Marion Essers

In vivo dosimetry during external photon beam radiotherapy

In this critical review of the current practice of patient dose verification, we first demonstrate that a high accuracy (about 1-2%, 1 SD) can be obtained. Accurate in vivo dosimetry is possible if diodes and thermoluminescence dosimeters (TLDs), the main detector types in use for in vivo dosimetry, are carefully calibrated and the factors influencing their sensitivity are taken into account. Various methods and philosophies for applying patient dose verification are then evaluated: the measurement of each field for each fraction of each patient, a limited number of checks for all patients, or measurements of specific patient groups, for example, during total body irradiation (TBI) or conformal radiotherapy. The experience of a number of centers is then presented, providing information on the various types of errors detected by in vivo dosimetry, including their frequency and magnitude. From the results of recent studies it can be concluded that in centers having modern equipment with verification systems as well as comprehensive quality assurance (QA) programs, a systematic error larger than 5% in dose delivery is still present for 0.5-1% of the patient treatments. In other studies, a frequency of 3-10% of errors was observed for specific patient groups or when no verification system was present at the accelerator. These results were balanced against the additional manpower and other resources required for such a QA program. It could be concluded that patient dose verification should be an essential part of a QA program in a radiotherapy department, and plays a complementary role to treatment-sheet double checking. As the radiotherapy community makes the transition from the conventional two-dimensional (2D) to three-dimensional (3D) conformal and intensity modulated dose delivery, it is recommended that new treatment techniques be checked systematically for a few patients, and to perform in vivo dosimetry a few times for each patient for situations where errors in dose delivery should be minimized.

Transmission dosimetry with a liquid-filled electronic portal imaging device

PURPOSEnTo assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data.nnnMETHODS AND MATERIALSnTransmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled ionization chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled ionization chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied.nnnRESULTSnThe increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid ionization chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm.nnnCONCLUSIONSnThe difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.

NEW METHOD TO OBTAIN THE MIDPLANE DOSE USING PORTAL IN VIVO DOSIMETRY

PURPOSEnThe 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.nnnMETHODS AND MATERIALSnThe 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.nnnRESULTSnOur 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.nnnCONCLUSIONSnUsing 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.

Dosimetric characteristics of a liquid-filled electronic portal imaging device

PURPOSEnTo determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled ionization chambers, for transmission dose measurements during patient treatment.nnnMETHODS AND MATERIALSnElectronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type ionization chamber in air in a miniphantom and in an extended water phantom.nnnRESULTSnThe warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the ionization chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the ionization chambers is small. The sensitivity increases about 0.5% for all ionization chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the ionization chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID ionization chambers were between those of an ionization chamber in a miniphantom and in a water phantom.nnnCONCLUSIONnThe matrix ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.

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

PURPOSEnTo identify thoracic structures that exhibit little internal motion during irradiation and to determine setup variations in patients with lung cancer.nnnMETHODS AND MATERIALSnIntrafractional 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.nnnRESULTSnMobility 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.nnnCONCLUSIONSnWe 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.

Commissioning of a commercially available system for intensity-modulated radiotherapy dose delivery with dynamic multileaf collimation

PURPOSEnTo commission commercially available equipment for intensity-modulated radiotherapy (IMRT) using dynamic multileaf collimation (DMLC).nnnMATERIALS AND METHODSnFirst, the stability in leaf positioning and in realized IMRT profiles on a Varian 2300 C/D machine were determined as a function of time and gantry angle, and as a result of treatment interrupts. Second, dose distributions calculated with the CadPlan (Varian) treatment planning system, using leaf trajectories calculated with the leaf motion calculator (LMC) algorithm, were compared with distributions realized at the 2300 C/D unit.nnnRESULTSnDay-to-day and gantry angle variations in leaf positioning and dose delivery were very small (less than 0.1-0.2 mm and 2%). The effect of treatment interrupts on measured dose distributions was less than 2%. The agreement between the final dose distribution calculated by CadPlan and the measured dose was generally within 2%, or 2 mm at steep dose gradients, using a leaf transmission value of 1.8% and a leaf separation value of 2 mm in LMC. For narrow peaks, deviations of up to 6% were observed. LMC does not synchronize adjacent leaf trajectories resulting in tongue-and-groove underdosages of up to 29% for extreme cases.nnnCONCLUSIONSnThe 2300 C/D machine is suitable for accurate and reproducible DMLC treatments. The agreement between dose predictions with LMC and CadPlan, and realized doses at this unit is clinically acceptable for most cases. However, differences between calculated and actual dose values may exist in peaked fluences or due to tongue-and-groove effects. Therefore, pretreatment dosimetric verification for each patient is recommended.

In vivo dosimetry during conformal therapy of prostatic cancer

In vivo dose measurements were performed during the simultaneous boost technique for prostatic cancer to check the accuracy of dose calculations by a monitor unit calculation program and a three-dimensional planning system. The dose of the large field and the boost field are given simultaneously using customized 10 mm thick Roses-metal blocks in which the boost field is cut out. Following the procedure of the quality assurance protocol for this technique, the dose at the specification point has been determined by in vivo dosimetry. The measured dose was initially too high for 5 out of 16 patients, due to unexpected differences in two beams with the same nominal beam quality and a different density correction for the femoral heads; the monitor unit calculation program was therefore checked and improved. The dose at the specification point was also compared with calculations performed by a CT-based three-dimensional (3-D) planning system. The average deviation of the 3-D planning system from the measurements is 0.1% +/- 1.2%. Entrance, midline and exit dose values in the central axial plane, in a cranial plane and in a plane under the transmission block have also been compared with calculations performed by the 3-D treatment planning system. The measured entrance dose is, on average, 3.4% higher than the calculated dose for the AP beam and up to 5.5% for the lateral beams. Phantom measurements were performed and showed that these differences were not related to patient set-up errors.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo dosimetry during conformal radiotherapy: Requirements for and findings of a routine procedure

PURPOSEnConformal radiotherapy requires accurate knowledge of the actual dose delivered to a patient. The impact of routine in vivo dosimetry, including its special requirements, clinical findings and resources, has been analysed for three conformal treatment techniques to evaluate its usefulness in daily clinical practice.nnnMATERIALS AND METHODSnBased on pilot studies, routine in vivo dosimetry quality control (QC) protocols were implemented in the clinic. Entrance and exit diode dose measurements have been performed during two treatment sessions for 378 patients having prostate, bladder and parotid gland tumours. Dose calculations were performed with a CT-based three-dimensional treatment planning system. In our QC-protocol we applied action levels of 2.5% for the prostate and bladder tumour group and 4.0% for the parotid gland patients. When the difference between the measured dose at the dose specification point and the prescribed dose exceeded the action level the deviation was investigated and the number of monitor units (MUs) adjusted. Since an accurate dose measurement was necessary, some properties of the on-line high-precision diode measurement system and the long-term change in sensitivity of the diodes were investigated in detail.nnnRESULTSnThe sensitivity of all diodes decreased by approximately 7% after receiving an integrated dose of 10 kGy, for 4 and 8 MV beams. For 34 (9%) patients the difference between the measured and calculated dose was larger than the action level. Systematic errors in the use of a new software release of the monitor unit calculation program, limitations of the dose calculation algorithms, errors in the planning procedure and instability in the performance of the accelerator have been detected.nnnCONCLUSIONSnAccurate in vivo dosimetry, using a diode measurement system, is a powerful tool to trace dosimetric errors during conformal radiotherapy in the range of 2.5-10%, provided that the system is carefully calibrated. The implementation of an intensive in vivo dosimetry programme requires additional staff for measurements and evaluation. The patient measurements add only a few minutes to the total treatment time per patient and guarantee an accurate dose delivery, which is a prerequisite for conformal radiotherapy.

The accuracy of CT-based inhomogeneity corrections and in vivo dosimetry for the treatment of lung cancer

PURPOSEnTo determine the accuracy of dose calculations based on CT-densities for lung cancer patients irradiated with an anterio posterior parallel-opposed treatment technique and to evaluate, for this technique, the use of diodes and an Electronic Portal Imaging Device (EPID) for absolute exit dose and relative transmission dose verification, respectively.nnnMATERIALS AND METHODSnDose calculations were performed using a 3-dimensional treatment planning system, using CT-densities or assuming the patient to be water-equivalent. A simple inhomogeneity correction model was used to take CT-densities into account. For 22 patients, entrance and exit dose calculations at the central beam axis and at several off-axis positions were compared with diode measurements. For 12 patients, diode exit dose measurements and exit dose calculations were compared with EPID transmission dose values.nnnRESULTSnUsing water-equivalent calculations, the actual exit dose value under lung was, on average, underestimated by 30%, with an overall spread of 10% (1 SD) in the ratio of measurement and calculation. Using inhomogeneity corrections, the exit dose was, on average, overestimated by 4%, with an overall spread of 6% (1 SD). Only 2% of the average deviation was due to the inhomogeneity correction model. The other 2% resulted from a small inaccuracy in beam fit parameters and the fact that lack of backscatter is not taken into account by the calculation model. Organ motion, resulting from the ventilatory or cardiac cycle, caused an estimated uncertainty in calculated exit dose of 2.5% (1 SD). The most important reason for the large overall spread was, however, the inaccuracy involved in point measurements, of about 4% (1 SD), which resulted from the systematic and random deviation in patient set-up and therefore in the diode position with respect to patient anatomy. Transmission and exit dose values agreed with an average difference of 1.1%. Transmission dose profiles also showed good agreement with calculated exit dose profiles.nnnCONCLUSIONSnThe study shows that, for this treatment technique, the dose in the thorax region is quite accurately predicted using CT-based dose calculations and a simple heterogeneity correction model. Point detectors such as diodes are not suitable for exit dose verification in regions with inhomogeneities. The EPID has the advantage that the dose can be measured in the entire irradiation field, thus allowing an accurate verification of the dose delivered to regions with large dose gradients.

Dosimetric control of conformal treatment of parotid gland tumours

The purpose of this study was to determine the dosimetric accuracy of the treatment of parotid gland tumours using 8 MV X-ray beams. These tumours are generally situated near the patients skin. Entrance in vivo dose measurements with diodes were obtained for 20 patients during 5 sessions per patient, in the anterior-oblique and posterior-oblique wedged fields, on the central beam axis as well as in points situated in a cranial plane 2 or 3 cm off-axis. Phantom measurements were performed in order to determine the actual position of the 95% isodose surface. The measurements were compared with calculations performed with our three-dimensional treatment planning system. The reproducibility of the diode measurements on patients was found to be 1.4% (1 SD). The total accuracy in the entrance dose determination for the average of 2 measurements was 1.8% (1 SD). The central axis entrance dose for the anterior field was on average 1.5% +/- 3.2% (1 SD) higher than the calculated value. For the posterior field, the difference was 0.9% +/- 3.1% (1 SD). The deviations for the off-axis points were somewhat smaller, mainly due to overestimation of the block effect in the calculations. The value of the dose at the isocentre obtained by extrapolation of the measured entrance dose values, differed 0.3% +/- 2.1% from the calculations. The accuracy in dose determination at the isocentre was 2% (1 SD). After correction for the difference in prescribed and actual source-to-skin distance, the results showed good agreement with phantom measurements on a polystyrene phantom without inhomogeneities, performed both with diodes and an ionization chamber.(ABSTRACT TRUNCATED AT 250 WORDS)