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

PURPOSE: Conformal 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. MATERIALS AND METHODS: Based 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. RESULTS: The 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. CONCLUSIONS: Accurate 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.

Differences in wedge factor determination in air using a PMMA mini-phantom or a brass build-up cap.

The head scatter dose contribution to the output of a treatment machine has been determined for an open and wedged 60Co gamma-ray beam and for open and wedged x-ray beams of 4, 8, and 16 MV. From those data wedge factor values "in air" have been deduced, expressed as the ratio of the dose to water, measured in air, for the situation with and without wedge, for the same number of monitor units (or treatment time for 60Co). The measurements have been performed using a polymethyl-metacrylate (PMMA) and a graphite-walled ionization chamber inserted in a brass build-up cap and in a PMMA mini-phantom, respectively. Absolute wedge factor values deduced with both detector systems and based on the ratio of ionization chamber readings, differ for the investigated photon beams, up to 3.5% for the 4 MV x-ray beam. The deviations results from the difference in composition between the detector materials and water and can be taken into account by conversion of the ionization chamber readings for both the open and wedged photon beams to the absorbed dose to water. For the brass build-up cap detector system the ratio of the conversion factors for the wedged and open beam changes the ratio of the ionization chamber readings up to about 3.6% for the 4 MV x-ray beam. For the mini-phantom the conversion factors for the wedged and open beam are almost equal for all photon beams. Consequently, for that system wedge factors based on ionization chamber readings or dose values are the same. With respect to the wedge factor variation with field size a somewhat larger increase has been determined for the 60Co and 4 MV photon beam using the brass build-up cap: about 1% for field sizes varying between 5 cm x 5 cm and 15 cm x 15 cm. This effect has to be related to an apparent more pronounced variation of the head scatter dose contribution with field size for the wedged photon beams if the brass build-up cap detection system is used. It can be concluded that determination of wedge factors "in air" under reference irradiation conditions, performed with both the mini-phantom and brass build-up cap yields within 0.5% the same result if the wedge factors are based on a dose to water ratio. However, by using high-Z build-up materials the determination is more complicated because appropriate conversion factors are then required, while similar conversion factors can be ignored if more water equivalent build-up materials such as PMMA are applied.

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

PURPOSE: To 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. MATERIALS AND METHODS: Dose 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. RESULTS: Using 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. CONCLUSIONS: The 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 patient's 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)

Wedge factor constituents of high-energy photon beams: head and phantom scatter dose components.

The head and phantom scatter contribution to the output of a treatment machine have been determined for open and wedged 60Co gamma-ray beams and 4, 8, 16 and 25 MV X-ray beams, using an extended and a small-sized phantom. The wedge factor variation with field size and phantom depth have been analysed as a function of both scatter components. For the wedged beams a stronger increase of the head scatter contribution with field size, i.e. 4-9% for field sizes increasing from 5 cm x 5 cm to 20 cm x 20 cm, has been observed compared with open beams. This result indicates that the wedge factor variation with field size is related to a change of the primary photon fluence. Our study shows that the ratio of the head and phantom scatter contribution for the wedged and open beams remains unchanged for all beams except the 4 and 25 MV X-ray beam. This implies that, except for these latter energies, the variation of the wedge factor with phantom depth is determined by the wedge-induced change of the primary photon energy fluence. For the 4 and 25 MV X-ray beam it is shown that the wedge factor is also influenced by a change of the phantom scatter contribution. The wedge factor for the 25 MV X-ray beam is strongly influenced by the electron contamination for phantom depths up to 6 cm. For the 60Co and the 4 MV photon beam it is shown that the wedge factor decreases slightly with increasing source-to-skin distance due to a reduced contribution to the total dose from photons scattered in the wedge. For clinical use, an algorithm is given to calculate the wedge factor variation with field size and phantom depth.

Wedge factor constituents of high energy photon beams: field size and depth dependence.

Wedge factors have been determined as a function of field size and phantom depth for a 60Co gamma-ray beam and X-ray beams in the range from 4 MV to 25 MV. The results show an increase of the wedge factor with field size, up to 9.1% for the 25 MV X-ray beam. The magnitude of this increase is a linear function of the product of that part of the irradiated wedge volume that can be observed from the point of measurement, its mass energy-absorption coefficient and mass density. This relationship is independent of the photon beam energy, the type of wedge material and the wedge angle. Differences in variation of the amount of backscatter to the monitor with field size for the open and wedged photon beam yielded only a minor influence, up to 0.7%. For the 4-16 MV X-ray beams the wedge factor increases linearly with phantom depth, almost independently of field size. For the 60Co gamma-ray beam and the 25 MV X-ray beam the wedge factor variation is a more complicated function of phantom depth for a particular field size.

Quality assurance of the simultaneous boost technique for prostatic cancer: dosimetric aspects.

Quality assurance (QA) in radiotherapy is of particular importance if a new irradiation technique is introduced. The dosimetric aspects of such a QA program concern the check of the dose calculation procedure, i.e. the prediction of the relative dose distribution, as well as the verification of the absolute value of the target absorbed dose specified at a particular point. In our institution a QA program has been developed for a new conformal irradiation technique of prostatic cancer: the simultaneous boost technique. With this technique the dose of the boost field and the large field are given simultaneously, using customized 10 mm thick Roses-metal plates in which the boost field has been cut out. The computation of the dose distribution, using a procedure adapted from a commercially available 2D treatment planning system, has been compared with isodose distributions measured in a water phantom. Good agreement, better than 3% or 3 mm, was observed for both open and wedged 8 MV X-ray beams. In vivo dose measurements have been performed on individual patients to check the dose delivery at the specification point. An agreement better than +/- 2% with the calculated dose value was required. The average ratio for 18 patients of the actual and expected dose value amounted to 1.005 +/- 0.017 (1 SD) after a correction of the number of monitor units for 2 patients during the treatment. Quality control of the dose transmission factor of the Roses-metal plates has been performed.(ABSTRACT TRUNCATED AT 250 WORDS)

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 pelvic treatment.

High precision in vivo entrance and exit dose measurements have been performed with p-type diodes on patients during 8 MV X-ray irradiation of the pelvis, to investigate the accuracy of dose calculations in this region. Based on phantom measurements the accuracy of the p-type diode measuring system itself, i.e. the agreement with ionisation chamber dose measurements, was shown to be better than 0.7% while the reproducibility in the dose determination was 1.1%, 1.5% and 1.6% (1 S.D.) at the entrance point, isocentre and exit point, respectively, for the wedged lateral fields. Patient movement and the uncertainty in the diode position increased these values to 1.7%, 1.5% and 3.1% (1 S.D.) for dose determinations on patients. From the entrance and exit in vivo dose values the dose actually delivered to the isocentre was determined. For the anterior-posterior beams a good correspondence for most patients was observed at the entrance and exit point and at the isocentre between the in vivo and calculated dose values. For the wedged lateral beams a systematic deviation of about 3% was observed. In addition to the in vivo dose measurements phantom dose measurements have been performed to quantify the accuracy of the dose calculation algorithms including the computation of the number of monitor units. These measurements also served to quantify the effects of the actual patient on the dose delivery. The measurements showed that accurate calculation of the dose requires a separation of the head and phantom scatter contribution of the output of the treatment machine. The dependence of the wedge factor on field size, depth and source-skin-distance has also to be considered for accurate dose calculations. The effect of the patient on the dose calculation is mainly related to the actual electron densities of fat and bone structures compared to water: neglecting these densities in the dose computation could yield deviations up to 8.5% for the exit point in wedged beams. Based on these results, improvements in the dose calculation algorithms and monitor unit calculation including the use of the actual electron densities will be implemented in the treatment planning procedure.

Should inhomogeneity corrections be applied during treatment planning of tangential breast irradiation?

Due to the inclusion of lung tissue in the treatment volume, some parts of the breast will get a higher dose during tangential breast irradiation because of the lower lung density. Data on the accuracy of dose calculation algorithms, investigated by phantom measurements, determinations of the geometry and density of the actual lung in the patient and the results of in vivo dose measurements, are presented. From this information it can be concluded that a lung correction varying between about 3% and 7% is needed but its magnitude is slightly overpredicted in a number of commercial treatment planning systems. Because this increase in dose is already in a high dose region, it is recommended that inhomogeneity corrections should be applied during tangential breast irradiation.

In vivo dosimetry during tangential breast treatment.

The 3-dimensional (3-D) dose distribution as calculated in clinical practice for tangential breast treatment was verified by means of in vivo dosimetry. Clinical practice in our institution implies the use of 8 MV X-ray beams, a 2-D treatment planning system, collimator rotation and a limited set of patient data for dose calculations. By positioning diodes at the central beam axes as well as in the periphery of the breast the magnitude of the dose values at the isocentre and in points situated in the high-dose regions behind the lung could be assessed. The position of the diodes was verified by means of an on-line portal imaging device. The reproducibility of these in vivo dose measurements was better than 2% (1 SD). Our study showed that on the average the dose delivery at the isocentre is 2% less and at the points behind the lung, 5.7% higher with respect to the calculated dose values. Detailed analysis of these in vivo dosimetry results, based on dose measurements performed with a breast shaped phantom, yielded the magnitudes of the errors in the predicted dose due to several limitations in the dose calculation algorithms and dose calculation procedure. These limitations are each introducing an error of several percent but are compensating each other for the dose calculation at the isocentre. We concluded that the dose distribution in a patient for our treatment technique and dose calculation procedure can be predicted with a 2-D treatment planning system in an acceptable way. A more accurate prediction of the dose distribution can be performed but requires an estimation of the lack of scatter due to missing tissue, the change in the dose distribution due to oblique incident beams and the incorporation of the actual output of the treatment machine in the assessment of the number of monitor units.

Accuracy in tangential breast treatment set-up: a portal imaging study.

To test the accuracy and reproducibility of the tangential breast treatment set-up used in The Netherlands Cancer Institute, a portal imaging study was performed in 12 patients treated for early stage breast cancer. With an on-line electronic portal imaging device (EPID) images were obtained of each patient in several fractions and compared with simulator films and with each other. In five patients, multiple images (on the average 7) per fraction were obtained to evaluate set-up variations due to respiratory movement. The central lung distance (CLD) and other set-up parameters varied within one fraction about 1 mm (1 SD). The average variation of these parameters between various fractions was about 2 mm (1 SD). The differences between simulator and treatment set-up over all patients and all fractions was on the average 2-3 mm for the central beam edge to skin distance and the central lung distance. It can be concluded that the tangential breast treatment set-up is very stable and reproducible and that respiration does not have a significant influence on treatment volume. The EPID appears to be an adequate tool for studies of treatment set-up accuracy like this.

Comparison of entrance and exit dose measurements using ionization chambers and silicon diodes.

A high precision patient dosimetry method has been developed, based on the use of p-type diodes. First, entrance as well as exit dose calibration factors have to be determined under reference irradiation conditions. Secondly, a set of correction factors must be added for situations deviating from the reference conditions, i.e. for different source-skin distances, phantom (patient) thicknesses, field sizes or for insertion of a wedge into the photon beam. Finally some other detector characteristics such as the temperature dependence of the response have to be taken into account. For most irradiation conditions this procedure is sufficiently accurate to allow entrance as well as exist dose determinations with a diode to be in good agreement with dose values measured by an ionization chamber. The main factors effecting the value of the correction factors, the dependence of the diode sensitivity on the energy and the dose per pulse, have been investigated to explain some of the observed phenomena. Despite a strong energy dependence of the sensitivity, the correction factors are, for a particular type of diode, the same for 4 and 8 MV x-ray beams. The variation in the values for the correction factors with integrated dose received by the diode is small. These findings indicate that the correction factors, once available, can be applied under a number of circumstances. Due to the difference in behaviour of various diodes, even from the same batch, it is, however, necessary to determine the characteristics for each diode individually.