Digitally Reconstructed Radiographs Research Articles

435 poster A linear-quadratic-offset-filter for contrast enhancement in digitally reconstructed radiographs (DRR) for virtual simulation in radiotherapy

435 poster A linear-quadratic-offset-filter for contrast enhancement in digitally reconstructed radiographs (DRR) for virtual simulation in radiotherapy

Use of digitally reconstructed radiographs in radiotherapy treatment planning and verification.

The authors present 3 years of experience in using digitally reconstructed radiographs (DRR) for radiotherapy planning and verification. Comparison is made with simulation film (SF), to illustrate the advantages of DRR over SF. Emphasis is placed on using the appropriate equipment and applying the correct technique. A brief discourse on the principle of CT imaging is presented to illustrate the operation of CT software and optimization of image display for axial slices and DRR. Emphasis is placed on the application of clinical knowledge to enhance the usefulness, as well as the technical quality, of the DRR. Illustrative examples are given.

Predictors of severe esophagitis include use of concurrent chemotherapy, but not the length of irradiated esophagus: a multivariate analysis of patients with lung cancer treated with nonoperative therapy

Purpose: To identify in a multivariate analysis treatment-related factors predisposing patients (pts) with lung cancer to acute esophagitis, expressed as a severity grade or Esophagitis Index (EI). Methods and Materials: Acute esophagitis is prospectively scored as an RTOG Grade in our institution during and after thoracic radiotherapy. Charts, toxicity forms and digitally reconstructed radiographs (DRRs) of all pts with lung cancer who received thoracic radiotherapy (RT) between 11/95 and 1/99 were reviewed. Esophagitis grades for each time point were verified by review of weekly physician and nursing treatment notes, hospital discharge summaries and referring physician notes and then plotted on graph against time. The area under the curve was calculated for each patient’s graph and was defined as an Esophagitis Index. The length of esophagus was measured on each anterior DRR while assuming that esophagus overlies the vertebral bodies on the anterior films and projects over the edge of the vertebral body on the oblique DRRs. This assumption was confirmed in 10 pts by digitizing esophagus on CT simulator-derived slices and visualizing its position on DRRs. To compare RT doses delivered with different fractionation schemes to standard fractionated doses, the equivalent RT doses were calculated using the linear–quadratic formula and α/β ratio of 10. Univariate and multivariate analyses of several factors potentially influencing the maximum esophagitis grade, as well as EI, were performed. Results: A total of 277 pts were identified. Pts were included in the analysis ( n = 105) if they fulfilled the following criteria: chart, toxicity form and DRRs were all available; parallel opposed fields (no multiple fields) were used for both the initial and off cord/cone down fields; and an equivalent dose of 45.0 Gy or more was delivered. Seventy-eight pts had Stage III; 32, Stage IV, and the remainder, Stages I, II, or recurrent lung cancer (85 non–small cell and 18, small cell). Seventy-four pts were treated with definitive intent. Chemotherapy was given concurrently with RT in 58 pts (in 7 pts, with twice daily, or b.i.d., RT) and as induction treatment, in 11. Only 2 pts required a treatment break of more than 1 week. Median total and equivalent RT doses, fraction size, and anterior esophageal length were as follows: 59.9 Gy, 59.9 Gy, 2.0 Gy, and 14 cm (range, 4.2–21). The following maximum grades of esophagitis were recorded: 1, in 54 pts; 2, in 17 pts; 3, in 13 pts, and 4, in 1 pt. The mean EI for all pts; pts treated with standard RT alone; induction chemotherapy and standard RT; concurrent chemotherapy and standard (QD) RT; and b.i.d. RT with concurrent chemotherapy, was 41.5 (range, 0–317); 13.6; 24.5; 52.4; and 132.1, respectively ( p < 0.001). Three pts developed an esophageal stricture within 3 months beginning RT. In multivariate analysis, the following factors were significantly associated with increasing EI: concurrent chemotherapy with QD RT and concurrent chemotherapy with b.i.d. RT ( p < 0.001, considered jointly). Both factors were also associated with increasing maximum esophagitis grade ( p = 0.011). Esophageal length was not associated with increasing EI or esophagitis grade in either univariate or multivariate analyses. Conclusion: Concurrent chemotherapy and twice daily radiotherapy, especially if combined together, were associated with the highest acute maximum esophagitis grade and esophagitis index in pts with lung cancer. The duration of acute esophagitis was also longest in the concurrent chemotherapy/twice daily radiotherapy group. Esophagitis Index appeared to be a more sensitive measure of acute esophagitis than the maximum esophagitis grade. The increasing length of esophagus in the radiation field did not predict for the severity of acute esophagitis.

An alternative mantle irradiation technique using 3D CT- based treatment planning for female patients with Hodgkin’s disease

Purpose: For female patients, radiotherapy treatment for Hodgkin’s disease invariably results in the irradiation of breast tissue that may lead to radiation induced secondary cancers. The risk for secondary breast cancer is correlated with dose. We have developed a technique in an attempt to increase breast sparing during mantle field irradiation for female patients. Material and Methods: To minimize the irradiated breast volume, a virtual simulation technique making use of a Styrofoam breast immobilization board has been developed whereby the patient lies prone with the breasts positioned in grooves within the board. The breast position is adjusted using Styrofoam wedges, and breast placement is verified using an AP CT-pilot view. A CT scan of the neck and thoracic regions is taken, and the lymph nodes, breast volume and critical structures are outlined. Virtual simulation of the mantle fields (typically AP/PA isocentric beams) is performed, and beam blocks are drawn on the digitally reconstructed radiographs (DRR) generated by the virtual simulation package. The shielding is designed to allow adequate margins around the lymph nodes while maximizing shielding of the lung and breast tissues. The para-aortic fields are also easily determined through virtual simulation, where multi-planar reconstructions (MPR) and 3D renderings of the patient’s CT data are used to determine the field limits and beam gaps. In addition to allowing for the geometric optimization of the positioning of the breasts under the lung shields, the virtual simulation technique provides the necessary information for a 3D dosimetric analysis, including dose-volume histograms (DVHs) of the irradiated breast volume. Results: The 3D breast sparing technique was qualitatively and quantitatively compared to non-CT-based techniques and other 3D techniques currently available to assess the protection of the breasts. In a preliminary analysis, virtual simulation images (DRRs, 3D rendering and multi-planar reconstruction) demonstrated the advantage of using the breast sparing technique. A further analysis of DVHs showed a reduction of at least 50% in the volume of breast tissue irradiated when using the breast positioning board and virtual simulation as compared to the conventional simulation techniques where a breast immobilization board was not used. Conclusions: The use of a breast immobilization board and of a virtual simulation technique is recommended for the planning and treatment of female patients with Hodgkin’s disease. DVH analysis has shown that this leads to a decrease in the volume of breast irradiated. It is hoped that this approach will reduce the risk of secondary breast malignancies in female patients with Hodgkin’s disease.

Designing attenuators for total-body irradiation using virtual simulation

In total-body photon irradiation, the lungs are the most commonly shielded organ. Lung compensators are often designed by using high-energy portal films. Other organs, such as the kidneys and liver, are poorly visualized in portal films due to their unit-density composition. A computed tomography-based technique to design kidney and liver attenuators involves outlining these organs in a virtual simulation. The position and the shape of the attenuator are then determined from a digitally-reconstructed radiograph. Appropriate attenuator thickness is determined from measured transmission curves. This article provides a summary of this technique for total-body photon irradiation in a 4-MV photon beam.

Intensity modulation for breast treatment using static multi-leaf collimators

Purpose: To achieve more uniform dose distributions in breast cancer treatment using multiple sets of multi-leaf collimator (MLC) defined fields. Dose uniformity for many breast cancer patients can be significantly improved by using two or more sets of portals and the “hot” regions of a traditional treatment can be significantly reduced. Methods and Materials: Patients for breast cancer treatment are immobilized with alpha cradle in the traditional arm-up position and have a CT scan in the treatment position. The target volume is delineated on the 5-mm thick CT slices that are obtained from the lower neck to well below the breast target volume. Medial and lateral tangential fields at conventional gantry angles are designed with the aid of digitally reconstructed radiographs (DRRs). The MLC, without collimator rotation, is used to shape the field to spare as much lung as possible. The wedges and relative weights of the beams are optimized to provide the best dose uniformity. For the patients with large dose inhomogeneity, a second set of fields is designed. The weight of the original set of fields is reduced (usually to ∼90%) so that the “original hot” regions receive the prescription dose; the second set of fields delivers a supplemental dose to the “cold” region, typically ∼10% of the total dose. The second set of fields has the same beam parameters but “treat” only the part of breast tissue that is “cool.” Presently, the design of the reduced field is an iterative process. The process can be extended to more than two sets of portals to obtain the desired dose uniformity. Results: With 3D planning and multiple MLC fields, dose uniformity in the treatment of breast patients was improved from 7%–22% to ∼7%–15%. The volume receiving these high doses decreased significantly and shifted from the lung to the target. By keeping the gantry angles and wedges the same for the multiple fields, treatments can be delivered quickly and reliably. The internal mammary nodes (IM) can also be treated without including significant amount of lung or heart in the field. Conclusion: Dose uniformity can be significantly improved by using this intensity modulation technique to treat certain breast patients. With these static MLC fields creating the intensity modulation, the dose uniformity to the breast can be significantly improved and the hot region in lung reduced. There is no increase in setup complexity. The small increase in treatment time is insignificant.

The role of CT simulation in whole-brain irradiation

Purpose: Evidence is growing that incorrect field-shaping is a major cause of treatment failure in whole-brain irradiation (WBI). To evaluate the potential benefits of CT simulation in WBI we compared field-shaping based on 3D CT simulation to conventional 2D simulation. Methods: CT head scans were obtained from 20 patients. Conventional 2D planning was imitated by drawing the block contours on digitally reconstructed radiographs (DRR) by four radiotherapists. Critical parts of the target and the eye lenses were subsequently marked and planning was repeated using 3D information (“3D planning”). The results of both methods were compared by evaluation of the minimal distance from the field edge according to each site. Results: In conventional planning using DRR, major geographic mismatches (< −3 mm) occurred in the subfrontal region and both eye lenses with 1% each location. Minor mismatches (−3 to 0 mm) predominantly occurred in the contralateral lens (21%), ipsilateral lens (10%), and subfrontal region (9%). Close margins (0–5 mm) were most frequently noted at the contralateral lens (49%), ipsilateral lens (35%), and the subfrontal region (28%). When 3D planning was used, mismatches were not found. However, close margins were inevitable at the ipsilateral lens (5%), subfrontal region (30%), and contralateral lens (70%). Conclusions: CT simulation in WBI is significantly superior to conventional simulation with respect to complete coverage of the target volume and protection of the eye lenses. The narrow passage between the ocular lenses and lamina cribrosa represents a serious limitation. These patients are safely identified with CT simulation and can be referred for modified irradiation techniques.

Clinical application of digitally-reconstructed radiographs generated from magnetic resonance imaging for intracranial lesions

Purpose: The purpose of this work is to demonstrate the clinical utility of magnetic resonance (MR) imaging-based digitally reconstructed radiographs (DRRs) for the setup and verification of patients with intracranial lesions. Methods and Materials: MR images of 16 patients with various intracranial lesions were obtained for treatment planning and virtual simulation. Five-millimeter-thick contiguous T1-weighted postcontrast transverse slices were obtained using a standard head coil in a General Electric Signa 1.5T MR scanner. MR-DRRs were generated using the “pseudo density” technique on an existing treatment planning computer without any special modifications. Anterior and lateral verification films were taken for each patient for visual comparison with MR-based DRRs. Results: Visual alignment with bony landmarks, including the orbits, frontal sinus, sphenoid sinus, auditory meatus, nasal bone, vomer bone, mastoid process, and the cranium were used by physicians, physicists, and therapists to verify patient positioning. Misalignments from 3 to 10 mm were visually identified and corrected using this technique. Conclusion: A method for visually utilizing MR-based DRRs during simulation has been developed and clinically implemented. The quality of MR-DRRs generated using this technique is such that physicians, physicists, and therapists can easily and routinely compare MR-DRRs side-by-side with simulation films.

Assessment of accuracy of daily set-ups in prostate radiotherapy using electronic imaging.

The purpose of the study was to assess the accuracy, using electronic portal imaging, of daily set-ups, in patients undergoing radiotherapy for prostate carcinoma. We used a scanning liquid ion chamber to assess the accuracy of set-ups in 25 consecutive patients undergoing a 6 1/2 week course of radiotherapy to the prostate. Electronic images (EPIs) were collected during 33 treatments to each of four ports. The positions of anatomical structures on the EPIs were compared with the same structures seen on digitally reconstructed radiographs (DRRs) made after CT simulation before beginning radiotherapy. Displacements of the EPIs compared with the DRRs were computer-calculated in millimetres in lateral, longitudinal and rotational directions for each port. 11 patients had ports moved because of discrepancies between the EPIs and the DRRs; eight required moves in the first five treatments to correct systematic (simulator) errors. In the right-left and anterior-posterior directions, nearly 95% of the EPIs were within 5 mm of the simulated port position. In the superior-inferior direction, 98% of the ports were within 5 mm of the simulated port position. Two patients had in-plane rotational errors on the lateral ports (8 degrees and 10 degrees respectively). It was concluded that daily electronic imaging is an effective technique for assessing the accuracy of set-ups in prostate radiotherapy.

Magnetic resonance imaging based digitally reconstructed radiographs, virtual simulation, and three-dimensional treatment planning for brain neoplasms.

Currently, patients with brain neoplasms must undergo both computed tomography (CT) and magnetic resonance (MR) imaging to take advantage of CT's density information and MR's soft tissue imaging capabilities. A method has been developed that allows virtual simulation, digitally reconstructed radiographs (DRRs), and 3-D treatment planning of patients with brain neoplasms to be generated using only one T1-weighted MR data set. DRRs of an anthropomorphic RANDO head phantom were generated using MR and CT imaging. The MR based DRRs provided structural information equivalent to CT based DRRs. The spatial linearity of CT and MR image sets was evaluated by measuring the percent distortion and spatial error. There was no statistical difference in spatial linearity or accuracy between the CT and MR image sets. MR and CT based treatment planning were compared using a variety of different treatment accessories, field sizes, photon energies, and gantry positions. Doses at various points throughout the head phantom were used as comparison points between CT based heterogeneous, CT based homogenous, and MR based homogenous treatment planning of the head phantom. Lithium fluoride thermoluminescent dosimeters were used to verify the dosimetric accuracy of MR based treatment planning by taking measurements at these points. For treatment plans with fields that pass through large air cavities, such as the maxillary sinus, homogenous treatment planning produces unacceptable dosimetric error (2%-4%). For treatment plans with fields that pass through the skull, MR homogenous treatment planning can be used with a dosimetric accuracy of +/- 2%.

Lung and heart dose volume analyses with CT simulator in radiation treatment of breast cancer

Purpose: Radiation pneumonitis and cardiac effects are directly related to the irradiated lung and heart volumes in the treatment fields. The central lung distance (CLD) from a tangential breast radiograph is shown to be a significant indicator of ipsilateral irradiated lung volume. Retrospective analysis of the pattern of dose volume of lung and heart with actual volume data from a CT simulator in the treatment of breast cancer is presented with respect to CLD. Methods and Materials: The heart and lung volumes in the tangential treatment fields were analyzed in 108 consecutive cases (52 left and 56 right breast) referred for CT simulation. All patients in this study were immobilized and placed on an inclined breast board in actual treatment setup. Both arms were stretched over head to avoid collision with the scanner aperture. Radiopaque marks were placed on the medial and lateral borders of the tangential fields. All patients were scanned in spiral mode with slice width and thickness of 3 mm each, respectively. The lung and heart structures as well as irradiated areas were delineated on each slice and respective volumes were accurately measured. The treatment beam parameters were recorded and the digitally reconstructed radiographs (DRRs) were generated for the measurement of the CLD and analysis. Results: Using CT data the mean volume and standard deviation of left and right lungs were 1307.7 ± 297.7 cm 3 and 1529.6 ± 298.5 cm 3, respectively. The magnitude of irradiated volume in left and right lung is nearly equal for the same CLD that produces different percent irradiated volumes (PIV). The left and right PIV lungs are 8.3 ± 4.7% and 6.6 ± 3.7%, respectively. The PIV data have shown to correlate with CLD with second- and third-degree polynomials; however, in this study a simple straight line regression is used to provide better confidence than the higher order polynomials. The regression lines for the left and right breasts are very different based on actual CT data. The slopes of regression lines for the left and right lung are 0.6%/mm and 0.5%/mm, respectively which is statistically different with the p value of 0.01. A maximum heart PIV of >3.0% is observed in 80% of the patients. The heart PIV is inversely correlated with gantry angle and weakly correlated with CLD. Conclusions: The CT-simulator provides accurate volumetric information of the heart and lungs in the treatment fields. The lung PIV is directly correlated to the CLD (0.6%/mm and 0.5%/mm for the left and right lungs). Left and right lungs have different volumes and hence, different regression lines are recommended. An additional 12% lung volume could be irradiated in the supraclavicular field. Heart volume is not correlated with the CLD. The heart PIV is associated to the beam angle. Heart volume may not be accurately visualized in a tangential radiograph; however, this can be easily seen in a DRR with contour delineation and can be minimized with proper beam parameters iteratively with a virtual simulator. Lung and heart PIV along with dose volume histograms (DVH) are essential in reducing pulmonary and cardiac complications.

Computed tomographic simulation of craniospinal fields in pediatric patients: improved treatment accuracy and patient comfort

Purpose: To reduce the time required for planning and simulating craniospinal fields through the use of a computed tomography (CT) simulator and virtual simulation, and to improve the accuracy of field and shielding placement. Methods and Materials: A CT simulation planning technique was developed. Localization of critical anatomic features such as the eyes, cribriform plate region, and caudal extent of the thecal sac are enhanced by this technique. Over a 2-month period, nine consecutive pediatric patients were simulated and planned for craniospinal irradiation. Four patients underwent both conventional simulation and CT simulation. Five were planned using CT simulation only. The accuracy of CT simulation was assessed by comparing digitally reconstructed radiographs (DRRs) to portal films for all patients and to conventional simulation films as well in the first four patients. Results: Time spent by patients in the CT simulation suite was 20 min on average and 40 min maximally for those who were noncompliant. Image acquisition time was <10 min in all cases. In the absence of the patient, virtual simulation of all fields took 20 min. The DRRs were in agreement with portal and/or simulation films to within 5 mm in five of the eight cases. Discrepancies of ≥5 mm in the positioning of the inferior border of the cranial fields in the first three patients were due to a systematic error in CT scan acquisition and marker contouring which was corrected by modifying the technique after the fourth patient. In one patient, the facial shield had to be moved 0.75 cm inferiorly owing to an error in shield construction. Conclusions: Our analysis showed that CT simulation of craniospinal fields was accurate. It resulted in a significant reduction in the time the patient must be immobilized during the planning process. This technique can improve accuracy in field placement and shielding by using three-dimensional CT-aided localization of critical and target structures. Overall, it has improved staff efficiency and resource utilization.

Determination of rotations in three dimensions using two-dimensional portal image registration.

The relative relationships among anatomic features visualized on planar radiographic images change due to rotations of the patient out of the imaging plane. These changes can be predicted a priori from a three-dimensional radiographic model of the patient. In this study we assess the feasibility of using that information together with a planar image feature alignment tool to account for out-of-plane rotations in the evaluation of subsequent clinical patient images. A series of digitally reconstructed radiographs (DRRs) with known patient rotations was generated from a computed tomography scan of an anthropomorphic head phantom. Fixed anatomic features were extracted, as seen in the DRRs of rotated anatomy and entered into a database. Alignment of features from test radiographs with those from an entry in this database yielded an estimate of rotation out of plane (database entry that resulted in the best fit via planar transformation) along with the planar components of setup errors in the rotated plane. Tests using DRRs and films show that it is possible to select anatomic features in AP skull radiographs with position and orientation sensitive to out-of-plane rotation.

Verification of the correspondence between CT-stimulated and treatment beams.

A single phantom technique has been developed to verify the full CT simulation and treatment-beam delivery procedures. The phantom consists of a target delineated by thin copper strips, affixed to therapy verification film, and inserted securely between two slices of water-equivalent material. The target is defined with the aid of the copper strips, and the position of the isocenter and beam parameters such as field size, and gantry and collimator angle are determined by CT simulation. With these parameters, the phantom is subsequently irradiated by the linear accelerator in the treatment position. Correspondence between the planned and the irradiated region is determined by the position of the copper strips on the film. The technique is a simple and practical method for verifying the entire CT simulation and treatment-beam delivery processes, and provides a permanent record of the correspondence between the planning digitally reconstructed radiographs (DRR) and the actual beam delivered.

Radiotherapy treatment verification.

During a radiotherapy treatment, a dosimetric verification or a geometric localization can be done, in order to assess the quality of the treatment. The dosimetric verification is generally performed measuring the dose at some points inside (natural cavities) or outside the patient, and comparing it to the dose at the same points calculated and predicted by the treatment planning system. This can be done either with thermoluminescent or diodes dosimeters or with ionization chambers. The geometric localization can be done acquiring a portal image of the patient. Portal imaging can be performed either with films placed between metallic screens, or with an electronic portal imaging device such as fluoroscopic systems, solid state devices or matrix ionization chamber systems. In order to assess possible field placement errors, the portal images have to be compared with images obtained with the simulator in the same geometric conditions and/or with the digitally reconstructed radiograph (DRR) obtained with the treatment planning system. In particular, when using matrix ionization chamber systems, the portal images contain also information regarding the exit dose. This means that this kind of imaging device can be used both for geometric localization and for dosimetric verification. In this case, the exit dose measured by the portal image can be compared with the exit dose calculated and predicted by the treatment planning system. Some "in-vivo" applications of this methodology are presented.

Portal Imaging Protocol for Radical Dose-Escalated Radiotherapy Treatment of Prostate Cancer

Purpose: The use of escalated radiation doses to improve local control in conformal radiotherapy of prostatic cancer is becoming the focus of many centers. There are, however, increased side effects associated with increased radiotherapy doses that are believed to be dependent on the volume of normal tissue irradiated. For this reason, accurate patient positioning, CT planning with 3D reconstruction of volumes of interest, clear definition of treatment margins and verification of treatment fields are necessary components of the quality control for these procedures. In this study electronic portal images are used to (a) evaluate the magnitude and effect of the setup errors encountered in patient positioning techniques, and (b) verify the multileaf collimator (MLC) field patterns for each of the treatment fields. Methods and Materials: The Phase I volume, with a planning target volume (PTV) composed of the gross tumour volume (GTV) plus a 1.5 cm margin is treated conformally with a three-field plan (usually an anterior field and two lateral or oblique fields). A Phase II, with no margin around the GTV, is treated using two lateral and four oblique fields. Portal images are acquired and compared to digitally reconstructed radiographs (DRR) and/or simulator films during Phase I to assess the systematic (CT planning or simulator to treatment error) and the daily random errors. The match results from these images are used to correct for the systematic errors, if necessary, and to monitor the time trends and effectiveness of patient imobilization systems used during the Phase I treatment course. For the Phase II, portal images of an anterior and lateral field (larger than the treatment fields) matched to DRRs (or simulator images) are used to verify the isocenter position 1 week before start of Phase II. The Portal images are acquired for all the treatment fields on the first day to verify the MLC field patterns and archived for records. The final distribution of the setup errors was used to calculate modified dose–volume histograms (DVHs). This procedure was carried out on 36 prostate cancer patients, 12 with vacuum-molded (VacFix) bags for immobilization and 24 with no immobilization. Results: The systematic errors can be visualized and corrected for before the doses are increased above the conventional levels. The requirement for correction of these errors (e.g., 2.5 mm AP shift) was demonstrated, using DVHs, in the observed 10% increase in rectal volume receiving at least 60 Gy. The random (daily) errors observed showed the need for patient fixation devices when treating with reduced margins. The percentage of fields with displacements of ≤5.0 mm increased from 82 to 96% with the use of VacFix bags. The rotation of the pelvis is also minimized when the bags are used, with over 95% of the fields with rotations of ≤2.0° compared to 85% without. Currently, a combination of VacFix and thermoplastic casts is being investigated. Conclusion: The systematic errors can easily be identified and corrected for in the early stages of the Phase I treatment course. The time trends observed during the course of Phase I in conjunction with the isocenter verification at the start of Phase II give good prediction of the accuracy of the setup during Phase II, where visibility of identifiable structures is reduced in the small fields. The acquisition and inspection of the portal images for the small Phase II fields has been found to be an effective way of keeping a record of the MLC field patterns used. Incorporation of the distribution of the setup errors into the planning system also gives a clearer picture of how the prescribed dose was delivered. This information can be useful in dose–escalation studies in determining the relationship between the local control or morbidity rates and prescribed dose.

A prospective, randomized study addressing the need for physical simulation following virtual simulation

To accurately implement a treatment plan obtained by virtual or CT simulation, conventional or physical simulation is still widely used. To evaluate the need for physical simulation, we prospectively randomized patients to undergo physical simulation or no additional simulation after virtual simulation. From July 1995 to September 1996, 75 patients underwent conformal four-field radiation therapy planning for prostate cancer with a commercial grade CT simulator. The patients were randomized to undergo either port filming immediately following physical simulation or port filming alone. The precision of implementing the devised plan was evaluated by comparing simulator radiographs and/or port films against the digitally reconstructed radiographs (DRRs) for x, y, and z displacements of the isocenter. Changes in beam aperture were also prospectively evaluated. Thirty-seven patients were randomized to undergo physical simulation and first day port filming, and 38 had first day treatment verification films only without a physical simulation. Seventy-eight simulator radiographs and 195 first day treatment port films were reviewed. There was no statistically significant reduction in treatment setup error (>5 mm) if patients underwent physical simulation following virtual simulation. No patient required a resimulation, and there was no significant difference in changes of beam aperture. Following virtual simulation, physical simulation may not be necessary to accurately implement the conformal four-field technique. Because port filming appears to be sufficient to assure precise and reliable execution of a devised treatment plan, physical simulation may be eliminated from the process of CT based planning when virtual simulation is available.

Simulation of 3D-treatment plans in head and neck tumors aided by matching of digitally reconstructed radiographs (DRR) and on-line distortion corrected simulator images

Background and purpose: Simulation of 3D-treatment plans for head and neck malignancy is difficult due to complex anatomy. Therefore, CT-simulation and stereotactic techniques are becoming more common in the treatment preparation, overcoming the need for simulation. However, if simulation is still performed, it is an important step in the treatment preparation/execution chain, since simulation errors, if not detected immediately, can compromise the success of treatment. A recently developed PC-based system for on-line image matching and comparison of digitally reconstructed radiographs (DRR) and distortion corrected simulator monitor images that enables instant correction of field placement errors during the simulation process was evaluated. The range of field placement errors with noncomputer aided simulation is reported. Materials and methods: For 14 patients either a primary 3D-treatment plan or a 3D-boost plan after initial treatment with opposing laterals for head and neck malignancy with a coplanar or non-coplanar two- or three-field technique was simulated. After determining the robustness of the matching process and the accuracy of field placement error detection with phantom measurements, DRRs were generated from the treatment planning CT-dataset of each patient and were interactively matched with on-line simulator images that had undergone correction for geometrical distortion, using a landmark algorithm. Translational field placement errors in all three planes as well as in-plane rotational errors were studied and were corrected immediately. Results: The interactive matching process is very robust with a tolerance of <2 mm when suitable anatomical landmarks are chosen. The accuracy for detection of translational errors in phantom measurements was <1 mm and for in-plane rotational errors the accuracy had a maximum of only 1.5 °. For patient simulation, the mean absolute distance of the planned versus simulated isocenter was 6.4 ± 3.9 mm. The in-plane rotational error in both planes was <3 ° with one exception. Three large field placement errors (two patients with 11.5 and 16.0 mm distances of the planned versus simulated isocenter, respectively and one patient with a 7 ° rotational error) were detected and, as with the smaller errors, were immediately corrected. Conclusion: On-line image matching of treatment planning CT-derived DRRs and distortion corrected treatment simulator images is a precise and reliable method to reduce field placement errors in the simulation of complex 3D-treatment plans for head and neck malignancy and thus enhances accuracy in the first step of the treatment preparation/execution chain. However, out-of-plane rotational errors could not be assessed and assumedly they are comparatively small since due to rigid fixation, detected in-plane errors were small.

453 - Image resolution and geometric accuracy of digital reconstructed radiographs (DRR) in conventional and helical computed tomography

453 - Image resolution and geometric accuracy of digital reconstructed radiographs (DRR) in conventional and helical computed tomography

Efficient CT simulation of the four-field technique for conformal radiotherapy of prostate carcinoma

Conformal radiotherapy of prostate carcinoma relies on contouring of individual CT slices for target and normal tissue localization. This process can be very time consuming. In the present report, we describe a method to more efficiently localize pelvic anatomy directly from digital reconstructed radiographs (DRRs). Ten patients with prostate carcinoma underwent CT simulation (the spiral mode at 3 mm separation) for conformal four-field "box" radiotherapy. The bulbous urethra and bladder were opacified with iodinated contrast media. On lateral and anteroposterior DRRs, the volume of interest (VOI) was restricted to 1.0-1.5 cm tissue thickness to optimize digital radiograph reconstruction of the prostate and seminal vesicles. By removing unessential voxel elements, this method provided direct visualization of those structures. For comparison, the targets of each patient were also obtained by contouring CT axial slices. The method was successfully performed if the target structures were readily visualized and geometrically corresponded to those generated by contouring axial images. The targets in 9 of 10 patients were reliable representations of the CT-contoured volumes. One patient had 18 mm variation due to the lack of bladder opacification. Using VOIs to generate thin tissue DRRs, the time required for target and normal tissue localization was on the average less than 5 min. In CT simulation of the four-field irradiation technique for prostate carcinoma, thin-tissue DRRs allowed for efficient and accurate target localization without requiring individual axial image contouring. This method may facilitate positioning of the beam isocenter and provide reliable conformal radiotherapy.