Transit Dosimetry Research Articles

Routine individualised patient dosimetry using electronic portal imaging devices

Background and purpose To analyse the results of routine EPID measurements for individualised patient dosimetry. Materials and methods Calibrated camera-based EPIDs were used to measure the central field dose, which was compared with a dose prediction at the EPID level. For transit dosimetry, dose data were calculated using patient transmission and scatter, and compared with measured values. Furthermore, measured transit dose data were back-projected to an in vivo dose value at 5 cm depth in water ( D 5) and directly compared with D 5 from the treatment planning system. Dose differences per treatment session were calculated by weighting dose values with the number of monitor units per beam. Reported errors were categorised and analysed for approximately 37,500 images from 2511 patients during a period of 24 months. Results Pre-treatment measurements showed a mean dose difference per treatment session of 0.0 ± 1.7% (1 SD). Transfer errors were detected and corrected prior to the first treatment session. An accelerator output variation of about 4% was found between two weekly QC measurements. Patient dosimetry showed mean transit and D 5 dose differences of −0.7 ± 5.2% (1 SD) and −0.3 ± 5.6% (1 SD) per treatment session, respectively. Dose differences could be related to set-up errors, organ motion, erroneous density corrections and changes in patient anatomy. Conclusions EPIDs can be used routinely to accurately verify treatment parameter transfer and machine output. By applying transit and in vivo dosimetry, more insight can be obtained with respect to the different error sources influencing dose delivery to a patient.

SU‐FF‐T‐189: Custom‐Made Ion Chamber for Daily QA in RT

Purpose: A computerized dosimetric system with air‐tight ion chambers and on‐board electrometer has been devised to measure daily output checks of 4/6/10/15 MV X‐ray of linear accelerators. It can be inserted into the blocking tray or can be setup on the treatment table of the linear accelerators. Method and Materials: The dosimetric systems consists of three parts : (1) air‐tight plane‐parallel ionization chamber, (2) on‐board electrometer, (3) computer interface. The ionization chambers (10mm diameter * 2mm width) are implemented on printed circuit boards (PCB) with acrylic plates. Three electrodes are fabricated using approximately 100um thick gold/lead/cupper coating on the PCB board. The buildup plates ranging from 1cm to 3cm depending upon x‐ray or electrons. The electrometer is designed using a typical current‐to‐voltage converter (ACF2101, Burbrown, USA) with 20‐bit ADC circuits (ADC1210, Burbrown, USA), (3) The ADC output is transmitted via RS‐232 interface to a personal computer. The power from rechargeable battery is supplied all the time to the electrometer, thus eliminating warm‐up time. Results: The short and long term stabilities of the ion chamber are found to be less than 0.2% (one standard deviation) with reference of standard dosimetric system. Also, it is extended to be a 9‐channel dosimetric system as a QA device of implementing transit dosimetry (±/−3% agreements between expected and measured values) to determine the overall performances of radiation therapy for the head/neck and breast treatments. Conclusion: The daily QA devices of measuring the output of linear accelerators (6EX and 21EX of Varian linear accelerators) are proven to be very effective. It will be modified to measure the output and the PDD (in a step of 1cm) of the electron beams as a weekly QA.

An investigation of a new amorphous silicon electronic portal imaging device for transit dosimetry

The relationship between the pixel value and exit dose was investigated for a new commercially available amorphous silicon electronic portal imaging device. The pixel to dose mapping function was established to be linear for detector distances between 116.5 cm to 150 cm from the source, radiation field sizes from 5 x 5 cm2 to 20 x 20 cm2 and beam energies of 6 to 18 MV. Coefficients in the mapping function were found to be dependent on beam energy and field size. Open and wedged field profiles measured with the device showed agreement to a maximum of 5% and 8%, respectively, as compared to film. A comparison of relative transmission measurements between the EPID and ion chamber indicate a maximum deviation of 6% and 2% at 6 and 18 MV, respectively, for an attenuator thickness of 21 cm and SDD > or = 130 cm. It was found that accuracies of better than 1% could be obtained if detector position and field size specific fitting parameters were used to generate unique mapping functions for each configuration.

202 invited In vivo estimation of midline dose maps by transit dosimetry:clinical applications

202 invited In vivo estimation of midline dose maps by transit dosimetry:clinical applications

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 ρ, 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.

Transit dosimetry with an electronic portal imaging device (EPID) for 115 prostate cancer patients

Purpose: Comparison of predicted portal dose images (PDIs) with PDIs measured with an electronic portal imaging device (EPID) may be used to detect errors in the dose delivery to patients. However, these comparisons cannot reveal errors in the MU calculation of a beam, since the calculated number of MU is used both for treatment (and thus affects the PDI measurement) and for PDI prediction. In this paper a method is presented that enables “ in vivo” verification of the MU calculation of the treatment beams. The method is based on comparison of the intended on-axis patient dose at 5 cm depth for each treatment beam, D 5, with D 5 as derived from the portal dose D p measured with an EPID. The developed method has been evaluated clinically for a group of 115 prostate cancer patients. Methods and Materials: The patient dose D 5 was derived from the portal dose measured with a fluoroscopic EPID using (i) the predicted beam transmission (i.e., the ratio of the portal dose with and without the patient in the beam) calculated with the planning CT data of the patient, and (ii) an empirical relation between portal doses D p and patient doses D 5. For each beam separately, the derived patient dose D 5 was compared with the intended dose as determined from the relative dose distribution as calculated by the treatment planning system and the prescribed isocenter dose (2 Gy). For interpretation of observed deviating patient doses D 5, the corresponding on-axis measured portal doses D p were also compared with predicted portal doses. Results: For three beams, a total of 7828 images were analyzed. The mean difference between the predicted patient dose and the patient dose derived from the average measured portal dose was: 0.4 ± 3.4% (1 SD) for the anterior–posterior (AP) beam and −1.5 ± 2.4% (1 SD) for the lateral beams. For 7 patients the difference between the predicted portal dose and the average measured portal dose for the AP beam and the corresponding difference in patient dose were both greater than 5%. All these patients had relatively large gas pockets (3–3.5 cm in AP direction) in the rectum during acquisition of the planning CT, which were not present during (most) treatments. Conclusions: An accurate method for verification of the MU calculation of an x-ray beam using EPID measurements has been developed. The method allows the discrimination of errors that are due to changes in patient anatomy related to appearance or disappearance of gas pockets in the rectum and errors due to a deviating cGy/MU-value.

In-vivo dosimetry by diode semiconductors in combination with portal films during TBI: reporting a 5-year clinical experience

Background and purpose: In-vivo dosimetry is vital to assure an accurate delivery of total body irradiation (TBI). In-vivo lung dosimetry is strongly recommended because of the risk of radiation-induced interstitial pneumonia (IP). Here we report on our 5-year experience with in-vivo dosimetry using diodes in combination with portal films and assessing the effectiveness of in-vivo dosimetry in improving the accuracy of the treatment. Moreover, we wished to investigate in detail the possibility of in-vivo portal dosimetry to yield individual information on the lung dose and to evaluate the impact of CT planning on the correspondence between stated and in-vivo measured doses. Materials and methods: From March 1994 to March 1999, 229 supine-positioned patients were treated at our Institute with TBI, using a 6 MV X-rays opposed lateral beam technique. 146 patients received 10 Gy given in three fractions, once a day (FTBI), shielding the lungs by the arms; 70 received 12–13.2 Gy, given in 6–11 fractions, 2–3 fractions per day (HFTBI): in this case about 2/3 of the lungs were shielded by moulded blocks (mean shielded lung dose equal to 9 or 9.5 Gy). Thirteen patients received 8 Gy given in a single fraction (SFTBI, lung dose: 7 Gy). For all HFTBI and FTBI patients, midline in-vivo dosimetry was performed at the first fraction by positioning two diodes pairs (one at entrance and one at the exit side) at the waist (umbilicus) and at the pelvis (ankles). If at least one of the two diodes doses (waist-pelvis) was outside ±5% from the prescribed dose, actions could be initiated, together with possible checks on the following fractions. Transit dosimetry by portal films was performed for most patients; for 165 of them (117 and 48, respectively for FTBI and HFTBI) the midline in-vivo dose distribution of the chest region was derived and mean lung dose assessed. As a CT plan was performed for all HFTBI patients, for these patients, the lung dose measured by portal in-vivo dosimetry was compared with the expected value. Results: Concerning all diodes data, 528 measurements were available: when excluding the data of the first fraction(s) of the patients undergoing corrections ( n=392), mean and SD were respectively 0.0% and 4.5% (FTBI: −0.3±4.8%; HFTBI: 0.4±3.9%). In total 105/229 patients had a change after the first fraction and 66/229 were controlled by in-vivo dosimetry for more than one fraction. Since January 1998 a CT plan is performed for FTBI patients too: when comparing the diodes data before and after this date, a significant improvement was found (i.e. rate of deviations larger than 5% respectively equal to 30.7% and 13.1%, P=0.007). When considering only the patients with a CT plan, the global SD reduced to 3.5%. Concerning transit dosimetry data, for FTBI, the mean (midline) lung dose was found to vary significantly from patient to patient (Average 9.13±0.81 Gy; range 7.4–11.4 Gy); for the HFTBI patients the mean deviation between measured and expected lung dose was 0.0% (1 SD=3.8%). Conclusions: In vivo dosimetry is an effective tool to improve the accuracy of TBI. The impact of CT planning for FTBI significantly improved the accuracy of the treatment delivery. Transit dosimetry data revealed a significant inter-patient variation of the mean lung dose among patients undergoing the same irradiation technique. For patients with partial lung shielding (HFTBI), an excellent agreement between measured and expected lung dose was verified.

Evaluation of an Electronic Portal Imaging Device for Transit Dosimetry

The possibility of using a commercially available electronic portal imaging device for transit dosimetry was investigated. The detection unit of the device comprises a metal plate/fluorescence screen and a camera. Basic parameters of this system were investigated: stability, detector uniformity, dose-response curve, field-size dependence and phantom-thickness dependence. It was found that in terms of relative dosimetry, portal images corrected for detector non-uniformity are in good agreement (within 5%) with transit dose distributions measured by film dosimetry. For dose determination, it was found that the use of the device is hampered by an important field-size dependence and phantom-thickness dependence. Both correction factors should be applied if the device is to be used for this purpose.

Calibration of a portal imaging device for high-precision dosimetry: a Monte Carlo study.

Today electronic portal imaging devices (EPID's) are used primarily to verify patient positioning. They have, however, also the potential as 2D-dosimeters and could be used as such for transit dosimetry or dose reconstruction. It has been proven that such devices, especially liquid filled ionization chambers, have a stable dose response relationship which can be described in terms of the physical properties of the EPID and the pulsed linac radiation. For absolute dosimetry however, an accurate method of calibration to an absolute dose is needed. In this work, we concentrate on calibration against dose in a homogeneous water phantom. Using a Monte Carlo model of the detector we calculated dose spread kernels in units of absolute dose per incident energy fluence and compared them to calculated dose spread kernels in water at different depths. The energy of the incident pencil beams varied between 0.5 and 18 MeV. At the depth of dose maximum in water for a 6 MV beam (1.5 cm) and for a 18 MV beam (3.0 cm) we observed large absolute differences between water and detector dose above an incident energy of 4 MeV but only small relative differences in the most frequent energy range of the beam energy spectra. It is shown that for a 6 MV beam the absolute reference dose measured at 1.5 cm water depth differs from the absolute detector dose by 3.8%. At depth 1.2 cm in water, however, the relative dose differences are almost constant between 2 and 6 MeV. The effects of changes in the energy spectrum of the beam on the dose responses in water and in the detector are also investigated. We show that differences larger than 2% can occur for different beam qualities of the incident photon beam behind water slabs of different thicknesses. It is therefore concluded that for high-precision dosimetry such effects have to be taken into account. Nevertheless, the precise information about the dose response of the detector provided in this Monte Carlo study forms the basis of extracting directly the basic radiometric quantities photon fluence and photon energy fluence from the detector's signal using a deconvolution algorithm. The results are therefore promising for future application in absolute transit dosimetry and absolute dose reconstruction.

A cone-beam megavoltage CT scanner for treatment verification in conformal radiotherapy

Purpose: A prototype scanner for large-volume megavoltage computed tomography (MVCT) in a clinical set-up is described. The ultimate aim is to improve treatment accuracy in conformal radiotherapy through patient set-up error reduction and transit dosimetry. Materials and methods: The scanner consists of a custom-built 2D CsI(Tl) crystal array viewed by a lens and a CCD camera. Image acquisition is synchronized with radiation pulses. The 2D projections resulting from a single continuous 360° gantry rotation are reconstructed using a cone-beam tomography algorithm. Prior to reconstruction, the raw projections are calibrated and corrected for centre of rotation movement and accelerator output fluctuation. The performance of the system has been evaluated by reconstructing projections of open fields, test objects and a humanoid phantom. Results: Hundreds of 2D projections can be acquired with a clinically-acceptable data collection time (about 2 min) and dose (approximately 40 cGy, with a possible four-fold reduction). A maximum density resolution of about 2% is achieved offering some soft tissue discrimination without using image enhancement tools. A spatial resolution of 2.5 mm is obtained. The reconstructed image intensity is linear with electron density over the range of interest. Coronal or sagittal slices through the 3D reconstruction of the humanoid phantom show a better delineation of structures than the corresponding portal images taken at the same orientation. Conclusions: A similar image quality to our current single-slice MVCT scanner is achieved with the advantage of providing tens of tomographic slices for a single gantry rotation. This work demonstrates the feasibility of clinical cone-beam MVCT and indicates how this prototype can be improved.

Linear accelerator output variations and their consequences for megavoltage imaging.

An experimental study of radiation output intensity fluctuations of a Philips SL25 linear accelerator is presented. Measurements are obtained using an electronic portal imaging device, and the consequences of the measured fluctuations for various different applications of megavoltage imaging including portal imaging, transit dosimetry and megavoltage computed tomography (MVCT) are discussed with examples. Fluctuations in output of +/- 0.7% (1 SD) are seen on every radiation pulse after photon noise and uncertainties caused by the detection system have been accounted for. Large fluctuations are also seen during the initial beam stabilization period (15%), during normal accelerator operation after the beam has been on for more than 1 min (4.5%) and during are therapy as a repeatable function of gantry angle (9%). Such output intensity fluctuations are shown to produce image artifacts in portal imaging devices with scanned detector readout and can also produce systematic errors in detector calibration that would lead to uncertainty in transit dose calculations. The propagation of these intensity fluctuations through MVCT image reconstruction is shown to produce ring artifacts in the reconstructed image. Sample portal and MVCT images are presented. All observed fluctuations in accelerator output are well within the manufacturer's specifications and do not affect the total dose delivered during normal treatment. Finally, megavoltage imaging is shown to be a powerful tool for accelerator quality assurance and treatment verification.

X-ray quantum limited portal imaging using amorphous silicon flat-panel arrays.

We have measured the linearity, spatial resolution (MTF), noise (NPS), and signal-to-noise characteristics (DQE) of an electronic portal imaging device (EPID) based on an amorphous silicon flat-panel array. The array has a 128 x 128-pixel matrix and each pixel is 0.75 x 0.75 mm2 in dimension so the array covers an area of 96 x 96 mm2. The array acts like a large area light sensor and records the optical signals generated in a metal plate/phosphor screen x-ray detector when the detector is irradiated by a megavoltage x-ray beam. In addition, approximately 0.5% of the total signal is generated by nonoptical processes. The noise measurements show that the device is quantum noise limited with the noise power generated by the x-ray quanta being up to 100 times greater than the noise added by the external readout electronics and flat-panel light sensor itself. However, the flat-panel light sensor does reduce the spatial resolution (compared to a perfect optical sensor with infinitesimal pixel size) because of its moderate pixel size and because optical spread can occur in the transparent glues used to attach the phosphor screen to the flat-panel light sensor. The response of the sensor is very linear and does not suffer from the glare phenomenon associated with TV camera-based EPIDs--characteristics which suggest that the amorphous silicon EPID will be well suited to transit dosimetry. Nevertheless, some limitations need to be overcome before these devices can be used clinically. These include developing larger flat-panel light sensors, the elimination of "noisy" pixels with high dark signal, and improvements in the uniform sensitivity of the sensors. This last requirement is only needed for transit dosimetry applications where it would greatly simplify calibration of the device. In addition, an image acquisition scheme must be developed to eliminate artifacts created by the pulsed x-ray beam generated by linear accelerators. Despite these limitations, our studies suggest that the amorphous silicon EPIDs are very well suited to portal imaging.

An electronic portal imaging device for transit dosimetry

An electronic portal imaging device has been designed and constructed. It consists of an array of 128 CsI scintillation crystals coupled to photodiodes which is scanned across the field in 4 seconds. The linac is operated at a dose rate of and the dose delivered for image acquisition is . The data acquisition controller is a stand-alone STE computer located within the scan arm. Sample images are presented showing contrast and spatial resolution of the system together with a humanoid phantom image and a clinical image of a breast cancer patient. The phantom images show the detector has a contrast resolution of 0.3% (at 15 mm diameter) and a spatial resolution of 2.5 - 3.2 mm. Images of uniform Perspex blocks have also been calibrated for thickness, indicating the system can measure radiological thickness to an accuracy of 2 - 3 mm of water. These results indicate the detector may be used for transit dosimetry applications including compensator design.

Electronic portal imaging for dosimetric purposes

Dosimetric aspects of Electronic Portal Imaging Devices (EPIDs) have been explored. Three models to extract the primary signal from EPIDs have been studied. The first simple model removes the scatter component using a method based on the scatter‐to‐primary ratio (SPR). This model has several limitations. Two more complex models based on convolution of Pencil Beam Spread Kernels (PBSK) have been studied, one analytical approximation and one iterative solution. Both models based on PBSK were tested on a large set of Monte Carlo generated EPID signals. The primary signal was extracted to within 2% of the true primary signal, using either model, even when SPR was 0.3 in the original image. On the basis of the primary signal of the EPID, generated from any of the above models, two new applications were developed and studied: Transit dosimetry,1 this is verification of the dose delivered using an EPID; and the design of intensity modulated beams (IMB) for tangential breast fields. The IMBs were implemented using either a physical compensator or a set of fields shaped with a multileaf collimator. Dose measurements in an anatomical phantom showed that both implementations gave better dose uniformity than the standard treatment. 1V. N. Hansen, P. M. Evans, and W. Swindell, Med. Phys. 23, 713–721 (1996).

On-line exit dose profile measurements by a diode linear array

The possibility of using a commercial diode linear array (BMS Schuster Inc.) in on-line portal dosimetry has been investigated. The system is composed of 88 equispaced diodes (distance between diodes, 0.5 cm; detection area, ). The ability to measure exit dose profiles has been demonstrated by comparing relative exit dose profiles measured by an ionization chamber (NE 2571) with `portal' profiles measured by the diode array on a number of homogeneous and inhomogeneous phantoms in 6 MV x-ray beams (from a Clinac Varian 6/100). The influence of the variation of the scattered radiation at the exit level with respect to the detector level has been discussed and investigated by varying the air gap (from 0 to 80 cm for a homogeneous phantom and from 0 to 25 cm for inhomogeneous phantoms) as well as the field width. A good agreement (maximum difference 3.8%; 2.5% when the array was positioned in contact with the phantoms) between `portal' profiles and exit dose profiles in both homogeneous and inhomogeneous phantoms has been found if the array - phantom distance is kept below 7.5 cm. Results indicate that the system should be suitable for applications in transit dosimetry.

The application of transit dosimetry to precision radiotherapy

A method of using electronic portal imaging (EPI) for transit dosimetry is described. In this method, a portal image of the treatment field is first aligned with a digitally reconstructed radiograph (DRR) to geometrically relate the computed tomography (CT) scan, used to generate the DRR, with the EPI. Then the EPI is corrected for scatter within the patient to yield a map of primary fluence striking the detector. This is backprojected through the planning CT data set to yield a distribution of primary fluence within the patient. This distribution is then convolved with dose deposition kemels to yield a map of dose delivery within the patient. Such a distribution may be compared with the dose distribution resulting from the original treatment plan in order to evaluate the adequacy of the treatment. This method has been evaluated using a humanoid phantom. We find the transit dosimetry relative dose distribution when compared with film and thermoluminescent dosimeter (TLD) measurements and compared with our planning system to agree within 2% in the pelvic region of a humanoid phantom.

Scattered radiation in portal images: A Monte Carlo simulation and a simple physical model

The scattered radiation in 6 MV radiotherapy portal images is analyzed. First, a quantity SPR* is studied, by means of Monte Carlo (MC) modeling. SPR* is defined as the ratio, on the central axis, of the signal due to scattered radiation to that due to the primary radiation. The detector model mimics a high-energy photon detector in the context of transit dosimetry. Second, a physical model of SPR* has been developed from first principles. For a cylindrical phantom, placed symmetrically about the isocenter, it predicts that SPR* depends on the area A at the isocenter of the circular field and the phantom thickness d as follows. SPR* = k0Ad(1 + k1d)(1 + k2A), where k0 = 0.0266(L1 + L2)2/(L1L2)2, k2 = - [L1(-2) + L2(-2) + (L1(-1) + L2(-1))2((2/3) + (3 kappa/2))]/2pi, L1 is the source-to-isocenter distance, L2 is the isocenter-to-detector distance, and kappa is the mean energy of the radiation beam (MeV/0.511). Constant k1, for which there is no simple expression, depends on L2. Comparison to the MC data shows that for 60 <or= L2 <or= 100 cm the dependence is weak and k1 approximately equal to 2 x 10(-3) cm-1. The root mean square (rms) agreement between the MC-derived values of SPR* and the physical model is better than 0.001 over a wide range of A and d values likely to be encountered in clinical practice for L2 >or= 50 cm. Third, experimental measurements of the scatter-to-primary ratio were obtained using our custom built imaging system mounted on a Philips SL25 linear accelerator. In the first experiment, A was varied from 40 to 400 cm2 with L1 = L2 = 100 cm with d = 20 cm. In the second experiment water depth d was varied from 0 to 28 cm with L1 = L2 = 100 cm and A = 200 cm2. The rms agreements between the MC data and the experiments were 0.0015 and 0.0045, respectively.

Evaluation of computed tomography assisted and transit dosimetry treatment planning with thermoluminescent dosimetry measurements

Transit dosimetry methods have been compared with computer plans which use computed tomography (CT) determined patient geometry to make corrections for inhomogeneities in radiation therapy treatment planning. These two planning methods have been applied to treatment sites in the head and chest of an anthropomorphic phantom. The results of the treatment plans are compared to delivered doses as measured with thermoluminescent dosimeters (TLD) in the phantom. The treatment planning results from two independent commercial computer systems have been found to agree with each other and with transit dosimetry calculations to within 2 %. In the head, the result of both inhomogeneity correction methods agree with measured doses to within 2%. In the chest the calculated doses disagree with measured values by 6%. This results from the inadequacy of the correction methods to account for the inhomogeneity distribution. When this effect is taken into account, calculated and measured doses agree to within 2 % even for the chest irradiations.

Evaluation of CT and transit dosimetry treatment planning with tld measurements

Evaluation of CT and transit dosimetry treatment planning with tld measurements

Cobalt-60 depth-dose correction as determined by transmission dose measurements.

Although the introduction of central-axis depthdose data, measured in tissue-equivalent phantoms, elevated clinical radiation therapy to a semiquantitative discipline, it is generally recognized that in many instances these data, however accurate, fall short of indicating the true dose distribution in the patient because of variations of tissue types within the treatment field. In order to determine the magnitude of the effect that tissues of different densities have upon central-axis depth dose, a technic based upon transit-dose measurements and physically similar to schemes reported earlier, has been developed. In addition to providing factors which correct depth-dose data for tissue variations, a second requirement of this investigation was that the technic evolved should be simple enough to be carried out as a routine procedure by the resident staff. Instrumentation consists of a simple straightbore collimator mounted to the primary beam shield of a rotating cobalt-60 unit. This collimator is designed to carry the standard 25-r Victoreen dosimeter. The patient's transmission is determined by making one measurement during the course of treatment and another, under identical conditions, with the patient removed. Next, the effective absorption coefficient is determined from a family of curves which relate per cent transmission and patient diameters. It is assumed that the contribution of scattered radiation to central-axis depth dose will not change appreciably when the treatment field includes tissues whose densities differ from unity. Centralaxis depth dose will therefore depend mainly upon primary beam absorption. Wilen measured absorption coefficients differ from that for unit density material, depth-dose data should be modified in accordance with the ratio e-µ1t/e-µ1t where µl is the experimentally determined linear absorption coefficient, µ1 is the coefficient for unit density material, and t is the tumor depth. Because of the asymmetrical location of tumors and tissues of different densities within the body, the depth-dose correction factors determined by transit dosimetry are applicable only to opposed portal treatment plans where the effects of asymmetry will average out. This has been experimentally verified with a variety of asymmetrical phantoms. Transit dosimetry applied to patients with lung tumors has resulted in depth-dose correction factors ranging from 1.00 to 1.18. It was not possible to correlate these data with the appearance of the lung on a diagnostic x-ray film. Measurements made for mediastinal fields resulted in correction factors between 0.90 and 1.00.