Source-skin Distance Research Articles

Role of adaptive radiation therapy for pediatric patients with diffuse pontine glioma

We investigate the role of adaptive radiation therapy in pediatric patients with diffuse pontine glioma and the impact of steroid‐related weight gain on treatment parameters utilizing cone‐beam CT. Fifteen patients with diffuse pontine glioma were treated with three‐dimensional conformal radiation therapy and enrolled on a daily localization protocol. The median age was 6 years (range: 2–13 years). Patient charts were examined to obtain the prescribed daily dose of dexamethasone and weight. The original treatment plan was recalculated based on the data obtained from the daily cone‐beam CT. The changes in target and critical structure doses were calculated using gEUD. Correlations between prescribed dexamethasone, weight gain, source‐to‐skin distance (SSD) changes and dosimetric changes were investigated. Eleven of the 15 patients gained weight during radiation therapy, with an average gain of 2.2 kg (8.0%). The mean gEUD decreased was 0.57 Gy (range: 0.24–1.4 Gy) for the PTV, and the mean gEUD increase for critical structures was 1.14%. No strong correlations between prescribed dexamethasone doses, weight gain and dosimetric changes were found. Change in SSD vs. dose to PTV was correlated (R2=0.51). Weight gain and changes to the external surface are apparent in these patients; however, the dosimetric changes to the target and critical structures were small and in most cases did not warrant an adaptive plan. The potential exists for a decrease in target dose in these patients; therefore, they should be monitored to assess for replanning when necessary.PACS number: 87.53Jw

Measurement and comparison of skin dose using OneDose MOSFET and Mobile MOSFET for patients with acute lymphoblastic leukemia

SummaryBackgroundTotal body irradiation is a protocol used to treat acute lymphoblastic leukemia in patients prior to bone marrow transplant. It is involved in the treatment of the whole body using a large radiation field with extended source-skin distance. Therefore measuring and monitoring the skin dose during the treatment is important. Two kinds of metal oxide semiconductor field effect transistor (OneDose MOSFET and mobile MOSEFT) dosimeter are used during the treatment delivery to measure the skin dose to specific points and compare it with the target prescribed dose.The objective of this study was to compare the variation of skin dose in patients with acute lymphatic leukemia (ALL) treated with total body irradiation (TBI) using OneDose MOSFET detectors and Mobile MOSFET, and then compare both results with the target prescribed dose.Material/MethodsThe measurements involved 32 patient’s (16 males, 16 females), aged between 14–30 years, with an average age of 22.41 years. One-Dose MOSFET and Mobile MOSFET dosimetry were performed at 10 different anatomical sites on every patient.ResultsThe results showed there was no variation between skin dose measured with OneDose MOSFET and Mobile MOSFET in all patients. Furthermore, the results showed for every anatomical site selected there was no significant difference in the dose delivered using either OneDose MOSFET detector or Mobile MOSFET as compared to the prescribed dose.ConclusionsThe study concludes that One-Dose MOSFET detectors and Mobile MOSFET both give a direct read-out immediately after the treatment; therefore both detectors are suitable options when measuring skin dose for total body irradiation treatment.

Evaluation of clinical use of OneDose metal oxide semiconductor field-effect transistor detectors compared to thermoluminescent dosimeters to measure skin dose for adult patients with acute lymphoblastic leukemia

Background:Total body irradiation is a protocol used to treat acute lymphoblastic leukemia in patients prior to their bone marrow transplant. It involves the treatment of the whole body using a large radiation field with extended source-skin distance. Therefore, it is important to measure and monitor the skin dose during the treatment. Thermoluminescent dosimeters (TLDs) and the OneDose™ metal oxide semiconductor field effect transistor (MOSFET) detectors are used during treatment delivery to measure the radiation dose and compare it with the target prescribed dose.Aims:The primary goal of this study was to measure the variation of skin dose using OneDose MOSFET detectors and TLD detectors, and compare the results with the target prescribed dose. The secondary aim was to evaluate the simplicity of use and determine if one system was superior to the other in clinical use.Material and Methods:The measurements involved twelve adult patients diagnosed with acute lymphoblastic leukemia. TLD and OneDose MOSFET dosimetry were performed at ten different anatomical sites of each patient.Results:The results showed that there was a variation between skin dose measured with OneDose MOSFET detectors and TLD in all patients. However, the variation was not significant. Furthermore, the results showed for every anatomical site there was no significant different between the prescribed dose and the dose measured by either TLD or OneDose MOSFET detectors.Conclusion:There were no significant differences between the OneDose MOSFET and TLDs in comparison to the target prescribed dose. However, OneDose MOSFET detectors give a direct read-out immediately after the treatment, and their simplicity of use to compare with TLD detectors may make them preferred for clinical use.

Conventional Craniospinal Irradiation with Patient Supine and Source-Skin Distance (SSD) 100 cm for Spinal Field

We describe a method of craniospinal irradiation (CSI) in the supine position and at a source-skin distance (SSD) of 100 cm for the spinal fields. The procedure is carried out with a 100-cm isocenter linear accelerator and conventional simulator, and the treatment is delivered with 2 opposed lateral cranial fields at source-axis distance (SAD) of 100 cm and 1 or 2 direct posterior spinal fields at SSD, 100 cm. The half beam–blocked cranial fields with a collimator rotation is used to match the superior border of the spinal field at the level of C2 vertebral body. The length of the spinal field is fixed, and is the same if 2 spinal fields are used. The position of the isocenter of the spine field is defined by longitudinally moving the couch a distance from the isocenter of the cranial fields and adjusting the SSD = 100 cm to the surface of the couch with the gantry rotated to the angle of 180° (posteroanterior position), and the distance can be calculated easily according to a few parameters. It only needs a simple calculation without couch rotation, extended SSD, or markers. The inferior and superior borders of the spinal field do not require visualization under fluoroscopy when it is beyond the visual field of the simulator. The entire simulation takes no more than 20 minutes. Supine craniospinal treatment using this technique may substitute the traditional prone position as a potentially beneficial alternative to CSI.

Beam configuration and physical parameters of clinical high energy photon beam for total body irradiation (TBI)

Purpose To start total body irradiation (TBI) treatments, physical parameters are measured for a magna field irradiation. Methods and Materials 6 MV photon beam from Clinac 600 CD linear accelerator (Varian, USA) with fully opened collimator at 45° and gantry at 270° provided a diamond shaped magna field with diagonal dimension 224 cm at 4.0 m source skin distance (SSD). The flatness of the radiation field was measured in the presence of locally designed acrylic beam spoiler and beam flatness filter. Central Axis Depth dose data (CADD), tissue maximum ratios and entrance dose pattern are measured using large phantoms. Methods for clinical dose estimation using semi-conductor diodes and TLD were standardized. Results PVC beam flattener at the shielding tray position and the presence of acrylic beam spoiler in the radiation field provided a flatness of 100.15% ± 0.44% compared to open beam flatness 101.6 ± 1.5%. A reduction of 2% in percentage depth dose was observed at 10 cm depth in the presence of 15 mm acrylic beam spoiler. However, no changes are observed in the TMRs with presence of beam spoiler. The measured ionization ratios clearly showed change of beam quality with the introduction of beam spoiler. The presence of 15 mm beam spoiler ensured entrance dose 100% at skin and remaining unchanged within 1% upto a depth of 10 mm. Phantom measurements show good agreement between calculated and measured doses. Conclusions The paper recommends use of modified CADD parameters for treatment planning, if calibration of output is carried out in the presence of beam spoiler.

Dosimetric validation of a commercial Monte Carlo based IMRT planning system

Recently a commercial Monte Carlo based IMRT planning system (Monaco version 1.0.0) was released. In this study the dosimetric accuracy of this new planning system was validated. Absolute dose profiles, depth dose curves, and output factors calculated by Monaco were compared with measurements in a water phantom. Different static on-axis and off-axis fields were tested at various source-skin distances for 6, 10, and 18 MV photon beams. Four clinical IMRT plans were evaluated in a water phantom using a linear diode detector array and another six IMRT plans for different tumor sites in solid water using a 2D detector array. In order to evaluate the accuracy of the dose engine near tissue inhomogeneities absolute dose distributions were measured with Gafchromic EBT film in an inhomogeneous slab phantom. For an end-to-end test a four-field IMRT plan was applied to an anthropomorphic lung phantom with a simulated tumor peripherally located in the right lung. Gafchromic EBT film, placed in and around the tumor area, was used to evaluate the dose distribution. Generally, the measured and the calculated dose distributions agreed within 2% dose difference or 2 mm distance-to-agreement. But mainly at interfaces with bone, some larger dose differences could be observed. Based on the results of this study, the authors concluded that the dosimetric accuracy of Monaco is adequate for clinical introduction.

Investigation of the dosimetric accuracy of the isocenter shifting method in prostate cancer patients with and without hip prostheses

The use of image guided radiation therapy (IGRT) enables compensation for prostate movement by shifting the treatment isocenter to track the prostate on a daily basis. Although shifting the isocenter can alter the source to skin distances (SSDs) and the effective depth of the target volume, it is commonly assumed that these changes have a negligible dosimetric effect, and therefore, the number of monitor units delivered is usually not adjusted. However, it is unknown whether or not this assumption is valid for patient with hip prostheses, which frequently contain high density materials. The authors conducted a retrospective study to investigate dosimetric effect of the isocenter shifting method for prostate patients with and without hip prostheses. For each patient, copies of the prostate volume were shifted by up to 1.5 cm from the original position to simulate prostate movement in 0.5 cm increments. Subsequently, 12 plans were created for each patient by creating a copy of the original plan for each prostate position with the isocenter shifted to track the position of the shifted prostate. The dose to the prostate was then recalculated for each plan. For patients with hip prostheses, plans were created both with and without lateral beam angles entering through the prostheses. Without isocenter shifting to compensate for prostate motion of 1.5 cm, the dose to the 95% of the prostate (D-95%) changed by an average of 30% and by up to 64%. This was reduced to less than 3% with the isocenter shifting method. It was found that for patients with hip prostheses, this technique worked best for treatment plans that avoided beam angles passing through the prostheses. The results demonstrated that the isocenter shifting method can accurately deliver dose to the prostate even in patients with hip prostheses.

Commissioning and evaluation of an extended SSD photon model forPINNACLE3: An application to total body irradiation

Total body irradiations (TBIs) are unusual radiation therapy techniques used to treat specific hematological diseases. Most TBI techniques use extended source to patient distances [source-to-skin distance (SSD)] to provide lateral or anteroposterior irradiations. Those techniques differ from one institution to the other since they need to be customized to accommodate for local material constraints. However, with those unusual techniques come additional challenges for dose calculation. The purpose of this study was to obtain an accurate (better than 4%) dose calculation model for extended source-to-skin distance (eSSD) treatment techniques, which will be used for TBI planning. The studied dynamic TBI technique has special aspects (eSSD, beam spoiler, large field, and out of field dose contribution) that need to be considered in dose calculation. The first part of this study presents an eSSD beam model commissioning in PINNACLE3 and its validation. The second part looks at the comparison between two dose calculation algorithms, the 3D pencil beam and the superposition-convolution algorithms implemented in THERAPLAN PLUS and PINNACLE3, respectively. A regular linac beam was commissioned in each treatment planning system and an additional dedicated TBI beam model was implemented in PINNACLE3. The comparison results indicate that the quality of the TBI treatment greatly depends on the treatment planning system and its beam commissioning. The superposition-convolution algorithm (PINNACLE3) provides a better dose calculation tool for TBI than the 3D pencil beam algorithm (THERAPLAN PLUS) with a maximum mean error of 2.2% on a dynamic treatment. The TBI specific beam model of PINNACLE3 (ESSP-P3) also improves the dose calculation. The maximum difference between calculations and measurements (depth doses and beam profiles) was 2% except for extreme cases (build-up region and depth of 20 cm) where the error was higher. Output factor determination and the dose contribution outside the primary beam weaknesses were found in PINNACLE3. Methods are proposed to overcome these limitations. With the correction method applied, the TBI specific beam model allows a maximum mean error of -0.68% on a dynamic treatment. Accurate TBI dose computation necessitates a good dose calculation algorithm combined with a realistic beam model. Inappropriate dose calculation could lead to an important over- or underdose estimation. No perfect algorithm and beam model were found, but methods are proposed to overcome some of the limitations. Those methods are simple and can be used for other eSSD treatment types.

The dependence of percentage depth dose on the source-to-skin distance for kilovoltage X-rays

The variation of percentage depth dose (PDD) with source-to-skin distance (SSD) for kilovoltage X-rays used in radiotherapy has been investigated. Based on physical parameters of photon fluence, absorption and scatter during interaction of radiation with tissue, a mathematical model was developed to predict the PDDs at different depths of treatment for various field sizes and at selected SSDs for radiation qualities Half Value Layers of 1 mm Al and 8 mm Al, using 10 cm SSD input data. Higher values of PDD were predicted for increasing SSD as compared to benchmark PDD data, and the differences were more pronounced at larger depths and lower PDDs. Within the range of clinical significance of 90 - 100 % PDD, the discrepancies between the calculated values and benchmark data were ≤ 0.5 % when the SSD was varied by 50 %, and increased to a maximum of 2.2 % with SSD difference of 200 %. With 1.0 % error acceptable in radiotherapy dosimetry, the model was found to be efficient, especially for applications involving small differences in SSD.

Craniospinal Radiotherapy in an Advanced Technique

Whole craniospinal irradiation cannot be achieved in one field at a normal treatment distance for adults. The aim of this newly developed technique is to minimize problems of matching fields and to maximize precision of craniospinal radiotherapy. Twelve patients (3-59 years) had craniospinal irradiation in supine position. The head was treated with lateral opposed isocentric fields with collimator rotation and isocentric table rotation. Using an extended source-skin distance of 160 cm only one dorsal field is necessary to cover the whole spinal axis. To avoid systematic under- or overdosage, junction field edges were moved twice by 1.5 cm. Treatment planning was performed based on CT scans. For visual verification of field matching an additional line laser was first adjusted to the caudal edge of one lateral light field and then checked against the light field of the spinal field under the table. Control films show good homogeneity in the junction between lateral and vertical fields. Reproducibility of table movements is acceptable. Total time needed for one fraction is about 15-20 min. The described technique is now well established, feasible and leads to less risk of dose uncertainties.

Peripheral dose measurements for 6 and photon beams on a linear accelerator with multileaf collimator

Peripheral dose (PD) to critical structures outside treatment volume is of clinical importance. The aim of the current study was to estimate PD on a linear accelerator equipped with multileaf collimator (MLC). Dose measurements were carried out using an ionization chamber embedded in a water phantom for 6 and 18 MV photon beams. PD values were acquired for field sizes from 5 x 5 to 20 x 20 cm2 in increments of 5 cm at distances up to 24 cm from the field edge. Dose data were obtained at two collimator orientations where the measurement points are shielded by MLC and jaws. The variation of PD with the source to skin distance (SSD), depth, and lateral displacement of the measurement point was evaluated. To examine the dependence of PD upon the tissue thickness at the entrance point of the beam, scattered dose was measured using thermoluminescent dosemeters placed on three anthropomorphic phantoms simulating 5- and 10-year-old children and an average adult patient. PD from 6 MV photons varied from 0.13% to 6.75% of the central-axis maximum dose depending upon the collimator orientation, extent of irradiated area, and distance from the treatment field. The corresponding dose range from 18 MV x rays was 0.09% to 5.61%. The variation of PD with depth and with lateral displacements up to 80% of the field dimension was very small. The scattered dose from both photon beams increased with the increase of SSD or tissue thickness along beam axis. The presented dosimetric data set allows the estimation of scattered dose outside the primary beam.

Dose measurement verification in solid state phantom in place of field connection for non-standard radiotherapy conditions

Summary Background This work describes chosen combinations of therapeutic fields during the patient's total body irradiation (TBI). The TBI technique requires as large a radiation field as one can achieve [1][2]. That is why the source-skin distance (SSD) is greater than 100 cm (SSD for standard treatment conditions). In such non-standard radiotherapy conditions all the measurements described in this paper were done. Materials/Methods All beam profiles were obtained by irradiating films with 20 MV (nominal energy) photon rays in non-standard conditions of radiotherapy. The method of measurement (film dosimetry) and used materials (self-developing GAFCHROMIC EBT films) are presented in the Material and Method section [3]. Aim Because in the therapy some areas must receive a greater dose than others, the size of the therapeutic fields must be adjusted. That is why the areas where two fields overlap inside the patient's body differ. The diversity of absorbed dose in these areas was measured and presented in the schemas in the Results section. Results The profiles presenting dose distribution in the areas where therapeutic fields overlap in most cases show increase of the delivered dose. For the most often used therapeutic fields the increase exceeds 180% of the planned dose in the sector about 2.5 – 10 cm. There were also two cases where the delivered dose was lower than the planned one (about 29 – 86% of the planned dose). Chosen measurements and combinations of the therapeutic fields are discussed in detail and all the results are collected in a table at the end of the section. Conclusions The profiles obtained from the scans of the irradiated GAFCHROMIC EBT films and their digital processing include 0.05 Gy noise. This means that the described method requires a high quality scanner, dedicated to RTG films. The spectrophotometer measurements showed high film dependency on light wavelength. It seems that using a monochromatic source of light may give better results.

Amifostine Use in Radiation-Induced Kidney Damage

PURPOSE To assess the degree of protective effects of amifostine on kidney functions via semiquantitative static renal scintigraphy and histopathologic analysis. MATERIAL AND METHODS 30 female albino rats were divided into three equal groups as control (CL), radiotherapy alone (RT), and radiotherapy + amifostine (RT+AMI). The animals in the CL and RT groups were given phosphate-buffered saline, whereas the animals in the RT+AMI group received amifostine (200 mg/kg) by intraperitoneal injection 30 min before irradiation. RT and RT+AMI groups were irradiated with a single dose of 6 Gy using a (60)Co unit at a source-skin distance of 80 cm to the whole right kidney. They were followed up for 6 months. CL, RT, and RT+AMI groups underwent static kidney scintigraphy at the beginning of the experiment and, again, on the day before sacrificing. Histopathologically, tubular atrophy and fibrosis of the kidney damage were evaluated. RESULTS After irradiation, the median value of right kidney function was 48% (44-49%) and 50.5% (49%-52%) in RT and RT+AMI groups, respectively (p = 0.0002). Grade 1 kidney fibrosis was observed to be 60% in the RT group, while it was only 30% in the RT+AMI group. Grade 2 kidney fibrosis was 30% and 0% in the RT and RT+AMI group, respectively. Grade 1 tubular atrophy was 70% and 50% in the RT and RT+AMI group, respectively. Grade 2 tubular atrophy effect was the same in both groups (10%). CONCLUSION Static kidney scintigraphy represents an objective and reproducible method to noninvasively investigate kidney function following irradiation. Amifostine produced a significant reduction in radiation-induced loss of renal function.

Performance characteristics and commissioning of MOSFET as an in-vivo dosimeter for high energy photon external beam radiation therapy

SUMMARY AIM: In vivo dosimetry is an essential tool of quality assurance programmes in radiotherapy. In fact, the assessment of the fi nal uncertainty between the prescribed dose and the dose actually delivered to the patient is an effective way of checking the entire dosimetric procedure. Metal oxide semiconductor fi eld effect transistors (MOSFETs) have recently been proposed for use in radiation therapy. The purpose of this work is to study the performance characteristics and to carry out the commissioning of MOSFET as an in-vivo dosimeter for high-energy photon external beam radiation therapy. MATERIAL AND METHODS: Characterization and commissioning of low sensitivity TN502RD and high sensitivity TN1002RD MOSFETs for entrance and exit dosimetry respectively for application in in-vivo dosimetry in radiotherapy was carried out. The MOSFETs were characterized in terms of reproducibility, short-term constancy, long-term constancy, linearity, angular dependence, energy dependence, source to skin distance (SSD) dependence and fi eld size dependence. RESULTS: The reproducibility of standard sensitivity MOSFET is about 1.4% (1 SD) and 1.98% (1 SD) for high sensitivity detectors. The linearity of both MOSFETs was excellent (R2 = 0.996). The response of MOSFETs varies linearly for square fi elds from 3 x 3 cm2 to 30 x 30 cm 2 . For beam incidence ranging from ±45° the MOSFET response varies within ±3%. Commissioning of both MOSFETs was carried out in terms of entrance dose calibration factor, exit dose calibration factor, SSD correction factor, fi eld size correction factor, wedge correction factor and shielding tray correction factor. The average calibration factor for low and high sensitivity MOSFET detectors is 0.9065 cGy/mV and 0.3412 cGy/mV respectively. The average SSD correction factors are quite small and vary between 0.968 and 1.027 for both types of detectors for the range of clinical SSDs from 80 cm to 120 cm. The fi eld size correction factor varies from 1.00 to 1.02 for both types of detectors. The wedge and the shielding tray correction factors for both the detectors also show quite small variation. MOSFET characteristics are suitable for in vivo dosimetry of entrance and exit dose measurement relevant to 6 MV treatment. CONCLUSION: It can be concluded that MOSFET dosimetry’s low energy dependence, high sensitivity and immediate readout make it a good replacement for TLD in radiation therapy dosimetry.

Radiation Therapy of Large Intact Breasts Using a Beam Spoiler or Photons with Mixed Energies

Radiation treatment of large intact breasts with separations of more than 24 cm is typically performed using x-rays with energies of 10 MV and higher, to eliminate high-dose regions in tissue. The disadvantage of the higher energy beams is the reduced dose to superficial tissue in the buildup region. We evaluated 2 methods of avoiding this underdosage: (1) a beam spoiler: 1.7-cm-thick Lucite plate positioned in the blocking tray 35 cm from the isocenter, with 15-MV x-rays; and (2) combining 6- and 15-MV x-rays through the same portal. For the beam with the spoiler, we measured the dose distribution for normal and oblique incidence using a film and ion chamber in polystyrene, as well as a scanning diode in a water tank. In the mixed-energy approach, we calculated the dose distributions in the buildup region for different proportions of 6- and 15-MV beams. The dose enhancement due to the beam spoiler exhibited significant dependence upon the source-to-skin distance (SSD), field size, and the angle of incidence. In the center of a 20 × 20-cm 2 field at 90-cm SSD, the beam spoiler raises the dose at 5-mm depth from 77% to 87% of the prescription, while maintaining the skin dose below 57%. Comparison of calculated dose with measurements suggested a practical way of treatment planning with the spoiler—usage of 2-mm “beam” bolus—a special option offered by in-house treatment planning system. A second method of increasing buildup doses is to mix 6- and 15-MV beams. For example, in the case of a parallel-opposed irradiation of a 27-cm-thick phantom, dose to D max for each energy, with respect to midplane, is 114% for pure 6-, 107% for 15-MV beam with the spoiler, and 108% for a 3:1 mixture of 15- and 6-MV beams. Both methods are practical for radiation therapy of large intact breasts.

SU-FF-T-398: The Implications of Monitor Unit and SSD Rounding Errors On Delivered Dose

Purpose: To investigate the individual and overall contributions of Monitor Unit and SSD rounding errors on delivered dose for different treatment regions. Method and Materials: Treatment planning computers are configured to calculate treatment monitor units (MUs) to at least one decimal place, but because the ion chambers in many linear accelerators can not measure radiation to this accuracy, treatment plan MUs are often rounded off to the nearest whole number. Further more, treatment planning systems can calculate radiation source-skin distances (SSDs) to millimeter accuracy, while the optical distance indicators (ODIs) on linear accelerators generally require rounding to the nearest half centimeter. Acceptable treatment plans were generated to obtain MUs and SSDs to at least one decimal places. Treatment MUs and SSDs from these plans were then rounded and put back into the plans to obtain the real world deliverable plans. The isodose distributions and Dose-Volume Histograms (DVHs) for individual and total contributions of these two rounding errors were then compared and analyzed for different treatment regions. Results: Our results show distortions of isodose distributions and Dose-Volume Histograms (DVH) as a result of the compounded rounding errors in MU calculations and ODI SSD readings. In particular, we found that normal tissue dose constraints, which appeared to be satisfied in the original computer plan, could be exceeded by up to 6% when rounding errors were taken into account, particularly in sites with relatively short radiation effective path-lengths, like the head and neck. The effects of MU rounding error appear to be greater than those ODI SSD rounding error and less significant for sites with long radiation effective path lengths such as lung tumors. Conclusions: This work indicates the potential overdose or underdose as a result of MU and SSD rounding errors.

Linearity of the Dose Monitor System at Low Monitor Units

The accuracy of the dose monitor system of a medical electron accelerator is of crucial importance for the precise delivery of the prescribed dose. Special attention is given to the case of low monitor units (MU). The long-term stability of the dose monitor system is analyzed. Photon and electron fields with monitor units between 1 and 500 MU were measured with field sizes of 10 x 10 cm(2) and 20 x 20 cm(2) at source-skin distances (SSDs) of 100 cm and 95 cm, respectively. The relative dose per MU [see text] was determined for an Elekta linear accelerator. The relative dose per MU was measured to be constant within about 1% down to fields with only 2 MU. The deviation remained below 2% down to 1 MU which is the minimum setting of the accelerator. The long-term stability is better than 0.2% per month. No therapeutically relevant deviation of the prescribed dose could be detected for the Elekta linear accelerator. This is a prerequisite for the implementation of intensity-modulated radiotherapy (IMRT) techniques.

Electron dosimetry of angular fields

Shaping electron fields through the use of lead cut-outs may result in there being acute angles in part of the field. Using both experimental techniques and EGSnrc Monte Carlo simulations an investigation was carried out to determine the dosimetric consequences of this. Measurements were made to investigate how the field dose was related to the angle between adjacent sides in the cut-outs. The study involved two electron energies (9 MeV and 12 MeV) and source-skin distances (SSDs) in the range 1000-1100 mm. For angles less than about 120 degrees the dose received in the angular region decreased significantly, the effect being more pronounced at 12 MeV than at 9 MeV, and at longer SSDs. The planar shapes of the Monte Carlo dose distributions agreed with those experimentally determined to within +/-1.5 mm at 9 MeV and +/-1.0 mm at 12 MeV, demonstrating the validity of using such calculations for this purpose. Graphs are presented which may help in the prospective assessment of the dose reductions likely to be incurred.

Total scalp irradiation using helical tomotherapy

Homogeneous irradiation of the scalp poses technical and dosimetric challenges due to the extensive, superficial, curved treatment volume. Conventional treatments on a linear accelerator use multiple matched electron fields or a combination of electron and photon fields. Problems with these techniques include dose heterogeneity in the target due to varying source-to-skin distance (SSD) and angle of beam incidence, significant dose to the brain, and the potential for overdose or underdose at match lines between the fields. Linac-based intensity-modulated radiation therapy (IMRT) plans have similar problems. This work presents treatment plans for total scalp irradiation on a helical tomotherapy machine. Helical tomotherapy is well-suited for scalp irradiation because it has the ability to deliver beamlets that are tangential to the scalp at all points. Helical tomotherapy also avoids problems associated with field matching and use of more than one modality. Tomotherapy treatment plans were generated and are compared to plans for treatment of the same patient on a linac. The resulting tomotherapy plans show more homogeneous target dose and improved critical structure dose when compared to state-of-the-art linac techniques. Target equivalent uniform dose (EUD) for the best tomotherapy plan was slightly higher than for the linac plan, while the volume of brain tissue receiving over 30 Gy was reduced by two thirds. Furthermore, the tomotherapy plan can be more reliably delivered than linac treatments, because the patient is aligned prior to each treatment based on megavoltage computed tomography (MVCT).

A simulation tool to support teaching and learning the operation of X-ray imaging systems

We present a software tool for the simulation of an X-ray imaging systems. It consists of three virtual objects: the X-ray source, the human body and the detector. The X-ray source is modeled as a radiological tube for which the user can modify the tube potential, the anode material, the tube load, the filtration and some geometric parameters, such as source-skin distance, orientation and field size. The virtual body consists of a 3D voxel matrix in which CT numbers for each point of the body are stored, obtained from tomographic slices. The interactions of X-rays passing through the body are evaluated using pencil beam technique. The image is obtained computing the dose absorbed by the detector and converting it into optical density by the use of a proper response function. The dose absorbed in each point of the body is also computed and can be visualized both in 2D and 3D representations. The influence of each parameter on the beam spectrum, on the image quality and on the dose to the patient can be observed interactively.