[en] Introduction: Craniospinal axis irradiation remains an important therapy in the management of several neoplastic diseases of the central nervous system. It is a technically demanding treatment; particularly at the junction of the cranial and spine fields where treatment fields overlap or gaps may cause unacceptable dosimetric heterogeneity. A method to accurately simulate and verify the 3-field junction is described. Methods and Materials: We use a comfortable supine position which minimizes patient movement in this time-consuming treatment. Although the supine position does reduce visual 'light field' confirmation of the 3 field match line, alternative methods of verifying this junction are described. The supine position provides airway access by anesthesiology in patients requiring sedation or anesthesia. Virtual simulation is performed with the Picker AcQsim. Multiplanar sagittal and coronal CT reconstructions allow visual confirmation of 3-field matching at the cervical region during virtual simulation. Definition of intracranial target volumes and normal critical organs, such as the eyes, facilitates block design. Digital placement of isocenters for each field and table and collimator angles are determined by calculation of field sizes and accommodation of beam divergence. At treatment, exact matching of the 3-fields is assured using digital couch readouts and record and verify confirmation of beam collimator settings, rotation, and gantry parameters. Mini-verification films (6x6 cm) are constructed from Kodak XOMAT V film. They are placed behind the patient's neck for each fraction. These films are exposed at each treatment by all treated fields (posterior flash from the lateral cranial fields and entrance from the PA spine field). These films are then developed and reviewed daily to assess field placement accuracy. Finally, review of standard portal radiographs is facilitated by the placement of radiopaque markers at the junction that are visualized in each clinical portal radiograph. Results: The supine position is readily accepted by the patients and treatment setup is facilitated. Adequate coverage of the cribriform plate and middle cranial fossa and protection of the eyes is accomplished by viewing the target volume and critical structures with beams-eye-view displays during block design and virtual simulation. Field placement using digital couch settings is easy, efficient, and accurate. Daily mini-verification films are simple, inexpensive, and allow real-time daily verification of each treatment field matching. Field placement errors of greater than 1 mm can be readily identified and corrected at subsequent treatment sessions. Conclusion: Virtual simulation and direct daily junction verification with mini-verification films allows for easy and quantitative evaluation of the junction associated with the 3-field craniospinal axis irradiation technique. The supine patient position does not present any difficulties in field matching or verification. On the contrary, it simplifies airway access for anesthesiology and allows a comfortable treatment position for older children and adults, thereby minimizing intratreatment motion and inexact treatment junctions

[en] Purpose: We evaluated the utility of three dimensional (3D) treatment planning in the management of children with parameningeal head and neck rhabdomyosarcomas. Methods and Materials: Five children with parameningeal rhabdomyosarcoma were referred for treatment at our radiation oncology center from May 1990 through January 1993. Each patient was evaluated, staged, and treated according to the Intergroup Rhabdomyosarcoma Study. Patients were immobilized and underwent a computed tomography scan with contrast in the treatment position. Tumor and normal tissues were identified with assistance from a diagnostic radiologist and defined in each slice. The patients were then planned and treated with the assistance of a 3D treatment planning system. A second plan was then devised by another physician without the benefit of the 3D volumetric display. The target volumes designed with the 3D system and the two-dimensional (2D) method were then compared. The dosimetric coverage to tumor, tumor plus margin, and normal tissues was also compared with the two methods of treatment planning. Results: The apparent size of the gross tumor volume was underestimated with the conventional 2D planning method relative to the 3D method. When margin was added around the gross tumor to account for microscopic extension of disease in the 2D method, the expected area of coverage improved relative to the 3D method. In each circumstance, the minimum dose that covered the gross tumor was substantially less with the 2D method than with the 3D method. The inadequate dosimetric coverage was especially pronounced when the necessary margin to account for subclinical disease was added. In each case, the 2D plans would have delivered substantial dose to adjacent normal tissues and organs, resulting in a higher incidence of significant complications. Conclusions: 3D conformal radiation therapy has a demonstrated advantage in the treatment of sarcomas of the head and neck. The improved dosimetric coverage of the tumor and its margin for subclinical extensions may result in improvement in local control of these tumors. In addition, lowering of radiation dose to adjacent critical structures may help lower the incidence of adverse late effects in children

[en] Purpose: Early-stage testicular seminoma is among the most radiosensitive tumors, with an overall cure rate of over 90%. Among those cured of the disease by orchiectomy and postoperative irradiation, there is a risk of having a second malignancy. We conducted a study to determine the relative risk of the occurrence of secondary malignancy. Methods and Materials: From 1964 through 1988, 128 patients with histologically confirmed early-stage seminoma of the testis underwent orchiectomy and postoperative irradiation at the Radiation Oncology Center, Mallinckrodt Institute of Radiology, and affiliate hospitals. The follow-up periods ranged from 5 to 29 years, with a median of 11.7 years. The expected rate of developing a second cancer was computed by the standardized incidence ratio using the Connecticut Tumor Registry Database. The rate is based on the number of person-years at risk, taking into account age, gender, and race. Results: Nine second nontesticular malignancies were found; the time of appearance in years is indicated in brackets: two squamous cell carcinomas of the lung [3, 11], one adenocarcinoma of the rectum [15], one chronic lymphocytic leukemia [2], one adenocarcinoma of the pancreas [14], one diffuse histiocytic lymphoma of the adrenal gland [7], one sarcoma of the pelvis [5], and two transitional cell carcinomas of the renal pelvis and ureter [14, 17]. One patient who developed a contralateral testicular tumor was excluded from risk assessment. The actuarial risk of second nontesticular cancer is 3%, 5%, and 20%, respectively, at 5, 10, and 15 years of follow-up. When compared with the general population, the overall risk of second nontesticular cancer in the study group did not reach the 0.05 significance level, with an observed/expected (O/E) ratio of 2.09 (95% confidence interval, 0.39-3.35). When analyzed by the latency period after radiation treatment, during the period of 11 to 15 years, the risk was higher (O/E ratio of 4.45, 95% confidence interval, 1.22-11.63) than expected. The median duration for developing a second cancer was 11 years for tumors arising from tissues outside the irradiated field and 14 years for those within or near the irradiated area. Conclusions: The overall observed incidence of second nontesticular malignancy among patients with early-stage testicular seminoma treated with adjuvant radiation therapy was not significantly increased in comparison with the expected incidence. Clinical implications are discussed

[en] Purpose: The purpose of this study was to assess the safety and efficacy of total lymphoid irradiation in a series of patients experiencing chronic rejection following bilateral lung transplantation. Patients and Materials: Eleven patients (10 males, 1 female) received total lymphoid irradiation for chronic allograft rejection (bronchiolitis obliterans syndrome) refractory to conventional treatment modalities. Treatment was delivered between March, 1995, and September, 1996. Mean patient age was 33 years (range 15-51). Indications for transplantation included cystic fibrosis (7 patients), alpha₁ anti-trypsin deficiency (2 patients), primary pulmonary hypertension (1 patient), and emphysema (1 patient). Radiation therapy was prescribed as 800 cGy delivered in ten 80 cGy fractions, 2 fractions per week, via AP/PA mantle and inverted-Y fields. Radiation was withheld for total wbc count <1,000/mm³, absolute neutrophil count <500/mm³, or platelets <100,000/mm³. Serial pre- and post-radiation therapy pulmonary function values, complete blood counts, and immunosuppressive augmentation requirements (use of methylprednisolone, azathioprine, mycophenolate mofetil, OKT3, and FK506) were monitored. Results: In the 3 months preceding total lymphoid irradiation, the average decrease in FEV₁ was 34% (range 0-75%) and the median number of immunosuppression augmentations was 3 (range 0-5). At initiation of radiation therapy, the average FEV₁ was 1.4 liters (range 0.77-2.28). Only (4(11)) patients completed all 10 treatment fractions. Reasons for discontinuation included unabated rejection (4 patients), worsening pulmonary infection (2 patients), and persistent thrombocytopenia (1 patient). No treatment course was discontinued because of persistent neutropenia or leukopenia. Seven of the 11 patients failed within 8 weeks of treatment cessation. One patient had unabated rejection and received bilateral living related donor transplants. He is alive and well. Six patients died. Two of these deaths were due to pulmonary infection from organisms isolated prior to the start of radiation treatment; the other 4 deaths were from progressive pulmonary decline. The four remaining patients had durable positive responses to total lymphoid irradiation (mean follow-up of 47 weeks, range 24-72). These four patients had an average 1% improvement in FEV₁ in the 3 months following radiation, compared to an average 40% decline in the 3 months preceding treatment. These patients also had a decrease in immunosuppressive requirements. In the 3 months following radiation, they required a median of 0 immunosuppression augmentations, compared to a median of 3.5 in the 3 months preceding radiation. None of these patients has developed lymphoproliferative disease nor has died. Features suggestive of a positive response to total lymphoid irradiation included longer interval from transplant to radiation, higher FEV₁ at initiation of radiation, and absence of pre-existing pulmonary infection. Conclusion: Total lymphoid irradiation for chronic allograft rejection refractory to conventional medical management following bilateral lung transplantation was tolerable. A subset of patients experienced durable preservation of pulmonary function and decreased immunosuppressive requirements. Earlier initiation of total lymphoid irradiation for patients experiencing chronic allograft rejection following bilateral lung transplantation is warranted

[en] Purpose: To compare postirradiation biochemical disease-free survival using the American Society of Therapeutic Radiology and Oncology (ASTRO) Consensus or elevation of postirradiation prostate-specific antigen (PSA) level beyond 1 ng/mL as an endpoint and correlate chemical failure with subsequent appearance of clinically detected local recurrence or distant metastasis. Methods and Materials: Records of 466 patients with histologically confirmed adenocarcinoma of the prostate treated with irradiation alone between January 1987 and December 1995 were analyzed; 339 patients were treated with bilateral 120 deg. arc rotation and, starting in 1992, 117 with three-dimensional conformal irradiation. Doses were 68-77 Gy in 1.8 to 2 Gy daily fractions. Minimum follow-up is 4 years (mean, 5.5 years; maximum, 9.6 years). A chemical failure was recorded using the ASTRO Consensus or when postirradiation PSA level exceeded 1 ng/mL at any time. Clinical failures were determined by rectal examination, radiographic studies, and, when clinically indicated, biopsy. Results: Six-year chemical disease-free survival rates using the ASTRO Consensus according to pretreatment PSA level for T1 tumors were: ≤4 ng/mL, 100%; 4.1-20 ng/mL, 80%; and >20 ng/mL, 50%. For T2 tumors the rates were: ≤4 ng/mL, 91%; 4.1-10 ng/mL, 81%; 10.1-20 ng/mL, 55%; 20.1-40 ng/mL, 63%; and >40 ng/mL, 46%. When postirradiation PSA levels higher than 1 ng/mL were used, the corresponding 6-year chemical disease-free survival rates for T1 tumors were 92% for pretreatment PSA levels of ≤4 ng/mL, 58-60% for levels of 4.1-20 ng/mL, and 30% for levels >20 ng/mL. For T2 tumors, the 6-year chemical disease-free survival rates were 78% in patients with pretreatment PSA levels of 4-10 ng/mL, 45% for 10.1-40 ng/mL, and 25% for >40 ng/mL. Of 167 patients with T1 tumors, 30 (18%) developed a chemical failure, 97% within 5 years from completion of radiation therapy; no patient has developed a local recurrence or distant metastasis. In patients with T2 tumors, overall 45 of 236 (19%) had chemical failure, 94% within 5 years of completion of radiation therapy; 4% have developed a local recurrence, and 10%, distant metastasis. In patients with T3 tumors, overall, 24 of 65 (37%) developed a chemical failure, 100% within 3.5 years from completion of radiation therapy; 4% of these patients developed a local recurrence within 2 years, and 12% developed distant metastasis within 4 years of completion of irradiation. The average time to clinical appearance of local recurrence or distant metastasis after a chemical failure was detected was 5 years and 3 years, respectively. Conclusion: There was a close correlation between the postirradiation nadir PSA and subsequent development of a chemical failure. Except for patients with T1 tumors and pretreatment PSA of 4.1-20 ng/mL, there is good agreement in 6-year chemical disease-free survival using the ASTRO Consensus or PSA elevations above 1 ng/mL as an endpoint. Although the ASTRO Consensus tends to give a higher percentage of chemical disease-free survival in most groups, the differences with longer follow-up are not statistically significant (p>0.05). It is important to follow these patients for at least 10 years to better assess the significance of and the relationship between chemical and clinical failures

[en] Purpose: To assess the safety and efficacy of total lymphoid irradiation (TLI) in patients experiencing chronic rejection following bilateral lung transplantation (BLT). Patients and Materials: Eleven patients received TLI for chronic allograft rejection (bronchiolitis obliterans syndrome) refractory to conventional treatment modalities. Radiation therapy (RT) was prescribed as 8 Gy delivered in 10 0.8-Gy fractions, 2 fractions/week, via mantle, paraaortic, and inverted-Y fields. Serial pre- and post-RT pulmonary function values, complete blood counts, and immunosuppressive augmentation requirements [use of methylprednisolone, murine anti-human mature T-cell monoclonal antibody (OKT3), polyclonal antithymocyte globulin (ATG), and tacrolimus] were monitored. Results: In the 3 months preceding TLI, the average decrease in forced expiratory volume in 1 s (FEV₁) was 34% (range 0-75%) and the median number of immunosuppression augmentations was 3 (range 0-5). Only 4 of 11 patients completed all 10 TLI treatment fractions. Reasons for discontinuation included progressive pulmonary decline (four patients), worsening pulmonary infection (two patients), and persistent thrombocytopenia (one patient). Seven of the 11 patients failed within 8 weeks of treatment cessation. One patient had unabated rejection and received bilateral living related-donor transplants; he is alive and well. Six patients died. Two of these deaths were due to pulmonary infection from organisms isolated prior to the start of RT; the other four deaths were from progressive pulmonary decline. The four remaining patients had durable positive responses to TLI (mean follow-up of 47 weeks; range 24-72). Comparing the 3 months preceding RT to the 3 months following treatment, these four patients had improvements in average FEV₁ (40% decline vs. 1% improvement) and fewer median number of immunosuppressive augmentations (3.5 vs. 0). None of these patients has developed lymphoproliferative disease or has died. Features suggestive of a positive response to TLI included longer interval from transplant to RT, higher FEV₁ at initiation of RT, and absence of preexisting pulmonary infection. Conclusion: Total lymphoid irradiation for chronic allograft rejection refractory to conventional medical management following BLT was tolerable. A subset of patients experienced durable preservation of pulmonary function and decreased immunosuppressive requirements. Patients with rapidly progressive allograft rejection, low FEV₁, or preexisting infection were least likely to benefit from irradiation. Early initiation of TLI for patients experiencing chronic allograft rejection following BLT may be warranted

[en] Purpose: To evaluate the relative frequency and magnitude of intratreatment and intertreatment displacements in the patient positioning for pelvic radiotherapy using electronic portal imaging. Methods and Materials: Five hundred ninety-four electronic portal images of seven patients treated with a four-field pelvic technique were evaluated. All patients were treated prone without an immobilization device. Two fields were treated per day, from which an average of two electronic portal images were obtained for each field. No treatment was interrupted or adjusted on the basis of these images. Each image was aligned to the corresponding simulation film to measure the displacements in the mediolateral, craniocaudal, and anteroposterior directions relative to the simulated center. The intertreatment displacement was the displacement measured from the initial image for each daily treated field. For each daily treated field the intratreatment displacement was calculated by subtracting the displacement measured on the initial image from the displacement measured on the final image. Results: The frequency of intertreatment displacements exceeding 10 mm was 3%, 16%, and 23% for the mediolateral, craniocaudal, and anteroposterior translations, respectively. There were no intratreatment displacements exceeding 10 mm (p < 0.001). The frequency of intertreatment displacements exceeding 5 mm was 40, 52, and 51% for the mediolateral, craniocaudal, and anteroposterior translations, respectively; whereas, the frequency of intratreatment displacements exceeding 5 mm was 1, 5, and 7% for the same translations, respectively (p < 0.001). The standard deviation of the intertreatment displacements was at least three times as great as the standard deviation of the intratreatment displacements for all translations. These deviations were greater than the precision limit of the measurement technique, which is approximately 1 mm. Each patient had one direction where systematic error predominated in intertreatment positioning. Random error predominated for intratreatment positioning and for the other two directions in intertreatment positioning. Conclusions: During a course of pelvic radiotherapy, the frequency of intertreatment displacements exceeding 5 and 10 mm is significantly greater than the frequency of intratreatment displacements of these magnitudes. Errors in intertreatment positioning are predominantly systematic in one direction for each patient, whereas intratreatment error is predominantly random. Because patients do not move considerably during the daily treatment of a pelvic field, a single electronic portal image per daily field may be considered representative of the treated position

[en] PURPOSE: 3-D conformal radiation therapy (3DCRT) holds promise in allowing safe escalation of radiation dose to increase the local control of prostate cancer. Prospective evaluation of this new modality requires strict quality assurance (QA). We report the results of QA review on patients receiving 3DCRT for prostate cancer on a cooperative group trial. MATERIALS and METHODS: In 1993 the NCI awarded the ACR/RTOG and nine institutions an RFA grant to study the use of 3DCRT in the treatment of prostate cancer. A phase I/II trial was developed to: a) test the feasibility of conducting 3DCRT radiation dose escalation in a cooperative group setting; b) establish the maximum tolerated radiation dose that can be delivered to the prostate; and c) quantify the normal tissue toxicity rate when using 3DCRT. In order to assure protocol compliance each participating institution was required to implement data exchange capabilities with the RTOG 3D QA center. The QA center reviews at a minimum the first five case from each participating center and spot checks subsequent submissions. For each case review the following parameters are evaluated: 1) target volume delineation, 2) normal structure delineation, 3) CT data quality, 4) field placement, 5) field shaping, and 6) dose distribution. RESULTS: Since the first patient was registered on August 23, 1994, an additional 170 patients have been accrued. Each of the nine original approved institutions has participated and three other centers have recently passed quality assurance bench marks for study participation. Eighty patients have been treated at the first dose level (68.4 Gy minimum PTV dose) and accrual is currently ongoing at the second dose level (73.8 Gy minimum PTV dose). Of the 124 cases that have undergone complete or partial QA review, 30 cases (24%) have had some problems with data exchange. Five of 67 CT scans were not acquired by protocol standards. Target volume delineation required the submitting institution's correction and resubmission in 7 of 67 (10.4%) reviewed cases. Normal tissues required correction in 6 of 67 (8.9%) of cases. Initial field shaping differed from the submitted treatment plan by more than 5 mm in significant regions of the field in only 2% of the cases. Isocenter shifts of more than 5 mm on at least one of the treated fields was identified in 7% of initial port films examined. Dosimetry review has demonstrated that 14 of 86 cases (16.3%) had minor variations in target volume coverage (<100% of the target volume coverage by the prescription isodose) and 3.4% had major variation in dose coverage (<95% coverage of target volume by prescription isodose). Nineteen of 93 cases (20%) had more than 7% heterogeneity of dose within the planning target volume. CONCLUSION: 3DCRT can be studied and implemented in a cooperative group setting. Although data exchange problems in this study have been frequent, most of these problems occurred early in the trial and have been resolved in most circumstances. A significant amount of variation has been identified in the definition of target volumes and organs at risk. Similarly, field shaping and port film evaluation showed occasional errors. It is our impression that quality assurance is a critical component of 3DCRT in the cooperative group setting. As experience in the planning of patients with 3DCRT increases, it is expected that the frequency of planning variations will diminish

[en] Purpose: Guidelines to conduct multi-institutional three-dimensional conformal radiation therapy (3-D CRT) clinical trials are needed as the modality emerges from a single institution procedure to a research tool in multi-institution clinical group trials. The guidelines are used (1) to ensure that participating institutions have the proper equipment and appropriate techniques to administer 3D CRT; (2) to define a standard data set to be submitted to a review center for each treated patient to assess protocol compliance; and (3) to establish a quality assurance (QA) review process of the submitted data. Materials and Methods: Computer hardware and software components have been implemented which allow the digital data transfer (via either the Internet or magnetic tape), display, manipulation, and storage of a 3D CRT protocol patient treatment planning and image data set for QA review. Each participating institution is required to complete a 3D CRT Facility Questionnaire and submit it to the RTOG 3-D QA Center prior to enrolling patients on a 3-D CRT protocol. In addition, a protocol 'dry run' test has been designed to demonstrate each participating institutions' ability to submit a protocol compliant data set prior to placing patients on a 3D CRT study. This dry run test involves the digital transfer of all protocol required data and the supporting hard copy documentation excepting simulation or portal films/images. Results: The 3D CRT Facility Questionnaire includes descriptions of: (1) linac model, collimation system and energies to be used; (2) isocenter accuracy for gantry, collimator, and couch rotations; (3) type of immobilization repositioning system and patient motion studies if required by protocol (set-up uncertainty, organ movement); and (4) treatment verification system(s). The 3-D RTP system must have the following capabilities: (1) ability to handle at least 40 axial CT slices; (2) beam's-eye-view (BEV) display; (3) calculate 3-D dose matrices; (4) display/hard copy of superimposed isodose distributions on 2-D CT axial, saggital, and coronal image planes (optionally, multiple axial plots with adequate internal structure detail without CT images); (5) calculate/display/hard copy dose-volume histograms (DVH); (6) non-coplanar beams capability for both beam geometry definition and dose computation (optional depending on site specific protocol); and (7) calculate and display digital reconstructed radiographs (DRRs) with superimposed target volume, critical structure contours and treatment aperture (optional depending on site specific protocol). The results of the dry run test have demonstrated some difficulty in compliance with the QA guidelines, primarily pertaining to digital data transfer, and which, as the process is better defined through usage, have influenced rational modification of these guidelines. See for example 3-D RTP system requirement No.4 above (display/hard copy of superimposed isodose curves). Digital data submitted for each protocol patient include: (1) volumetric patient CT image data; (2) patient contours including target volume(s) and critical normal tissues; (3) volumetric 3-D dose distribution data including fractionation; (4) beam modality/geometry specification; (5) DVHs; and (6) digital simulator, DRRs, and portal images (optional). A 3-D QA Center staff radiation oncologist and physicist review all target volumes and designated critical structures contours superimposed on CT display, first day portal films on all patients, and the 3D dose distribution. The case is classified as per protocol if the prescription dose covers 100% of the planning target volume; as a minor variation (marginal coverage) if the prescription dose covers between ≥95% to <100% of the planning target volume; or as a major variation (miss) if the prescription dose covers less than 95% of the planning target volume. Conclusion: The technology and methodology to conduct multi-institutional 3-D CRT clinical trials is now in place. Quality assurance guidelines which address technical capability requirement s, data reporting, and treatment compliance issues are being implemented into active and developing 3D CRT protocols

[en] Purpose: We recently replaced our university developed CT simulator prototype with a commercial grade spiral CT simulator (Picker AcQsim) that is networked with three independent virtual simulation workstations and our 3D radiation therapy planning (3D-RTP) system multiple workstations. This presentation will report our initial experience with this CT simulation device and define criteria for optimum clinical use as well as describe some potential drawbacks of the current system. Methods and Materials: Over a 10 month period, 210 patients underwent CT simulation using the AcQsim. An additional 127 patients had a volumetric CT scan done on the device with their CT data and target and normal tissue contours ultimately transferred to our 3D-RTP system. We currently perform the initial patient localization and immobilization in the CT simulation suite by using CT topograms and a fiducial laser marking system. Immobilization devices, required for all patients undergoing CT simulation, are constructed and registered to a device that defines the treatment table coordinates. Orthogonal anterior and lateral CT topograms document patient alignment and the position of a reference coordinate center. The volumetric CT scan with appropriate CT contrast materials administered is obtained while the patient is in the immobilization device. On average, more than 100 CT slices are obtained per study. Contours defining tumor, target, and normal tissues are drawn on a slice by slice basis. Isocenter definition can be automatically defined within the target volume and marked on the patient and immobilization device before leaving the initial CT simulation session. Virtual simulation is then performed on the patient data set with the assistance of predefined target volumes and normal tissue contours displayed on rapidly computed digital reconstructed radiographs (DRRs) in a manner similar to a conventional fluoroscopic radiotherapy simulator. Lastly, a verification simulation is performed with the patient on a conventional simulator in which portal radiographs are compared against DRRs. Results: Several important issues have been identified that impact on clinical utilization of the CT simulator. Thin, finely spaced CT slices improve the DRR quality but potentially degrade the quality of cross sectional images used for image segmentation. Large data sets also increase the workload for anatomic image segmentation (contouring) and raise concerns regarding data storage and easy network access. Software for image segmentation has been significantly improved allowing rapid drawing of contours around tumor/target volumes and normal tissues and improved edit functions that allow interpolation, copying, and correction of contours. These tools have reduced the time for defining all the normal tissues and target volumes for some sites (e.g. prostate) to less than 30 minutes. Acquisition of this large volume CT data currently takes less than 30 minutes potentially reducing patient time in the simulation/planning process. Difficult simulations, such as mantle/periaortic and craniospinal fields that typically require multiple 2-3 hour simulation sessions, now take half as much time with the spiral CT scanner. Subsequent field reductions or secondary fields can be planned without the physical presence of the patient. A comparison of predicted isocenter shift coordinates and actual coordinates used for verification simulation showed that the average change in isocenter position was less than a few mm indicating that the verification process can likely be eliminated. Disadvantages of current device include limited CT ring size (70 cm) and reconstruction window (48 cm) which prevent universal application of technique to all patients. Conclusion: The AcQsim offers significant advantages over a conventional simulator in terms of patient compliance and fatigue, as well as departmental throughput. In virtual simulation, after the initial acquisition of CT data, the CT scanner portion of the CT simulator can be used to acquire other patient data sets. Furthermore, as the plan and treatment course evolve the patient need not necessarily return to the scanner. A major advantage of the CT simulation process is the display of defined target volumes and critical structures on a high quality DRR that allows the choice of optimal beam projection to maximize target volume coverage and minimize treatment of adjacent tissues. The optimum choice of CT scan spacing and thickness is a function of desired DRR quality and the available space or image storage. The verification simulation process is likely to be eliminated as confidence in the software mounts and other security systems, such as record and verify, are added to our treatment units