Dose Penalty Research Articles

Feasibility and sensitivity study of helical tomotherapy for dose painting plans

Important limitations for dose painting are due to treatment planning and delivery constraints. The purpose of this study was to develop a methodology for creating voxel-based dose painting plans that are deliverable using the clinical TomoTherapy Hi-Art II treatment planning system (TPS). Material and methods. Uptake data from a head and neck patient who underwent a [61Cu]Cu-ATSM (hypoxia surrogate) PET/CT scan was retrospectively extracted for planning. Non-uniform voxel-based prescriptions were converted to structured-based prescriptions for compatibility with the Hi-Art II TPS. Optimized plans were generated by varying parameters such as dose level, structure importance, prescription point normalization, DVH volume, min/max dose, and dose penalty. Delivery parameters such as pitch, jaw width and modulation factor were also varied. Isodose distributions, quality volume histograms and planning target volume percentage receiving planned dose within 5% of the prescription (Q0.95–1.05) were used to evaluate plan conformity. Results. In general, the conformity of treatment plans to dose prescriptions was found to be adequate for delivery of dose painting plans. The conformity was better as the dose levels increased from three to nine levels (Q0.95–1.05: 69% to 93%), jaw decreased in width from 5.0cm to 1.05cm (Q0.95–1.05: 81% to 93%), and modulation factor increased up to 2.0 (Q0.95–1.05: 36% to 92%). The conformity was invariant to changes in pitch. Plan conformity decreased as the prescription DVH constraint (Q0.95–1.05: 93% vs. 89%) or the normalization point (Q0.95–1.05: 93% vs. 90%) deviated from the means. Conclusion. This investigation demonstrated the ability of the Hi-Art II TPS to create voxel-based dose painting plans. Results indicated that agreement in prescription dose and planned dose distributions for all plans were sensitive to physical delivery parameter changes in jaw width and modulation factors, but insensitive to changes in pitch. Tight constraints on target structures also resulted in decreased plan conformity while under a relaxed set of optimization parameters, plan conformity was increased.

TU-A-201B-05: Radiation Dose Penalty in Axial Mode CT

Purpose: To determine whether a dose penalty exists when scanning patients in axial scan mode on General Electric (GE)Computed Tomography(CT)scanners. A 10‐millisecond‐long rise of the x‐ray tube output occurs immediately before image acquisition and is suspected to contribute additional dose near the start angle of the x‐ray tube. This dose penalty may exist for axial scanning because the rise time occurs for every 360° rotation of the x‐ray tube, whereas rise time only occurs once in helical scan mode. Method and Materials: 10‐, 15‐, and 32‐cm CTDI phantoms were scanned on a GE VCT scanner for a single axial rotation in service mode. Exposure was measured at a constant peripheral location (12:00) and recorded for various x‐ray tube start angles. These methods were repeated on the 32‐cm phantom, changing its vertical position in 2‐cm increments from 6‐cm below to 12‐cm above iso‐center. Results: Exposure, measured at the 12:00 peripheral chamber position, was 5.5%, 1.8%, and 1.0% higher in the 32‐, 15‐, and 10‐cm phantom, respectively, when a start angle of 0° was used versus a start angle of 270° (this start angle was used for comparison to avoid attenuation from the patient table). The exposure penalty ranged from 2.9% to 12.7% when the 32‐cm phantom was 6‐cm below and 12‐cm above iso‐center, respectively. Conclusion: In light of the dose penalty observed in this study, axial acquisitions should feature a start angle of 180° to avoid imparting this penalty on superficial radiosensitive organs (i.e. breast, testes, and thyroid), which are near the anterior side of patients. Also, dose is not consistent around the periphery of a phantom scanned in axial mode and contiguous helical scanning (pitch = 1.0) provides a dose advantage over axial scanning (all other technique factors being equal).

SU-GG-T-113: Impact of Treatment Planning Optimization Parameters on Dose Painting Plans

Purpose: To develop a methodology for creating voxel-based dose painting plans that are clinically deliverable using a commercialized, structure-based treatment planning system (TPS). Methods and Materials: Uptake data from head and neck patients who underwent [61Cu]Cu-ATSM (hypoxia surrogate) PET/CT scans were retrospectively extracted for treatment planning. Non-uniform voxel-based prescriptions were converted to structure-based prescriptions for compatibility with the TomoTherapy Hi-ART II™ TPS. Optimized treatment plans were generated using fixed pitch, jaw width, and modulation factor delivery parameters. The effects of structure importance, prescription point normalization, iteration number, DVH volume, Min/Max dose, and dose penalty were investigated by varying these constraints one at a time in each plan. Avoidance structures were constrained to clinical tolerances. Quality-volume histograms (QVH) as percent receiving within 5 percent of the prescription (Q0.95–1.05) were used to evaluate treatment plan conformity. Results: Conformity of the plans to their non-uniform dose prescription was insensitive to the optimization parameters when the prescription DVH constraint and the dose normalization point were set to the mean PTV volume and dose. Plan conformity decreased as the prescription DVH constraint (Q0.95–1.05: 88% vs. 84%) or the normalization point (Q0.95–1.05: 88% vs. 85%) deviated from the means. Plan conformity was also negatively impacted by increasing integral boost doses from 10 Gy to 30 Gy (Q0.95–1.05: 97% vs 78%), decreasing iteration number from 1000 to 100 (Q0.95–1.05: 86% vs 80%), and applying conventional planning strategies (Q0.95–1.05: 88% vs 59%). Conclusions: This investigation demonstrates the ability of a structure-based TPS to create voxel-based dose painting plans using a relaxed set of optimization parameters and a dose normalization point set to the mean PTV dose. Under these conditions, plan conformity is dominated by few optimization parameters. However, tightening constraints on target structures via conventional planning strategies results in decreased plan conformity.

Performance evaluation of an algorithm for fast optimization of beam weights in anatomy-based intensity modulated radiotherapy

This study aims to evaluate the performance of a new algorithm for optimization of beam weights in anatomy-based intensity modulated radiotherapy (IMRT). The algorithm uses a numerical technique called Gaussian-Elimination that derives the optimum beam weights in an exact or non-iterative way. The distinct feature of the algorithm is that it takes only fraction of a second to optimize the beam weights, irrespective of the complexity of the given case. The algorithm has been implemented using MATLAB with a Graphical User Interface (GUI) option for convenient specification of dose constraints and penalties to different structures. We have tested the numerical and clinical capabilities of the proposed algorithm in several patient cases in comparison with KonRad® inverse planning system. The comparative analysis shows that the algorithm can generate anatomy-based IMRT plans with about 50% reduction in number of MUs and 60% reduction in number of apertures, while producing dose distribution comparable to that of beamlet-based IMRT plans. Hence, it is clearly evident from the study that the proposed algorithm can be effectively used for clinical applications.

Current and Upcoming Erythropoiesis-Stimulating Agents, Iron Products, and Other Novel Anemia Medications

Treatment for anemia has come a long way in the last 20 years since the first recombinant human erythropoietins were licensed for the management of anemia in chronic kidney disease. The first-generation epoetins were succeeded by the development and production of a longer-acting erythropoietin (EPO) analog, darbepoetin alpha, which allowed less frequent dosing, usually once weekly or once every 2 weeks. More recently, another EPO-related molecule has been manufactured called Continuous Erythropoietin Receptor Activator with an even longer half-life, and although for patent reasons this is not available in the United States, it is licensed and is already being used in Europe. Other molecules are in development or are becoming licensed in Europe, including biosimilar epoetin products/follow-on biologics, and elsewhere in the world there are cheaper-production "copy" epoetins. Indeed, it is estimated that up to 80 such products may be sold in countries with less stringent regulatory control of pharmaceutical products. Two different biosimilar epoetins have already been licensed in Europe, one under 2 different brand names and one under 3 different brand names, and others may follow. Hematide is a synthetic peptide-based EPO receptor agonist that, interestingly, has no structural homology with EPO, and yet is still able to activate the EPO receptor and stimulate erythropoiesis. This agent is currently in phase III clinical trials. Research continues for orally active antianemic therapies, and several strategies are being investigated, although none is imminently available. Two new intravenous iron preparations have recently been developed, one in the United States (Ferumoxytol; AMAG Pharmaceuticals, Inc., Cambridge, MA) and one recently licensed in Europe (ferric carboxymaltose [Ferinject; Vifor Pharma, Zurich, Switzerland]). In conclusion, the development of effective therapies for the treatment of anemia has been a highly active field, both scientifically and economically, over the last two decades.

Randomized Equivalence Study Evaluating the Possibility of Switching Hemodialysis Patients Receiving Subcutaneous Human Erythropoietin Directly to Intravenous Darbepoetin Alfa

Darbepoetin alfa is an erythropoiesis-stimulating agent (ESA) used either intravenously or subcutaneously with no dose penalty; however, the direct switch from subcutaneous recombinant human erythropoietin (rHuEPO) to intravenous darbepoetin has barely been studied. To establish the equivalence of a direct switch from subcutaneous rHuEPO to intravenous darbepoetin versus an indirect switch from subcutaneous rHuEPO to intravenous darbepoetin after 2 months of subcutaneous darbepoetin in patients undergoing hemodialysis. In this open, randomized, 6-month, prospective study, patients with end-stage kidney disease who were on hemodialysis were randomized into 2 groups: direct switch from subcutaneous rHuEPO to intravenous darbepoetin (group 1) and indirect switch from subcutaneous rHuEPO to intravenous darbepoetin after 2 months of subcutaneous darbepoetin (group 2). A third, nonrandomized group (control), consisting of patients treated with intravenous rHuEPO who were switched to intravenous darbepoetin, was also studied to reflect possible variations of hemoglobin (Hb) levels due to change from one type of ESA to the other. The primary outcome was the proportion of patients with stable Hb levels at month 6. Secondary endpoints included Hb stability at month 3, dosage requirements for darbepoetin, and safety of the administration route. Among 154 randomized patients, the percentages with stable Hb levels were equivalent in groups 1 and 2, respectively, at month 3 (86.0% vs 91.3%) and month 6 (82.1% vs 81.6%; difference -0.5 [90% CI -12.8 to 11.8]). Mean Hb levels between baseline and month 6 remained stable in both groups, with no variation in mean darbepoetin dose. Mean ferritin levels remained above 100 microg/L in the 3 groups during the whole study, and darbepoetin was well tolerated. This study has shown equivalent efficacy on Hb stability without the need for dosage increase in patients switched directly from subcutaneous rHuEPO to intravenous darbepoetin.

Normal tissue dose-effect models in biological dose optimisation

Sophisticated radiotherapy techniques like intensity modulated radiotherapy with photons and protons rely on numerical dose optimisation. The evaluation of normal tissue dose distributions that deviate significantly from the common clinical routine and also the mathematical expression of desirable properties of a dose distribution is difficult. In essence, a dose evaluation model for normal tissues has to express the tissue specific volume effect. A formalism of local dose effect measures is presented, which can be applied to serial and parallel responding tissues as well as target volumes and physical dose penalties. These models allow a transparent description of the volume effect and an efficient control over the optimum dose distribution. They can be linked to normal tissue complication probability models and the equivalent uniform dose concept. In clinical applications, they provide a means to standardize normal tissue doses in the face of inevitable anatomical differences between patients and a vastly increased freedom to shape the dose, without being overly limiting like sets of dose-volume constraints.

The influence of patient centering on CT dose and image noise

Although x-ray intensity shaping filters (bowtie filters) have been used since the introduction of some of the earliest CT scanner models, the clinical implications on dose and noise are not well understood. To achieve the intended dose and noise advantage requires the patient to be centered in the scan field of view. In this study we explore the implications of patient centering in clinical practice. We scanned various size and shape phantoms on a GE LightSpeed VCT scanner using each available source filter with the phantom centers positioned at 0, 3, and 6 cm below the center of rotation (isocenter). Surface doses were measured along with image noise over a large image region. Regression models of surface dose and noise were generated as a function of phantom size and centering error. Methods were also developed to determine the amount of miscentering using a scout scan projection radiograph (SPR). These models were then used to retrospectively evaluate 273 adult body patients for clinical implications. When miscentered by 3 and 6 cm, the surface dose on a 32 cm CTDI phantom increased by 18% and 41% while image noise also increased by 6% and 22%. The retrospective analysis of adult body scout SPR scans shows that 46% of patients were miscentered in elevation by 20-60 mm with a mean position 23 mm below the center of rotation (isocenter). The analysis indicated a surface dose penalty of up to 140% with a mean dose penalty of 33% assuming that tube current is increased to compensate for the increased noise due to miscentering. Clinical image quality and dose efficiency can be improved on scanners with bowtie filters if care is exercised when positioning patients. Automatically providing patient specific centering and scan parameter selection information can help the technologist improve workflow, achieve more consistent image quality and reduce patient dose.

SU‐FF‐T‐59: Accuracy of Heterogeneity Correction Algorithms Used with Step and Shoot and Sliding Window IMRT

Purpose: To evaluate the accuracy of tissue heterogeneity dose correction algorithms for IMRT delivery using step and shoot and sliding window techniques. Method and Materials: A solid water phantom incorporating a lung equivalent heterogeneity layer along with several planning targets was constructed. The phantom was CT scanned and contouring of targets was done. Several anatomical dose constraints and penalty functions were defined. A commercial treatment planning system was used to evaluate the optimal fluence for several IMRT field setups. Each fluence was then run through a leaf motion calculator to provide deliverable fluences using both step and shoot and sliding window techniques. Heterogeneity correction algorithms were used in both the dose optimization process and the final dose calculations. GafChromic EBT film was strategically placed within the phantom. The phantom was irradiated with each set of IMRT treatment fields and each film was then processed. The measured dose distribution was compared to the predicated distributions. Results: Preliminary studies with static fields (1×10 cm2 through 10×10 cm2) showed that heterogeneity algorithms became less accurate with decreasing field width. The largest difference between the predicted and the delivered dose on the beam axis inside the lung equivalent material was 28% found for the EqTAR algorithm for the 1×10 cm2 field size. Not surprisingly, calculated dose distributions were most accurate for step and shoot delivery where field widths are typically larger than those used with sliding window delivery. Conclusion: Current heterogeneity correction algorithms were found to become less accurate with decreasing field widths. The accuracy of IMRT dose delivery in regions of tissue heterogeneity is therefore dependent on the delivery technique. Since sliding window techniques typically irradiate with narrower fields, heterogeneity algorithms are more accurate for step and shoot delivery where fewer but larger fields are typically used.

SU‐FF‐I‐29: Optimizing Radiation Dose and Image Quality in a Multi‐Detector Computed Tomography

Purpose: The study was carried out to find optimal scanning parameters for an MDCT at Ramathibodi hospital. Method and materials: Two types of phantom were employed to assess image quality and standard ionization chamber was used to measure radiation dose. Tube current, slice thickness, pixel size and pitch were varied from routine techniques to evaluate factors affecting image quality (spatial resolution, contrast detectability and image noise) and dose (CTDIair) with aim of optimization. Results: From our study showed that, optimization of radiation dose and image quality could be achieved by proper selection of scanning parameters. Reduction of tube current could be made 22 – 28%, pitch could be doubled from 0.75 to 1.5 (50% dose reduction) in slice thickness less than 7.5 mm without any significant effects on noise and contrast. In examinations that do not require high spatial resolution, larger pixel size could be made and could result in 15 – 17% dose reduction. Conclusions: We concluded from our study that, optimal scanning parameters were 115, 105, 150 and 170 mA for head (above/below posterior fossa) and abdomen (pre‐ and post‐ contrast) protocols, respectively. Slice thickness should be made thinnest possible before the penumbra dose penalty becomes significant. Pitch and pixel size should be selected based on clinical requirements.

SU-FF-T-226: Evaluation of Radiation Therapy Treatment Plans Through Conventional IMRT and Inversely Planned Intensity Modulated ARC Therapy (IMAT), A Clinical Comparison

Purpose: The goals of this study were to evaluate the difference in target dose uniformity and dose distribution through mean DVH and OAR analysis for a given PTV. Dosimetric analysis were also extended to compare sparing of normal tissue and critical structures between two treatment planning techniques. Methods and Material: The ADAC Pinnacle inverse planning system was used to generate conventional IMRT inverse plans for a few head & neck patients. These plans were then compared with treatment plans generated using the IMAT software package manufactured by 3D line USA Medical System Corp. Using axial CT for the same patients, targets, and numerous critical structures were delineated and “best” plans were developed in each environment. In IMAT planning, continuous and dynamically shaped arcs were planned through aperture optimization and leaf sequencing. The dose delivery was accomplished through a specially designed MICRO DMLC with 3 mm leaf width and double focus focalization. Results: For IMRT plans, 5 or 7 co‐planar arcs were used and treatment plans were generally optimized using a predefined set of dose objectives, penalties, min and max dose and percentage of volume required. The IMAT plans were designed with typically two co‐planar arcs of 180 degrees around the patient with similar dosimetry objective parameters. The percentage coverage for V95 in targets 1&2 were 94.9 ± 2.8 and 95.1 ± 2.1 for IMRT and 98.4 ± 2.6 & 98.1 ± 2.8 for IMAT respectively. Sparing of critical structures was generally better in IMAT, a detail of which will be presented at the meeting. Conclusions: In the IMAT technique with direct aperture optimization,dose distribution within the target was found to be better by 8%. Superior critical structure sparing was achieved as compared with similar plan using IMRT technique. The IMAT technique may be considered as an alternative to tomotherapy.

SU‐FF‐I‐25: Optimizing Dose and Image Quality in Pediatric Computed Tomography

Purpose: The goal of this work was to accurately quantify the doses delivered to pediatric patients during computed tomography (CT) exams while simultaneously evaluating the image quality of images obtained with the same protocols, and thus identify potential dose‐saving protocols that maintain adequate image quality. Method and Materials: A tomographic newborn physical phantom was constructed from tissue‐equivalent materials for use in evaluating the doses delivered to pediatric patients as a result of diagnostic imaging. Fiber optic‐coupled (FOC) dosimeters were used along with the physical phantom to measure average organ doses during CT exams across a wide range of protocol parameters (80–120 kV, 50–150 mAs, 12 mm and 24 mm collimated beam widths, pitches of 0.75, 1.0, and 1.25, for both head and body protocols). Then, images of the Catphan CTP515 low contrast module were acquired using the same protocols, and scored automatically with a custom‐written scoring routine with threshold contrast‐to‐noise ratios (CNR) based on radiologists' scoring of similar phantom images. Results: Measured effective dose values for head exams ranged from 0.33 mSv to 4.3 mSv, depending on protocol selection. These effective dose values are driven by doses to the bone marrow, bone surface, brain, and thyroid gland. Measured effective doses for CAP exams ranged from 1 mSv to 14.3 mSv, depending on protocol selection. A collimated beam width of 24 mm (16 × 1.5) was determined to be the optimal setting for both head and CAP imaging in terms of both image quality and dose. Conclusions: Tube potential and tube current‐time product are the two major contributors to both dose and image quality. However, the use of pitch values less than 1.0 does involve a substantial dose penalty (approximately 30 percent greater dose) without providing any significant gains in image quality for general imaging tasks.

SU‐FF‐I‐42: The Influence of Bowtie Filter Selection, Patient Size and Patient Centering On CT Dose and Image Noise

Purpose: Bowtie filters have been used on CT scanners for many years to reduce patient dose. In this study we rigorously characterized bowtie filter dose and noise performance and discovered some significant clinical implications for the CT scanner technologist. Methods and Materials: Various size and shape phantoms were scanned on a GE LightSpeed VCT scanner using each available bowtie filter with phantoms positioned at 0 mm, 30 mm and 60 mm below isocenter. Surface and CTDI doses were measured along with image noise. Relationships for dose and noise were determined using regression methods and compared to computer simulated results. An algorithm was developed to determine the centering error from scout scans and subsequently used to evaluate the clinical implications for 273 adult body patients. Results: The measured noise and dose performance agreed with simulations and indicated an optimum bowtie filter can be selected as a function of phantom size. The 32cm CTDI phantom scanned on the large, medium and small bowtie filters produced a dose savings of 29%, 24%, and 17% respectively compared to the flat filter for similar image noise. However, the surface dose increased by 18% and 41% for the miscentered cases while image noise increased by 2% and 13% with the optimum CTDI phantom bowtie. The noise increased even more (5% and 20%) for a miscentered nominal body shaped phantom. The clinical scout scan analysis indicated that 54% of patients were miscentered by 20 mm to 60 mm. With this miscentering, the surface dose penalty for the 32 cm CTDI phantom would range from 22 % to 71% assuming mAs is also increased to compensate for noise,. Conclusions: Clinical image quality and dose efficiency can be significantly improved on scanners with bowtie filters if technologists can nominally center the patient region to be scanned.

Optical Reflectance Spectroscopy for Prospective Studies on Breast Cancer Risk in adolescent Girls

Fifth AACR International Conference on Frontiers in Cancer Prevention Research, Nov 12-15, 2006 A15 Prospective studies on diet and lifestyle effects on future breast cancer risk in young girls typically rely on establishing correlations between risk factors, and serum sex or growth hormones. Quantitative physical information on developing breast tissue is limited to breast density derived from DEXA at a cost of ionizing radiation exposure and the associated dose penalty. Near-infrared Optical Reflectance Spectroscopy (ORS) uses light to access structural and metabolic bulk tissue properties, providing quantitative physical and physiological information. Here we want to evaluate the ability of ORS to detect physiological and chromophore changes between different Tanner stages of breast development. Measurements were obtained from 7 adolescent girls ages 10 - 14 years. Spectrally resolved reflected light from 2 radial distances, 1.5 and 3.0cm, providing different sample depth, is detected. Principle Component Analysis (PCA) was used to reduce the spectral data and generate individual scores, representing physiologic and morphologic tissue information. Four principle components were identified accounting for 99.92% of the spectral variance. Radial distances and hence tissue volume interrogation, had an impact on ORS parameters t1, t2 and t4 (all at p<0.05, 2-tail independent t-test). Pearson correlation analysis indicated a positive correlation between t3 (r=-0.462, p<0.001) and increasing breast Tanner stages; t4 (r=0.240, p<0.011) showed a reverse relationship. r values were strengthened when adjusted for menarche status and radial distances. ORS may provide an alternative tool to DEXA to identify morphologic and physiologic changes to the breast tissue resulting from an intervention during the adolescent years, and will enable a much-improved sampling rate throughout the intervention.

Polyp Detection with MDCT: A Phantom-Based Evaluation of the Impact of Dose and Spatial Resolution

The objective of our study was to evaluate the impact of dose and spatial resolution on the detection of colonic polyps using a 4-MDCT scanner. Twenty-four latex phantoms that simulate the large bowel and contain artificial polyps of different sizes and shapes were constructed. The polyps were divided into three size groups (diameter, 0-2, 2-5, and 5-10 mm) and were classified into four shape groups: pedunculated; broad-based; ulcerated or depressed; and sessile or flat. The colon phantoms were submerged in a water tank and scanned on a 4-MDCT scanner using 12 protocols with various settings of slice thickness, pitch, and tube current. The images were independently evaluated by three radiologists using axial 2D multiplanar reconstruction images and a 3D surface-rendering technique (fly-through). At a constant dose (i.e., dose-length product [DLP]), the polyp detection rate increased with increasing axial spatial resolution. For the standard protocol (2.50-mm slice thickness, 1.5 pitch), the detection rate for all polyp sizes decreased from approximately 70% at 100 mA to 55% at 40 mA. Between a 60- and 100-mA tube current, the detection rate for the largest polyps (> 5 mm) was almost constant, close to 90%. The detection of polyps in the large bowel using a standard protocol can be improved without dose penalty by increasing the axial spatial resolution of the image acquisition and adjusting the tube current setting. If the analysis can be restricted to polyps larger than 5 mm, the dose can be substantially reduced without compromising the detection rate.

Dosimetric evaluation of an online image guidance strategy for prostate cancer

The recently available kV cone-beam imaging capability on board an accelerator provides excellent soft tissue information in treatment condition, thus offers many opportunities for image guided radiotherapy of prostate cancer. The objective of this study is to investigate the feasibility of an online image guidance strategy to facilitate hypo-fractionated treatment of prostate cancer in terms of optimal margin design and treatment benefits evaluations. Twenty-eight (28) patients with at least 16 CT scans each (one planning and 15 treatment CT scans) from a conventional CT scanner were used in this study. For each patient, all CT datasets were initially registered based on bony landmark. It was thus assumed that the setup errors were eliminated. The CTV is the prostate gland only (i.e., low risk patient). Conformal 3D plans of 4-field box with 18 MV photons were created. The prescription dose is 3.9 Gy per fraction for 15 fractions and the total dose is 58.5 Gy to the isocenter, usually the center of the CTV. A second registration was performed for each treatment CT based on the position of prostate gland. In addition to the translations in all directions, the rotation was allowed in the sagittal plane to accommodate the more pronounced prostate motion in that plane. The online correction procedure includes isocenter shift and the rotation of MLC for lateral beams. Various CTV to PTV margins (0 to 10 mm) were used in the plan to evaluate the effect of the online correction and to determine the optimal parameters. Two additional dose distributions were computed for each patient (1) the “Uncorrected” is the one delivered to the patient based on the original plan, and (2) the “Corrected” is the one after correction is made. The cumulative dose distributions were obtained based on a finite element method that models soft tissue deformation. Dosimetric indices used for comparison include D99 (minimum dose) for prostate, EUD for rectal wall and CI95 (conformity index at 95% of prescription dose) for normal tissues. Intra-fraction organ motion was also simulated to account for the variations between the time of imaging and dose delivery in the treatment session. The benefits of online correction procedure depend on the magnitude of the motion, which is patient and treatment day specific. There are three types of scenarios: (1) motion is negligible and no correction is necessary, (2) motion is significant, but can be corrected to maintain dose conformation, and (3) motion is significant, but cannot be fully corrected due to organ deformation. Patient in scenario 2 would benefit most from this guidance strategy. Scenario 3 can be improved when margins were added to the plan. The results are summarized in the table. Basically, organ motion is bad for prostate and good for rectal wall (both inter- and intra- fraction motions). Increasing margin improves prostate coverage at the expense of higher doses to the rectal wall and normal tissue. The acceptance of the plan also depends on the dose penalty that one is willing to accept. If the criterion is 2% dose reduction on prostate D99, only 22% patient pass at 0 margin without correction, and this number improves to 33% when online correction is applied. When margin is increased to 3 mm, 64% patients pass without correction, and 86% pass with correction, this is comparable to the plan of 10 mm margin without online corrections. In addition to removing setup errors, online image guidance can improve dose conformation by correcting for inter-fraction rigid-body motion. The benefits depend on the acceptance criteria and the planning margins. Based on the analysis of all organs of interest, the benefit of online image guidance can be maximized with a planning margin of 2–3 mm to satisfy the 1–2% minimum dose reduction criteria. To further reduce the effect of non-rigid organ motion, a combination of offline and online planning strategy is necessary.

Evaluation of concave dose distributions created using an inverse planning system

Purpose : To evaluate and develop optimum inverse treatment planning strategies for the treatment of concave targets adjacent to normal tissue structures. Methods and Materials : Optimized dose distributions were designed using an idealized geometry consisting of a cylindrical phantom with a concave kidney-shaped target (PTV) and cylindrical normal tissues (NT) placed 5–13 mm from the target. Targets with radii of curvature from 1 to 2.75 cm were paired with normal tissues with radii between 0.5 and 2.25 cm. The target was constrained to a prescription dose of 100% and minimum and maximum doses of 95% and 105% with relative penalties of 25. Maximum dose constraint parameters for the NT varied from 10% to 70% with penalties from 10 to 1000. Plans were evaluated using the PTV uniformity index (PTV D max/PTV D 95) and maximum normal tissue doses (NT D max/PTV D 95). Results : In nearly all situations, the achievable PTV uniformity index and the maximum NT dose exceeded the corresponding constraints. This was particularly true for small PTV-NT separations (5–8 mm) or strict NT dose constraints (10%–30%), where the achievable doses differed from the requested by 30% or more. The same constraint parameters applied to different PTV-NT separations yielded different dose distributions. For most geometries, a range of constraints could be identified that would lead to acceptable plans. The optimization results were fairly independent of beam energy and radius of curvature, but improved as the number of beams increased, particularly for small PTV-NT separations or strict dose constraints. Conclusion : Optimized dose distributions are strongly affected by both the constraint parameters and target-normal tissue geometry. Standard site-specific constraint templates can serve as a starting point for optimization, but the final constraints must be determined iteratively for individual patients. A strategy whereby NT constraints and penalties are modified until the highest acceptable PTV uniformity index is achieved is discussed. This strategy can be used, in simple patient geometries, to ensure the lowest possible normal tissue dose. Strategies for setting the optimum dose constraints and penalties may vary for different optimization algorithms and objective functions. Increasing the number of beams can significantly improve normal tissue dose and target uniformity in situations where the PTV-NT separation is small or the normal tissue dose limits are severe. Setting unrealistically severe constraints in such situations often results in dose distributions that are inferior to plans achieved with more lenient constraints.

Reducing dose at barium enema: radiographers do it digitally.

It has been previously shown that, whilst radiographers in our hospital can undertake barium enema examinations with the same degree of diagnostic accuracy as consultant radiologists, there was a dose penalty to the patient arising from the use of a restrictive protocol requiring radiographers to take a series of plain radiographs for reporting purposes. For the past 3 years radiographers at this hospital have worked to a new protocol that replaces all routine radiographs with digital spot films. In the present study, dose-area product (DAP) measurements for 801 barium enema examinations performed by consultant radiologists and radiographers, using the revised protocol, were analysed and compared to ascertain whether there were still significant differences in radiation dose to the patient depending on the category of staff performing the examination. All examinations were reported by a consultant radiologist. The radiologists' reports were analysed against the known outcomes to compare the diagnostic accuracy of the examination when carried out by the two categories of staff. This study shows that using a modified protocol, in which digital spot films replace the series of overcouch radiographs for reporting, our radiographers are able to perform barium enemas without dose penalty to the patient, and without compromizing diagnostic accuracy. Means with 95% confidence intervals for DAP in the two groups were 9.8 Gycm(2) (9.4-10.3 Gycm(2)) and 10.7 Gycm(2) (10.2-11.1 Gycm(2)) for radiographers and radiologists, respectively.

Towards an evidence-based 3D conformal radiotherapy prostate protocol: what is the role of spiral CT?

Aim: To define the role of spiral CT in evolving an evidence-based 3D Conformal Radiotherapy (CRT) prostate protocol at Lincoln, UK.Discussion: Tumour doses traditionally prescribed at this centre to the prostate planning target volume (PTV) (64 Gy in 32 fractions) cannot be further escalated without modification of existing technique and may currently be inadequate to obtain the highest probability of local control. Prostate CRT has been demonstrated to be well tolerated with both conventional and escalated doses, however as 3D CRT PTV margins are tightened, prostate position has to be reliably predicted to avoid geographic misses or unacceptable normal tissue toxicity. The question of prostate position variability might be addressed by sequential on-treatment spiral CT scans at this centre. Spiral CT offers specific advantages of speed, small detail conspicuity, and arbitrary axial reconstruction compared to conventional CT with no attached dose penalty. Spiral CT coupled to the next generation of radiotherapy treatment planning systems (RTPs) may soon replace the CT virtual-simulator. There are significant hardware discrepancies between some present generation CT couch tops and linac couch tops. Recently published CT studies that consider prostate position variability may be fundamentally and significantly flawed due to these couch top differences. Due to a paucity of reported evidence regarding immobilisation methods, a spiral CT study is warranted to assess efficacy of immobilization method for an evidence-based prostate protocol. Confirmative spiral CT research at this centre into prostate position variability is required to select adequate margins to form the PTV for an evidence-based 3D CRT prostate protocol. Such a spiral CT study could be integrated with the immobilisation study and may separate or define the correlation (which at present is both unclear and unreported) between pelvic immobilization and prostate position variability. Initial PTV margins defined by expanding the CTV in three dimensions using an ellipsoid with major axes 1.65 times one standard deviation of prostatic displacement reported in initial studies to obtain margins of 0.7 cm laterally, 0.7 cm cranio-caudally and 1.1 cm in the AP direction are presently indicated for this centre's evidence-based prostate protocol.Conclusion: Spiral CT will provide the essential data set for 3D CRT planning for an evidence-based prostate protocol at Lincoln. Confirmative research using spiral CT is also warranted to assess daily prostate position variability and help define the prostate PTV for an evidence based prostate protocol at this centre.

Influence of anode/filter material and tube potential on contrast, signal-to-noise ratio and average absorbed dose in mammography: a Monte Carlo study.

The comparative performance of mammographic X-ray systems that use different anode/filter combinations has been assessed for screen-film and digital imaging. Monte Carlo techniques have been used to calculate average glandular dose as well as contrast and signal-to-noise ratio for imaging two test details. Five anode/filter combinations have been studied to establish the potential for dose saving or image quality improvement. For screen-film mammography, it was found that little benefit is gained by changing from a standard 28 kV molybdenum/molybdenum spectrum for breasts up to 6 cm thick. For thicker breasts, where the tube potential for the standard technique might be increased, 20% improvement in contrast can be achieved without dose penalty using molybdenum/rhodium or rhodium/rhodium spectra, whereas dose savings of more than 50% can be attained whilst maintaining contrast using tungsten/rhodium or rhodium/aluminium spectra. In digital mammography, a molybdenum/molybdenum spectrum delivers the lowest dose for a 2 cm breast, but gives the highest dose for thicker breasts. Tungsten/rhodium or rhodium/aluminium spectra provide the lowest doses at greater thicknesses. It is concluded that for screen-film mammography, molybdenum/molybdenum is the spectrum of choice for all but the thickest or most glandular breasts. In digital mammography, an alternative spectrum is preferable for breasts thicker than 2 cm.