Tracking tumor boundary using point correspondence for adaptive radio therapy.

BACKGROUND AND OBJECTIVE: Tracking mobile tumor regions during the treatment is a crucial part of image-guided radiation therapy because of two main reasons which negatively affect the treatment process: (1) a tiny error will lead to some healthy tissues being irradiated; and (2) some cancerous cells may survive if the beam is not accurately positioned as it may not cover the entire cancerous region. However, tracking or delineation of such a tumor region from magnetic resonance imaging (MRI) is challenging due to photometric similarities of the region of interest and surrounding area as well as the influence of motion in the organs. The purpose of this work is to develop an approach to track the center and boundary of tumor region by auto-contouring the region of interest in moving organs for radiotherapy. METHODS: We utilize a nonrigid registration method as well as a publicly available RealTITracker algorithm for MRI to delineate and track tumor regions from a sequence of MRI images. The location and shape of the tumor region in the MRI image sequence varies over time due to breathing. We investigate two approaches: the first one uses manual segmentation of the first frame during the pretreatment stage; and the second one utilizes manual segmentation of all the frames during the pretreatment stage. RESULTS: We evaluated the proposed approaches over a sequence of 600 images acquired from 6 patients. The method that utilizes all the frames in the pretreatment stage with moving mesh based registration yielded the best performance with an average Dice Score of 0.89 ±â€¯0.04 and Hausdorff Distance of 3.38 ±â€¯0.10 mm. CONCLUSIONS: This study demonstrates a promising boundary tracking tool for delineating the tumor region that can deal with respiratory movement and the constraints of adaptive radiation therapy.

Prediction-Based Compensation for Gate On/Off Latency during Respiratory-Gated Radiotherapy.

During respiratory-gated radiotherapy (RGRT), gate on and off latencies cause deviations of gating windows, possibly leading to delivery of low- and high-dose radiations to tumors and normal tissues, respectively. Currently, there are no RGRT systems that have definite tools to compensate for the delays. To address the problem, we propose a framework consisting of two steps: (1) multistep-ahead prediction and (2) prediction-based gating. For each step, we have devised a specific algorithm to accomplish the task. Numerical experiments were performed using respiratory signals of a phantom and ten volunteers, and our prediction-based RGRT system exhibited superior performance in more than a few signal samples. In some, however, signal prediction and prediction-based gating did not work well, maybe due to signal irregularity and/or baseline drift. The proposed approach has potential applicability in RGRT, and further studies are needed to verify and refine the constituent algorithms.

Re-Irradiation of Locoregional NSCLC Recurrence Using Robotic Stereotactic Body Radiotherapy.

BACKGROUND: We evaluated the efficacy, toxicity, and dose responses of re-irradiation with stereotactic body radiotherapy (SBRT) in patients with recurrent non- small cell lung cancer (NSCLC) after previous irradiation. PATIENTS AND METHODS: 28 patients were included. Previous median radiation doses were 54 and 66 Gy. The median interval time between previous radiotherapy and SBRT was 14 months. The median follow-up time after SBRT was 9 months (range 3-93 months). To evaluate the effectiveness of SBRT, local control, overall survival, and treatment-related toxicity were reported. RESULTS: SBRT doses and fractionation ranged from 60 to 30 Gy and from 3 to 8, respectively, according to previous doses, location of the recurrence, and interval time. 65% of tumor recurrences overlapped with previous treatment, while 35% of tumors recurred outside of the previous treatment. 4 patients had local progression after SBRT at their first follow-up. The Kaplan-Meier estimates of the 1- and 2-year actuarial overall survival were 71 and 42%, respectively. The mean survival following SBRT was 32.8 months, and the median survival was 21 months. No grade 3 or higher toxicities were observed. CONCLUSION: Robotic SBRT is a tolerable treatment option with manageable toxicity which can be used with radical or palliative intent in carefully selected patients with locally recurrent tumors after previous irradiation.

Education and Training Needs in Radiation Oncology in India: Opportunities for Indo-US Collaborations.

PURPOSE: To conduct a survey of radiation oncologists in India, to better understand specific educational needs of radiation oncology in India and define areas of collaboration with US institutions. METHODS AND MATERIALS: A 20-question survey was distributed to members of the Association of Indian Radiation Oncologists and the Indian Brachytherapy Society between November 2013 and May 2014. RESULTS: We received a total of 132 responses. Over 50% of the physicians treat more than 200 patients per day, use 2-dimensional or 3-dimensional treatment planning techniques, and approximately 50% use image guided techniques. For education needs, most respondents agreed that further education in intensity modulated radiation therapy, image guided radiation therapy, stereotactic radiation therapy, biostatistics, and research methods for medical residents would be useful areas of collaboration with institutions in the United States. Other areas of collaboration include developing a structured training module for nursing, physics training, and developing a second-opinion clinic for difficult cases with faculty in the United States. CONCLUSION: Various areas of potential collaboration in radiation oncology education were identified through this survey. These include the following: establishing education programs focused on current technology, facilitating exchange programs for trainees in India to the United States, promoting training in research methods, establishing training modules for physicists and oncology nurses, and creating an Indo-US. Tumor Board. It would require collaboration between the Association of Indian Radiation Oncologists and the American Society for Radiation Oncology to develop these educational initiatives.

A time-varying seasonal autoregressive model-based prediction of respiratory motion for tumor following radiotherapy.

To achieve a better therapeutic effect and suppress side effects for lung cancer treatments, latency involved in current radiotherapy devices is aimed to be compensated for improving accuracy of continuous (not gating) irradiation to a respiratory moving tumor. A novel prediction method of lung tumor motion is developed for compensating the latency. An essential core of the method is to extract information valuable for the prediction, that is, the periodic nature inherent in respiratory motion. A seasonal autoregressive model useful to represent periodic motion has been extended to take into account the fluctuation of periodic nature in respiratory motion. The extended model estimates the fluctuation by using a correlation-based analysis for adaptation. The prediction performance of the proposed method was evaluated by using data sets of actual tumor motion and compared with those of the state-of-the-art methods. The proposed method demonstrated a high performance within submillimeter accuracy. That is, the average error of 1.0 s ahead predictions was 0.931 ± 0.055 mm. The accuracy achieved by the proposed method was the best among those by the others. The results suggest that the method can compensate the latency with sufficient accuracy for clinical use and contribute to improve the irradiation accuracy to the moving tumor.

Patterns of practice in intensity-modulated radiation therapy and image-guided radiation therapy for prostate cancer in Japan.

BACKGROUND: The purpose of this study was to compare the prevalence of treatment techniques including intensity-modulated radiation therapy and image-guided radiation therapy in external-beam radiation therapy for prostate cancer in Japan. METHODS: A national survey on the current status of external-beam radiation therapy for prostate cancer was performed in 2010. We sent questionnaires to 139 major radiotherapy facilities in Japan, of which 115 (82.7%) were returned. RESULTS: Intensity-modulated radiation therapy was conducted at 67 facilities (58.3%), while image-guided radiation therapy was conducted at 70 facilities (60.9%). Simulations and treatments were performed in the supine position at most facilities. In two-thirds of the facilities, a filling bladder was requested. Approximately 80% of the facilities inserted a tube or encouraged defecation when the rectum was dilated. Some kind of fixation method was used at 102 facilities (88.7%). Magnetic resonance imaging was routinely performed for treatment planning at 32 facilities (27.8%). The median total dose was 76 Gy with intensity-modulated radiation therapy and 70 Gy with three-dimensional radiation therapy. The doses were prescribed at the isocenter at the facilities that conducted three-dimensional radiation therapy. In contrast, the dose prescription varied at the facilities that conducted intensity-modulated radiation therapy. Of the 70 facilities that could perform image-guided radiation therapy, 33 (47.1%) conducted bone matching, 28 (40.0%) conducted prostate matching and 9 (12.9%) used metal markers. Prostate or metal marker matching tended to produce a smaller margin than bone matching. CONCLUSIONS: The results of the survey identified current patterns in the treatment planning and delivery processes of external-beam radiation therapy for prostate cancer in Japan.

Survey of the availability and use of advanced radiotherapy technology in the UK.

AIMS: To determine the availability of intensity-modulated radiotherapy (IMRT) treatment in the UK and to assess the magnitude of the shortfall in terms of patient treatments. In addition, the availability of image-guided radiotherapy (IGRT) was also reviewed. MATERIALS AND METHODS: A survey was carried out between July and September 2008 of the use of advanced technology in radiotherapy. RESULTS: In total, 50 centres responded out of the 58 National Health Service centres canvassed, representing about 89% of patients treated in the UK. Forty-six centres had at least two machines capable of IMRT and 26 centres had at least one machine capable of IGRT. Thirty-two centres were carrying out forward-planned IMRT and 18 centres were carrying out the more complex inverse-planned IMRT. In all, 38 centres (76% of respondents) were offering either forward- or inverse-planned IMRT to some of their patients. All the centres with IGRT capability were using IGRT for at least some of their patients. Respondents were asked to list the total number of radical and palliative patients being treated according to the treatment site. Forty-two per cent of respondents took the option to list the total number of radical and palliative patients only. Based on these data, 10.7% of radical patients are currently being given forward-planned IMRT, mainly for breast cancer (18.6% of such patients) and 2.2% of radical patients are being given inverse-planned IMRT, mainly for prostate (7.5% of such patients) and head and neck cancer (6.7% of such patients). Whereas at present only 18 centres are able to treat with inverse-planned IMRT, 45 centres expected to be able to do so by 2010. Respondents were asked to estimate the percentage of patients who should be given IMRT for each site and this was used to estimate the shortfall in IMRT provision. CONCLUSIONS: Based on the consensus of opinion, 32% of radically treated patients should receive inverse-planned IMRT and 22% forward-planned IMRT, making a total of 55%. In fact, 2% receive inverse-planned IMRT and 11% the less complex forward-planned IMRT. Thus, with an estimated 75,948 radical treatments being carried out with megavoltage radiotherapy, the professional opinion is that 41,421 of patients would benefit from treatment with IMRT. In fact, only 9775 were so treated in 2008; a shortfall of 32,497 patients treated instead with conventional radiotherapy.

A survey of image-guided radiation therapy use in the United States.

BACKGROUND: Image-guided radiation therapy (IGRT) is a novel array of in-room imaging modalities that are used for tumor localization and patient setup in radiation oncology. The prevalence of IGRT use among US radiation oncologists is unknown. METHODS: A random sample of 1600 radiation oncologists was surveyed by Internet, e-mail and fax regarding the frequency of IGRT use, clinical applications, and future plans for use. The definition of IGRT included imaging technologies that are used for setup verification or tumor localization during treatment. RESULTS: Of 1089 evaluable respondents, 393 responses (36.1%) were received. The proportion of radiation oncologists using IGRT was 93.5%. When the use of megavoltage (MV) portal imaging was excluded from the definition of IGRT, the proportion using IGRT was 82.3%. The majority used IGRT rarely (in <25% of their patients; 28.9%) or infrequently (in 25%-50% of their patients; 33.1%). The percentages using ultrasound, video, MV-planar, kilovoltage (kV)-planar, and volumetric technologies were 22.3%, 3.2%, 62.7%, 57.7%, and 58.8%, respectively. Among IGRT users, the most common disease sites treated were genitourinary (91.1%), head and neck (74.2%), central nervous system (71.9%), and lung (66.9%). Overall, 59.1% of IGRT users planned to increase use, and 71.4% of nonusers planned to adopt IGRT in the future. CONCLUSIONS: IGRT is widely used among radiation oncologists. On the basis of prospective plans of responders, its use is expected to increase. Further research will be required to determine the safety, cost efficacy, and optimal applications of these technologies.

Utilization of advanced imaging technologies for target delineation in radiation oncology.

PURPOSE: The aim of this study was to evaluate the utilization of advanced imaging technologies for target delineation among radiation oncologists in the United States. METHODS: A random sample of 1,600 radiation oncologists was contacted by Internet, e-mail, and fax and questioned regarding the use of advanced imaging technologies, clinical applications, and future plans for use. Advanced imaging technologies were defined as any of the following that were directly incorporated into radiation therapy planning: MRI, PET, single-photon emission CT, 4-D CT, functional MRI, and MR spectroscopy. RESULTS: Of 1,089 contactable physicians, 394 (36%) responded. Of respondents, 65% were in private practice and 35% were in academic practice. The proportion using any advanced imaging technology for target delineation was 95%. However, the majority reported only rare (in <25% of their patients; 46.6%) or infrequent (in 25%-50% of their patients; 26.0%) utilization. The most commonly used technologies were 2-[(18)F]fluoro-2-deoxyglucose PET (76%), MRI (72%), and 4-D CT (44%). The most common cancers treated using image-guided target delineation were those of the lung (83%), central nervous system (79%), and head and neck (79%). Among users of advanced imaging technologies, 66% planned to increase use; 30% of nonusers planned to adopt these technologies in the future. CONCLUSIONS: Advanced imaging technologies are widely used by US radiation oncologists for target delineation. Although the majority of respondents used them in <50% of their patients, the frequency of utilization is expected to increase. Studies determining the optimal application of these technologies in radiation therapy planning are needed.

When is CT-based postoperative seroma most useful to plan partial breast radiotherapy? Evaluation of clinical factors affecting seroma volume and clarity.

PURPOSE: To evaluate the effect of the time from surgery and other clinical factors on seroma volume and clarity and establish the optimal time to use the computed tomography (CT)-based seroma to plan partial breast irradiation (PBI). METHODS AND MATERIALS: A total of 205 women with early-stage breast cancer underwent planning CT after breast-conserving surgery. One radiation oncologist contoured the seroma volume and scored the seroma clarity, using a standardized Seroma Clarity Score scale, from 0 (not detectable) to 5 (clearest). Univariate and multivariate analyses were performed to evaluate the associations between the seroma characteristics and the interval from surgery and other clinical factors. RESULTS: The mean interval from surgery to CT was 84 days (standard deviation 59). During postoperative Weeks 3-8, the mean seroma volume decreased from 47 to 30 cm(3), stabilized during Weeks 9-14 (mean 21) and was involuted beyond 14 weeks (mean 9 cm(3)). The mean seroma clarity score was 3.4 at Weeks 3-8, 2.5 at Weeks 9-14, and 1.6 after 14 weeks. The seroma clarity was greater in patients aged >or=70 years. The seroma volume and clarity correlated significantly with the volume of excised breast tissue but not with the maximal tumor diameter, surgical re-excision, or chemotherapy use. CONCLUSION: The optimal time to obtain the planning CT scan for PBI is within 8 weeks after surgery. During Weeks 9-14, the seroma might remain adequately defined in some patients; however, after 14 weeks, alternate strategies are needed to identify the PBI target. The lack of correlation between the seroma volume and tumor size suggests that the CT-based seroma should not be the sole guide for PBI target volume definition.

Patient-assessed late toxicity rates and principal component analysis after image-guided radiation therapy for prostate cancer.

PURPOSE: The aims of this study were to determine the incidence of patient-assessed late toxicity after high-dose, image-guided radiation therapy in a cohort of men with prostate cancer; and to correlate toxicity with conventional dosimetric parameters and rectal and bladder dose-volume histograms (DVH) reduced using principal component analysis. METHODS AND MATERIALS: Toxicity questionnaires were sent to 690 men treated for localized prostate cancer to 75.6 Gy or 79.8 Gy using three-dimensional conformal radiation therapy (3DCRT) or intensity-modulated radiation therapy (IMRT) between 1997 and 2003 at the Princess Margaret Hospital. Toxicity was graded according to the modified Radiation Therapy Oncology Group (RTOG)-late effects normal tissue (LENT) scoring system. Late rectal and bladder toxicity scores were dichotomized as < Grade 2 and > or = Grade 2, and correlated with dosimetric parameters and with the first three principal components of rectal and bladder DVHs. RESULTS: In all, 63% of the patients completed the questionnaire. At a median follow-up of 37 months, the incidence of late rectal toxicity RTOG Grades 1, 2, and 3 was 25.2%, 2.5%, and 0.7% respectively. The incidence of late urinary toxicity RTOG Grade 1, 2, and 3 was 16.5%, 8.8%, and 0.9% respectively. Maintenance of erectile function sufficient for intercourse was reported in 68%. No dosimetric parameter analyzed, including principal component analysis reduction of DVHs, correlated with late toxicity. CONCLUSIONS: Postal questionnaire was effective for collection of patient-assessed late toxicity data. The incidence of late toxicity was low, with a lack of correlation to dosimetric parameters. We attribute this to the use of conformal techniques and daily image guidance.

Clinical implementation and efficiency of kilovoltage image-guided radiation therapy.

This paper describes measurements of clinical efficiency and time requirements associated with image-guided radiation therapy (IGRT). In June 2004, the authors' institution installed an integrated kilovoltage (kV) imaging system attached to a medical linear accelerator for radiographic target localization. Over the past year, 242 patients have been localized with the kV radiographic imaging system for a total of 2,700 fractions. Data were analyzed by reviewing the time required for each patient's IGRT session, broken into both image acquisition and image analysis time. Average IGRT procedure time was reviewed pertaining to months, treatment sessions, disease sites, and radiation therapists. Results showed that the average IGRT procedure time was reduced from 450 to 237 seconds from June 2004 to June 2005. Further analysis revealed that each therapist showed improvement in reducing the IGRT procedure time from the first month of use to the month of June 2005. The routine use of IGRT may ultimately be performed within 3 to 4 minutes, with minimal disruption to the clinical treatment process.

Considerations of cost-effectiveness for new radiation oncology technologies.

PURPOSE: The additional equipment and personnel costs of supplying image-guided radiation therapy (IGRT) technology have caused many to question if the marginal gains in patients' health-related quality of life are worth the additional cost. Novel IGRT technologies, including cone-beam computed tomography and helical tomotherapy, provide the opportunity to study cost and effectiveness for patients. MATERIALS AND METHODS: This methodologic study proposes to evaluate the cost and effectiveness of treating conventional radiotherapy versus IGRT patients prospectively among several institutions. The cost of treating patients varies among institutions depending on personnel, equipment, and overhead costs, but the nature and quality of services provided are expected to be consistent. RESULTS: The study will track cost information at a single institution and simultaneously as the median from multiple institutions. Effectiveness measures will include both standard quality-adjusted life-year instruments completed by patients and performance status measures completed by institutional personnel. In addition, disease-specific effectiveness measures will be accommodated in the study. Each participating institution will use the same effectiveness measures to track patients with similar diseases. CONCLUSION: The resulting cost and effectiveness data will be available to investigators at any point during the study, immediately on the completion of a trial, or when statistical acceptability is achieved. These considerations are being incorporated into a high-level information model under development.

Computed tomographic simulation in palliative radiotherapy: the Princess Margaret Hospital experience.

AIM: To examine the pattern of palliative radiation planning and the use of computed tomographic simulation (CTSIM) for this purpose. MATERIALS AND METHODS: We reviewed our department's external radiotherapy database for all courses of treatment with a palliative intent during the period of April to June 2002. Patient characteristics and treatment details were compared based on whether CTSIM had been used or not. RESULTS: During the above period, 593 courses of external radiation treatment were delivered with palliative intent in our department. Of these, 100 treatments (17%) were planned with the help of CTSIM. The mean age of patients with CTSIM (62.9 years) was not significantly different with the patients planned without CTSIM (63.6 years). CTSIM use varied by treatment location, being highest in mediastinum/oesophagus (48%) and pancreas/stomach (47%) treatments, and lowest in spine (6%), lung (3%) and long bones (4%) (P < 0.01). Only 3% of palliative treatments without CTSIM were prescribed using multiple/complex fields (all field arrangements more complex than a single field or two opposed parallel fields). Although significantly higher (P < 0.001), this proportion was also only 24% in the cases planned with CTSIM. Only 12% of treatments without CTSIM were prescribed with more than 5 fractions, whereas 32% of CT-simulated treatments included more than 5 fractions (P < 0.001). CONCLUSION: CTSIM was used much less frequently in our department's palliative radiotherapy compared with its use in radical treatments. The relatively low rate of multiple/complex fields planned in CT-simulated cases suggested that CTSIM was mostly used to improve tumour localisation. The optimal role of CTSIM in palliative radiotherapy will most probably evolve, based on an enhanced understanding of the implications from improved localisation and optimal planning techniques on clinical outcomes, patient convenience and resource accessibility.

Radioimmunoscintigraphy for postprostatectomy radiotherapy: analysis of toxicity and biochemical control.

UNLABELLED: Our goal was to evaluate the role of radioimmunoscintigraphy (RIS) directed against prostate-specific membrane antigen (PSMA) in influencing postprostatectomy radiotherapy (RT) toxicity and biochemical control. METHODS: The records of 107 postprostatectomy RT patients were reviewed. The group for whom no RIS scan was obtained (group A, n = 54) was identified as was the group for whom a RIS scan was obtained (group B, n = 53). Group B was further subdivided into those who had a RIS and CT-scan correlation to aid in treatment planning (subgroup B1, n = 40) versus those who did not (subgroup B2, n = 13). Gastrointestinal (GI) and genitourinary (GU) toxicities were reviewed for each of these groups and subgroups and compared. Biochemical failures (defined as 2 successive PSA rises after a nadir of >or=0.2 ng/mL) were identified to generate biochemical failure-free survival (BFFS) curves for each of the groups and subgroups. RESULTS: No significant differences in late toxicity were observed between any group or subgroup. However, acute GI toxicity was higher in group B versus group A (P = 0.026), and acute GU toxicity was higher in subgroup B2 versus subgroup B1 (P = 0.050). Overall, most toxicity was grade 1 or 2; only one case of grade 3 toxicity and no cases of grade 4 or 5 toxicity were observed. Three-year BFFS was higher for group B versus group A (80.7% vs. 75.5%) and for subgroup B1 versus subgroup B2 (84.5% vs. 71.6%). On multivariate analysis of pretreatment (age, race), surgical/staging (stage, grade, margin status, extracapsular extension, lymph node status, seminal vesicle invasion, post-radical retropubic prostatectomy [RRP] prostate-specific antigen [PSA] nadir, maximum post-RRP PSA, and RRP-to-RT interval), and treatment (hormone therapy, RT dose, RT technique, RIS scan, and RIS/CT correlation) factors on BFFS, the only covariate reaching significance was RIS/CT correlation (P = 0.042). CONCLUSION: A small BFFS advantage was observed in patients for whom RIS was used to guide RT decision making and treatment planning; however, this advantage only reached significance in this study for those for whom the RIS/CT correlation was used to guide target definition. The improved PSA control using RIS was achieved with a small increase in acute toxicity but with no observed change in late toxicity. These findings can serve as the basis for prospective studies in this area of investigation.

Patterns of care study: comparison of process of post-mastectomy radiotherapy (PMRT) in Japan and the USA.

BACKGROUND: The Patterns of Care Study (USPCS) by the American College of Radiology (ACR) has made significant contributions to improvements in the procedures of care for patients with breast cancer in the USA. The purpose of this study was to identify problems associated with the process of care for patients undergoing post-mastectomy radiotherapy (PMRT) in Japan compared with those in the USA. METHODS: The Japanese Patterns of Care Study Subgroup (JPCS) conducted a national survey in 1998-2000, involving 79 institutions and using two-stage cluster sampling of institutions and patients, which showed that between 1995 and 1997 PMRT was performed on 258 patients. The survey of the USPCS, involving 55 institutions, found that 407 patients received PMRT between 1998 and 1999. RESULTS: More than three axillary positive nodes were detected in 54% of the patients covered by the JPCS and in 46% of those covered by the USPCS. The clinical set-up of radiation treatment was planned without the aid of computed tomography or X-ray simulation for 25% of the JPCS patients and for 6% of the USPCS patients. The chest wall of 31% of the JPCS patients and of 98% of the USPCS patients was irradiated. The JPCS showed that inappropriate radiation techniques such as parallel opposed fields for chest wall irradiation were used for 3% of the patients in academic facilities, but for 25% of those in non-academic facilities (P = 0.0002). CONCLUSION: There is ample room for improvement in radiation treatment planning and chest wall irradiation techniques in Japan.

Reference photon dosimetry data: a preliminary study of in-air off-axis factor, percentage depth dose, and output factor of the Siemens Primus linear accelerator.

The dosimetric characteristics for modern computer-controlled linear accelerators with the same make, model, and nominal energy are known to be very similar, as long as the machines are unaltered from the manufacturer's original specifications. In this preliminary study, a quantitative investigation of the similarity in the basic photon dosimetry data from the Siemens Primus linear accelerators at eight different institutions is reported. The output factor, percentage depth dose (PDD), and in-air off-axis factor (OAF) for the 6 and 18 MV photon beams measured or verified by the Radiological Physics Center (RPC) were analyzed. The RPC-measured output factors varied by less than about 2% for each field size. The difference between the maximum and minimum RPC-verified PDD values at each depth was less than about 3%. The difference between the maximum and minimum RPC-measured in-air OAF was no more than 4% at all off-axis distances considered in this study. These results strongly suggest that it is feasible to establish a reference photon dosimetry data set for each make, model, and nominal energy, universally applicable to those machines unaltered from the manufacturers' original specifications, within a clinically acceptable tolerance (e.g., approximately +/-2%).