New Method to Obtain the Midplane Dose Using Portal In Vivo Dosimetry

Purpose: The aim of this study was to develop a method to derive the midplane dose [i.e., the two-dimensional (2D) dose distribution in the middle of a patient irradiated with high-energy photon beams] from transmission dose data measured with an electronic portal imaging device (EPID). A prerequisite for this method was that it could be used without additional patient information (i.e., independent of a treatment-planning system). Second, we compared the new method with several existing (conventional) methods that derive the midline dose from entrance and exit dose measurements.Methods and Materials: The proposed method first calculates the 2D contribution of the primary and scattered dose component at the exit side of the patient or phantom from the measured transmission dose. Then, a correction is applied for the difference in contribution for both dose components between exit side and midplane, yielding the midplane dose. To test the method, we performed EPID transmission dose measurements and entrance, midplane, and exit dose measurements using an ionization chamber in homogeneous and symmetrical inhomogeneous phantoms. The various methods to derive the midplane dose were also tested for asymmetrical inhomogeneous phantoms applying two opposing fields. A number of combinations of inhomogeneities (air, cork, and aluminum), phantom thicknesses, field sizes, and a few irregularly shaped fields were investigated, while each experiment was performed in 4-, 8-, and 18-MV open and wedged beams.Results: Our new method can be used to assess the midplane dose for most clinical situations within 2% relative to ionization chamber measurements. Similar results were found with other methods. In the presence of large asymmetrical inhomogeneities (e.g., lungs), discrepancies of about 8% have been found (for small field sizes) using our transmission dose method, owing to the absence of lateral electron equilibrium. Applying the other methods, differences between predicted and measured midplane doses were even larger, up to 10%. For large field sizes, the agreement between measured and predicted midplane dose was within 3% using our transmission dose method.Conclusions: Using our new method, midplane doses were estimated with a similar or higher accuracy compared with existing conventional methods for in vivo dosimetry. The advantage of our new method is that the midplane dose can be determined in the entire (2D) field. With our method, portal in vivo dosimetry is an accurate alternative for conventional in vivo dosimetry.     

Variation of relative transit dose profiles with patient-detector distance.

BACKGROUND AND PURPOSE: In view of using portal images for exit dosimetry, an experimental study is performed of relative transit dose profiles at different distances behind patients (and phantoms) and of their relation to the exit dose profile. MATERIALS AND METHODS: Irregular, homogeneous polystyrene phantoms with a variable thickness to simulate head and neck (H&N) treatments (6-MV photon beam) are investigated by ionization chamber measurements performed close to the exit surface and at various distances behind the phantom (10, 20 and 30 cm). Similar measurements are performed for a rectangular phantom with large inhomogeneities (A1 and air). For one irregular homogeneous phantom and an irregular phantom containing an A1 inhomogeneity, ionization chamber measurements are performed at the exit surface, and a portal film image is taken at 30 cm behind the phantom. Portal films of a patient treated for a head and neck malignancy are evaluated for different air gaps behind the patient. RESULTS: For the irregular phantoms, deviations up to 15% and more are observed between the exit dose profile (along the shaped surface of the phantom) and the transit profile close to the phantom (perpendicular to the beam axis). There is, however, a good agreement--within 3%--between the exit profile and the transit profile at 30 cm. For the rectangular, inhomogeneous phantom, the deviation between the exit profile and the transit dose profile at 30 cm does not exceed 5%; transit dose profiles overestimate the exit dose for the air cavity and underestimate the dose for the A1 inhomogeneity. Measurements on portal films of a H&N patient for different air gaps confirm the order of magnitude of the difference observed between transit dose profiles close to the patient and transit dose profiles at some distance behind the patient. CONCLUSIONS: For 6-MV photon beam treatments with significant thickness variations (H&N), large variations (> 10%) are observed in transit dose profiles as a function of the air gap between the patient and the portal film. For this energy, a good agreement is found between the exit profile and the transit profile at about 30 cm behind the patient.     

In vivo dosimetry and radiation therapy of breast cancer]

INTRODUCTION: Verification of absorbed dose in target volume is a key factor for quality assurance in radiotherapy. In vivo measurements allow evaluation of the variations in dose with time and variations between measured doses and calculated doses by TPS. The aim of this work were to evaluate reproducibility of patient positioning and to compare calculated doses by 2 different TPS. PATIENTS AND METHODS: Twenty patients were divided in 2 groups according to the thickness of their breast (mean SSD = 92.9 cm). In vivo measurement was performed within the first two sessions. RESULTS: Reproducibility of SSD evaluation was made on 12 beams between 2 fractions. With a tolerance margin of 0.5 cm, positioning errors were present in 33% (4/12). The 2 TPS were in agreement in 75% (30/40). CONCLUSION: In vivo dosimetry can be a very interesting tool to assess patients positioning variations and TPS dose calculation.     

Quality control in interstitial brachytherapy of the breast using pulsed dose rate: treatment planning and dose delivery with an Ir-192 afterloading system.

BACKGROUND AND PURPOSE: In the Radiotherapy Department of Leuven, about 20% of all breast cancer patients treated with breast conserving surgery and external radiotherapy receive an additional boost with pulsed dose rate (PDR) Ir-192 brachytherapy. An investigation was performed to assess the accuracy of the delivered PDR brachytherapy treatment. Secondly, the feasibility of in vivo measurements during PDR dose delivery was investigated. MATERIALS AND METHODS: Two phantoms are manufactured to mimic a breast, one for thermoluminescent dosimetry (TLD) measurements, and one for dosimetry using radiochromic films. The TLD phantom allows measurements at 34 dose points in three planes including the basal dose points. The film phantom is designed in such a way that films can be positioned in a plane parallel and orthogonal to the needles. RESULTS: The dose distributions calculated with the TPS are in good agreement with both TLD and radiochromic film measurements (average deviations of point doses <+/-5%). However, close to the interface tissue-air the dose is overestimated by the TPS since it neglects the finite size of a breast and the associated lack of backscatter (average deviations of point doses -14%). CONCLUSION: Most deviations between measured and calculated doses, are in the order of magnitude of the uncertainty associated with the source strength specification, except for the point doses measured close to the skin. In vivo dosimetry during PDR brachytherapy treatment was found to be a valuable procedure to detect large errors, e.g. errors caused by an incorrect data transfer.     

A simple and robust method for in vivo midline dose map estimations using diodes and portal detectors.

INTRODUCTION: This work investigates the possibility of using a pair of diodes on the beam axis in conjunction with a portal imaging detector to estimate in vivo midline dose distributions, without any additional patient information, related to the external body contour. MATERIALS AND METHODS: In the proposed method, the patient is considered equivalent to a parallelepiped phantom with a thickness z equal to the patient's physical thickness on the field axis with a variable electronic density rho, depending on the water-equivalent thickness. Based on this assumption, if the air gap between portal detector and patient is kept small (within 10-15 cm), the relative exit dose map may be assumed to be equal to the corresponding map measured at the portal detector level by geometrical back projection to the corresponding exit points. The relative exit dose map is then normalized at the on-axis value measured by the exit diode. The entrance dose map is derived by correcting the absolute dose value measured with the diode at the entrance surface by the off-axis ratios. For each pair of entrance and exit doses, the midline dose may be estimated by applying algorithms reported in literature. The method was tested in 6 MV beams using portal film as detector and the Huyskens and Rizzotti algorithms for midline dose estimation. Tests on homogeneous cubic phantoms, homogeneous phantoms with varying thickness symmetrically (simulating head and neck regions) and asymmetrically (simulating abdomen/pelvis region), and a half-sphere phantom with simulating the breast, were performed. Midline doses estimated with the proposed method have been compared with corresponding ones measured by ionisation chamber. RESULTS AND DISCUSSION: Results confirm that the proposed method can be used to estimate midplane dose maps within 2-3% for most clinically suitable situations. For homogeneous symmetrical phantoms the agreement between estimated and measured midline doses decreases with the phantom-portal film distance, the field sizes and the thickness. For homogeneous asymmetrical phantoms the percentage deviations are generally within 3%. Discrepancies larger than 3% (up to 5-6%) are found only for "stressed" irradiation geometries which are not linked with any clinical condition. CONCLUSIONS: The obtained results not only show the accuracy of the proposed method but, due to its simplicity, suggest a rapid clinical implementation of this method in relevant clinical situations such as head-neck, breast and abdomen/pelvis irradiation. Previous investigations which confirmed the possibility of using portal detectors for transit dosimetry in inhomogeneous regions suggest the further exploration of the accuracy and the limits of the proposed method in such cases.     

Total body irradiation, toward optimal individual delivery: dose evaluation with metal oxide field effect transistors, thermoluminescence detectors, and a treatment planning system

PURPOSE: To predict the three-dimensional dose distribution of our total body irradiation technique, using a commercial treatment planning system (TPS). In vivo dosimetry, using metal oxide field effect transistors (MOSFETs) and thermoluminescence detectors (TLDs), was used to verify the calculated dose distributions. METHODS AND MATERIALS: A total body computed tomography scan was performed and loaded into our TPS, and a three-dimensional-dose distribution was generated. In vivo dosimetry was performed at five locations on the patient. Entrance and exit dose values were converted to midline doses using conversion factors, previously determined with phantom measurements. The TPS-predicted dose values were compared with the MOSFET and TLD in vivo dose values. RESULTS: The MOSFET and TLD dose values agreed within 3.0% and the MOSFET and TPS data within 0.5%. The convolution algorithm of the TPS, which is routinely applied in the clinic, overestimated the dose in the lung region. Using a superposition algorithm reduced the calculated lung dose by approximately 3%. The dose inhomogeneity, as predicted by the TPS, can be reduced using a simple intensity-modulated radiotherapy technique. CONCLUSIONS: The use of a TPS to calculate the dose distributions in individual patients during total body irradiation is strongly recommended. Using a TPS gives good insight of the over- and underdosage in a patient and the influence of patient positioning on dose homogeneity. MOSFETs are suitable for in vivo dosimetry purposes during total body irradiation, when using appropriate conversion factors. The MOSFET, TLD, and TPS results agreed within acceptable margins.     

Are port films reliable for in vivo exit dose measurements?

The possibility of using conventional port films as an in vivo method for obtaining relative exit dose distribution in patients is evaluated. Kodak "Verification" films in "Localization" cassettes are positioned in the usual clinical conditions behind an homogeneous polystyrene phantom as well as behind a phantom containing air, wood and aluminium inhomogeneities. Taking beam divergency into account the densitometric profiles are projected back to the exit side of the phantom. They are compared to the profiles obtained with an ionization chamber used under full backscatter conditions. The agreement between the off-axis ratios determined with either method are mostly better than 2% and never exceed 5%. These phantom measurements are completed by a comparison between off-axis ratios determined on a port film for a head and neck patient and those obtained by diode dosimetry applied on the patient at the exit side of the beam. A similar agreement as between film and ion chamber on the phantoms is obtained. These encouraging results illustrate the possibilities of using conventional port films for in vivo dosimetry.     

Two-dimensional mapping of underdosed areas using radiochromic film for patients undergoing total skin electron beam radiotherapy

PURPOSE: To demonstrate the viability of radiochromic film as an in vivo, two-dimensional dosimeter for the measurement of underdosed areas in patients undergoing total skin electron beam (TSEB) radiotherapy. The results were compared with thermoluminescent dosimeter measurements. METHODS AND MATERIALS: Dosimetry results are reported for an inframammary fold of 2 patients treated using a modified version of the Stanford six-position (i.e., six-field and dual-beam) TSEB technique. The results are presented as contour plots of film optical density and percentage of dose. A linear dose profile measured from film was compared with the thermoluminescent dosimeter measurements. RESULTS: The results showed that the percentage doses as measured by film are in good agreement with those measured by the thermoluminescent dosimeters. The isodose contour plots provided by film can be used as a two-dimensional dose map for a patient when determining the size of the supplemental patch fields. CONCLUSION: Radiochromic film is a viable dosimetry tool that the radiation oncologist can use to understand the surface dose heterogeneity better across complex concave regions of skin to help establish more appropriate margins to patch underdosed areas. Film could be used for patients undergoing TSEB for disorders such as mycosis fungoides or undergoing TSEB or regional skin electron beam for widespread skin metastases from breast cancer and other malignancies.     

Quality assurance in radiotherapy by in vivo dosimetry. 2. Determination of the target absorbed dose

Combined entrance and exit dose measurements were performed with semiconductor detectors on patients, treated for neck and oral cavity malignancies. Transmission measurements showed the important influence of contour inaccuracies and tissue inhomogeneities. In 39.6% (21/53) of the checked contours, the discrepancy between the contour diameter used for routine treatment planning and the actual patient diameter was 1 cm or more, and in this group a systematic tendency for patient diameter underestimation due to the procedure was detected. When the X-ray beam passed through important bone structures such as the mandibular bones or the vertebral body, large discrepancies of 10% and more between the measured and the expected transmission were found. The target absorbed dose was determined from the transmission and entrance dose measurement. A systematic underdosage of about 2% at midline level was found to be due to an inaccuracy in the algorithms of the treatment planning system. Underdosages of 5% or more at midline were detected in more than 20% (47/230) of the measurements. In all cases, the reason for erroneous dose delivery was identified. Entrance dose measurements were previously demonstrated to be useful for the assessment of uncertainties related to treatment machine, patient set-up and treatment planning system (part 1). Transmission measurements (the ratio of the exit to the entrance dose measurement) are shown to be very useful to evaluate uncertainties related to patient data such as contour errors and tissue inhomogeneities as well as to the algorithms of the planning system. The influence of these errors on the target absorbed dose can be estimated and corrections can be applied for each individual patient.     

A phantom study of dose compensation behind hip prosthesis using portal dosimetry and dynamic MLC

BACKGROUND AND PURPOSE: A dose compensation method is presented for patients with hip prosthesis based on Dynamic Multi Leaves Collimator (DMLC) planning. Calculations are done from an exit Portal Dose Image (PDI) from 6 MV Photon beam using an Electronic Portal Imaging Device (EPID) from Varian. Four different hip prostheses are used for this work. METHODS: From an exit PDI the fluence needed to yield a uniform dose distribution behind the prosthesis is calculated. To back-project the dose distribution through the phantom, the lateral scatter is removed by deconvolution with a point spread function (PSF) determined for depths from 10 to 40cm. The dose maximum, D(max), is determined from the primary plan which delivers the PDI. A further deconvolution to remove the dose glare effect in the EPID is performed as well. Additionally, this calculated fluence distribution is imported into the Treatment Planning System (TPS) for the final calculation of a DMLC plan. The fluence file contains information such as the relative central axis (CAX) position, grid size and fluence size needed for correct delivery of the DMLC plan. GafChromic EBT films positioned at 10cm depth are used as verification of uniform dose distributions behind the prostheses. As the prosthesis is positioned at the phantom surface the dose verifications are done 10cm from the prosthesis. CONCLUSION: The film measurement with 6 MV photon beam shows uniform doses within 5% for most points, but with hot/cold spots of 10% near the femoral head prostheses.     

Three-dimensional tumor dosimetry for radioimmunotherapy using serial autoradiography.

Three-dimensional dose distributions have been calculated for LS174T human colon cancer xenografts in athymic nude mice treated with 131I-labeled 17-1A monoclonal antibody. Autoradiographs were made for fifteen to twenty 32-micron-thick representative serial sections of tumors removed 1 and 4 days postinjection. Film density readings were converted to activity density and entered into a radiotherapy treatment planning system. Three-dimensional dose distributions were obtained by summing the dose contributions due to each voxel of uniform activity. Isodoserate distributions and dose-rate-volume histograms for representative tumors at 1 and 4 days following 131I-labeled 17-1A injection showed a progressive change from a predominantly surface deposition (day 1) to a more volumetric deposition (day 4). Average tumor doses calculated using the assumptions of uniform source distribution and local dose deposition resulted in a poor estimation of the cumulative dose because of the significant time-dependent dose-rate nonuniformities.     

Accuracy in radiation field alignment in head and neck cancer: a prospective study

A prospective study has been performed to determine the accuracy of radiation field alignment for a group of 22 patients with tumors in the head and neck. The accuracy was assessed by an analysis of 138 megavolt portal films in comparison to 55 simulation films. The distance (at the patient midplane) between corresponding points at the field edges on verification film and simulation film appeared to be 5 mm on the average and the standard deviation 5 mm. The analysis was extended by translational and rotational matching of the fields in order to separate each error in a translation error of the field with respect to the patient and an error in field size or shape. Translation errors appear to be somewhat larger than field size or shape errors. From an analysis of a series of megavolt films taken every third radiotherapy session, it was concluded that treatment-to-treatment variations are as large as the errors due to the transition from simulation to treatment situation. Further analysis showed that variation of the patient's position within the cast is clearly one of the error sources.     

Accuracy of out-of-field dose calculation of tomotherapy and cyberknife treatment planning systems: A dosimetric study

PurposeLate toxicities such as second cancer induction become more important as treatment outcome improves. Often the dose distribution calculated with a commercial treatment planning system (TPS) is used to estimate radiation carcinogenesis for the radiotherapy patient. However, for locations beyond the treatment field borders, the accuracy is not well known. The aim of this study was to perform detailed out-of-field-measurements for a typical radiotherapy treatment plan administered with a Cyberknife and a Tomotherapy machine and to compare the measurements to the predictions of the TPS.Materials and methodsIndividually calibrated thermoluminescent dosimeters were used to measure absorbed dose in an anthropomorphic phantom at 184 locations. The measured dose distributions from 6 MV intensity-modulated treatment beams for CyberKnife and TomoTherapy machines were compared to the dose calculations from the TPS.ResultsThe TPS are underestimating the dose far away from the target volume. Quantitatively the Cyberknife underestimates the dose at 40 cm from the PTV border by a factor of 60, the Tomotherapy TPS by a factor of two. If a 50% dose uncertainty is accepted, the Cyberknife TPS can predict doses down to approximately 10 mGy/treatment Gy, the Tomotherapy-TPS down to 0.75 mGy/treatment Gy. The Cyberknife TPS can then be used up to 10 cm from the PTV border the Tomotherapy up to 35 cm.ConclusionsWe determined that the Cyberknife and Tomotherapy TPS underestimate substantially the doses far away from the treated volume. It is recommended not to use out-of-field doses from the Cyberknife TPS for applications like modeling of second cancer induction. The Tomotherapy TPS can be used up to 35 cm from the PTV border (for a 390 cm³ large PTV).     

Dosimetry of intracavitary placements for uterine and cervical carcinoma: results of orthogonal film, TLD, and CT-assisted techniques.

A total of 720 192Ir high-dose-rate (HDR) applications in 331 patients with gynecological tumors were analyzed to evaluate the dose to normal tissues from brachytherapy. Based on the calculations of bladder base, bladder neck, and rectal doses derived from orthogonal films the planned tumor dose or fractionation was altered in 20.4% of intracavitary placements (ICP) for cervix carcinoma and 9.2% of ICP for treatment of the vaginal vault. In 13.8% of intracervical and 8.1% of intravaginal treatments calculated doses to both the bladder and rectum were greater than or equal to 140% of the initially planned dose fraction. Doses at the bladder base were significantly higher than at the bladder neck (p less than 0.001). In 17.5% of ICP the dose to the bladder base was at least twice as high as to the bladder neck. The ratio of bladder base dose to the bladder neck was 1.5 (+/- 1.19 SD) for intracervical and 1.46 (+/- 1.14 SD) for intravaginal applications. The comparison of calculated doses from orthogonal films with in-vivo readings showed a good correlation of rectal doses with a correlation coefficient factor of 0.9556. CT-assisted dosimetry, however, revealed that the maximum doses to bladder and rectum were generally higher than those obtained from films with ratios of 1-1.7 (average: 1.44) for the bladder neck, 1-5.4 (average: 2.42) for the bladder base, and 1.1-2.7 (average: 1.37) for the rectum. When doses to the specified reference points of bladder neck and rectum from orthogonal film dosimetry were compared with the corresponding points on CT scans, similar values were obtained for both methods with a maximum deviation of +/- 10%. Despite the determination of multiple reference points our study revealed that this information was inadequate to predict doses to the entire rectum and bladder. If conventional methods are used for dosimetry it is recommended that doses to the bladder base should be routinely calculated, since single point measurements at the bladder neck seriously underestimate the dose to the bladder. Also the rectal dose should be determined at several points over the length of the implant due to the wide range of anatomic variations possible.     

Dose verification in clinical IMRT prostate incidents

PURPOSE: In view of the need for dose-validation procedures on each individual intensity-modulated radiation therapy (IMRT) plan, dose-verification measurements by film, by ionization chamber, and by polymer gel-MRI dosimetry were performed for a prostate-treatment plan configuration. Treatment planning system (TPS) calculations were evaluated against dose measurements. METHODS AND MATERIALS: Intensity-modulated radiation therapy (IMRT) treatments were planned on a commercial TPS. Kodak EDR-2 films were used for the verification of two-dimensional (2D) dose distributions at 1 coronal and 5 axial planes in a water-equivalent phantom. Full three-dimensional (3D) dose distributions were measured by use of a novel polymer gel formulation and a 3D magnetic resonance imaging (MRI) readout technique. Calculations were compared against measurements by means of isocontour maps, gamma-index maps (3% dose difference, 3-mm distance to agreement) and dose-volume histograms. RESULTS: A good agreement was found between film measurements and TPS predictions for points within the 60% isocontour, for all the examined plans (gamma-index <1 for 96% of pixels). Three-dimensional dose distributions obtained with the polymer gel-MRI method were adequately matched with corresponding TPS calculations, for measurements in a gel phantom covering the planning-target volume (PTV). CONCLUSIONS: Measured 2D and 3D dose distributions suggest that, for the investigated prostate IMRT plan configuration, TPS calculations provide clinically acceptable accuracy.     

Calibration of a MOSFET Detection System for 6-MV In Vivo Dosimetry

Purpose: Metal oxide semiconductor field-effect transistor (MOSFET) detectors were calibrated to perform in vivo dosimetry during 6-MV treatments, both in normal setup and total body irradiation (TBI) conditions.Methods and Materials: MOSFET water-equivalent depth, dependence of the calibration factors (CFs) on the field sizes, MOSFET orientation, bias supply, accumulated dose, incidence angle, temperature, and spoiler-skin distance in TBI setup were investigated. MOSFET reproducibility was verified. The correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was studied. MOSFET midplane dosimetry in TBI setup was compared with thermoluminescent dosimetry in an anthropomorphic phantom. By using ionization chamber measurements, the TBI midplane dosimetry was also verified in the presence of cork as a lung substitute.Results: The water-equivalent depth of the MOSFET is about 0.8 mm or 1.8 mm, depending on which sensor side faces the beam. The field size also affects this quantity; Monte Carlo simulations allow driving this behavior by changes in the contaminating electron mean energy. The CFs vary linearly as a function of the square field side, for fields ranging from 5 × 5 to 30 × 30 cm². In TBI setup, varying the spoiler-skin distance between 5 mm and 10 cm affects the CFs within 5%. The MOSFET reproducibility is about 3% (2 SD) for the doses normally delivered to the patients. The effect of the accumulated dose on the sensor response is negligible. For beam incidence ranging from 0° to 90°, the MOSFET response varies within 7%. No monotonic correlation between the sensor response and the temperature is apparent. Good correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was found (the correlation coefficient is about 1). The MOSFET midplane dosimetry relevant to the anthropomorphic phantom irradiation is in agreement with TLD dosimetry within 5%. Ionization chamber and MOSFET midplane dosimetry in inhomogeneous phantoms are in agreement within 2%.Conclusion: MOSFET characteristics are suitable for the in vivo dosimetry relevant to 6-MV treatments, both in normal and TBI setup. The TBI midplane dosimetry using MOSFETs is valid also in the presence of the lung, which is the most critical organ, and allows verifying that calculation of the lung attenuator thicknesses based only on the density is not correct. Our MOSFET dosimetry system can be used also to determine the surface dose by using the water-equivalent depth and extrapolation methods. This procedure depends on the field size used.     

A method to estimate the transit dose on the beam axis for verification of dose delivery with portal images.

PURPOSE AND BACKGROUND: A feasibility study is performed to evaluate the possibility of using the transit dose of portal images on the beam axis to measure the accuracy in dose delivery. The algorithm and the method are tested on a breast phantom and on patients with a breast disease. MATERIALS AND METHODS: To estimate the transit dose at various air gaps behind the patient, a method is proposed which applies, for a given air gap, the inverse square law to the primary component of the exit dose and an experimentally determined function for the scatter component of the exit dose. It is assumed that the primary component and the scattered component of the exit dose are given by the treatment planning system. The experimental function for the variation of the scattered component with the air gap, determined by phantom measurements, is modelled by an analytical function which contains only field size, air gap and one energy-dependent parameter. RESULTS: The measurements on the breast phantom yield a maximum deviation between measured and estimated transit doses of 4.5%. The mean deviation is 0.9% with a standard deviation of the distribution of 2.3%. In vivo diode measurements on the same phantom yield a maximum deviation of 2.7%. Transit dose measurements on the beam axis for 45 portal images of breast patients show a mean deviation of 0.0% between the measured transit dose and the estimated transit dose. The standard deviation of the distribution is 4.4%. The method seems to be very sensitive to patient positioning and to discrepancies in breast thicknesses used for treatment planning. CONCLUSION: Preliminary results on breast patients show that the method proposed to evaluate transit doses on the beam axis from portal images may be a valuable alternative to conventional in vivo exit dosimetry. The method can be implemented in a simple way and does not require additional time during the irradiation session, as exit dosimetry with diodes does. The transit dose is only considered in one point. Nevertheless, in the framework of quality assurance of treatment delivery, this study is an example of the possibilities of monitoring at the same time the visual evaluation of the irradiated volume as well as the dosimetric control (i.e. in Gy) of treatment delivery with portal images.     

Assessment of dose inhomogeneity at target level by in vivo dosimetry: can the recommended 5% accuracy in the dose delivered to the target volume be fulfilled in daily practice?

The inhomogeneity of the dose delivered to the target volume due to irregular body surface and tissue densities remains in many cases unknown, since the dose distribution is calculated for most radiation treatments in only one transverse section and assuming the patient to be water equivalent. In the present study, the transmission and the target absorbed dose homogeneity is assessed for 11 head-and-neck cancer treatments by in vivo measurements with silicon diodes. Besides the dose to the specification point, the dose delivered to 2-4 off-axis points in the midline sagittal plane is estimated from entrance and exit dose measurements. Simultaneously made portal films allow to identify the anatomical structures passed by the beam before reaching the exit diode. The mean deviation from the expected transmission is -6.8% for bone, +6% for air cavities and -2.5% for soft tissue. At the midplane, the mean deviations from the expected target dose are respectively -3.5%, +2.3% and -1.9%. The deviations from the prescribed dose are larger than 5% in 12 out of the 39 target points. The accuracy requirement in target dose delivery of plus or minus 5%, as proposed by ICRU, cannot be fulfilled in 7 out of the 11 patients and is mostly due to irregular body contour and tissue densities. As only a limited number of points are considered, the inhomogeneity in the dose delivered throughout the whole irradiated volume is underestimated as is illustrated from the exit dose profiles obtained from the portal image.(ABSTRACT TRUNCATED AT 250 WORDS)