Prescription, precision, and decision in treatment planning.

The goal of radiotherapy is to eradicate a tumor without causing severe damage to healthy tissues. Various published experiences have lead to the conclusion that an overall precision of +/- 5% on the absorbed doses, at any point in the patient, is required to meet this goal. Clear definitions of the method for specifying the absorbed dose and dose homogeneity throughout the target volume are essential, to facilitate communication, to improve the knowledge of dose-effect relations and to establish the necessary criteria for the optimization of treatment plans. Determination of the optimal energy is one of the most controversial problems in treatment plan optimization. It is clearly related to the criteria selected. A brief review of some criteria is proposed according to the tumor site. Computers may provide three-dimensional dose calculations for treatment conditions for which manual calculations are not feasible. Because the random errors are very small, computer calculations are often considered as exact although large risks of error are associated with each step of the calculation. The reduction of the overall uncertainty to the stated level of +/- 5% requires a constant effort from both radiotherapists and physicists at each step of treatment planning from basic definitions to dose distribution calculations.

Importance of in vivo dosimetry as part of a quality assurance program in tangential breast treatments.

PURPOSE: The investigation of the accuracy and reproducibility in the daily dose delivered in tangential breast treatments with in vivo dose measurements. METHODS AND MATERIALS: In vivo dose measurements performed on the tangential treatment fields of 35 breast cancer patients are analysed for three units: a 6 MV linear accelerator, an old Cobalt unit and a new Cobalt unit. The results are plotted in frequency distributions. Deviations on the mean are often the expression of a systematic error in one of the core procedures of a department. A large spread of the results around the mean indicates a high burden of random set-up errors and/or systematic errors in individual patients. The reproducibility in dose delivery is studied by comparing repetitive checks to their respective mean for investigation of random day-to-day variation. RESULTS: A small systematic error on the entrance dose (+ 1.4%) is detected on the old Cobalt unit due to a discrepancy between measured and published percentage depth dose values. An unexpected systematic overdosage (+ 6%) is detected after implementation of a new software for dose calculation, proving that treatment quality is a process needing continuous monitoring. The transmission measurements demonstrate a systematic error in dose delivery of 1.5 to 3% due to the assumption that the breast is water equivalent when calculating the dose. The large spread of the transmission measurements (sa = 7.7%) shows that the weakest point in the treatment preparation chain is inaccurate acquisition of external body contours, leading to systematic errors in dose delivery for specific patients. The standard deviation for the reproducibility is 3.1% for the old Cobalt unit, vs. 1.6% on the other units, demonstrating the influence of staffing and mechanical characteristics of the units on daily precision in dose delivery. CONCLUSION: In vivo dosimetry is an important tool in a departmental quality assurance program to detect systematic errors in dose delivery, to identify inadequate treatment situations, to investigate weak points in the chain of treatment preparation and to ensure accurate dose delivery for individual patients. The predictive value of a single check for the accuracy in dose delivery during the whole treatment series is high for reproducible treatment methodologies.

Can we compare systems for interstitial therapy?

Three main dosimetry systems are used for interstitial brachytherapy: Manchester, Quimby and Paris Systems. A comparison is made of the rules of source distributions recommended by the three systems stating the advantages and disadvantages of each system. The comparison shows up the differences in the size of the treated volumes for similar implanted volumes. The next sections emphasize the differences in dose specification which make very difficult any comparison of clinical results obtained with different systems. Furthermore, the recent development of computers has lead some radiotherapists to specify the dose individually for each patient without reference to any published system. Very large variations in the dose delivered to a patient for a given implant and for the same prescribed dose, may results from this subjective procedure. In the last section one considers the various quality criteria proposed in the literature.

Dosimetry for total body irradiation.

Basic dosimetry as well as patient dosimetry are considered. The following items concerning the basic beam dosimetry are discussed: dosimetry calibration, phantom material, beam quality and depth dose measurements in TBI conditions. Dose to the patient should be specified to the midplane of the abdomen but dose to the lung should be estimated for each patient. In vivo, dosimetry is strongly recommended for determination of dose homogeneity, as well as to check patient position, reproducibility of treatment and instabilities in dose rate during TBI. Many physical problems are associated with in vivo dosimetry. All influences on the detector response have to be considered and the detectors must be calibrated for TBI conditions.

An approach to the interpretation of clinical data on the tumour control probability-dose relationship.

A conventional statistical model allows predicting the sterilisation rate as a function of dose. However, the computation requires data on biological parameters (proportion of clonogenic cells, survival per fraction, multiplication rate) which are inaccessible for human tumours. The curative dose 50% (TCD50) can be used as a synthesis of these parameters and its significance for the response-dose relationship of a population of tumours of uniform radiosensitivity is discussed. The slope of the dose-control curve provides vital information regarding the variation in radiocurability of the various individual tumours. The model allows the analysis of the clinical data and the separation of tumour subsets with different radiation responsiveness. It provides an evaluation of the benefit which could be obtained from the identification of the subsets and a guidance for the clinical, pathological and biological studies which relate to this identification. The change in the response-dose relationships with the tumour size cannot (usually) be explained by the cell number increase alone. Other possible factors of reduced radiocurability are discussed.

Characteristics of contamination electrons in high energy photon beams.

A simple method to estimate the contribution of contaminating electrons to the dose, and to evaluate their dosimetric characteristics is proposed. The method is based on a normalisation of the tissue--maximum ratio curves to a constant primary photon fluence. The contribution of the contaminating electrons to the dose is calculated by subtracting the dose relative to a small field from the dose relative to the field under consideration. The method includes the determination of the mean energy, the linear apparent attenuation coefficient, the 50% range and the maximum range of the contaminating electrons. The extrapolated surface dose normalised to a constant primary photon fluence has been found to be constant for a constant collimator opening whatever may be the source distance.

On the use of a quality index to specify high energy photon beams.

It is important to specify the beam quality in a simple and nonambiguous way in order on one hand to make comparisons easier between treatments performed in various hospitals, or at different times in the same hospital and on the other hand to facilitate the choice of numerical values for factors like restricted mass-collision stopping-power ratios and perturbation correction factors used in the conversion of ionization measurements into absorbed dose. We have adopted for high-energy photon beam specification a quality index (I) defined by the ratio (I20/I10) of ionizations measured with a constant source-detector distance for a reference field size 10 X 10 cm2. We have found that this quality index is independent of the source detector distance. On the other hand, the apparent linear attenuation coefficient measured on the exponential part of the tissue-maximum ratio curve can be calculated for any field size from the value of I for most high energy photon beams. In order to check the validity of the quality index for other linacs from other manufacturers, we have compared our results to published data related to various photon beams in a wide energy range: 2.5 to 45 MV.

Tangential breast irradiation: influence of technique of set-up on transfer errors and reproducibility.

Using conventional portal films, the influence of the technique of set-up on the transfer error from simulator to treatment couch and on the subsequent reproducibility was made for the irradiated volume in the treatment of breast cancer. A total number of 376 portal films have been performed on 14 patients. All the patients were treated on a 6 MV Linac supplied with an automatic verification system excluding, however, the couch parameters. The overall precision of the treatment delivery is evaluated by the global analysis of the discrepancies between the simulator films and the different portal films. For the patient group lying on a inclined plane with (group 2) or without (group 1) fixed arm support, a narrow gaussian distribution is obtained in the anteroposterior (AP) direction with a SD of 4 mm. In the craniocaudal (CC) direction, the frequency of distribution of the patients treated without fixed arm support is much larger than in the other group: the SD is respectively 15.5 mm for the first and 5.5 mm for the second one. The reproducibility of the series of set-ups of the two groups estimated by reference to the mean value are similar in the AP direction. The SD within the series of portal films in the CC direction is 5.8 mm for the first group and 3.7 mm for the second group. Their comparison with the discrepancies of 15.5 mm and 5.5 mm when assessing the deviation between the simulated and portal films shows the importance of systematic errors. The localisation of the irradiation port can be improved by using support systems with fixed arm indicators.

A model for calculating the costs of in vivo dosimetry and portal imaging in radiotherapy departments.

The costs of in vivo dosimetry and portal imaging in radiotherapy are estimated, on the basis of a detailed overview of the activities involved in both quality assurance techniques. These activities require the availability of equipment, the use of material and workload. The cost calculations allow to conclude that for most departments in vivo dosimetry with diodes will be a cheaper alternative than in vivo dosimetry with TLD-meters. Whether TLD measurements can be performed cheaper with an automatic reader (with a higher equipment cost, but lower workload) or with a semi-automatic reader (lower equipment cost, but higher workload), depends on the number of checks in the department. LSP-systems (with a very high equipment cost) as well as on-line imaging systems will be cheaper portal imaging techniques than conventional port films (with high material costs) for large departments, or for smaller departments that perform frequent volume checks.

Black and white in accuracy assessment of megavoltage images: the medical decision is often grey.

Using different criteria for acceptance of the portal film taken at the first treatment session, a comparison was made of the relevance of the information obtained from such a single check for the subsequent irradiations. A total number of 234 verification films have been taken on 29 fields for 27 head and neck patients. Patients were immobilised with individual plastic masks fixed to the couch and treated on a 6-MV linac fitted with an automatic verification system. Field alignment was checked with a measurement in the anteroposterior (AP) and craniocaudal (CC) direction on each film. Referring to the simulated field, this group of patients was treated with excellent average precision (mean, -0.7 mm) and reasonable spread (s = 5 mm). The percentage of 'large' deviations (> or = 6 mm) occurring during the whole treatment course is proportional to the upper limit of deviation accepted in the first assessed field (for an upper limit < 6 mm): it goes progressively up from 5% (AP-CC direction) to 17% (AP) and 13% (CC) for accepted magnitudes of deviation going from 2 mm to 6 mm in the first film. As the reproducibility of the different treatment series (s = 2 mm) is independent of the upper level of error accepted on the first film, this means that errors are mainly systematic errors coming from the transfer of the simulation unit to the treatment unit. Precision in a series of set-ups is always expressed by a Gaussian curve.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between temperature and dose rate dependence of semiconductor response; influence of accumulated dose.

The temperature and dose per pulse dependence of 20 Therados semiconductor detectors have been investigated. The rise in relative response observed when increasing temperature from 20 degree C to 32 degree C ranges between 1.00 and 1.07, that observed when increasing dose per pulse from 9.8 x 10(-6) to 6.5 x 10(-4) Gy/pulse between 1.01 and 1.11. The diodes with the most pronounced dose per pulse dependence have also the most pronounced temperature dependence. Accumulated dose has been found to increase both temperature and dose per pulse dependence until a level off dose (less than 4 kGy for one of the diodes) is reached. However, no level off has been observed concerning sensitivity decrease with dose: after an accumulated dose of 29 kGy a sensitivity decrease of 65% was still observed when giving another 25 kGy to one of the diodes.

Late effects of total body irradiation in correlation with physical parameters.

Clear and complete documentation of the physical parameters of total body irradiation (TBI) is one of the essential requirements for the evaluation and improvement of the clinical results of TBI. Concerning the dosimetric aspects of TBI, a number of recommendations have been formulated with emphasis on basic dosimetry, patient dosimetry and dose specification. The dosimeters should be calibrated regularly with reference to the absorbed dose in water. Depth dose measurements should be performed in water equivalent phantoms of specified dimensions. It has been strongly suggested to measure the absorbed dose at the surface of the patient at 8 different regions at the entry and exit of the beam under TBI conditions. The reference dose to the patient should be specified as the total dose to mid abdomen at the height of the umbilicus. As an independent parameter, the lung dose should be specified as the mean dose in the central region of the shielded part of both lungs. Recent, more complete, information on the physical and dosimetric aspects of TBI will be incorporated in the registry of the European Bone Marrow Transplant Group (EBMT). A cooperation has been established between the EBMT and the European Late Effects Project Group (EULEP) to study the development of late effects in man caused by ionising radiation.

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

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

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

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

A comparison of build-up and depth dose characteristics of different photon beams for the treatment of Hodgkin's disease.

Measurements have been performed of build-up and depth-dose characteristics of photon beams under Hodgkin's disease treatment conditions as applied in two hospitals (AVL, Amsterdam and IGR, Villejuif). Although different types of accelerators, photon energies, field sizes and SSD are employed, similar dose distributions along the beam axis have been obtained in both centers. In order to explain this unexpected good agreement, the influence of the geometrical conditions of irradiation on the build-up and depth-dose distribution has been studied in detail for five photon beams (8 MV-25 MV) of three types of accelerators.

Preliminary results of a quality assurance network for radiotherapy centres in Europe.

Based on the IAEA/WHO experience in mailed dosimetry, a Quality Assurance (QA) Network, sponsored by the EC committee "Europe Against Cancer", has been set up in 1991 for European centres, not involved in clinical research. Besides a survey of radiotherapy infrastructure, the project includes three measurement steps: primarily a check of beam output and quality in reference conditions with a mailed TLD-procedure, in a second step the mailed verification of other beam data and dose calculation procedures with a multipurpose phantom and finally in vivo dosimetry at the individual patient levels with mailed dosimeters. The results of infrastructure show good agreement with the data previously published by EORTC for research centres. Until January 1993, a total number of 37 centres (49 X-ray beams and 28 cobalt beams) from Belgium, France, Italy, The Netherlands, Sweden and The Czech and Slovak Republics have participated in the first measurement step. Deviations > 3% and < 6% from the stated dose are found in 17 out of the 79 checked beams. A deviation < 6% from the stated dose is detected in only 1 out of the 28 cobalt beams, but in 7 of the 49 X-ray beams. The large majority of the beams (23/25) with deviations > 3% belong to centres not having participated in external audits in the previous 5 years. A pilot study for the second step shows larger deviations in non reference conditions, especially for off-axis points and for oblique phantom surface. The third step has yet to be implemented.(ABSTRACT TRUNCATED AT 250 WORDS)

Quality control of radiotherapy centres in Europe: beam calibration.

A European Quality Assurance Network for external radiotherapy was set up. In this, several countries participated on a voluntary basis to elaborate common protocols, using joint infrastructure. The support for the co-ordination of the project was given by the EC committee 'Europe Against Cancer'. In the first step of the programme, beam outputs and beam qualities are checked with mailed thermoluminescent dosimeters following the protocol prepared by the IAEA. The results concerning 125 beams from 66 centres are analysed. Twenty-two beams presented minor deviations (3-6%) and 15 beams (4/48 60Co beams and 11/77 X-ray beams) from 11 centres presented major deviations (> or = 6%). The analysis shows that 16/22 minor deviations and all major deviations have been detected in centres which have not benefited from an external check during the last 5 years. In 14 out of 15 large deviations, the measured dose was smaller than the stated dose. In most centres with major deviation the physicists did not have the necessary experience and did not regularly calibrate the beams. In 6 out of 11 centres there was no dosimeter or the dosimeter available had not been calibrated recently. In 3 centres, the physicist did not give any explanation.

Quality assurance in radiotherapy by in vivo dosimetry. 1. Entrance dose measurements, a reliable procedure.

Entrance dose measurements were performed with semiconductor detectors on patients treated for head and neck and brain tumors with a 6 MV X-ray beam. A total number of 554 treatment set-ups were measured. The results showed a gaussian distribution with a mean value of 97.8% and a standard deviation of 2.8%. A systematic error of 2.2% on the mean value was shown to be due to a systematic deficiency in the algorithm used in the planning system and to a systematic error in the application of the dosimetry protocol. Two treatment techniques were identified leading to an erroneous dose delivery. Finally, large deviations (more than 2 S.D.) of the measured dose from the expected dose were detected in 3% of the measured treatment set-ups, the sources of the errors could in all cases be identified and eliminated in the further treatment sessions. This study demonstrated the reliability of the use of semiconductor detectors for in vivo dosimetry and its usefulness as part of a departmental quality assurance program.

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

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

Influence of shielding blocks on the output of photon beams as a function of energy and type of treatment unit.

The influence of field-defining shielding blocks on the output of a cobalt unit and of seven different accelerators (one with dual energy output) has been investigated. The quality indices range from 0.57 (cobalt-60) to 0.79. The loss in output due to shielding blocks has been calculated taking into account loss in phantom scatter only. Comparison with experimental results shows that the calculation algorithm is correct in most of the clinical conditions. However, for quality indices of 0.70 and higher, for blocks close to the central beam axis, an overestimation of the output by the algorithm has been found. The maximum deviation observed is about 5% for the highest energy and for block positions corresponding to those applied, e.g. for inverted Y-fields with narrow lumbo-aortic block spacing.