Abstract. Dosimetric accuracies at low monitor units are evaluated for linear accelerators from various manufacturers. A large error is observed in the majority of ...
1991, The British Journal of Radiology, 64, 808-811
Dosimetric accuracy at low monitor unit settings By Indra J . Das, MSc, MS, PhD, Kenneth R. Kase, PhD, FACR and 'Victor M. Tello, ME, MBA Department of Radiation Oncology, University of Massachusetts Medical Center, Worcester, MA 01655, USA and 'Radiological Physics Center-235, M. D. Anderson Hospital and Tumor Institute, Houston, TX 77030, USA
{Received April 1990 and in revised form February 1991)
Keywords: Dose linearity, Low monitor unit, Accelerator, Dosimetry
Abstract. Dosimetric accuracies at low monitor units are evaluated for linear accelerators from various manufacturers. A large error is observed in the majority of the accelerators. The error can be positive or negative. Although the error can exceed 20% for the first few monitor units, it is usually less than 5% when more than 10 monitor units are delivered. When low doses are required proper precautions should be taken for dosimetric accuracy including the beam energy, beam flatness and dose per monitor unit.
To optimize tumour-cure probability an uncertainity of ±5% in tumour dose and + 2% in radiation dosimetry is often recommended in clinical situations (Task Group 21, 1983; WHO, 1988). Achieving this level of accuracy depends on the understanding of various uncertainties in the dosimetric parameters. Most of them have been discussed in the literature. One consideration that is usually ignored, even in the recent publications (IPSM, 1988; WHO, 1988), is the accuracy of dose delivery at low monitor unit (MU) settings from medical linear accelerators. For MU settings < 50, the actual delivered dose could vary significantly for some linear accelerators and may fall beyond the limit of acceptable accuracy. Ideally, dose delivered per unit MU should be constant. Since MU are integer numbers and can be set only to the nearest unit, the uncertainty increases as fewer MU are set. However, for most accelerators there seems to be an additional definite non-linearity. Situations do arise where < 50 MU are delivered during dosimetric applications, research and patient treatment. While using fast film for radiation protection dosimetry low MU are typically used. For thermoluminescent dosemeter (TLD) dosimetry, often low doses (MU) are used for calibration. In patient treatments using multiple fields, the MU/field may be < 50. Ohara et al (1989) used time gated low exposure of 3, 5, and 10 MU to avoid geometric miss in thoracic tumours resulting from diaphragm movement caused by respiration. In other clinical situations like chronic myelocytic leukaemia (CML) and polycythemia vera the dose per fraction is as low as 25 cGy. This means the MU per field is in the range of 8-15 MU depending upon patient thickness and technique used. Johnson (1977) showed that in CML small doses can cause appreciable cytopenias leading to patient death. Hence, to avoid any serious complications the dose delivery must be accurate. Finally when an accelerator is not functioning properly, multiple restarts may be attempted to complete the prescribed dose. In such situations only a few MU are delivered at a time. 808
Whenever low monitor units are used it is important to know the uncertainty in the actual dose. This project was undertaken to evaluate the dosimetric accuracy at low MU settings for several medical linear accelerators. Methods
To study the dosimetric linearity (dose/MU) at low MU of a linear accelerator, measurements were performed as described in Task Group 21 (1983). Dose in a medium at a point is given as: = Q
p * ion
p rep
p A
»
(1)
where Q is the average measured charge corrected for temperature and pressure, 7Vgas is the cavity gas calibration factor, (L/p) is the restricted stopping power ratio of medium to chamber gas, PlOD is a correction for ion recombination, Prep is the replacement correction factor, and Pwall is a correction for the chamber wall. All of these parameters are discussed in Task Group 21 (1983). If we assume all the parameters are constant for a specific machine and energy, then Dmed/Q should be constant. The validity of this assumption is explained in the discussion section. For a few selected units, beam energy, Pion, and (L/p) were evaluated at various MU settings. Any deviation from a constant value of the measured dose/MU of a machine could be due to: (i) the ion chamber; (ii) the electrometer and (iii) the machine. Dosemeter linearity is essential and has been discussed in the literature (Massey, 1970; Orton & Seibert, 1972; IPSM, 1988; WHO, 1988). It is expected that for a beam energy the dosemeter linearity should be within +1 % over the working range. For a small volume ion chamber operating at high collecting voltage ( > 300 V) the limit is easily achievable. In this study it was assumed that the 0.6 cm3 Farmer-type ion chambers are very linear with dose rate and charge collection (see discussion section). For the high precision electrometers, corrections were made for non-linearity in the different The British Journal of Radiology, September 1991
Dosimetric accuracy at low monitor unit settings
range settings. It was accounted for by accumulating charges with different range settings for the same MU. All measurements were performed in a standard calibration set-up at the respective reference points as outlined in Task Group 21 (1983). Measurements were taken from the lowest allowed MU setting up to 400 MU at the clinically used dose rates (200-300 MU/min) for a variety of accelerators from several manufacturers. In most cases a 0.6 cm3 PTW cylindrical ion chamber (Nuclear Associates, Carle Place, NY) was used as the dosemeter and Keithley (Keithley Instrument Co., Cleveland, OH) high precision electrometers were used for the measurement of the charge. For the lower MU settings, high sensitivity scales were used. Several sets of readings were taken for each data point to achieve suitable statistics. The per-
centage uncertainty in any data set was < +0.5% and it was not plotted in figures. From the above assumption, Q/MU was taken as a measure of dose per MU. Different accelerators with the same stated beam energy were included in this study. The beam energies studied were 4, 6, 10 and 18 MV. Results
The dose non-linearity cannot be visualized if the normalized charge (normalized to 1.0 at 100 MU) is plotted vs. MU on a log-log scale. However, if the same data are plotted differently the non-linearity becomes obvious. Parts (a), (b), (c) and (d) of Fig. 1 are semilog plots of dose/MU vs. MU for 4, 6, 10 and 18 MV beams, respectively. There seems to be no correlation between the photon energies and the behaviour of the 6 MV Beams
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Figure 1. Plot of dose/MU vs. machine MU for (a) 4 MV, (b) 6 MV, (c) 10 MV and (d) 18 MV photon beams. Vol. 64, No. 765
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/. J. Das, K. R. Kase and V. M. Tello
dose/MU curve. The results are machine specific. In most cases the error is greatest at lowest MU.
Stopping Power of water to air 1.16
The large deviation in dose/MU from 1.0 at low MU could be due to machine non-linearity. This deviation is the dosimetric error caused by the integer nature of MU and the machine performance at low MU. This error can be called monitor unit error, 0.4 MU. For some units 5 is not constant, hence usual methods for determining S cannot be used and the prediction of error is inaccurate. Machines having a large S value require that a greater number of MU be set to achieve an acceptable limit of accuracy. While the individual beam dose may be in error, in terms of total target dose, the error may be insignificant. Only where the total treatment is made up of small MU exposures, the error in total target dose is of the same order. Therefore, in clinical situations the error introduced by dose non-linearity is of minimal concern as most treatments are > 10 MU. Where S is large, a correction factor may be appropriate for accurate dosimetry. If a machine is not functioning properly and terminates, multiple attempts to complete the treatment should be avoided or determined with direct consultation with a physicist. Caution should be exercised when using small MU settings especially for TLD and film dosimetry. Determination of S should also be included in the machine acceptance testing and usual quality control. It is not clear that acceptance criteria
Vol. 64, No. 765
can be established for the stability of dose/MU. However, the magnitude and constancy of the error should be determined so that approximate corrections can be made when needed. If lower doses are desired other methods such as extended distance and beam attenuation should be used. Acknowledgment We greatly acknowledge help from Walter Tang and Bruce Gerbi of Minneapolis, MN, Frank Ascoli of Worcester and Paul Nizin of Framingham, MA, for the data on their machines. References BARISH, R. J. FLEISCHMAN, R. C. & PIPMAN, Y. M., 1987.
Teletherapy beam characteristics: The first second. Medical Physics, 14, 657-661. CHENG, P. & KUBO, H., 1988. Unexpectedly large dose rate dependent output from a linear accelerator. Medical Physics, 15, 766-767. JOHNSON, R. E., 1977. Role of radiation therapy in management of adult leukemia. Cancer, 39 (Suppl. 2), 852-855. MASSEY, J. B., 1970. Manual of Dosimetry in Radiotherapy, Technical Report Series Number 110 (International Atomic Energy Agency, Vienna). OHARA, K., OKUMURA, T., AKISADA, M., INADA, T., MORI, T., YOKOTA, H. & CALAGUAS, M. J. B., 1989. Irradiation
synchronized with respiration gate. International Journal of Radiation Oncology, Biology, Physics, 17, 853-857. ORTON, C. G. & SEIBERT, J. B., 1972. The measurement of
teletherapy unit timer errors. Physics in Medicine and Biology, 17, 198-205. IPSM, 1988. Report No. 54: Commissioning and Quality Assurance of Linear Accelerators (The Institute of Physical Sciences in Medicine, York, England). TASK GROUP 21, 1983. A protocol for the determination of
absorbed dose from high energy photon and electron beams. Medical Physics, 10, 741-771. TUOHY, J. B. & MORGAN, A. M., 1990. Temporary enlargement
of penumbra on linear accelerators. British Journal of Radiology, 63, 911. WORLD HEALTH ORGANIZATION, 1988. Quality Assurance in
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