J Kruse

On the insensitivity of single field planar dosimetry to IMRT inaccuracies

PURPOSE To report on the sensitivity of single field planar measurements in identifying IMRT plans with poor calculational accuracy. METHODS Three IMRT plans for head and neck cancer were subjected to extensive quality assurance. The plans were recalculated on a cylindrical phantom and between eight and 18 low gradient points were measured in each plan with an ion chamber. Every point measured in these plans agreed to within 4% of the dose predicted by the planning system and the plans were judged acceptable for clinical use. Each plan was then reoptimized with aggressive dose constraints so that the new treatment fields were more highly modulated than the ones from the original plans. Very complex fields can be calculated less accurately and ion chamber measurements of these plans in the cylindrical phantom confirmed significant dosimetric errors-Several of the measured points in each plan differed from the calculated dose by more than 4%, with a maximum single deviation of 10.6%. These three plans were judged unacceptable for clinical use. All six plans (three acceptable, three unacceptable) were then analyzed with two means of individual field planar dosimetry: Portal imaging with an electronic portal imaging device (EPID) and an ion chamber array. Gamma analysis was performed on each set of planar measurements with 2%/2 mm distance to agreement (DTA) and 3%/3 mm DTA criteria to try to determine a gamma analysis threshold which would differentiate the flawed plans from the acceptable ones. RESULTS With the EPID and 2%/2 mm DTA criteria, between 88.2% and 92.8% of pixels from the acceptable IMRT plans passed the gamma analysis, and between 87.5% and 91.9% passed for the unacceptable IMRT plans. With the ion chamber array and 2%/2 mm DTA criteria, between 92.4% and 94.9% of points in the acceptable plans passed the gamma analysis, while 86.8% to 98.3% of the points in the unacceptable plans passed the gamma analysis. The difference between acceptable and unacceptable plans was diminished further when gamma criteria were expanded to 3%/3 mm DTA. A fraction of pixels passing the gamma analysis was found to be a poor predictor of dosimetric accuracy with both planar dosimeters, as well as both sets of gamma criteria. CONCLUSIONS Deconstruction of an IMRT plan for field-by-field QA requires complex analysis methods such as the gamma function. Distance to agreement, a component of the gamma function, has clinical relevance in a composite plan but when applied to individual, highly modulated fields, it can mask important dosimetric errors. While single field planar dosimetry may comprise one facet of an effective QA protocol, gamma analysis of single field measurements is insensitive to important dosimetric inaccuracies of the overall plan.

TECHNICAL ASPECTS OF DAILY ONLINE POSITIONING OF THE PROSTATE FOR THREE-DIMENSIONAL CONFORMAL RADIOTHERAPY USING AN ELECTRONIC PORTAL IMAGING DEVICE

PURPOSE To develop a real-time electronic portal imaging device (EPID) procedure to identify intraprostatic gold markers and correct daily variations in target position during external beam radiotherapy for prostate cancer. METHODS AND MATERIALS Pretherapy electronic portal images (EPIs) were acquired with a small portion of the therapeutic 18-MV dose from an orthogonal pair of treatment fields. The position of the intraprostatic gold markers on the EPIs was aligned with that on the simulation digitally reconstructed radiographs. If the initial three-dimensional target displacement (3DI) exceeded 5 mm or rotations exceeded 3 degrees, the beam was realigned before the remainder of the dose was delivered. Field-only EPIs were then acquired for all fields and offline analysis was performed to determine the final 3D target placement (3DF). RESULTS Twenty patients completed protocol-specified treatment, and all markers were identified on 99.6% of the pretherapy EPIs. Overall, 53% of treatment fractions were realigned. The mean 3DI was 5.6 mm in all patients (range 3.7-9.3), and the mean 3DF was 2.8 mm (range 1.6-4.0), which was statistically significant (p < 0.001). Rotational corrections were made on 15% of treatments. Mean treatment duration was 1.4 min greater for protocol patients than for similar patients in whom localization was not performed. CONCLUSIONS Frequent field misalignment occurs when external fiducial marks are used for patient alignment. Misalignments can be readily and rapidly identified and corrected with an EPID-based online correction procedure that integrates commercially available equipment and software.

Proton Beam Radiotherapy Versus Three-Dimensional Conformal Stereotactic Body Radiotherapy in Primary Peripheral, Early-Stage Non–Small-Cell Lung Carcinoma: A Comparative Dosimetric Analysis

PURPOSE Proton radiotherapy (PT) and stereotactic body radiotherapy (SBRT) have the capacity to optimize the therapeutic ratio. We analyzed the dosimetric differences between PT and SBRT in treating primary peripheral early-stage non-small-cell lung cancer. METHODS AND MATERIALS Eight patients were simulated, planned, and treated with SBRT according to accepted techniques. SBRT treatments were retrospectively planned using heterogeneity corrections. PT treatment plans were generated using single-, two-, and three-field passively scattered and actively scanned proton beams. Calculated dose characteristics were compared. RESULTS Comparable planning target volume (PTV) median minimum and maximum doses were observed between PT and SBRT plans. Higher median maximum doses 2 cm from the PTV were observed for PT, but higher median PTV doses were observed for SBRT. The total lung mean and V5 doses were significantly lower with actively scanned PT. The lung V13 and V20 were comparable. The dose to normal tissues was lower with PT except to skin and ribs. Although the maximum doses to skin and ribs were similar or higher with PT, the median doses to these structures were higher with SBRT. Passively scattered plans, compared with actively scanned plans, typically demonstrated higher doses to the PTV, lung, and organs at risk. CONCLUSIONS Single-, two-, or three-field passively or actively scanned proton therapy delivered comparable PTV dose with generally less dose to normal tissues in these hypothetic treatments. Actively scanned beam plans typically had more favorable dose characteristics to the target, lung, and other soft tissues compared with the passively scanned plans. The clinical significance of these findings remains to be determined.

Electronic and film portal images: a comparison of landmark visibility and review accuracy.

PURPOSE To quantitatively compare a scanning liquid ion chamber electronic portal imaging device (SLIC-EPID) and an amorphous silicon flat panel (aSi) EPID with portal film in clinical applications using measures of landmark visibility and treatment review accuracy. METHODS AND MATERIALS Six radiation oncologists viewed 39 electronic portal images (EPIs) from the SLIC-EPID, 36 EPIs from the aSi-EPID, and portal films of each of these treatment fields. The physicians rated the clarity of anatomic landmarks in the portal images, and the scores were compared between EPID and film. Nine hundred portal image reviews were performed. EPID and film portal images were acquired with known setup errors in either phantom or cadaver treatments. Physicians identified the errors visually in portal films and with computerized analysis for EPID. RESULTS There were no statistically significant (p < 0.05) differences between film and SLIC-EPID in ratings of landmark clarity. Eleven of 12 landmarks were more visible in aSi-EPID than in film. Translational setup errors were identified with an average accuracy of 2.5 mm in film, compared to 1.5 mm with SLIC-EPID and 1.3 mm with aSi-EPID. CONCLUSIONS Both EPIDs are clinically viable replacements for film, but aSi-EPID represents a significant advancement in image quality over film.

Guide to clinical use of electronic portal imaging

The Electronic Portal Imaging Device (EPID) provides localization quality images and computer‐aided analysis, which should in principal, replace portal film imaging. Modern EPIDs deliver superior image quality and an array of analysis tools that improve clinical decision making. It has been demonstrated that the EPID can be a powerful tool in the reduction of treatment setup errors and the quality assurance and verification of complex treatments. However, in many radiation therapy clinics EPID technology is not in routine clinical use. This low utilization suggests that the capability and potential of the technology alone do not guarantee its full adoption. This paper addresses basic considerations required to facilitate clinical implementation of the EPID technology and gives specific examples of successful implementations.

The clinical case for proton beam therapy

AbstractOver the past 20 years, several proton beam treatment programs have been implemented throughout the United States. Increasingly, the number of new programs under development is growing. Proton beam therapy has the potential for improving tumor control and survival through dose escalation. It also has potential for reducing harm to normal organs through dose reduction. However, proton beam therapy is more costly than conventional x-ray therapy. This increased cost may be offset by improved function, improved quality of life, and reduced costs related to treating the late effects of therapy. Clinical research opportunities are abundant to determine which patients will gain the most benefit from proton beam therapy. We review the clinical case for proton beam therapy.Summary sentenceProton beam therapy is a technically advanced and promising form of radiation therapy.

Utilizing an electronic portal imaging device to monitor light and radiation field congruence

A method to investigate light and radiation field congruence utilizing a commercially available amorphous silicon electronic portal imaging device (EPID) was developed. This method employed an EPID, the associated EPI software, and a diamond‐shaped template. The template was constructed from a block tray in which Sn/Pb wires, 1 mm in diameter, were embedded into a diamond shaped groove milled into the tray. The collimator jaws of the linac were aligned such that the light field fell directly on the corners of the diamond. A radiation detection algorithm within the EPI software determined the extent of the radiation field. The light and radiation field congruence was evaluated by comparing the vertexes of the diamond reference structure to the detected radiation field. In addition, the digital jaw settings were recorded and later compared to the light field detected on the films and EPIs. Three linear accelerators were tracked for a period ranging from 2–8 months. Light radiation field congruence tests with films and EPIs were comparable, yielding a difference of less than 0.6 mm, well within the allowed 2‐mm tolerance. A disparity was observed in the magnitude of the detected light field. The X and Y dimensions of the light field measured with film differed by less than or equal to 1.4 mm from the digital collimator settings, whereas the values extracted from the EPIs differed by up to 2.5 mm. Based on these findings, EPIs were found to be a quick and reliable alternative to film for qualitative and relative analyses. PACS number(s): 87.53.Xd, 87.56.Fc, 87.53.Oq, 87.52.–g, 87.53.–j

Optimizing Normal Tissue Sparing in Ion Therapy Using Calculated Isoeffective Dose for Ion Selection

PURPOSE To investigate how the selection of ion type affects the calculated isoeffective dose to the surrounding normal tissue as a function of both normal tissue and target tissue α/β ratios. METHODS AND MATERIALS A microdosimetric biologic dose model was incorporated into a Geant4 simulation of parallel opposed beams of protons, helium, lithium, beryllium, carbon, and neon ions. The beams were constructed to give a homogeneous isoeffective dose to a volume in the center of a water phantom for target tissues covering a range of cobalt equivalent α/β ratios of 1-20 Gy. Concomitant normal tissue isoeffective doses in the plateau of the ion beam were then compared for different ions across the range of normal tissue and target tissue radiosensitivities for a fixed isoeffective dose to the target tissue. RESULTS The ion type yielding the optimal normal tissue sparing was highly dependent on the α/β ratio of both the normal and the target tissue. For carbon ions, the calculated isoeffective dose to normal tissue at a 5-cm depth varied by almost a factor of 5, depending on the α/β ratios of the normal and target tissue. This ranges from a factor of 2 less than the isoeffective dose of a similar proton treatment to a factor of 2 greater. CONCLUSIONS No single ion is optimal for all treatment scenarios. The heavier ions are superior in cases in which the α/β ratio of the target tissue is low and the α/β ratio of normal tissue is high, and protons are superior in the opposite circumstances. Lithium and beryllium appear to offer dose advantages similar to carbon, with a considerably lower normal tissue dose when the α/β ratio in the target tissue is high and the α/β ratio in the normal tissue is low.

3D calculation of radiation‐induced second cancer risk including dose and tissue response heterogeneities

PURPOSE Tools for comparing relative induced second cancer risk, to inform choice of radiotherapy treatment plan, are becoming increasingly necessary as the availability of new treatment modalities expands. Uncertainties, in both radiobiological models and model parameters, limit the confidence of such calculations. The aim of this study was to develop and demonstrate a software tool to produce a malignant induction probability (MIP) calculation which incorporates patient-specific dose and allows for the varying responses of different tissue types to radiation. METHODS The tool has been used to calculate relative MIPs for four different treatment plans targeting a subtotally resected meningioma: 3D conformal radiotherapy (3DCFRT), volumetric modulated arc therapy (VMAT), intensity-modulated x-ray therapy (IMRT), and scanned protons. RESULTS Two plausible MIP models, with considerably different dose-response relationships, were considered. A fractionated linear-quadratic induction and cell-kill model gave a mean relative cancer risk (normalized to 3DCFRT) of 113% for VMAT, 16% for protons, and 52% for IMRT. For a linear no-threshold model, these figures were 105%, 42%, and 78%, respectively. The relative MIP between plans was shown to be significantly more robust to radiobiological parameter uncertainties compared to absolute MIP. Both models resulted in the same ranking of modalities, in terms of MIP, for this clinical case. CONCLUSIONS The results demonstrate that relative MIP is a useful metric with which treatment plans can be ranked, regardless of parameter- and model-based uncertainties. With further validation, this metric could be used to discriminate between plans that are equivalent with respect to other planning priorities.

Patient-specific daily pretreatment setup protocol using electronic portal imaging for radiation therapy

The purpose of this study was to evaluate electronic portal imaging (EPI) as a means of identifying and correcting field displacement in patients with problematic external beam radiotherapy setups. Fourteen patients with problematic setups were identified for pretreatment daily EPI beam monitoring as part of a physician‐directed therapist intervention protocol. Pretreatment EPIs were used to realign fields as necessary to bring the setup within the physician‐prescribed tolerance level. For comparison, daily EPIs were available for 12 control patients who had no particular setup difficulties and for whom online beam realignment was not made. Anatomy‐matching software was used to measure setup variation along medial‐lateral, superior‐inferior, and anterior‐posterior axes. Online field realignment yielded a significant (p=0.001) improvement when comparing initial and final setup variations. The mean standard deviation of setup displacement averaged over three axes was reduced from 6.4 mm to 3.1 mm after realignment. The final variation of protocol patients was comparable to that of control patients. In conclusion, EPI provided effective means to perform online beam realignment in a group of difficult‐to‐position patients. This procedure resulted in a reduction in setup displacement that was statistically significant, clinically relevant, and approached that of a more typical patient group. PACS number: 87.53.Oq