The stability of liquid-filled matrix ionization chamber electronic portal imaging devices for dosimetry purposes.

This study was performed to determine the stability of liquid-filled matrix ionization chamber (LiFi-type) electronic portal imaging devices (EPID) for dosimetric purposes. The short- and long-term stability of the response was investigated, as well as the importance of factors influencing the response (e.g., temperature fluctuations, radiation damage, and the performance of the electronic hardware). It was shown that testing the performance of the electronic hardware as well as the short-term stability of the imagers may reveal the cause of a poor long-term stability of the imager response. In addition, the short-term stability was measured to verify the validity of the fitted dose-response curve immediately after beam startup. The long-term stability of these imagers could be considerably improved by correcting for room temperature fluctuations and gradual changes in response due to radiation damage. As a result, the reproducibility was better than 1% (1 SD) over a period of two years. The results of this study were used to formulate recommendations for a quality control program for portal dosimetry. The effect of such a program was assessed by comparing the results of portal dosimetry and in vivo dosimetry using diodes during the treatment of 31 prostate patients. The improvement of the results for portal dosimetry was consistent with the deviations observed with the reproducibility tests in that particular period. After a correction for the variation in response of the imager, the average difference between the measured and prescribed dose during the treatment of prostate patients was -0.7%+/-1.5% (1 SD), and -0.6%+/-1.1% (1 SD) for EPID and diode in vivo dosimetry, respectively. It can be concluded that a high stability of the response can be achieved for this type of EPID by applying a rigorous quality control program.

Impact of simple tissue inhomogeneity correction algorithms on conformal radiotherapy of lung tumours.

BACKGROUND AND PURPOSE: Conformal radiotherapy requires accurate dose calculation at the dose specification point, at other points in the planning target volume (PTV) and in organs at risk. To assess the limitations of treatment planning of lung tumours, errors in dose values, calculated by some simple tissue inhomogeneity correction algorithms available in a number of currently applied treatment planning systems, have been quantified. MATERIALS AND METHODS: Single multileaf collimator-shaped photon beams of 6, 8, 15 and 18 MV nominal energy were used to irradiate a 50 mm diameter spherical solid tumour, simulated by polystyrene, which was located centrally inside lung tissue, simulated by cork. The planned dose distribution was made conformal to the PTV, which was a 15 mm three-dimensional expansion of the tumour. Values of both the absolute dose at the International Commission on Radiation Units and Measurement (ICRU) reference point and relative dose distributions inside the PTV and in the lung were calculated using three inhomogeneity correction algorithms. The algorithms investigated in this study are the pencil beam algorithm with one-dimensional corrections, the modified Batho algorithm and the equivalent path length algorithm. The calculated data were compared with measurements for a simple beam set-up using radiographic film and ionization chambers. RESULTS: For this specific configuration, deviations of up to 3.5% between calculated and measured values of the dose at the ICRU reference point were found. Discrepancies between measured and calculated beam fringe values (distance between the 50 and 90% isodose lines) of up to 14 mm have been observed. The differences in beam fringe and penumbra width (20-80%) increase with increasing beam energy. Our results demonstrate that an underdosage of the PTV up to 20% may occur if calculated dose values are used for treatment planning. The three algorithms predict a considerably higher dose in the lung, both along the central beam axis and in the lateral direction, compared with the actual delivered dose values. CONCLUSIONS: The dose at the ICRU reference point of such a tumour in lung geometry is calculated with acceptable accuracy. Differences between calculated and measured dose distributions are primarily due to changes in electron transport in the lung, which are not adequately taken into account by the simple tissue inhomogeneity correction algorithms investigated in this study. Particularly for high photon beam energies, clinically unacceptable errors will be introduced in the choice of field sizes employed for conformal treatments, leading to underdosage of the PTV. In addition, the dose to the lung will be wrongly predicted which may influence the choice of the prescribed dose level in dose-escalation studies.

The effect of breathing and set-up errors on the cumulative dose to a lung tumor.

BACKGROUND AND PURPOSE: To assess the impact of both set-up errors and respiration-induced tumor motion on the cumulative dose delivered to a clinical target volume (CTV) in lung, for an irradiation based on current clinically applied field sizes. MATERIALS AND METHODS: A cork phantom, having a 50 mm spherically shaped polystyrene insertion to simulate a gross tumor volume (GTV) located centrally in a lung was irradiated with two parallel opposed beams. The planned 95% isodose surface was conformed to the planning target volume (PTV) using a multi leaf collimator. The resulting margin between the CTV and the field edge was 16 mm in beam's eye view. A dose of 70 Gy was prescribed. Dose area histograms (DAHs) of the central plane of the CTV (GTV+5 mm) were determined using radiographic film for different combinations of set-up errors and respiration-induced tumor motion. The DAHs were evaluated using the population averaged tumor control probability (TCP(pop)) and the equivalent uniform dose (EUD) model. RESULTS: Compared with dose volume histograms of the entire CTV, DAHs overestimate the impact of tumor motion on tumor control. Due to the choice of field sizes a large part of the PTV will receive a too low dose resulting in an EUD of the central plane of the CTV of 68.9 Gy for the static case. The EUD drops to 68.2, 66.1 and 51.1 Gy for systematic set-up errors of 5, 10 and 15 mm, respectively. For random set-up errors of 5, 10 and 15 mm (1 SD), the EUD decreases to 68.7, 67.4 and 64.9 Gy, respectively. For similar amplitudes of respiration-induced motion, the EUD decreases to 68.8, 68.5 and 67.7 Gy, respectively. For a clinically relevant scenario of 7.5 mm systematic set-up error, 3 mm random set-up error and 5 mm amplitude of breathing motion, the EUD is 66.7 Gy. This corresponds with a tumor control probability TCP(pop) of 41.7%, compared with 50.0% for homogeneous irradiation of the CTV to 70 Gy. CONCLUSION: Systematic set-up errors have a dominant effect on the cumulative dose to the CTV. The effect of breathing motion and random set-up errors is smaller. Therefore the gain of controlling breathing motion during irradiation is expected to be small and efforts should rather focus on minimizing systematic errors. For the current clinically applied field sizes and a clinically relevant combination of set-up errors and breathing motion, the EUD of the central plane of the CTV is reduced by 3.3 Gy, at maximum, relative to homogeneous irradiation of the CTV to 70 Gy, for our worst case scenario.

Accurate in vivo dosimetry of a randomized trial of prostate cancer irradiation.

PURPOSE: To guarantee an accurate dose delivery, within +/- 2.5%, in a Phase III randomized trial of prostate cancer irradiation (68 vs. 78 Gy) by means of a comprehensive in vivo dosimetry program. METHODS AND MATERIALS: Prostate patients are generally treated in our clinic with a 3-field isocentric technique: an 8-MV anteroposterior beam and 2 18-MV wedged laterals. All fields are shaped conformally to the PTV. Patients were randomized between two dose levels of 68 Gy and 78 Gy. During treatment, the entrance and exit dose were measured for each patient with diodes. Special 2.5-mm thick steel build-up caps were applied to make the diodes appropriate for measurements in 18-MV photon beams as well. Portal images were used to verify the correct position of the diodes and to detect and correct for gas filling in the rectum that may influence the exit dose reading. Entrance and exit dose measurements were converted to midplane dose, which was used in combination with a depth dose correction to obtain the dose at the specification point. An action level of 2.5% was applied. RESULTS: The added build-up for the diodes in the 18-MV beams resulted in correction factors that were only slightly sensitive to changes in beam setup and comparable to the corrections used in the 8-MV beams for diodes without extra build-up. The calibration factor increased almost linearly with cumulative dose: 0.7%/kGy for the 8-MV and 1.2%/kGy for the 18-MV photon beams. The introduction of average correction factors made the analysis easier, while keeping the accuracy within acceptable limits. In a period of 3 years, 225 patients were analyzed, from which 8 patients needed to be corrected. The average ratio of measured and prescribed dose was 1.009 (standard deviation [SD] 0.012) for the total group treated on two linear accelerators. When the results were analyzed per accelerator, the ratios were 1.002 (SD, 0.001) for Accelerator A and 1.015 (SD, 0.001) for Accelerator B. This difference could be attributed to the cumulative effect of three small imperfections in the performance of Accelerator B that were well within the limits of our quality assurance program. CONCLUSION: Diodes can be used for accurate in vivo dosimetry during prostate irradiation in high-energy photon beams. The dose delivery in this randomized trial is guaranteed within the 2.5% limits on an individual patient basis. This could not be achieved without the in vivo dosimetry program, despite our high-standard quality assurance program of treatment delivery.