q- syndrome in a patient with chronic exposure to ionizing radiation

A report is presented on an interstitial deletion of the long arm of chromosome #5(5q-), associated with refractory anemia in a patient who had been exposed to chronic ionizing radiation. Six years of follow-up did not disclose any evolution toward leukemia.     

Using patient data similarities to predict radiation pneumonitis via a self-organizing map

This work investigates the use of the self-organizing map (SOM) technique for predicting lung radiation pneumonitis (RP) risk. SOM is an effective method for projecting and visualizing high-dimensional data in a low-dimensional space (map). By projecting patients with similar data (dose and non-dose factors) onto the same region of the map, commonalities in their outcomes can be visualized and categorized. Once built, the SOM may be used to predict pneumonitis risk by identifying the region of the map that is most similar to a patient's characteristics. Two SOM models were developed from a database of 219 lung cancer patients treated with radiation therapy (34 clinically diagnosed with Grade 2+ pneumonitis). The models were: SOM(all) built from all dose and non-dose factors and, for comparison, SOM(dose) built from dose factors alone. Both models were tested using ten-fold cross validation and Receiver Operating Characteristics (ROC) analysis. Models SOM(all) and SOM(dose) yielded ten-fold cross-validated ROC areas of 0.73 (sensitivity/specificity = 71%/68%) and 0.67 (sensitivity/specificity = 63%/66%), respectively. The significant difference between the cross-validated ROC areas of these two models (p < 0.05) implies that non-dose features add important information toward predicting RP risk. Among the input features selected by model SOM(all), the two with highest impact for increasing RP risk were: (a) higher mean lung dose and (b) chemotherapy prior to radiation therapy. The SOM model developed here may not be extrapolated to treatment techniques outside that used in our database, such as several-field lung intensity modulated radiation therapy or gated radiation therapy.     

Verification of absolute ionization chamber dosimetry in a proton beam using carbon activation measurements

Reference ionization chamber dosimetry implemented in a clinical proton beam and based on the ICRU 59 recommendations has been verified with an independent carbon activation method. The 12C(p,pn)11C nuclear reaction was used to measure the beam fluence and entrance dose. A method to transfer from the entrance dose to the dose at the ion chamber calibration position has been developed. Measurements performed in a monochromatic 200 MeV beam show that the ratio of absolute doses measured using the carbon activation and the ion chamber methods is 1.017 +/- 0.03 (type A uncertainty). This result is within the uncertainties of both methods employed, which are estimated at +/- 4.3% (carbon activation) and +/- 2.7% (ion chamber calibration).     

A study of the characteristics of radiation contaminants within a clinically useful photon beam

Contaminant radiation within a therapeutic beam has been studied for accelerator-produced 24-MV x rays by a direct measurement utilizing independent jaws. A carefully positioned diode was exposed to secondary radiation for various collimator settings that project field sizes between [10 X (0 + 5)] and [26 X (0 + 13)] cm at a source-to-axis distance of 1 m. [The notation [L X (W1 + W2)] means the projected field area due to lower (L) and upper (W1, W2) jaws.] Measurements were taken by placing polystyrene sheets with density thicknesses ranging from 0.21 to 4.53 g cm-2 in front of the detector. The data strongly demonstrate that with increasing field size, the ratio of the dose due to electrons to that due to photons increases rapidly. The characteristic feature of the electron depth dose curves may be linked to the observed shift in the depth of dose maximum (dmax) with field size. Data taken with a magnetic field of 0.15 T permit analysis of photons and electrons with regard to their intensity, energy, and behavior in a phantom. From an analysis of 12.5 X 12.5 cm data, various radiation components have been studied and are correlated with the observed contaminants.     

Verification of dynamic intensity-modulated beam deliveries in canine subjects.

Our objective in this work was to assess the precision and degree of accuracy with which intensity modulated radiation therapy (IMRT) can deliver highly localized dose distributions to tumors near critical structures using the dynamic sliding window technique. Measurements of dose distribution were performed both in vivo and in vitro using a combination of dosimeters [thermoluminescent dosimeters (TLD's), films, and diodes]. In vivo measurements were performed in two groups of purpose-bred dogs: one receiving four-field three-dimensional (3D) conformal treatment and the other receiving IMRT. The algorithms used in the inverse planning process included the Macro Pencil Beam (MPB) model and Projections onto Convex Sets (POCS). Single beam measurements were performed in phantoms to verify the accuracy of monitor unit settings required for delivering the desired doses. The composite doses from the delivery of the seven beam intensity modulated plans were measured in phantoms and cadavers, Biological end points (spinal cord toxicity and neurologic deficits due to irradiation) were evaluated at the end of one year to determine the spatial accuracy of the IMRT treatments over a fractionated course in live subjects. Results in single beam measurements were used at first to improve the dose calculation and translation algorithms. Results of the measurements for the delivery of all seven beams in phantoms confirmed that the system was capable of accurate spatial and dosimetric IMRT delivery. The in vivo results showed dramatic differences between control and IMRT-treated dogs, with the IMRT group showing no adverse effects and the control animals showing severe spinal cord injuries due to irradiation. The measurements presented in this paper have helped to verify the successful and accurate delivery of IMRT in a clinically related model using the University of Washington Medical Center (UWMC) system.     

Spatial fragment distribution from a therapeutic pencil-like carbon beam in water

The latest heavy ion therapy tends to require information about the spatial distribution of the quality of radiation in a patient's body in order to make the best use of any potential advantage of swift heavy ions for the therapeutic treatment of a tumour. The deflection of incident particles is described well by Molière's multiple-scattering theory of primary particles; however, the deflection of projectile fragments is not yet thoroughly understood. This paper reports on our investigation of the spatial distribution of fragments produced from a therapeutic carbon beam through nuclear reactions in thick water. A DeltaE-E counter telescope system, composed of a plastic scintillator, a gas-flow proportional counter and a BGO scintillator, was rotated around a water target in order to measure the spatial distribution of the radiation quality. The results revealed that the observed deflection of fragment particles exceeded the multiple scattering effect estimated by Molière's theory. However, the difference can be sufficiently accounted for by considering one term involved in the multiple-scattering formula; this term corresponds to a lateral 'kick' at the point of production of the fragment. This kick is successfully explained as a transfer of the intra-nucleus Fermi momentum of a projectile to the fragment; the extent of the kick obeys the expectation derived from the Goldhaber model.