Conformal photon-beam radiotherapy of prostate carcinoma.

Three-dimensional conformal radiotherapy is the recommended radiation technique for localized or locally advanced prostate cancer. In the past decades, external beam irradiation procedures have evolved in the context of technical developments of radiation and imaging equipment. The article summarizes these developments and gives a definition of new techniques and their potential advantages over conventional irradiation. It is meant to provide urologists and medical and radiation oncologists with a better comprehension of modern radiation treatment of prostate cancer and its possible improvements in the future.

Electronic portal image assisted reduction of systematic set-up errors in head and neck irradiation.

PURPOSE: To quantify systematic and random patient set-up errors in head and neck irradiation and to investigate the impact of an off-line correction protocol on the systematic errors. MATERIAL AND METHODS: Electronic portal images were obtained for 31 patients treated for primary supra-glottic larynx carcinoma who were immobilised using a polyvinyl chloride cast. The observed patient set-up errors were input to the shrinking action level (SAL) off-line decision protocol and appropriate set-up corrections were applied. To assess the impact of the protocol, the positioning accuracy without application of set-up corrections was reconstructed. RESULTS: The set-up errors obtained without set-up corrections (1 standard deviation (SD)=1.5-2mm for random and systematic errors) were comparable to those reported in other studies on similar fixation devices. On an average, six fractions per patient were imaged and the set-up of half the patients was changed due to the decision protocol. Most changes were detected during weekly check measurements, not during the first days of treatment. The application of the SAL protocol reduced the width of the distribution of systematic errors to 1mm (1 SD), as expected from simulations. A retrospective analysis showed that this accuracy should be attainable with only two measurements per patient using a different off-line correction protocol, which does not apply action levels. CONCLUSIONS: Off-line verification protocols can be particularly effective in head and neck patients due to the smallness of the random set-up errors. The excellent set-up reproducibility that can be achieved with such protocols enables accurate dose delivery in conformal treatments.

Clinical implications of incomplete repair parameters for rat spinal cord: the feasibility of large doses per fraction in PDR and HDR brachytherapy.

PURPOSE: To evaluate the clinical implications of the repair parameters determined experimentally in rat spinal cord and to test the feasibility of large doses per fraction or pulses in daytime high-dose-rate (HDR) or pulsed-dose-rate (PDR) brachytherapy treatment schedules as an alternative to continuous low-dose-rate (CLDR) brachytherapy. METHODS AND MATERIALS: BED calculations with the incomplete repair LQ-model were performed for a primary CLDR-brachytherapy treatment of 70 Gy in 140 h or a typical boost protocol of 25 Gy in 50 h after 46-Gy conventional external beam irradiation (ERT) at 2 Gy per fraction each day. Assuming biphasic repair kinetics and a variable dose rate for the iridium-192- (192Ir) stepping source, the LQ-model parameters for rat spinal cord as derived in three different experimental studies were used: (a) two repair processes with an alpha/beta ratio = 2.47 Gy and repair half-times of 0.2 h (12 min) and 2.2 h (Pop et. al.); (b) two repair processes with an alpha/beta ratio = 2.0 Gy and repair half-times of 0.7 h (42 min) and 3.8 h (Ang et al.); and (c) two repair processes with an alpha/beta ratio = 2.0 Gy and repair half-times of 0.25 h (15 min) and 6.4 h (Landuyt et al.). For tumor tissue, an alpha/beta ratio of 10 Gy and a monoexponential repair half time of 0.5 h was assumed. The calculated BED values were compared with the biologic effect of a clinical reference dose of conventional ERT with 2 Gy/day and complete repair between the fractions. Subsequently, assuming a two-catheter implant similar to that used in our experimental study and with the repair parameters derived in our rat model, BED calculations were performed for alternative PDR- and HDR-brachytherapy treatment schedules, in which the irradiation was delivered only during daytime. RESULTS: If the repair parameters of the study of Pop et al., Ang et al., or Landuyt et al. are used, for a CLDR-treatment of 70 Gy in 140 h, the calculated BED values were 117, 193, or 216 Gy(sc) (Gy(sc) was used to express the BED value for the spinal cord), respectively. These BED values correspond with total doses of conventional ERT of 65, 96, or 104 Gy. The latter two are unrealistic high values and illustrate the danger of a straightforward comparison of BED values if repair parameters are used in situations quite different from those in which they were derived. For a brachytherapy boost protocol, the impact of the different repair parameters is less, due to the fact that the percentage increase in total BED value by the brachytherapy boost is less than 50%. If a primary treatment with CLDR brachytherapy delivering 70 Gy in 140 h has to be replaced, high doses per fraction or pulses (> 1 Gy) during daytime can only be used if the overall treatment time is prolonged with 3-4 days. The dose rate during the fraction or pulse should not exceed 6 Gy/h. For a typical brachytherapy boost protocol after 46 Gy ERT, it seems to be safe to replace CLDR delivering a total dose of 25 Gy in 50 h by a total dose of 24 Gy in 4 days with HDR or PDR brachytherapy during daytime only. Total dose per day should be limited to 6 Gy, and the largest time interval as possible between each fraction or pulse should be used. CONCLUSION: Extrapolations based on longer repair half-times in a CLDR reference scheme may lead to the calculation of unrealistically high BED values and dangerously high doses for alternative HDR and PDR treatment schedules. Based on theoretical calculations with the IR model and using the repair parameters derived in our rat spinal cord model, it is estimated that with certain restrictions, large doses per fraction or pulses can be used during daytime schedules of HDR or PDR brachytherapy as an alternative to CLDR brachytherapy, especially for those treatment conditions in which brachytherapy is used after ERT for only less than 50% of the total dose.

Equivalence of hyperfractionated and continuous brachytherapy in a rat tumor model and remarkable effectiveness when preceded by external irradiation.

PURPOSE: In clinical brachytherapy, there is a tendency to replace continuous low-dose-rate (LDR) irradiation by either single-dose or fractionated high-dose-rate (HDR) irradiation. In this study, the equivalence of LDR treatments and fractionated HDR (2 fractions/day) or pulsed-dose-rate (PDR, 4 fractions/day) schedules in terms of tumor cure was investigated in an experimental tumor model. METHODS AND MATERIALS: Tumors (rat rhabdomyosarcoma R1M) were grown s.c. in the flank of rats and implanted with 4 catheters guided by a template. All interstitial radiation treatment (IRT) schedules were given in the same geometry. HDR was given using an (192)Ir single-stepping source. To investigate small fraction sizes, part of the fractionated HDR and PDR schedules were applied after an external irradiation (ERT) top-up dose. The endpoint was the probability of tumor control at 150 days after treatment. Cell survival was estimated by excision assay. RESULTS: Although there was no fractionation effect for fractionated HDR given in 1 or 2 fractions per day, TCD(50)-values were substantially lower than that for LDR. A PDR schedule with an interfraction interval of 3 h (4 fractions/day), however, was equivalent to LDR. The combination of ERT and IRT resulted in a remarkably increased tumor control probability in all top-up regimens, but no difference was found between 2 or 4 fractions/day. Catheter implantation alone decreased the TCD(50) for single-dose ERT already by 17.4 Gy. Cell viability assessed at 24 h after treatment demonstrated an increased effectiveness of interstitial treatment, but, after 10 Gy ERT followed by 10 Gy IRT (24-h interval), it was not less than that calculated for the combined effect of these treatments given separately. CONCLUSION: In full fractionation schedules employing large fractions and long intervals, the sparing effect of sublethal damage repair may be significantly counteracted by reoxygenation. During 3-h intervals, however, repair may be largely completed with only partial reoxygenation causing PDR schedules to be less effective than fractionated HDR, and equivalent to LDR. Brachytherapy with clinically sized fractions after a large external top-up dose showed a remarkable increase in tumor control rate with no effect of fractionation (up to 4 fractions/day), which could not be fully explained by differences in dose distribution or in the cell viability assessed after treatment. This suggests a longer lasting effect on cell survival or radiosensitivity associated with catheter implantation shortly after the top-up dose.

Clinical thermometry, using the 27 MHz multi-electrode current-source interstitial hyperthermia system in brain tumours.

BACKGROUND AND PURPOSE: In interstitial hyperthermia, temperature measurements are mainly performed inside heating applicators, and therefore, give the maximum temperatures of a rather heterogeneous temperature distribution. The problem of how to estimate lesion temperatures using the multi-electrode current-source interstitial hyperthermia (MECS-IHT) system in the brain was studied. MATERIALS AND METHODS: Temperatures were measured within the electrodes and in an extra catheter at the edge of a 4 x 4 x 4.5 cm(3) glioblastoma multiforme resection cavity. From the temperature decays during a power-off period, information was obtained about local maximum and minimum tissue temperatures. The significance of these data was examined through model calculations. RESULTS: Maximum tissue temperatures could be estimated roughly by switching off all electrodes for about 5 s. Model calculations showed that the minimum tissue temperatures near a certain afterloading catheter correspond well with the temperature of the applicator inside, about 1 min after this applicator was switched off. CONCLUSIONS: Although the electrode temperatures read during heating are not suitable to assess the temperature distribution, it is feasible to heat the brain adequately using the MECS-IHT system with extra sensors outside the electrodes and/or application of decay methods.

Analysis and reduction of 3D systematic and random setup errors during the simulation and treatment of lung cancer patients with CT-based external beam radiotherapy dose planning.

PURPOSE: To determine the magnitude of the errors made in (a) the setup of patients with lung cancer on the simulator relative to their intended setup with respect to the planned treatment beams and (b) in the setup of these patients on the treatment unit. To investigate how the systematic component of the latter errors can be reduced with an off-line decision protocol for setup corrections. METHODS AND MATERIALS: For 39 patients with CT planning, digitally-reconstructed radiographs (DRRs) were calculated for anterior-posterior and lateral beams. Retrospectively, the position of the visible anatomy relative to the planned isocenter was compared with the corresponding position on the digitized simulator radiographs using contour match software. The setup accuracy at the treatment unit relative to the simulator setup was measured for 40 patients for at least 5 fractions per patient in 2 orthogonal beams with the aid of an electronic portal imaging device (EPID). Setup corrections were applied, based on an off-line decision protocol, with parameters derived from knowledge of the random setup errors in the studied patient group. RESULTS: The standard deviations (SD) of the simulator setup errors relative to the CT planning setup in the lateral, longitudinal, and anterior-posterior directions were 4.0, 2.8, and 2.5 mm, respectively. The SD of rotations around the anterior-posterior axis was 1.6 degrees and around the left-right axis 1.3 degrees. The setup error at the treatment unit had a small random component in all three directions (1 SD = 2 mm). The systematic components were larger, particularly in the longitudinal direction (1 SD = 3.6 mm), but were reduced with the decision protocol to 1 SD < 2 mm with, on average, 0.6 setup correction per patient. CONCLUSION: Setup errors at the simulator, which become systematic errors if the simulation defines the reference setup, were comparable to the systematic setup errors at the treatment unit in case no off-line protocol would have been applied. Hence, the omission of a separate simulation step can reduce systematic errors as efficiently as the application of an off-line correction protocol during treatment. The random errors were sufficiently small to make an off-line protocol feasible.

Single dose irradiation response of pig skin: a comparison of brachytherapy using a single, high dose rate iridium-192 stepping source with 200 kV X-rays.

An experimental brachytherapy model has been developed to study acute and late normal tissue reactions as a tool to examine the effects of clinically relevant multifractionation schedules. Pig skin was used as a model since its morphology, structure, cell kinetics and radiation-induced responses are similar to human skin. Brachytherapy was performed using a microSelectron high dose rate (HDR) afterloading machine with a single stepping source and a custom-made template. In this study the acute epidermal reactions of erythema and moist desquamation and the late dermal reactions of dusky mauve erythema and necrosis were evaluated after single doses of irradiation over a follow-up period of 16 weeks. The major aims of this work were: (a) to compare the effects of iridium-192 (192Ir) irradiation with effects after X-irradiation; (b) to compare the skin reactions in Yorkshire and Large White pigs; and (c) to standardize the methodology. For 192Ir irradiation with 100% isodose at the skin surface, the 95% isodose was estimated at the basal membrane, while the 80% isodose covered the dermal fat layers. After HDR 192Ir irradiation of Yorkshire pig skin the ED50 values (95% isodose) for moderate/severe erythema and moist desquamation were 24.8 Gy and 31.9 Gy, respectively. The associated mean latent period (+/- SD) was 39 +/- 7 days for both skin reactions. Late skin responses of dusky mauve erythema and dermal necrosis were characterized by ED50 values (80% isodose) of 16.3 Gy and 19.5 Gy, with latent periods of 58 +/- 7 days and 76 +/- 12 days, respectively. After X-irradiation, the incidence of the various skin reactions and their latent periods were similar. Acute and late reactions were well separated in time. The occurrence of skin reactions and the incidence of effects were comparable in Yorkshire and Large White pigs for both X-irradiation and HDR 192Ir brachytherapy. This pig skin model is feasible for future studies on clinically relevant multifractionation schedules in a brachytherapy setting.

Tumor hypoxia--a confounding or exploitable factor in interstitial brachytherapy? Effects of tissue trauma in an experimental rat tumor model.

PURPOSE: To evaluate the potential effects of tumor hypoxia induced by afterloading catheter implantation on the effectiveness of brachytherapy in a rat tumor model. METHODS AND MATERIALS: Afterloading catheters (4) were implanted in subcutaneously growing R1M rhabdomyosarcoma in female Wag/Rij rats. A MicroSelectron (Nucletron) was used for interstitial high-dose-rate irradiation ((192)Ir). Tumor oxygenation, perfusion, and cell survival were assessed by pO(2) histography (Eppendorf), Tc-99m injection, and excision assay, respectively. RESULTS: Tumor perfusion was markedly reduced at 1 h after catheter implantation (33.9 +/- 6.0% (SEM, n = 9) of control) and partly recovered after 5 h (61.5 +/- 12.2%). At 24 h, the perfusion level reached control values (100.6 +/- 25.7%), but was highly variable with some of the tumors showing hardly any recovery at all. Tumor oxygenation showed a similar pattern, but with less recovery. Median pO(2) readings were 13.5, 1.2, and 5.3 mm Hg before and at 1 and 24 h after implantation, respectively (7 tumors). The percentages of pO(2) readings

On-line set-up corrections during radiotherapy of patients with gynecologic tumors.

PURPOSE: Positioning of patients with gynecologic tumors for radiotherapy has proven to be relatively inaccurate. To improve the accuracy and reduce the margins from clinical target volume (CTV) to planning target volume (PTV), on-line set-up corrections were investigated. METHODS AND MATERIALS: Anterior-posterior portal images of 14 patients were acquired using the first six monitor units (MU) of each irradiation fraction. The set-up deviation was established by matching three user-defined landmarks in portal and simulator image. If the two-dimensional deviation exceeded 4 mm, the table position was corrected. A second portal image was acquired using 30 MU of the remaining dose. This image was analyzed off-line using a semiautomatic contour match to obtain the final set-up accuracy. To verify the landmark match accuracy, the contour match was retrospectively performed on the six MU images as well. RESULTS: The standard deviation (SD) of the distribution of systematic set-up deviations after correction was < 1 mm in left-right and cranio-caudal directions. The average random deviation was < 2 mm in these directions (1 SD). Before correction, all standard deviations were 2 to 3 mm. The landmark match procedure was sufficiently accurate and added on average 3 min to the treatment time. The application of on-line corrections justifies a CTV-to-PTV margin reduction to about 5 mm. CONCLUSIONS: On-line set-up corrections significantly improve the positioning accuracy. The procedure increases treatment time but might be used effectively in combination with off-line corrections.

Characterization of a high-elbow, fluoroscopic electronic portal imaging device for portal dosimetry.

The application of a newly developed fluoroscopic (CCD-camera based) electronic portal imaging device (EPID) in portal dosimetry is investigated. A description of the EPID response to dose is presented in terms of stability, linearity and optical cross-talk inside the mechanical structure. The EPID has a relatively large distance (41 cm on-axis) between the fluorescent screen and the mirror (high-elbow), which results in cross-talk with properties quite different from that of the low-elbow fluoroscopic EPIDs that have been studied in the literature. In contrast with low elbow systems, the maximum cross-talk is observed for points of the fluorescent screen that have the largest distance to the mirror, which is explained from the geometry of the system. An algorithm to convert the images of the EPID into portal dose images (PDIs) is presented. The correction applied for cross-talk is a position dependent additive operation on the EPID image pixel values, with a magnitude that depends on a calculated effective field width. Deconvolution with a point spread function, as applied for low-elbow systems, is not required. For a 25 MV beam, EPID PDIs and ionization chamber measurements in the EPID detector plane were obtained behind an anthropomorphic phantom and a homogeneous absorber for various field shapes. The difference in absolute dose between the EPID and ionization chamber measurements, averaged over the four test fields presented in this paper, was 0.1 +/- 0.5% (1 SD) over the entire irradiation field, with no deviation larger than 2%.

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, D5, with D5 as derived from the portal dose Dp 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 D5 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 Dp and patient doses D5. For each beam separately, the derived patient dose D5 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 D5, the corresponding on-axis measured portal doses Dp 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.

Dosimetric verification of intensity modulated beams produced with dynamic multileaf collimation using an electronic portal imaging device.

Dose distributions can often be significantly improved by modulating the two-dimensional intensity profile of the individual x-ray beams. One technique for delivering intensity modulated beams is dynamic multileaf collimation (DMLC). However, DMLC is complex and requires extensive quality assurance. In this paper a new method is presented for a pretreatment dosimetric verification of these intensity modulated beams utilizing a charge-coupled device camera based fluoroscopic electronic portal imaging device (EPID). In the absence of the patient, EPID images are acquired for all beams produced with DMLC. These images are then converted into two-dimensional dose distributions and compared with the calculated dose distributions. The calculations are performed with a pencil beam algorithm as implemented in a commercially available treatment planning system using the same absolute beam fluence profiles as used for calculation of the patient dose distribution. The method allows an overall verification of (i) the leaf trajectory calculation (including the models to incorporate collimator scatter and leaf transmission), (ii) the correct transfer of the leaf sequencing file to the treatment machine, and (iii) the mechanical and dosimetrical performance of the treatment unit. The method was tested for intensity modulated 10 and 25 MV photon beams; both model cases and real clinical cases were studied. Dose profiles measured with the EPID were also compared with ionization chamber measurements. In all cases both predictions and EPID measurements and EPID and ionization chamber measurements agreed within 2% (1 sigma). The study has demonstrated that the proposed method allows fast and accurate pretreatment verification of DMLC.

Temperature measurement errors with thermocouples inside 27 MHz current source interstitial hyperthermia applicators.

The multielectrode current source (MECS) interstitial hyperthermia (IHT) system uses thermocouple thermometry. To obtain a homogeneous temperature distribution and to limit the number of traumas due to the implanted catheters, most catheters are used for both heating and thermometry. Implications of temperature measurement inside applicators are discussed. In particular, the impact of self-heating of both the applicator and the afterloading catheter were investigated. A one-dimensional cylindrical model was used to compute the difference between the temperature rise inside the applicators (deltaTin) and in the tissue just outside the afterloading catheter (deltaTout) as a function of power absorption in the afterloading catheter, self-heating of the applicator and the effective thermal conductivity of the surrounding tissue. Furthermore, the relative artefact (ERR), i.e. (deltaTin - deltaTout)/deltaTin, was measured in a muscle equivalent agar phantom at different positions in a dual-electrode applicator and for different catheter materials. A method to estimate the tissue temperature by power-off temperature decay measurement inside the applicator was investigated. Using clinical dual-electrode applicators in standard brachytherapy catheters in a muscle-equivalent phantom, deltaTin is typically twice as high as deltaTout. The main reason for this difference is self-heating of the thin feeder wires in the centre of the applicator. The measurement error caused by energy absorption in the afterloading catheter is small, i.e. even for materials with a high dielectric loss factor it is less than 5%. About 5 s after power has been switched off, Tin in the electrodes represents the maximum tissue temperature just before power-off. This delay time (t(delay)) and ERR are independent of Tin. However, they do depend on the thermal properties of the tissue. Therefore, ERR and t(delay) and their stability in perfused tissues have to be investigated to enable a reliable estimation of the tissue temperatures around electrodes in clinical practice.

Verification of compensator thicknesses using a fluoroscopic electronic portal imaging device.

A method is presented for verification of compensator thicknesses using a fluoroscopic electronic portal imaging device (EPID). The method is based on the measured transmission through the compensator, defined by the ratio of the portal dose with the compensator in the beam and the portal dose without the compensator in the beam. The transmission is determined with the EPID by dividing two images, acquired with and without compensator inserted, which are only corrected for the nonlinear response of the fluoroscopic system. The transmission has a primary and a scatter component. The primary component is derived from the measured transmission by subtracting the predicted scatter component. The primary component for each point is only related to the radiological thickness of the compensator along the ray line between the focus and that point. Compensator thicknesses are derived from the primary components taking into account off-axis variations in beam quality. The developed method has been tested for various compensators made of a granulate of stainless steel. The compensator thicknesses could be determined with an accuracy of 0.5 mm (1 s.d.), corresponding to a change in the transmitted dose of about 1% for a 10 MV beam. The method is fast, accurate, and insensitive to long-term output and beam profile fluctuations of the linear accelerator.

An analysis of anatomic landmark mobility and setup deviations in radiotherapy for lung cancer.

PURPOSE: To identify thoracic structures that exhibit little internal motion during irradiation and to determine setup variations in patients with lung cancer. METHODS AND MATERIALS: Intrafractional images were generated with an electronic portal-imaging device from the AP fields of 10 patients, during several fractions. To determine the intrafractional mobility of thoracic structures, visible structures were contoured in every image and matched with a reference image by means of a cross-correlation algorithm. Setup variations were determined by comparing portal images with the digitized simulator films using the stable structures as landmarks. RESULTS: Mobility was limited in the lateral direction for the trachea, thoracic wall, paraspinal line, and aortic notch, and in the craniocaudal direction for the clavicle, aortic notch, and thoracic.wall. Analysis of patient setup revealed random deviations of 2.0 mm (1 SD) in the lateral direction and 2.8 mm in the craniocaudal direction, while the systematic deviations were 2.5 and 2.0 mm (1 SD) respectively. CONCLUSIONS: We have identified thoracic structures that exhibit little internal motion in the frontal plane, and recommend that these structures be used for verifying patient setup during radiotherapy. The daily variation in the setup of lung cancer patients at our center appears to be acceptable.

Inclusion of geometrical uncertainties in radiotherapy treatment planning by means of coverage probability.

PURPOSE: Following the ICRU-50 recommendations, geometrical uncertainties in tumor position during radiotherapy treatments are generally included in the treatment planning by adding a margin to the clinical target volume (CTV) to yield the planning target volume (PTV). We have developed a method for automatic calculation of this margin. METHODS AND MATERIALS: Geometrical uncertainties of a specific patient group can normally be characterized by the standard deviation of the distribution of systematic deviations in the patient group (Sigma) and by the average standard deviation of the distribution of random deviations (sigma). The CTV of a patient to be planned can be represented in a 3D matrix in the treatment room coordinate system with voxel values one inside and zero outside the CTV. Convolution of this matrix with the appropriate probability distributions for translations and rotations yields a matrix with coverage probabilities (CPs) which is defined as the probability for each point to be covered by the CTV. The PTV can then be chosen as a volume corresponding to a certain iso-probability level. Separate calculations are performed for systematic and random deviations. Iso-probability volumes are selected in such a way that a high percentage of the CTV volume (on average > 99%) receives a high dose (> 95%). The consequences of systematic deviations on the dose distribution in the CTV can be estimated by calculation of dose histograms of the CP matrix for systematic deviations, resulting in a so-called dose probability histogram (DPH). A DPH represents the average dose volume histogram (DVH) for all systematic deviations in the patient group. The consequences of random deviations can be calculated by convolution of the dose distribution with the probability distributions for random deviations. Using the convolved dose matrix in the DPH calculation yields full information about the influence of geometrical uncertainties on the dose in the CTV. RESULTS: The model is demonstrated to be fast and accurate for a prostate, cervix, and lung cancer case. A CTV-to-PTV margin size which ensures at least 95% dose to (on average) 99% of the CTV, appears to be equal to about 2Sigma + 0.7sigma for three all cases. Because rotational deviations are included, the resulting margins can be anisotropic, as shown for the prostate cancer case. CONCLUSION: A method has been developed for calculation of CTV-to-PTV margins based on the assumption that the CTV should be adequately irradiated with a high probability.

Accurate portal dose measurement with a fluoroscopic electronic portal imaging device (EPID) for open and wedged beams and dynamic multileaf collimation.

Measuring portal dose with an electronic portal imaging device (EPID) in external beam radiotherapy can be used to perform routine dosimetric quality control checks on linear accelerators and to verify treatments (in vivo dosimetry). An accurate method to measure portal dose images (PDIs) with a commercially available fluoroscopic EPID has been developed. The method accounts for (i) the optical 'cross talk' within the EPID structure, (ii) the spatially nonuniform EPID response and (iii) the nonlinearity of the EPID response. The method is based on a deconvolution algorithm. Measurement of the required input data is straightforward. The observed nonlinearity of the EPID response was largely due to the somewhat outdated EPID electronics. Nonlinearity corrections for more modern systems are expected to be smaller. The accuracy of the method was assessed by comparing PDIs measured with the EPID with PDIs measured with a scanning ionization chamber in a miniphantom, located at the same position as the fluorescent screen. For irradiations in open, wedged and intensity modulated 25 MV photon beams (produced with dynamic multileaf collimation) EPID and ionization chamber measurements agreed to within 1% (1 SD).

Multiple two-dimensional versus three-dimensional PTV definition in treatment planning for conformal radiotherapy.

PURPOSE: To demonstrate the need for a fully three-dimensional (3D) computerized expansion of the gross tumour volume (GTV) or clinical target volume (CTV), as delineated by the radiation oncologist on CT slices, to obtain the proper planning target volume (PTV) for treatment planning according to the ICRU-50 recommendations. MATERIALS AND METHODS: For 10 prostate cancer patients two PTVs have been determined by expansion of the GTV with a 1.5 cm margin, i.e. a 3D PTV and a multiple 2D PTV. The former was obtained by automatically adding the margin while accounting in 3D for GTV contour differences in neighbouring slices. The latter was generated by automatically adding the 1.5 cm margin to the GTV in each CT slice separately; the resulting PTV is a computer simulation of the PTV that a radiation oncologist would obtain with (the still common) manual contouring in CT slices. For each patient the two PTVs were compared to assess the deviations of the multiple 2D PTV from the 3D PTV. For both PTVs conformal plans were designed using a three-field technique with fixed block margins. For each patient dose-volume histograms and tumour control probabilities (TCPs) of the (correct) 3D PTV were calculated, both for the plan designed for this PTV and for the treatment plan based on the (deviating) 2D PTV. RESULTS: Depending on the shape of the GTV, multiple 2D PTV generation could locally result in a 1 cm underestimation of the GTV-to-PTV margin. The deviations occurred predominantly in the cranio-caudal direction at locations where the GTV contour shape varies significantly from slice to slice. This could lead to serious underdosage and to a TCP decrease of up to 15%. CONCLUSIONS: A full 3D GTV-to-PTV expansion should be applied in conformal radiotherapy to avoid underdosage.

Portal dose image (PDI) prediction for dosimetric treatment verification in radiotherapy. I. An algorithm for open beams.

A method is presented for calculation of transmission functions for high energy photon beams through patients. These functions are being used in our clinic for prediction of portal dose images (PDIs) which are compared with PDIs measured with an electronic portal imaging device (EPID). The calculations are based on the planning CT-scan of the patient and on the irradiation geometry as determined in the treatment planning process. For each beam quality, the required input data for the algorithm for transmission prediction are derived from a limited number of measured beam data. The method has been tested for a PDI-plane at 160 cm from the focus, in agreement with the fixed focus-to-detector distance of our fluoroscopic EPIDs. For 6, 23 and 25 MV photon beams good agreement (approximately 1%) has been found between calculated and measured transmissions through anthropomorphic phantoms.