Precision Of Dose Delivery Research Articles

Beam characteristics in two different proton uniform scanning systems: A side‐by‐side comparison

To compare clinically relevant dosimetric characteristics of proton therapy fields produced by two uniform scanning systems that have a number of similar hardware components but employ different techniques of beam spreading. This work compares two technologically distinct systems implementing a method of uniform scanning and layer stacking that has been developed independently at Indiana University (IU) and by Ion Beam Applications, S. A. (IBA). Clinically relevant dosimetric characteristics of fields produced by these systems are studied, such as beam range control, peak-to-entrance ratio (PER), lateral penumbra, field flatness, effective source position, precision of dose delivery at different gantry angles, etc. Under comparable conditions, both systems controlled beam range with an accuracy of 0.5 mm and a precision of 0.1 mm. Compared to IBA, the IU system produced pristine peaks with a slightly higher PER (3.23 and 3.45, respectively) and smaller, symmetrical, lateral in-air penumbra of 1 mm compared to about 1.9/2.4 mm in the inplane/crossplane (IP/CP) directions for IBA. Large field flatness results in the IP/CP directions were similar: 3.0/2.4% for IU and 2.9/2.4% for IBA. The IU system featured a longer virtual source-to-isocenter position, which was the same for the IP and CP directions (237 cm), as opposed to 212/192 cm (IP/CP) for IBA. Dose delivery precision at different gantry angles was higher in the IBA system (0.5%) than in the IU system (1%). Each of the two uniform scanning systems considered in this work shows some attractive performance characteristics while having other features that can be further improved. Overall, radiation field characteristics of both systems meet their clinical specifications and show comparable results. Most of the differences observed between the two systems are clinically insignificant.

Dosimetric characteristics of the small diameter BrainLab™ cones used for stereotactic radiosurgery

The purpose was to study the dosimetric characteristics of the small diameter (≤10.0 mm) BrainLAB cones used for stereotactic radiosurgery (SRS) treatments in conjunction with a Varian Trilogy accelerator. Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields is completely understood. Although a number of investigators have published the dosimetric characteristics of SRS cones, to our knowledge, there is no generally accepted value for the relative output factor (ROF) for the 5.0 mm diameter cone. Therefore, we have investigated the dosimetric properties of the small (≤10.0 mm) diameter BrainLAB SRS cones used in conjunction with the iPlan TPS and a Trilogy linear accelerator with a SRS beam mode. Percentage depth dose (PDD), off‐axis ratios (OAR), and ROF were measured using a SRS diode and verified with Monte Carlo (MC) simulations. The dependence of ROF on detector material response was studied. The dependence of PDD, OAR, and ROF on the alignment of the beam CAX with the detector motion line was also investigated using MC simulations. An agreement of 1% and 1 mm was observed between measurements and MC for PDD and OAR. The calculated ROF for the 5.0 mm diameter cone was 0.692±0.008 — in good agreement with the measured value of 0.683±0.007 after the diode response was corrected. Simulations of the misalignment between the beam axis and detector motion axis for angles between 0.5°–1.0° have shown a deviation > 2% in PDD beyond a certain depth. We have also provided a full set of dosimetric data for BrainLAB SRS cones. Monte Carlo calculated ROF values for cones with diameters less than 10.0 mm agrees with measured values to within 1.8%. Care should be exercised when measuring PDD and OAR for small cones. We recommend the use of MC to confirm the measurement under these conditions.PACS numbers: 87.53.Ly, 87.55.‐x, 87.53.Bn, 87.55.K‐

SU-E-T-536: SRS Diode and Diamond Detector Signal Response Correction Factors in Small Diameter Stereotactic Radiosurgery Fields

Purpose: To evaluate the magnitude of correction factors that need to be applied to the response of a SRS diode and a diamond detector used to measure the relative output factor of the 5.0 mm diameter BrainLab stereotactic cone in conjunction with a Varian Trilogy accelerator. Method and Materials: Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields are completely understood. We have previously reported a 3% difference between the relative output factor (ROF) for the 5.0mm diameter cone measured with uncompensated Scanditronix/IBA Si‐ diode and ROF calculated by Monte Carlo simulations using MCSIM code. However, the reported values for ROF did not account for differences in response of Si under conditions lacking lateral electronic equilibrium. A Si‐ response correction factor has been calculated using MCSIM code and was applied to the measured value for ROF. In addition PTW 60003 diamond detector was used to verify previous measurements. Corrections for dose rate dependence and volume averaging have been made using MC simulations. Results: ROF obtained for SRS diode (0.683±0.011) and diamond detector (0.685±0.008) are in excellent agreement (within 0.2%) with each other and within 1–1.3% of previously reported MC calculated value of ROF (0.692±0.011). Conclusions: We have performed an experimental and theoretical investigation of ROF for radiation field created by BrainLab 5.0 mm SRS cone in conjunction with a Trilogy accelerator equipped with SRS mode. Methods of obtaining ROF using Scanditronix/IBA Si‐diode and PTW Diamond 60003 detector have been outlined. Monte Carlo simulations were used to correct for various effects contributing to potential detector response errors. We conclude that corrections for detector signal response up to ±4% were needed for both SRS diode and diamond detectors.

On the use of polymer gels for assessing the total geometrical accuracy in clinical Gamma Knife radiosurgery applications

The nearly tissue equivalent MRI properties and the unique ability of registering 3D dose distributions of polymer gels were exploited to assess the total geometrical accuracy in clinical Gamma Knife applications, taking into account the combined effect of the unit's mechanical accuracy, dose delivery precision and the geometrical distortions inherent in MR images used for irradiation planning. Comparison between planned and experimental data suggests that the MR-related distortions due to susceptibility effects dominate the total clinical geometrical accuracy which was found within 1 mm. The dosimetric effect of the observed sub-millimetre uncertainties on single shot GK irradiation plans was assessed using the target percentage coverage criterion, and a considerable target dose underestimation was found.

SU‐GG‐T‐517: Dosimetric Characteristics of the Small Diameter BrainLab Cones for Stereotactic Radiosurgery

Purpose: To study the dosimetric characteristics of the small diameter (≤10.0mm) BrainLab cones used for stereotactic radiosurgery (SRS) treatments in conjunction with a Varian Trilogy accelerator. Method and Materials: Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields is completely understood. Although a number of investigators have published the dosimetric characteristics of SRS cones, to our knowledge, there is no generally accepted value for the relative output factor (ROF) for the 5.0mm diameter cone. Therefore we have investigated the dosimetric properties of the small (≤10.0mm) diameter BrainLab SRS cones used in conjunction with the iPlan TPS and a Trilogy linear accelerator with a SRS beam mode. Percentage depth dose (PDD), off‐axis ratios (OAR) and ROF were measured using a SRS diode and verified with Monte Carlo (MC) simulations. The dependence of PDD, OAR and ROF on the alignment of the beam CAX with the detector motion line was also investigated using MC simulations. Results: An agreement of 1% and 1mm was observed between measurements and MC for PDD and OAR. The calculated ROF for the 5.0mm diameter cone was 0.695±0.008 in good agreement with the measured value of 0.711. For 7.5 and 10.0mm diameter cones the agreement was within 0.5%. Simulations of the misalignment of the beam axis and detector motion axis for angles between 0.5°–1.0° show a deviation >2% in PPD beyond certain depth. Conclusions: Monte Carlo calculated ROF values for 5.0 mm diameter cone agrees with our measurement to within 2.3%. Care should be exercised when measuring PDD and OAR for small cones. We recommend the use of MC to confirm the measurement under these conditions.

Stereotactic Body Radiotherapy for Primary Management of Early-Stage, Low- to Intermediate-Risk Prostate Cancer: Report of the American Society for Therapeutic Radiology and Oncology Emerging Technology Committee

Stereotactic Body Radiotherapy for Primary Management of Early-Stage, Low- to Intermediate-Risk Prostate Cancer: Report of the American Society for Therapeutic Radiology and Oncology Emerging Technology Committee

Craniospinal radiotherapy in children: Electron- or photon-based technique of spinal irradiation

Background The prone position and electron-based technique for craniospinal irradiation (CSI) have been standard in our department for many years. But this immobilization is difficult for the anaesthesiologist to gain airway access. The increasing number of children treated under anaesthesia led us to reconsider our technique. Aim The purpose of this study is to report our new photon-based technique for CSI which could be applied in both the supine and the prone position and to compare this technique with our electron-based technique. Materials and methods Between November 2007 and May 2008, 11 children with brain tumours were treated in the prone position with CSI. For 9 patients two treatment plans were created: the first one using photons and the second one using electron beams for spinal irradiation. We prepared seven 3D-conformal photon plans and four forward planned segmented field plans. We compared 20 treatment plans in terms of target dose homogeneity and sparing of organs at risk. Results In segmented field plans better dose homogeneity in the thecal sac volume was achieved than in electron-based plans. Regarding doses in organs at risk, in photon-based plans we obtained a lower dose in the thyroid but a higher one in the heart and liver. Conclusions Our technique can be applied in both the supine and prone position and it seems to be more feasible and precise than the electron technique. However, more homogeneous target coverage and higher precision of dose delivery for photons are obtained at the cost of slightly higher doses to the heart and liver.

Control of brain metastases using frameless image-guided radiosurgery

Radiosurgery is an important and well-accepted method in the management of brain metastases. Using conventional frame-based techniques, high lesional control rates are expected. The introduction of image-guided techniques allows for improved patient comfort and workflow. Some controversy exists as to the accuracy of imageguided techniques and consequently the impact they might have on control of brain metastases (as opposed to the level of control achieved with frame-based methods). The authors describe their initial 15-month experience with image-guided radiosurgery (IGRS) using Novalis with ExacTrac for management of brain metastases. The authors reviewed the cases of brain metastasis treated by means of IGRS in their tertiary regional radiation oncology service over a 15-month period. During the study period 54 patients (median age 57.9 years) harboring 108 metastases were treated with IGRS. The median time from cancer diagnosis to development of brain metastasis was 12 months (range 0-144 months). The median tumor volume was 0.98 cm(3) (range 0.03-19.07 cm(3)). The median prescribed dose was 18 Gy to the 80% isodose line (range 14-20 Gy). Lesions were followed with postradiosurgery MR imaging every 2-3 months following treatment. The median follow-up period was 9 months (range 0-20 months). Median actuarial survival was 8.6 months following IGRS. Eight patients with 18 lesions died within the first 2 months after the procedure, before scheduled follow-up imaging. Thus 90 lesions (in 46 patients) were followed up with imaging studies. Lesions that were unchanged or reduced in size were considered to be under control. The 6-month actuarial lesion control rate was 88%. Smaller lesions (< 1 cm(3)) had a statistically improved likelihood of complete imaging response (loss of all contrast-enhancement p = 0.01). Image-guided radiosurgical treatment of brain metastases resulted in high rates of tumor control comparable to control rates reported for frame-based methods. High control rates were seen for small lesions in which spatial precision in dose delivery is critical. These data suggests that in regard to lesion control, IGRS using Novalis with ExacTrac is equivalent to frame-based radiosurgery methods.

SU‐GG‐T‐157: Commission of 2.5 Mm ModuLeaf MLC for Stereotactic Radiation Therapy

Purpose: Improvements of MLC technology in radiotherapy, including smaller leaf sizes, allow greater accuracy in delivering radiation doses to the target with better conformality. For example, the high ablative doses used in hypo‐fractionated stereotactic body radiotherapy (SBRT) require high precision in dose delivery, thus high precision in commissioning the MLC system to verifiy MLC accuracy. The purpose of this study was to evaluate the procedure and challenges in the commissioning and application of 2.5‐mm small‐leaf MLC. Method and Materials: Siemens' newly‐developed Moduleaf MLC (MMLC) with 2.5mm leaf‐size was commissioned for SBRT. Step‐by‐step QA and application procedures for SBRT commission guidelines were followed. Error tolerance and QA results were analyzed through the entire commission process from beam data preparation, to treatment planning system commission, to treatment plan verification. The commission was divided into 3 phases: mechanical commission, software commission, and comprehensive commission. Isocenter accuracy was assured for mechanical precision. For software commission, the effect of small field factor was analyzed. In comprehensive phase, a single field and a head‐and‐neck case were tested with the QA standard. Results: The commission and clinical implementation of 2.5mm MMLC were described in this study, with provided guidelines for QA procedures from beam data collection and modeling to treatment planning methodology. The accuracy of the MLC mechanic isocenter was within 0.2 mm. At the same time, the treatment modalities with or without the flatten filter were compared, and they were consistent to each other except for the dose‐rate effect. Different MMLC clinical protocols, from SBRT to head‐and‐neck and intracranial treatments, have been suggested. Conclusion: After strictly following the physical QA requirements, we successfully commissioned the new mounted‐on 2.5mm MMLC for SRS/SBRT. In the follow‐up studies, clinical applications with MMLC will be further compared to other on‐site SRS systems, including Gamma‐Knife, and Tomotherapy system.

SU‐GG‐J‐73: Dose Distribution Improvements for Prostate Cancer Patients Using Hybrid Image Guidance Systems

Purpose: Daily setup variations may compromise the precision of dose delivery, tumor control and normal tissue avoidance. This study analyzed the setup variations predicted by kilo‐voltage Cone Beam CT (CBCT) combined with 3D Ultrasound image guidance. The overall integrity of prostate dose distribution was considered to define an offset threshold above which image guidance becomes crucial for dose delivery. Method and Materials: Initial immobilization was performed using three‐point in‐room lasers. The residual positional errors were assessed using CBCT from Varian Trilogy™ and Ultrasound guidance by Sonarray™ system. Superposition of offset vectors on the initial prostate plans enabled detailed evaluation of resultant changes in dosimetric and biological aspects for target and organs at risk. On‐line CBCT images were used for calculation. Dose volume histograms (DVH), Gamma Index (GI), equivalent uniform dose (EUD) and tumor control probability (TCP) were calculated. Other indices such as mean total dose (mTd) and maximal total dose (MTD) for normal structures were also considered in defining the threshold. Results and Conclusion: After initial laser alignment, the mean vector of residual displacements over all prostate patients is 6.2mm with a 5.0mm standard deviation. The decrease in EUD for defined tumor volumes was up to 66% when the largest offset was applied and 10% in the composite plan which incorporated overall offset histories. Results show that if patient's offsets were smaller than the applied PTV margin (5mm), unappreciable changes in dose coverage and tumor control were revealed. However, if offsets were larger than the PTV margin, tumor control and normal tissue sparing can be significantly compromised if proper couch shifts are not made under the imaging guidance system. In this situation, special attention is needed especially on organs at risk and even the adaptive radiotherapy might be necessary.

SU‐GG‐T‐468: Image‐Guided Stereotactic Spinal Radiosurgery with a Conventional Linear‐Accelerator

Purpose: As large experiences of spinal radiosurgery were reported by using CyberKnife, this study evaluates our experiences with spinal radiosurgery given by a conventional linear‐accelerator, utilizing the features of KV‐OBI and CBCT. Method and Materials: Image‐guided stereotactic radiosurgery is used in our clinic to treat spinal metastases with 16–18 Gy in a single‐fraction, or 20–30 Gy in 5‐fractions. Treatment is delivered using nine co‐planar IMRT‐beams in 20–25 minutes. The success of spinal radiosurgery requires sparing of the proximal spinal cord and high precision of dose delivery. With proper beam arrangement and dose optimization, the maximal cord dose can be minimized to 60–65% of the prescription dose. Stereotactic localization of the metastatic vertebrae‐body is achieved by KV‐OBI and CBCT guidance before and during the treatment. The shifts based on OBI♯1 are made from the skin marks after initial setup. Next, CBCT further verifies the patient alignment. Two more OBIs before every three fields ensure patient immobility and allow iso shifts if necessary. Results: Couch shifts were recorded for a sample of 12 patients, a total of 27 fractions in this evaluation. It is found that the ranges of the shifts guided by OBI♯1 from skin marks are: 1.4 cm longitudinal, 0.9 cm vertical, 0.7 cm lateral, and 2° in‐plane couch rotation, while CBCT and consecutive OBI♯2 and ♯3 before every three fields found little patient movements. Conclusion: The conventional linear‐accelerator is able to deliver stereotactic radiosurgery treatment of spinal metastases with a high degree of accuracy, as guided by OBI and CBCT image techniques, in combination with proper patient setup, immobilization, and skin marks. No body‐wrap or fiducials are necessary in our approach. The study also finds that OBI and CBCT are equivalent due to the proper patient setup procedures in our approach. CBCT is beneficial for localizing C7/T1 for treatment.

Random coincidences during in-beam PET measurements at microbunched therapeutic ion beams

Abstract At the experimental carbon ion tumour therapy facility at GSI Darmstadt, in-beam positron emission tomography (PET) is used to monitor the dose delivery precision. A dual head positron camera has been assembled from commercial detector components in order to measure the β + -activity, induced by the irradiation, simultaneously to the dose application. Despite the positive clinical impact, the image quality is limited by the low counting statistics, orders of magnitude below that in standard PET applications to nuclear medicine. This paper investigates the origin for the noisy acquisition during particle extraction from the synchrotron of GSI. The results demonstrate the failure of standard random correction techniques due to a γ -ray background correlated in time with the carbon ion beam microstructure. This prevents the use of data acquired during beam extraction for imaging. The loss of counting statistics is expected to rise further at the future hospital-based facility at Heidelberg, due to a more efficient utilisation of the accelerator resulting in shorter beam pauses and a reduced treatment time. In this respect, this paper provides the basis for a new data acquisition concept tailored to the unconventional application of in-beam PET imaging to therapy monitoring at radiofrequency pulsed radiation sources.

Radiation-activated antitumor vectors.

Radiotherapy (RT) is a primary treatment modality for the majority of solid tumors. The objective is to destroy the tumor mass by exposure to ionizing radiation (IR) from an external beam or isotopic source. IR causes DNA damage directly and indirectly via the production of reactive oxygen intermediates (ROIs). Accumulation of sufficient damage leads to tumor cell death. The efficacy of RT is usually governed by the radiation dose given, the main limitation being the need to avoid injury to the surrounding normal tissues. To address the latter problem, physical techniques such as conformal and intensity-modulated radiotherapy have been developed to improve the precision of dose delivery to the tumor volume. However, some tumors prove refractory to conventional radiotherapy treatments, as insufficient dose can be delivered to the tumor. In such cases, other therapeutic strategies, such as chemotherapy, can be used in combination, particularly if such drugs lead to increased tumor radiosensitization. Nevertheless, many tumor types, (e.g., glioblastoma) are often resistant to even these combined approaches. Consequently, there is a need for new strategies that can improve the effectiveness of current radiotherapy regimens. Gene therapy offers the exciting possibility of significantly improving the efficacy of radiotherapy without the need for IR-dose escalation or undue increases in normal tissue morbidity. Furthermore, the potential to spatially and temporally target the activation of gene therapy vectors using clinically relevant IR doses provides a particularly attractive prospect.

Radiosurgery performed with the aid of a 3-mm collimator in the subthalamic nucleus and substantia nigra of the vervet monkey.

Radiosurgery for functional neurosurgery performed using a linear accelerator (LINAC) has not been extensively characterized in preclinical studies. In the present study, the properties of a newly designed 3-mm-diameter collimator were evaluated in a dedicated LINAC, which produced lesions in the basal ganglia of vervet monkeys. Lesion formation was determined in vivo in three animals by examining magnetic resonance (MR) images to show the dose-delivery precision of targeting and the geometry and extent of the lesions. Postmortem immunohistochemical studies were conducted to determine the extent of lesion-induced radiobiological effects. In three male vervet monkeys, the subthalamic nucleus (STN; one animal) and the pars compacta of the lateral substantia nigra (SN; two animals) were targeted by a Novalis Shaped Beam Surgery System that included a 3-mm collimator and delivered a maximum dose of 150 Gy. Magnetic resonance images obtained 4, 5, and 9 months posttreatment were reviewed, and the animals were killed so that immunohistological characterizations could be made. The generation of precise radiosurgical lesions by a 3-mm collimator was validated in studies that targeted the basal ganglia of the vervet monkey. The extent of the lesions created in all animals remained restricted in diameter (< 3 mm) throughout the duration of the studies, as assessed by reviewing MR images. Histological studies showed that the lesions were contained within the STN and SN target areas and that there were persistent increases in glial fibrillary acidic protein immunoreactivity. Increases in immunoreactivity for tyrosine hydroxylase, the serotonin transporter, and the GluR1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate glutamate receptor in penumbral regions of the lesion were suggestive of compensatory neuronal adaptations. This radiosurgical approach may be of particular interest for the induction of lesions of the STN and SN in studies of experimental parkinsonism, as well as for the development of potential radiosurgical treatments for Parkinson disease.

Improvement in dose escalation using the process of adaptive radiotherapy combined with three-dimensional conformal or intensity-modulated beams for prostate cancer

Purpose: Advances in technology allow the creation of complex treatment plans with tightly conforming doses. However, variations in positioning of the organ/patient with respect to treatment beams necessitate the use of an appreciable margin, potentially limiting dose escalation in many patients. To ( 1) reduce this margin and ( 2) test the hypothesis that the achievable level of dose escalation is patient dependent, a patient-specific, confidence-limited planning target volume (cl-PTV) was constructed using an adaptive radiotherapy (ART) process for prostate cancer treatment developed in-house. The potential dose escalation achievable with this ART process is quantified for both conformal radiotherapy (CRT) delivery and intensity-modulated radiotherapy (IMRT) delivery. Material and Methods: Patients with organ confined prostate cancer were entered prospectively into an ART process developed in-house. This ART process has been designed to improve accuracy and precision of dose delivery, consequently enhancing dose escalation. In this process, a cl-PTV is constructed for each patient in the second week of treatment based upon on-line portal and CT images acquired during the first week of treatment. The treatment prescription dose, defined as the minimum dose to the cl-PTV, is selected based on predefined dose-volume constraints for rectum/bladder and derived from the pretreatment planning CT image. In addition, the treatment modality (CRT or IMRT) is determined based on the level of dose escalation achievable and the risk of inaccurate targeting. The potential for both dose escalation and the application of IMRT was evaluated by comparing the prescription doses delivered using the ART process, with the cl-PTV, to those in the traditional treatment process, with a conventional generic PTV. In addition, the distributions of potential geometric target underdosing and normal tissue overdosing were also calculated to evaluate the quality of the conventional treatment plans. Results: One hundred and fifty patients have been treated with the ART process. When compared to the treatment dose delivered with the conventional treatment process (generic PTV), an average 5% (2.5–10%) more dose could be delivered using the ART process with CRT, and 7.5% (2.5–15%) more dose could be delivered with IMRT. Of the 150 patients, 70% were treated to a minimum cl-PTV dose ≥ 77.4 Gy (81.3 Gy ICRU isocenter dose). Dosimetric analysis revealed that 81 Gy to the cl-PTV (or 86.7 Gy ICRU) could be prescribed to at least 50% of patients if IMRT was applied using the ART process. In contrast, IMRT did not yield an obvious dose escalation gain if patients were treated using the generic PTV. Our results also demonstrate that the cl-PTV is significantly smaller than the conventional generic PTV for most patients, with a mean volume reduction of 24% (range, 5–43%). Conclusion: These results support our hypothesis that the achievable level of dose escalation using ART is patient dependent. By using the ART process to develop a cl-PTV, one can ( 1) optimize the dose level, ( 2) increase the applicability of IMRT, and ( 3) improve the quality of dose delivery. The ART process provides the foundation to identify a suitable option (CRT or IMRT) for the delivery of a safe treatment and dose escalation. It is now our standard of practice for prostate cancer treatment.

Methodological issues in the administration of multiple doses of smoked cocaine-base in humans

Many methodological issues exist in human laboratory research with smoked cocaine-base that include safety, precision of dose delivery of smoked cocaine, and the lack of an adequate placebo. All of these issues are particularly apparent with studies involving multiple doses of cocaine. Addressing these concerns is important in conducting parametric studies that require examining dose-response effects. The purposes of this study were to determine: 1) the safest interval between doses to deliver smoked cocaine; 2) the accuracy or reproducibility of administering precise and multiple doses of cocaine; 3) the potential for using a control dose of cocaine; and 4) the influence of multiple doses on these parameters. Six black males were given 10 doses of either 5 or 35 mg of cocaine-base at 15-, 30-, and 45-min intervals. The dependent measures included physiological, subjective, and performance responses. These measures were taken prior to dosing and at specific time intervals after each dose of smoked cocaine. The results showed: 1) dosing at 30-min intervals allowed sufficient time for recovery of blood pressure and heart rate to permit up to 10 doses to be safely administered; 2) reproducible blood cocaine levels were obtained with repeated dosing using a heated wire-coil device; 3) significant differences were observed between the 5- and 35-mg dose with 5 mg being a low enough dose to produce minimal effects; 4) acute tolerance was evidenced with multiple doses of cocaine for most of the measures; and 5) considerable between- and within-subject variability was observed in the pattern of responses to cocaine.

Importance of in vivo dosimetry as part of a quality assurance program in tangential breast treatments

Purpose : The investigation of the accuracy and reproducibility in the daily dose delivered in tangential breast treatments with in vivo dose measurements. Methods and Materials : In vivo dose measurements performed on the tangential treatment fields of 35 breast cancer patients are analysed for three units: a 6 MV linear accelerator, an old Cobalt unit and a new Cobalt unit. The results are plotted in frequency distributions. Deviations on the mean are often the expression of a systematic error in one of the core procedures of a department. A large spread of the results around the mean indicates a high burden of random set-up errors and/or systematic errors in individual patients. The reproducibility in dose delivery is studied by comparing repetitive checks to their respective mean for investigation of random day-to-day variation. Results : A small systematic error on the entrance dose (+ 1.4%) is detected on the old Cobalt unit due to a discrepancy between measured and published percentage depth dose values. An unexpected systematic overdosage (+ 6%) is detected after implementation of a new software for dose calculation, proving that treatment quality is a process needing continuous monitoring. The transmission measurements demonstrate a systematic error in dose delivery of 1.5 to 3% due to the assumption that the breast is water equivalent when calculating the dose. The large spread of the transmission measurements (s a = 7.7%) shows that the weakest point in the treatment preparation chain is inaccurate acquisition of external body contours, leading to systematic errors in dose delivery for specific patients. The standard deviation for the reproducibility is 3.1% for the old Cobalt unit, vs. 1.6% on the other units, demonstrating the influence of staffing and mechanical characteristics of the units on daily precision in dose delivery. Conclusion : In vivo dosimetry is an important tool in a departmental quality assurance program to detect systematic errors in dose delivery, to identify inadequate treatment situations, to investigate weak points in the chain of treatment preparation and to ensure accurate dose delivery for individual patients. The predictive value of a single check for the accuracy in dose delivery during the whole treatment series is high for reproducible treatment methodologies.

Tolerances in setup and dosimetric errors in the radiation treatment of breast cancer

Purpose : Treatment failure in radiation therapy, as well as unexpected complications, can be associated with set up changes or variations that can cause deviations from the prescribed radiation dose distribution both inside and outside the target volume. The effect of various deviations from the planned setup on the delivery of the prescribed radiation dose to the desired treatment volume was studied. Methods and Materials : Adding a second simulation was investigated as means of minimizing setup changes on treatment. The first simulation was used for planning the treatment and the second simulation was essentially a mock treatment. Dosimetric evaluations based on dose volume histograms were analyzed for each deviation in the setup. Results : In 95% of the patients, the frequency of the changes in the setup parameters between the second simulation and the treatment setup were reduced significantly from the changes that occurred between the first simulation and the second simulation. The changes in isocenter coordinates up to ±1.0 cm have minimal effects (±2%) on the dose distributions. Gantry angle variations up to ±4° produce a change of less than ±5% in the dose distribution within the target volume. However, this angular variation resulted in additional tissue irradiation outside of the desired treatment field (about 10 cm 3 for a large patient). A gantry angle variation of ±6° can change the volume of tissue that receives the prescribed dose by at least ±10%. In addition, such a change can increase the volume of tissue outside the desired treatment field that is irradiated. Conclusion : It is concluded that individually, deviations in one of the parameters from the planned setup of ±1.0 cm in isocenter position and ±4° in gantry angle do not produce significant deviations from the planned dose distribution. However, a significant change in dose distribution is observed if the setup parameters are concurrently changed. A second simulation may minimize the deviations of the treatment setup from the planned setup and maximize the precision in dose delivery to the target volume.

In vivo dosimetry for tangential breast irradiation: role of the equipment in the accuracy of dose delivery

Entrance dose measurements (920) have been performed on both tangential treatment fields for 105 breast cancer patients. About half of them (52) were treated on a modern unit (Mevatron, Siemens), supplied with an automatic verification system, the other half (53) were treated on an old therapy unit (cobalt-60). A team of three radiographers worked on the modern unit, while on the old unit only one radiographer was responsible for the treatment set-up. A small systematic error ( + 1.4%) has been detected on the old unit: a small discrepancy existed in the region of the maximum dose between the measured and the published percentage depth-dose values (Br. J. Radiol., Suppl. 17), the latter being used for dose calculation. A striking difference has been observed for the rate of large deviations (deviations of 5% and more from the mean dose): 2.3% (10/430) for the modern unit versus 15% (75/490) for the old one. This shows clearly that the precision in dose delivery is strongly dependent on the characteristics and staffing of the therapy unit, the modern unit being supplied with a check-and-confirm system and with the beam accessory devices in a fixed position in the beam and a team of radiographers being responsible for the set-up.

Radiosurgery with high energy photon beams: A comparison among techniques

The presently known radiosurgical techniques with high energy photon beams are based either on the commercially available Gamma unit utilizing 201 stationary cobalt beams or on isocentric linear accelerators. The techniques using linear accelerators are divided into the single plane rotation, the multiple non-coplanar arcs, and the dynamic rotation. A brief description of these techniques is given, and their physical characteristics, such as precision of dose delivery, dose fall-off outside the target volume, and isodose distributions are discussed. It is shown that the multiple non-coplanar arcs technique and the dynamic rotation give dose distributions similar to those of the Gamma unit, which makes these two linear accelerator based techniques attractive alternatives to radiosurgery with the Gamma unit.