Search

Purpose: Test and comparison of various 2-D real-time detectors for dosimetric quality assurance (QA) of intensity-modulated radiotherapy (IMRT) with the vision to replace radiographic films for 2-D dosimetry., Material and Methods: All IMRT treatment plans were created with the Konrad software (Siemens OCS). The final dose calculation was also carried out in Konrad. A Mevatron Primus (Siemens OCS) linear accelerator which provides 6-MV and 15-MV highenergy photon beams was used for the delivery of segmented multileaf-modulated IMRT. Three different 2-D detectors, each based on a different physical (interaction) principle, were tested for the field-related IMRT verification: (1) the MapCheck diode system (Sun Nuclear), (2) the I'mRT QA scintillation detector (Scanditronix/Wellhöfer), and the Seven29 ionization chamber array (PTW). The performance of these detector arrays was evaluated against IMRT dose distributions created and calculated with Konrad and the results obtained were compared with film measurements performed with radiographic films (EDR2, Kodak). Additionally, measurements were performed with point detectors, such as diamond, diodes (PTW) and ionization chambers (PTW, Scanditronics/ Wellhöfer) and radiochromic films (GafChromic film MD55, ISP)., Results: The results obtained with all three 2-D detector systems were in good agreement with calculations performed with the treatment-planning system and with the standard dosimetric tools, i.e., films or various point dose detectors. It could be shown that all three systems offer dosimetric characteristics required for performing field-related IMRT QA with relative dose measurements. The accuracy of the 2-D detectors was mostly +/- 3% normalized to dose maximum for a wide dynamic range. The maximum deviations did not exceed +/- 5% even in regions with a steep dose gradient. The main differences between the detector systems were the spatial resolution, the maximal field size, and the ability to perform absolute dosimetric measurements., Conclusion: Commercial 2-D detectors have the potential to replace films as an "area detector" for field-related verification of IMRT. The on-line information provided by the respective systems can even improve the efficiency of the QA procedures.

The dosimetric characteristics for modern computer-controlled linear accelerators with the same make, model, and nominal energy are known to be very similar, as long as the machines are unaltered from the manufacturer's original specifications. In this preliminary study, a quantitative investigation of the similarity in the basic photon dosimetry data from the Siemens Primus linear accelerators at eight different institutions is reported. The output factor, percentage depth dose (PDD), and in-air off-axis factor (OAF) for the 6 and 18 MV photon beams measured or verified by the Radiological Physics Center (RPC) were analyzed. The RPC-measured output factors varied by less than about 2% for each field size. The difference between the maximum and minimum RPC-verified PDD values at each depth was less than about 3%. The difference between the maximum and minimum RPC-measured in-air OAF was no more than 4% at all off-axis distances considered in this study. These results strongly suggest that it is feasible to establish a reference photon dosimetry data set for each make, model, and nominal energy, universally applicable to those machines unaltered from the manufacturers' original specifications, within a clinically acceptable tolerance (e.g., approximately +/-2%)., ((c) 2003 American College of Medical Physics.)

A new integrated CT/LINAC combination, in which the CT scanner is inside the radiation therapy treatment room and the same patient couch is used for CT scanning and treatment (after a 180-degree couch rotation), should allow for accurate correction of interfractional setup errors. The purpose of this study was to evaluate the sources of uncertainties, and to measure the overall precision of this system. The following sources of uncertainty were identified: (1) the patient couch position on the LINAC side after a rotation, (2) the patient couch position on the CT side after a rotation, (3) the patient couch position as indicated by its digital readout, (4) the difference in couch sag between the CT and LINAC positions, (5) the precision of the CT coordinates, (6) the identification of fiducial markers from CT images, (7) the alignment of contours with structures in the CT images, and (8) the alignment with setup lasers. The largest single uncertainties (one standard deviation or 1 SD) were found in couch position on the CT side after a rotation (0.5 mm in the RL direction) and the alignment of contours with the CT images (0.4 mm in the SI direction). All other sources of uncertainty are less than 0.3 mm (1 SD). The overall precision of two setup protocols was investigated in a controlled phantom study. A protocol that relies heavily on the mechanical integrity of the system, and assumes a fixed relationship between the LINAC isocenter and the CT images, gave a predicted precision (1 SD) of 0.6, 0.7, and 0.6 mm in the SI, RL and AP directions, respectively. The second protocol reduces reliance on the mechanical precision of the total system, particularly the patient couch, by using radio-opaque fiducial markers to transfer the isocenter information from the LINAC side to the CT images. This protocol gave a slightly improved predicted precision of 0.5, 0.4, and 0.4 mm in the SI, RL and AP directions, respectively. The distribution of phantom position after CT-based correction confirmed these results. Knowledge of the individual sources of uncertainty will allow alternative setup protocols to be evaluated in the future without the need for significant additional measurements.

Purpose: To develop a quality assurance (QA) procedure to assess the intensity profile and dosimetry for intensity-modulated (IM) treatment fields using electronic portal imaging devices (EPIDs)., Methods and Materials: A series of rapidly acquired (approximately 1/sec) portal images are summed and converted to dose. For relative intensity QA, the intended profile is subtracted point-by-point from the measured profile forming a series of error values. The standard deviation, sigma, of the errors, a measure of the goodness of the match, is minimized by applying a normalization and uniform scatter subtraction from the measured profile. For dose verification (dose to isocenter), an empirically determined phantom-correction factor is added to incorporate the effect of patient presence on EPID readings. Seventy prostate treatment fields were used in a phantom study to verify these approaches. Sensitivity was studied by creating artificial mismatches., Results: The average sigma for relative profile verification is 3.3% (percentage of average intended intensity) whereas artificial mismatches resulted in sigma values from 5% to 27%. The average isocentric dose calculated from EPID readings is 1.001 relative to the planned dose with a standard deviation of 0.018., Conclusions: An EPID can be used for profile verification and absolute isocentric dose measurement for IM fields.

Purpose: In vivo dosimetry has been shown in a number of evaluation studies, generally carried out in larger academic centres, to be a reliable method of checking the overall treatment accuracy. The object of this study was to investigate whether it was possible and useful to perform in vivo dosimetry in a small radiotherapy department and to detect if there were any systematic errors in the overall treatment set-up., Materials and Methods: All patients were treated on a cobalt-60 unit equipped with a verification system. Six hundred fifty entrance dose measurements were performed with silicon diodes., Results: The analysis showed a mean deviation of -1.3%. This negative deviation was mainly due to the mean deviation obtained in the treatment of head and neck (-1.6%) or breast (-2.5%) cancer patients. The results for pelvic or lung irradiation showed almost no deviation (-0.8 to 0.43%). Further investigation showed that the negative values for head and neck or breast irradiation were due to the irradiation technique; the lack of scattering material causes a reduction of the dose at the reference point, which is not taken into consideration by the treatment planning system. By performing in vivo dosimetry, we were also able to detect two large errors in 650 measurements and could prevent erroneous treatment., Conclusion: Even when the overall treatment set-up is very accurate, in vivo dosimetry is very useful in a small department since only a small effort can detect and prevent errors.

Variation in the table height position for 175 treatments of 167 patients was calculated as a measure for day-to-day set-up precision in 2063 treatment sessions and resulted in a median standard deviation of 1 mm. The median standard deviations of table longitudinal and lateral position were 3 and 5 mm, respectively.

Aim and Background: The aim of this experimental study was to correlate the thickness of acquired CT slices (2, 4 and 8 mm) or MR slices (4 and 7 mm) with the accuracy of three-dimensional volume reconstruction as performed by a commercially available radiation therapy planning system., Methods: We used a cylindrical phantom, with a 15-cm diameter and 20-cm height, containing 5 spheres (12.7-31.8 mm diameter) of solid Plexiglas sunk in a 3% agar jelly solution. The phantom was scanned by the CT scan with 3 different slice thicknesses (2, 4 and 8 mm and a distance of 0 mm between the slices). Two different acquisition techniques (slice thickness of 4 and 7 mm with 0.8 and 1.4 mm slice distance, respectively) were compared in the MR study. The volume values calculated from measurements were compared with the known true volume values of the spheres., Results: The average percentage volume difference between calculated and true values for the smaller spheres reconstructed with CT images 2 and 4 mm thick was generally less than 8%, whereas the error for volumes reconstructed with 8-mm-thick CT slices was more than 20%. For the large spheres, the error was generally less than 5%. The data produced by MR acquisition agreed with those obtained using CT sections., Conclusions: For targets less than 1.5 cm in diameter on our system it is reasonable to acquire CT images with the smallest thickness available. For targets between 1.5 and 3 cm, it seems sufficient to acquire the localization images with a slice thickness of 4 mm. For targets more than 4 cm in diameter, considering that with our radiation therapy planning system the time spent for manual contouring and for isodose calculation highly increased with the number of acquired images, we suggest that the acquisition of CT-MR slices 8-10-mm thick is totally adequate even for conformal radiotherapy treatments.

Due to the large number of steps and the number of persons involved in the preparation of a radiation treatment, the transfer of information from one step to the next is a very critical point. Errors due to inadequate transfer of information will be reflected in every next step and can seriously affect the final result of the treatment. We studied the frequency and the sources of the transfer errors. A total number of 464 new treatments has been checked over a period of 9 months (January to October 1990). Erroneous data transfer has been detected in 139/24,128 (less than 1%) of the transferred parameters; they affected 26% (119/464) of the checked treatments. Twenty-five of these deviations could have led to large geographical miss or important over- or underdosage (much more than 5%) of the organs in the irradiated volume, thus increasing the complications or decreasing the tumour control probability, if not corrected. Such major deviations, only occurring in 0.1% of the transferred parameters, affected 5% (25/464) of the new treatments. The sources of these large deviations were nearly always human mistakes, whereas a considerable number of the smaller deviations were, in fact, consciously taken decisions to deviate from the intended treatment. Nearly half of the major deviations were introduced during input of the data in the check-and-confirm system, demonstrating that a system aimed to prevent accidental errors, can lead to a considerable number of systematic errors if used as an uncontrolled set-up system. The results of this study show that human mistakes can seriously affect the outcome of patient treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

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.