Towards high spatial resolution tissue-equivalent dosimetry for microbeam radiation therapy using organic semiconductors.

Spatially fractionated ultra-high-dose-rate beams used during microbeam radiation therapy (MRT) have been shown to increase the differential response between normal and tumour tissue. Quality assurance of MRT requires a dosimeter that possesses tissue equivalence, high radiation tolerance and spatial resolution. This is currently an unsolved challenge. This work explored the use of a 500 nm thick organic semiconductor for MRT dosimetry on the Imaging and Medical Beamline at the Australian Synchrotron. Three beam filters were used to irradiate the device with peak energies of 48, 76 and 88 keV with respective dose rates of 3668, 500 and 209 Gy s-1. The response of the device stabilized to 30% efficiency after an irradiation dose of 30 kGy, with a 0.5% variation at doses of 35 kGy and higher. The calibration factor after pre-irradiation was determined to be 1.02 ±â€…0.005 µGy per count across all three X-ray energy spectra, demonstrating the unique advantage of using tissue-equivalent materials for dosimetry. The percentage depth dose curve was within ±5% of the PTW microDiamond detector. The broad beam was fractionated into 50 microbeams (50 µm FHWM and 400 µm centre-to-centre distance). For each beam filter, the FWHMs of all 50 microbeams were measured to be 51 ±â€…1.4, 53 ±â€…1.4 and 69 ±â€…1.9 µm, for the highest to lowest dose rate, respectively. The variation in response suggested the photodetector possessed dose-rate dependence. However, its ability to reconstruct the microbeam profile was affected by the presence of additional dose peaks adjacent to the one generated by the X-ray microbeam. Geant4 simulations proved that the additional peaks were due to optical photons generated in the barrier film coupled to the sensitive volume. The simulations also confirmed that the amplitude of the additional peak in comparison with the microbeam decreased for spectra with lower peak energies, as observed in the experimental data. The material packaging can be optimized during fabrication by solution processing onto a flexible substrate with a non-fluorescent barrier film. With these improvements, organic photodetectors show promising prospects as a cost-effective high spatial resolution tissue-equivalent flexible dosimeter for synchrotron radiation fields.

Effectiveness of rectal displacement devices in managing prostate motion: a systematic review.

PURPOSE: To determine whether rectal displacement devices (RDDs) have a prostate-stabilizing effect during prostate external beam radiotherapy (EBRT). METHODS: A systematic literature search using the PubMed database from January 1, 2000 to December 30th, 2019 was conducted. The effect of RDDs on inter- and intra-fractional prostate displacements was extracted. RESULTS: From 356 articles identified via the PubMed database and hand search, 21 articles were included in the systematic review. There was no randomized study. Twelve studies evaluated the role of the endorectal balloon (ERB) in managing prostate motion. Four studies reported the effect of hydrogel spacer on prostate motion. Four studies examined the effect of the rectal retractor (RR) on intra-fractional prostate motion, and only one study assessed the impact of ProSpare (Nottinghamshire, UK) in reducing prostate motion. CONCLUSION: Using an ERB significantly reduces intra-fractional prostate motion. This prostate-stabilizing effect of the ERB can translate into reduced planning target volume (PTV) margins and additional rectal dose sparing. Even with an ERB in place, inter-fractional prostate displacements are seen. As a consequence, ERB application does not obviate daily verification; however, this is not a crucial topic because pretreatment imaging is always done nowadays. As compared with ERB, the hydrogel spacer significantly reduces rectal dose and toxicity without influencing prostate immobilization. The RR can increase prostate and rectal inter- and intra-fractional stability without a clear influence on the reduction of rectal toxicity. Finally, it is unclear whether ProSpare is a suitable device reducing prostate motion. Further study will be required to clarify whether the prostate-stabilizing effects of the ERB and RR can result in a safe reduction of PTV margins and further sparing of organs at risks, especially the rectum.

Radiotherapy activities and technological equipment in Veneto, Italy: a report from the Rete Radioterapica Veneta.

BACKGROUND: Despite the pivotal role of radiotherapy in oncology, the provision of radiation treatments remains inadequate in many areas of the world. The present report is an assessment conducted among Radiation Oncology centers of Veneto region with the aim to collect information concerning radiotherapy assets and technological equipment availability. METHODS: Data concerning Veneto Radiation Oncology departments about radiotherapy activities, number of treatments, techniques used and radiotherapy machines available were collected. The reference time period was 2018. Reimbursement system databases and business intelligence systems were used. Extra-regional attraction and migration were evaluated. When available, data were compared to previous years. RESULTS: Veneto in 2018 was endowed with 1 megavolt unit for about 153,000 inhabitants. The number of megavolt machines per million inhabitants resulted to be 6.72. In 51% of radiotherapy treatments, intensity-modulated techniques were performed. Six percent of treatments were administered to extra-regional patients. CONCLUSION: Radiotherapy assets and equipment in Veneto seem to be appropriate to standard requests in terms of availability and technology.

McKesson® lubricating jelly and Aquasonic® ultrasound gel can be used as bolus materials for radiation therapy.

Bolus materials are commonly used for both human and veterinary radiation therapy (RT). Commercially available bolus materials often leave an air gap between the bolus and the skin which can lead to underdosing of the tumor. This prospective exploratory study evaluated the 6 MV X-ray and electron beam (6, 9, and 12 MeV) attenuating properties for two alternative bolus materials: McKesson lubricating jelly® (MLJ) and Aquasonic 100 Ultrasound gel® (AUG). The results comparing MLJ and water for 12 MeV and 9 MeV electron beams showed <3% difference, however, no other significant differences in radiation dose between water and MLJ nor AUG were seen. Findings demonstrated that both AUG and MLJ have radiation dose attenuating properties similar to water and supported use of these materials as alternative bolus materials for veterinary radiation therapy applications.

The versatile X-ray beamline of the Munich Compact Light Source: design, instrumentation and applications.

Inverse Compton scattering provides means to generate low-divergence partially coherent quasi-monochromatic, i.e. synchrotron-like, X-ray radiation on a laboratory scale. This enables the transfer of synchrotron techniques into university or industrial environments. Here, the Munich Compact Light Source is presented, which is such a compact synchrotron radiation facility based on an inverse Compton X-ray source (ICS). The recent improvements of the ICS are reported first and then the various experimental techniques which are most suited to the ICS installed at the Technical University of Munich are reviewed. For the latter, a multipurpose X-ray application beamline with two end-stations was designed. The beamline's design and geometry are presented in detail including the different set-ups as well as the available detector options. Application examples of the classes of experiments that can be performed are summarized afterwards. Among them are dynamic in vivo respiratory imaging, propagation-based phase-contrast imaging, grating-based phase-contrast imaging, X-ray microtomography, K-edge subtraction imaging and X-ray spectroscopy. Finally, plans to upgrade the beamline in order to enhance its capabilities are discussed.

Radiation-induced DNA double-strand breaks in peripheral leukocytes and therapeutic response of heel spur patients treated by orthovoltage X-rays or a linear accelerator.

PURPOSE: Biodosimetric assessment and comparison of radiation-induced deoxyribonucleic acid (DNA) double-strand breaks (DSBs) by γH2AX immunostaining in peripheral leukocytes of patients with painful heel spur after radiation therapy (RT) with orthovoltage X­rays or a 6-MV linear accelerator (linac). The treatment response for each RT technique was monitored as a secondary endpoint. PATIENTS AND METHODS: 22 patients were treated either with 140-kV orthovoltage X­rays (n = 11) or a 6-MV linac (n = 11) with two weekly fractions of 0.5 Gy for 3 weeks. In both scenarios, the dose was prescribed to the International Commission on Radiation Units and Measurements (ICRU) dose reference point. Blood samples were obtained before and 30 min after the first RT session. γH2AX foci were quantified by immunofluorescence microscopy to assess the yield of DSBs at the basal level and after radiation exposure ex vivo or in vivo. The treatment response was assessed before and 3 months after RT using a five-level functional calcaneodynia score. RESULTS: RT for painful heel spurs induced a very mild but significant increase of γH2AX foci in patients' leukocytes. No difference between the RT techniques was observed. High and comparable therapeutic responses were documented for both treatment modalities. This trial was terminated preliminarily after an interim analysis (22 patients randomized). CONCLUSION: Low-dose RT for painful heel spurs with orthovoltage X­rays or a 6-MV linac is an effective treatment option associated with a very low and comparable radiation burden to the patient, as confirmed by biodosimetric measurements.

The influence of radiotherapy techniques on the plan quality and on the risk of secondary tumors in patients with pituitary adenoma.

BACKGROUND: This planning study compares different radiotherapy techniques for patients with pituitary adenoma, including flatness filter free mode (FFF), concerning plan quality and secondary malignancies for potentially young patients. The flatness filter has been described as main source of photon scatter. MATERIAL AND METHODS: Eleven patients with pituitary adenoma were included. An Elekta Synergy™ linac was used in the treatment planning system Oncentra® and for the measurements. 3D plans, IMRT, and VMAT plans and non-coplanar varieties were considered. The plan quality was evaluated regarding homogeneity, conformity, delivery time and dose to the organs at risk. The secondary malignancy risk was calculated from dose volume data and from measured dose to the periphery using different models for carcinoma and sarcoma risk. RESULTS: The homogeneity and conformity were nearly unchanged with and without flattening filter, neither was the delivery time found substantively different. VMAT plans were more homogenous, conformal and faster in delivery than IMRT plans. The secondary cancer risk was reduced with FFF both in the treated region and in the periphery. VMAT plans resulted in a higher secondary brain cancer risk than IMRT plans, but the risk for secondary peripheral cancer was reduced. Secondary sarcoma risk plays a minor role. No advantage was found for non-coplanar techniques. The FFF delivery times were not shortened due to additional monitor units needed and technical limitations. The risk for secondary brain cancer seems to depend on the irradiated volume. Secondary sarcoma risk is much smaller than carcinoma risk in accordance to the results of the atomic bomb survivors. The reduction of the peripheral dose and resulting secondary malignancy risk for FFF is statistically significant. However, it is negligible in comparison to the risk in the treated region. CONCLUSION: Treatments with FFF can reduce secondary malignancy risk while retaining similar quality as with flattening filter and should be preferred. VMAT plans show the best plan quality combined with lowest peripheral secondary malignancy risk, but highest level of second brain cancer risk. Taking this into account VMAT FFF seems the most advantageous technique for the treatment of pituitary adenomas with the given equipment.

Dosimetric characteristics of the INTRABEAM ® system with spherical applicators in the presence of air gaps and tissue heterogeneities.

The main aim of this study was to investigate the dosimetric characteristics of the INTRABEAM ® system in the presence of air gaps between the surface of applicators (APs) and tumor bed. Additionally, the effect of tissue heterogeneities was another focus. Investigating the dosimetric characteristics of the INTRABEAM® system is essential to deliver the required dose to the tumor bed correctly and reduce the delivered dose to the ribs and lung. Choosing the correct AP size and fitting it to the lumpectomy cavity is essential to remove the effect of air gaps and avoid inaccurate dose delivery. Consequently, the Geant4 toolkit was used to simulate the INTRABEAM ® system with spherical APs of various sizes. The wall effect of the ion chamber (IC) PTW 34013 used in the present study was checked. The simulations were validated in comparison with measurements, and then used to calculate any inaccuracies in dose delivery in the presence of 4- and 10-mm air gaps between the surface of the APs and the tumor bed. Also, the doses received due to tissue heterogeneities were characterized. It turned out that measurements and simulations were approximately in agreement (± 2%) for all sizes of APs. The perturbation factor introduced by the IC due to differences in graphite-coated polyethylene and air as compared to the phantom material was approximately equal to one for all AP. The greatest relative dose delivery difference was observed for an AP with a diameter of 1.5 cm, i.e., 44% and 70% in the presence of 4- and 10-mm air gaps, respectively. In contrast, the lowest relative dose delivery difference was observed for an AP with a diameter of 5 cm, i.e., 24% and 42% in the presence of 4- and 10-mm air gaps, respectively. Increasing APs size showed a decrease in relative dose delivery difference due to the presence of air gaps. In addition, the undesired dose received by the ribs turned out to be higher when a treatment site closer to the ribs was assumed. The undesired dose received by the ribs increased as the AP size increased. The lung dose turned out to be decreased due to the shielding effect of the ribs, small lung density, and long separation distance from the AP surface.

[Discussion on the Different Design Ideas of Medical Accelerator(I)].

This discussion attempts to organize and analyze the clinical purpose of various technologies developed by medical electron accelerators from the development history and clinical needs of radiotherapy products, so as to avoid the troubles caused by specific technical details and summarize the development of medical accelerators. Directly, the study provides differentiated development ideas for the development of domestic medical accelerators and ways and means to determine the dimensions of differentiated development.

The use of hydrogel spacer in men undergoing high-dose prostate cancer radiotherapy: results of a prospective phase 2 clinical trial.

PURPOSE: The purpose of this study was to determine whether the degree of prostate to rectal separation using a hydrogel spacer (HS) and its effect on decreasing rectal dose can be reproduced in the community setting. METHODS: Thirty one patients with cT1-3aN0M0 prostate adenocarcinoma receiving radical radiotherapy to 78 Gy were recruited to the study. The primary endpoint was the proportion of patients achieving at least 25% reduction in volume of rectum receiving 70 Gy (rV70). Other endpoints included degree of prostate to rectum separation, HS insertion-related adverse events and the proportion of patients with grade 1 or worse acute or late gastrointestinal (GI) and genitourinary (GU) toxicity. RESULTS: All patients had successful insertion of their HS with no peri-operative toxicity. The mean prostate-rectal separation achieved was 10.5 mm. Twenty nine (93.5%) patients achieved a reduction in rV70 of at least 25%. Acute grade 1 GI toxicity was reported in 3 patients. All symptoms had resolved by 3 months post RT. Late grade 1 GI toxicity was reported in one patient (3.2%) with bowel frequency occurring at 6 months and resolving by 12 months post RT. There was no grade 2 or 3 acute or late GI toxicity seen. CONCLUSION: In conclusion, this study illustrates that the application and benefits of HS on reducing GI rectal dose endpoints and toxicities during prostate cancer RT can be reliably replicated in a community setting similar to centres participating in the randomised trial under high quality assurance trial monitoring.

A Usability Study of 3 Radiotherapy Systems: A Comparative Evaluation Based on Expert Evaluation and User Experience.

BACKGROUND The complex user interface design of radiotherapy treatment delivery systems can lead to use error and patient harm. In this study, we present the results of a comparison of 3 radiotherapy treatment delivery systems now used in China. MATERIAL AND METHODS We conducted a comprehensive usability study of 3 radiotherapy treatment delivery systems. Expert evaluation was performed through heuristic evaluation with 3 human-factors experts and 1 experienced radiation therapist for each system. User experience was assessed through perceived system usability and workload, using the National Aeronautics and Space Administration Task Load Index and the Post-Study System Usability Questionnaire. RESULTS For the expert evaluation, 47 usability problems were identified for Varian Trilogy, 75 for Elekta Precise, and 37 for Shinva XHA600E. Most problems were classified as major and minor usability problems, and were found in the process of patient setup and setup verification. For the user experience, radiation therapists presented a lower workload for Varian Trilogy compared to Elekta Precise (P<0.01) and Shinva XHA600E (P<0.01), and a lower workload for Elekta Precise compared to Shinva XHA600E (P=0.020). Radiation therapists perceived a higher system usability for Varian Trilogy compared to Shinva XHA600E (P<0.01), and a higher system usability for Elekta Precise compared to Shinva XHA600E (P<0.01). CONCLUSIONS This research provides valuable data on how 3 radiotherapy treatment delivery systems compare. The results of this study may be useful for hospital equipment procurement decisions, and designing next-generation products to improve patient safety.

ÖGRO survey on radiotherapy capacity in Austria : Status quo and estimation of future demands.

BACKGROUND: A comprehensive evaluation of the current national and regional radiotherapy capacity in Austria with an estimation of demands for 2020 and 2030 was performed by the Austrian Society for Radiation Oncology, Radiobiology and Medical Radiophysics (ÖGRO). MATERIALS AND METHODS: All Austrian centers provided data on the number of megavoltage (MV) units, treatment series, fractions, percentage of retreatments and complex treatment techniques as well as the daily operating hours for the year 2014. In addition, waiting times until the beginning of radiotherapy were prospectively recorded over the first quarter of 2015. National and international epidemiological prediction data were used to estimate future demands. RESULTS: For a population of 8.51 million, 43 MV units were at disposal. In 14 radiooncological centers, a total of 19,940 series with a mean number of 464 patients per MV unit/year and a mean fraction number of 20 (range 16-24) per case were recorded. The average re-irradiation ratio was 14%. The survey on waiting times until start of treatment showed provision shortages in 40% of centers with a mean waiting time of 13.6 days (range 0.5-29.3 days) and a mean maximum waiting time of 98.2 days. Of all centers, 21% had no or only a limited ability to deliver complex treatment techniques. Predictions for 2020 and 2030 indicate an increased need in the overall number of MV units to a total of 63 and 71, respectively. CONCLUSION: This ÖGRO survey revealed major regional differences in radiooncological capacity. Considering epidemiological developments, an aggravation of the situation can be expected shortly. This analysis serves as a basis for improved public regional health care planning.

Emerging role of MRI in radiation therapy.

Advances in multimodality imaging, providing accurate information of the irradiated target volume and the adjacent critical structures or organs at risk (OAR), has made significant improvements in delivery of the external beam radiation dose. Radiation therapy conventionally has used computed tomography (CT) imaging for treatment planning and dose delivery. However, magnetic resonance imaging (MRI) provides unique advantages: added contrast information that can improve segmentation of the areas of interest, motion information that can help to better target and deliver radiation therapy, and posttreatment outcome analysis to better understand the biologic effect of radiation. To take advantage of these and other potential advantages of MRI in radiation therapy, radiologists and MRI physicists will need to understand the current radiation therapy workflow and speak the same language as our radiation therapy colleagues. This review article highlights the emerging role of MRI in radiation dose planning and delivery, but more so for MR-only treatment planning and delivery. Some of the areas of interest and challenges in implementing MRI in radiation therapy workflow are also briefly discussed. Level of Evidence: 5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2018;48:1468-1478.

Illustrated instructions for mechanical quality assurance of a medical linear accelerator.

PURPOSE: The purpose of this study was to develop and test a set of illustrated instructions for effective training for mechanical quality assurance (QA) of medical linear accelerators (linac). METHODS: Illustrated instructions were created for mechanical QA and underwent several steps of review, testing, and refinement. Eleven testers with no recent QA experience were then recruited from our radiotherapy department (one student, two computational scientists, and eight dosimetrists). This group was selected because they have experience of radiation therapy but no preconceived ideas about how to do QA. The following parameters were progressively decalibrated on a Varian C-series linac: Group A = gantry angle, ceiling laser position, X1 jaw position, couch longitudinal position, physical graticule position (five testers); Group B = Group A + wall laser position, couch lateral and vertical position, collimator angle (three testers); Group C = Group B + couch angle, wall laser angle, and optical distance indicator (three testers). Testers were taught how to use the linac and then used the instructions to try to identify these errors. An experienced physicist observed each session, giving support on machine operation as necessary. RESULTS: Testers were able to follow the instructions. They determined gantry, collimator, and couch angle errors within 0.4°, 0.3°, and 0.9° of the actual changed values, respectively. Laser positions were determined within 1 mm and jaw positions within 2 mm. Couch position errors were determined within 2 mm and 3 mm for lateral/longitudinal and vertical errors, respectively. Accessory-positioning errors were determined within 1 mm. Optical distance indicator errors were determined within 2 mm when comparing with distance sticks and 6 mm when using blocks, indicating that distance sticks should be the preferred approach for inexperienced staff. CONCLUSIONS: Inexperienced users were able to follow these instructions and catch errors within the criteria suggested by AAPM TG-142 for linacs used for intensity-modulated radiation therapy. These instructions are, therefore, suitable for QA training.

COMP report: CPQR technical quality control guidelines for treatment planning systems.

The Canadian Organization of Medical Physicists (COMP), in close partnership with the Canadian Partnership for Quality Radiotherapy (CPQR) has developed a series of Technical Quality Control (TQC) guidelines for radiation treatment equipment. These guidelines outline the performance objectives that equipment should meet in order to ensure an acceptable level of radiation treatment quality. The TQC guidelines have been rigorously reviewed and field tested in a variety of Canadian radiation treatment facilities. The development process enables rapid review and update to keep the guidelines current with changes in technology. This article contains detailed performance objectives and safety criteria for Treatment Planning Systems (TPS) for External Beam Radiotherapy.

COMP report: CPQR technical quality control guidelines for kilovoltage X ray radiotherapy machines.

The Canadian Organization of Medical Physicists (COMP) in close partnership with the Canadian Partnership for Quality Radiotherapy (CPQR) has developed a series of Technical Quality Control (TQC) guidelines for radiation treatment equipment. These guidelines outline the performance objectives that equipment should meet in order to ensure an acceptable level of radiation treatment quality. The TQC guidelines have been rigorously reviewed and field tested at various Canadian radiation treatment facilities. The development process enables rapid review and update to keep the guidelines current with changes in technology (the most updated version of this guideline can be found on the CPQR website). This particular TQC details recommended quality control for kilovoltage X Ray radiotherapy machines.

Ion chambers compliance results of Brazilian radiation therapy facilities.

The Brazilian Nuclear Energy Commission (cnen) has been making a constant effort to keep up to date with international standards and national needs to strengthen the status of radiological protection of the country. The guidelines related to radiation therapy facilities have been revised in the last five years in order to take into consideration the most relevant aspects of the growing technology as well as to mitigate the accidents or incidents observed in practice. Hence, clinical dosimeters have gained special importance in this matter. In the present work, we discuss the effectiveness of regulation and inspections to the enforcement of instrument calibration accuracy for the improvement of patient dosimetry and quality control. As a result, we observed that the number of calibrated instruments, mainly well chambers, is increasing each year. The same behavior is observed for instruments employed in technologically advanced radiation treatments such as intensity modulated radiotherapy, volumetric therapy and stereotatic radiosurgery. We ascribe this behavior to the new regulation.

[Structure Design of the Supporting Device of Proton Radiotherapy Equipment Based on Workbench].

To reduce contact deformation between the supporting device and the contact area of the rotating gantry rollers, this paper mainly uses the workbench to perform finite element analysis on the contact deformation from two aspects of the layout of the ribs of the rotating gantry roller and the contact width with the supporting roller of the supporting device. The results show that for the proton therapy equipment corresponding to this article, the contact deformation is relatively small and the structure is more reasonable when the ribs are close to the symmetrical position and the contact width between the rotating gantry roller and the supporting roller of the supporting device is 200 mm. This article provides theoretical basis for the design of the proton radiotherapy equipment supporting device.

Risk Factors Leading to Complications in Early-Stage Breast Cancer Following Breast-Conserving Surgery and Intraoperative Radiotherapy.

OBJECTIVE: The aim of this study was to evaluate outcomes after breast-conserving surgery (BCS) and intraoperative radiotherapy (IORT), and to identify risk factors associated with complications. MATERIALS/METHODS: We evaluated patients with early-stage breast cancer treated from January 1, 2011 to January 31, 2014 with BCS and IORT at a single institution. The presence of breast cancer recurrences, complications, or fat necrosis were assessed at subsequent follow-up visits using physical examination and breast imaging. RESULTS: Overall, 113 patients, of whom three were undergoing bilateral treatments, were identified. The median length of time for IORT was 29 min and 36 s (range 15:50-59:00). Fifteen patients received additional external beam radiotherapy (EBRT), and the median follow-up was 40.3 months (range 1.6-58.3) for all patients. To date, one biopsy-proven ipsilateral recurrence has been noted (0.9%), for which the patient elected to undergo a mastectomy. Nine patients were found to have wound complications (7.7%) and two had fat necrosis (1.7%) on follow-up. Of all the evaluated risk factors, only applicator size (p < 0.01) had a statistically significant association with an increase in complications. CONCLUSIONS: With a short follow-up, IORT appears to be a safe treatment modality for a select group of patients, leading to a reasonable increase in operating room time and complication rates following BCS. The utilization of larger applicators at the time of IORT was associated with an increase in wound complications and fat necrosis.

Commissioning, clinical implementation, and performance of the Mobetron 2000 for intraoperative radiation therapy.

The Mobetron is a mobile electron accelerator designed to deliver therapeutic radiation dose intraoperatively while diseased tissue is exposed. Experience with the Mobetron 1000 has been reported extensively. However, since the time of those publications a new model, the Mobetron 2000, has become commercially available. Experience commissioning this new model and 3 years of data from historical use are reported here. Descriptions of differences between the models are emphasized, both in physical form and in dosimetric characteristics. Results from commissioning measurements including output factors, air gap factors, percent depth doses (PDDs), and 2D dose profiles are reported. Output factors are found to have changed considerably in the new model, with factors as high as 1.7 being measured. An example lookup table of appropriate accessory/energy combinations for a given target dimension is presented, and the method used to generate it described. Results from 3 years of daily QA measurements are outlined. Finally, practical considerations garnered from 3 years of use are presented.