- Email: [email protected]
The Siemens Virtual Wedge
Copyright
ELSEVIER
Medrcal Dosimetry, Vol. 22, No. 1, pp. 39-41. 1997 0 1997 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/97 $17.00 + .OO
PI1 SO958-3947( 96)00155-O
THE
SIEMENS
VIRTUAL
WEDGE
PETER MC GHEE, PH .D.+, TERRY CHU, M.Sc ., KONRAD and PETER DUNSCOMBE, PH.D.
LESZCZYNSKI,
PH .D.,
Department of Medical Physics, Northeastern Ontario Regional Cancer Centre, Sudbury, Ontario, Canada P3E 5Jl Abstract-A Siemens Virtual Wedge has recently been installed and commissioned at the Northeastern Ontario Regional Cancer Centre. Measurements reported below show that 1) Virtual Wedge factors are within 1.5% of 1; 2) percentage depth doses down to 15 cm for open and virtually wedged fields are identical to within 0.7%; 3) relative cross beam profiles for 60” virtual and physical wedges are very similar except at the toe end where a 5% difference in relative dose has been observed and 4) the peripheral dose from the 60” Virtual Wedge is about half of that from the 60” physical wedge. A clinical protocol requiring combined open and 60” wedged fields has been developed and validated. This protocol, which does not impair the utility of the Virtual Wedge, facilitates the use of on-line portal imaging and signilicantly reduces the effort required to commission the system. 0 1997 American Association of Medical Dosimetrists Key
Words: Virtual wedge, Dynamic wedge, Treatment planning.
up to 20 cm (wedged direction) X 40 cm long are possible and, with an appropriate collimator rotation, the wedge can be applied in any orientation. This remains true if the system is commissioned for the motion of one jaw only as the collimator is capable of -t 180” rotation. The wedge generated by the motion of the Al jaw only on our Siemens MX2 6MV accelerator has been commissioned. This simplification does not restrict the clinical utility of the Virtual Wedge.
INTRODUCTION The idea of generating wedge shaped isodoses through the motion of a collimating jaw during irradiation was first suggested by Kijewski et al.’ The advantages of such an approach over the use of a conventional physical wedge to tilt the isodoses include 1) elimination of the potential hazard to patients and staff of a dropped wedge; 2) continuously variable wedge angle; and 3) lower doses peripheral to the treatment field.’ In this communication we describe our experience with commissioning and introducing clinically the Virtual Wedge supplied by Siemens Medical Systems, Inc., Concord, California.
Beam data acquisition A linear array of 25 diodes (Scanditronix RFA300 3-D Waterphantom System with LDA-25 (Linear Detector Array-25)) was used to characterize the profiles for open and wedged fields. As the spacing of the diodes at 1 cm was considered too coarse, each profile is the sum of two measurements with an offset of 0.5 cm between them. Point dose measurements for wedge factor determination were made with a single diode in the water tank. Validation of the dosimetry of the Virtual Wedge, which was carried out prior to its clinical introduction, was performed with a 0.6 cc ion chamber (FT’W, Freiburg, Germany) in 35 X 35 x 35 cm3 water tank.
MATERIALS Virtual Wedge The equations upon which the operation of the Virtual Wedge is based have been presented.3 The wedge profile is generated by moving a collimating jaw at constant speed while varying the dose rate within the limits of lo-360 MU/min. Any of the four jaws can be used to generate the wedge, although in practice only the A (in plane) jaws are used as they can travel over the central axis by 10 cm, whereas the B jaws are limited to 2 cm overtravel. During delivery of a virtually wedged field the moving jaw moves in the direction of increasing field size. Fully wedged fields
Treatment planning Theraplan Version 5.OB (Theratronics, Kanata, Canada) is used to generate isodose distributions for treatments involving the Virtual Wedge. METHODS
Reprint requests to: Dr. Peter Dunscombe, Department
of Medical Physics, Northeastern Ontario Regional Cancer Centre, 41 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E5Jl. E-mail: [email protected] ’ Present address: Department of Medical Physics, Thunder Bay Regional Cancer Centre, 290 Munro Street, Thunder Bay, Ontario, Canada, P7A 7Tl
Wedge factors were determined at the isocentre 5 cm deep in water for field sizes 6 x 6, 10 X 10, 15 X 15 and 20 X 20 cm2 and for 15”, 30”, 45” and 60 Virtual Wedges. We define the wedge factor as the ratio of the central axis dose rate measured at a refer39
40
Medical Dosimetry
Table 1. The average wedge factor and range for each of four Virtual Wedges Virtual wedge angle degree (“I
Wedge factor
Range
15 30 45 60
0.9950 0.9882 0.9937 1.0024
.9942-.995? .9854-.9904 .9864- 1.000 .9912-1.014
ence depth (in our case 5 cm) for a wedged field to that for the equivalent open field. The linear diode array was used to measure lateral profiles of fields modified by the Virtual Wedge and each of the four physical wedges available ( 15”, 30”, 45” and 60”). Profiles were determined for 20 x 20 cm2 fields and at depths of 1.5, 5.0, 10.0 and 15.0 cm. Central axis profiles were not measured directly for the Virtual Wedge. Possible alteration of the central axis percentage depth dose due to the Virtual Wedge was studied by comparing the wedged and open field dose/MU on the central axis at the four depths at which profiles were determined. The monitor unit dependence of the Virtual Wedge profiles was examined by generating 60” wedged fields for 9, 50 and 200 MU. To expedite the introduction of the Virtual Wedge without compromising clinical utility it was decided to commission only lVW60 (a 60” Virtual Wedge generated by motion of the Al jaw). Theraplan version 5.OB is not designed to simulate the process by which a virtual or dynamically wedged field is generated. Therefore, profiles for lVW60 were fitted in Theraplan in the usual manner for a physical wedge, i.e., by defining a geometry for an attenuator with a specified linear attenuation coefficient. Final validation of the Virtual Wedge dosimetry consisted of generating a plan in which a wedged 60’ field was combined with an open field in the monitor unit ratio of 0.34 to 0.66. The field size was 12 (wedged direction) x 21 cm’. The relative beam weights and field size were selected as they had been approved by an oncologist for use with the first patient to receive a Virtual Wedge treatment in this centre. Using a 0.6 cc ion chamber in a water tank, point doses were measured at ten locations within the irradiated volume, and these were compared with predicted doses based on Theraplan calculations and associated dosimetry.
Volume 22, Number 1, 1997
ited by a similar device supplied by another manufacturer.4 In view of the very small field size dependence, our clinical dosimetry is based on a wedge factor of 1 for the lVW60 wedge. At the time of writing, the Virtual Wedge has been in clinical use for three months and the wedge factor for lVW60 has been constant to within 1.5%. It has been shown that the wedge factor is independent of gantry angle.3 Percentage depth dose Comparing the relative central axis doses measured during the acquisition of beam profiles at 1.5, 5.0, 10.0 and 15.0 cm with the open field depth dose measured in the conventional manner by a scanning single diode in water we conclude that the percentage depth doses along the central axis for 20 X 20 cm2 open and virtually wedged (60”) fields are identical to within 0.7%. Beam projile It is important to compare the Virtual Wedge profile with that generated by the corresponding physical wedge. If sufficient similarity exists it will be possible to substitute a physical wedge for a Virtual Wedge in the event of equipment failure without having to recompute the dose distribution. Figure 1 shows such a
RESULTS
Wedge factors Table 1 shows the average wedge factor and its range for field sizes of 6 x 6, 10 X 10, 15 X 15 and 20 x 20 cm2. Smaller wedge factors were associated with smaller fields although the field size dependence is 2% or less. This behaviour contrasts with that exhib-
Distance
from
Central
Axis
(mm)
Fig. 1. Profiles for 60” wedged fields generated by a physical ( -) and virtual wedge (w). The field size was 20 X 20 cm2 and the depth 5 cm. Profiles have been normalized to 100% on the central axis.
The Siemens
Virtual
Wedge
Table 2. Comparison of calculated and measured doses in water for combined open and 60 degree virtual wedged fields Measured
Position (mm) Depth 1.5 1.5 1.5 5 5 5 7.5 10 10 10
Offset -3 0 3 -3 0 3 0 -3 0 3
Fields are 12 X
Calculated
Ratio
Cc@)
(CGY)
(CM
146.4 128.9 120.6 125.8 112.7 104.7 100.0 96.7 87.9 81.4
149 130 123 127 113 106 101 98 89 83
1.018 1.008 1.020 1.009 1.002 1.012 1.010 1.014 1.013 1.019
21 (A X B).
are those used for our first clinical application of the Virtual Wedge (0.34 wedged; 0.66 open) Field
weightings
comparison between profiles at 5 cm deep for lVW60 and the 60” physical wedge. The physical wedge profile has been divided by the wedge factor to give a common normalization on the central axis. The largest discrepancy between the profiles occurs at the hot end of the distribution and is less than 5% of the local dose. As a result of the similarity of the profiles shown in Fig. 1 our policy permits substitution of the 60” physical wedge for the 60” Virtual Wedge without replanning should this, for technical reasons, be necessary. The only change in the dosimetry calculations is that the different wedge factors for the two wedges must be taken into account. A comparison was also made of lVW60 profiles taken for 9, 50 and 200 MU. The three profiles, measured in water at a depth of 5 cm agreed to within 1% over the distance examined (40 cm) except at the toe end where discrepancies of up to t 2% were observed. It is apparent from Fig. 1 that with the same dose delivered to a given reference point the peripheral dose in the virtually wedged field is lower than that measured when a physical wedge is present. A similar observation has been made using a comparable device of different design2 Validation With an open/wedged field combination based on that to be used as part of the first clinical Virtual Wedge treatment, experimental confirmation of the computed dose distribution was carried out. Table 2 shows computed and measured point doses together with the locations of each of the ten points relative to the central axis and the surface. All points are in the central axis plane in the wedged dimension. The average and maximum discrepancies observed are 1.3% and 2% respcc-
0 P. MCCHEE
41
et al.
tively. These are well within Van Dyk et al5
the criteria suggested by
DISCUSSION The Siemens Mevatron MX2 accelerator on which the Virtual Wedge has been implemented is also equipped with a Beamview Plus portal imager. The radiation therapists operating the machine rely on the imager to confirm that the patient is correctly positioned with respect to the radiation beam. An early concern was that the portal imager could not be used effectively for virtual wedged fields as the full field periphery is not visible before a significant dose has been delivered as the dynamic jaw starts from the closed position. It was this potential difficulty that was the primary motivation for planning clinical irradiations for combined open and wedged fields. The open field is delivered first and Beamview Plus is used to confirm field location and shape. This is followed by the virtually wedged (lVW60) field. There is no impairment of the clinical utility of the Virtual Wedge facility resulting from the use of only one wedge. In addition to facilitating the use of Beamview Plus such an approach has two further advantages. Firstly, commissioning only one wedge, in our case lVW60, minimizes the workload involved. Secondly, with the wedge in a fixed orientation with respect to the accessory holder the possibility of virtually wedging in the opposite direction to that intended is reduced. A visual check of the collimator rotation is sufficient to confirm wedge orientation. The Virtual Wedge described in this article has been enthusiastically accepted by all those involved in its use.
Acknowledgements-The purchase of the Virtual Wedge was made possible through the generosity of The Royal Canadian Legion Ontario Provincial Command Branches and Lgdies AuxiliariesCharitable Foundation. The assistance of Mark McCarthy of Siemens Medical Systems, Inc. is gratefully acknowledged.
REFERENCES 1. Kijewski, P.K.; Chin, L.M.; Bjamgard, B.E. Wedge-shaped dose distributions by computer controlled collimator motion. Med. Phys. 5426-429; 1978. 2. Weides, CD.; Mok, E.C.; Chang, W.C.; Findlay, D.O.; Shostak, CA. Evaluating the dose to the contralateral breast when using a dynamic wedge versus a regular wedge. Med. Dosim. 20:287293; 1995. 3. van Santvoort, J.P.C.; Huizenga, H.; van Battum, L.J. Dosimetric evaluation of the Siemens virtual wedge. (Abstract) Med. Phys. 22:976; 1995. 4. Klein, E.E.; Low, D.A.; Meigooni, A.S.; Purdy, J.A. Dosimetry and clinical implementation of dynamic wedge. Int. J. Radial. Oncol. Biol. Phys. 31:583-592; 1995. 5. Van Dyk, J.; Bamet, R.B.; Cygler, J.E.; Shragge, P.C. Commissioning and quality assurance of treatment planning computers. Int. J. Radiat. Oncol. Biol. Phys. 26:261-273; 1993.
ELSEVIER
Medrcal Dosimetry, Vol. 22, No. 1, pp. 39-41. 1997 0 1997 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/97 $17.00 + .OO
PI1 SO958-3947( 96)00155-O
THE
SIEMENS
VIRTUAL
WEDGE
PETER MC GHEE, PH .D.+, TERRY CHU, M.Sc ., KONRAD and PETER DUNSCOMBE, PH.D.
LESZCZYNSKI,
PH .D.,
Department of Medical Physics, Northeastern Ontario Regional Cancer Centre, Sudbury, Ontario, Canada P3E 5Jl Abstract-A Siemens Virtual Wedge has recently been installed and commissioned at the Northeastern Ontario Regional Cancer Centre. Measurements reported below show that 1) Virtual Wedge factors are within 1.5% of 1; 2) percentage depth doses down to 15 cm for open and virtually wedged fields are identical to within 0.7%; 3) relative cross beam profiles for 60” virtual and physical wedges are very similar except at the toe end where a 5% difference in relative dose has been observed and 4) the peripheral dose from the 60” Virtual Wedge is about half of that from the 60” physical wedge. A clinical protocol requiring combined open and 60” wedged fields has been developed and validated. This protocol, which does not impair the utility of the Virtual Wedge, facilitates the use of on-line portal imaging and signilicantly reduces the effort required to commission the system. 0 1997 American Association of Medical Dosimetrists Key
Words: Virtual wedge, Dynamic wedge, Treatment planning.
up to 20 cm (wedged direction) X 40 cm long are possible and, with an appropriate collimator rotation, the wedge can be applied in any orientation. This remains true if the system is commissioned for the motion of one jaw only as the collimator is capable of -t 180” rotation. The wedge generated by the motion of the Al jaw only on our Siemens MX2 6MV accelerator has been commissioned. This simplification does not restrict the clinical utility of the Virtual Wedge.
INTRODUCTION The idea of generating wedge shaped isodoses through the motion of a collimating jaw during irradiation was first suggested by Kijewski et al.’ The advantages of such an approach over the use of a conventional physical wedge to tilt the isodoses include 1) elimination of the potential hazard to patients and staff of a dropped wedge; 2) continuously variable wedge angle; and 3) lower doses peripheral to the treatment field.’ In this communication we describe our experience with commissioning and introducing clinically the Virtual Wedge supplied by Siemens Medical Systems, Inc., Concord, California.
Beam data acquisition A linear array of 25 diodes (Scanditronix RFA300 3-D Waterphantom System with LDA-25 (Linear Detector Array-25)) was used to characterize the profiles for open and wedged fields. As the spacing of the diodes at 1 cm was considered too coarse, each profile is the sum of two measurements with an offset of 0.5 cm between them. Point dose measurements for wedge factor determination were made with a single diode in the water tank. Validation of the dosimetry of the Virtual Wedge, which was carried out prior to its clinical introduction, was performed with a 0.6 cc ion chamber (FT’W, Freiburg, Germany) in 35 X 35 x 35 cm3 water tank.
MATERIALS Virtual Wedge The equations upon which the operation of the Virtual Wedge is based have been presented.3 The wedge profile is generated by moving a collimating jaw at constant speed while varying the dose rate within the limits of lo-360 MU/min. Any of the four jaws can be used to generate the wedge, although in practice only the A (in plane) jaws are used as they can travel over the central axis by 10 cm, whereas the B jaws are limited to 2 cm overtravel. During delivery of a virtually wedged field the moving jaw moves in the direction of increasing field size. Fully wedged fields
Treatment planning Theraplan Version 5.OB (Theratronics, Kanata, Canada) is used to generate isodose distributions for treatments involving the Virtual Wedge. METHODS
Reprint requests to: Dr. Peter Dunscombe, Department
of Medical Physics, Northeastern Ontario Regional Cancer Centre, 41 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E5Jl. E-mail: [email protected] ’ Present address: Department of Medical Physics, Thunder Bay Regional Cancer Centre, 290 Munro Street, Thunder Bay, Ontario, Canada, P7A 7Tl
Wedge factors were determined at the isocentre 5 cm deep in water for field sizes 6 x 6, 10 X 10, 15 X 15 and 20 X 20 cm2 and for 15”, 30”, 45” and 60 Virtual Wedges. We define the wedge factor as the ratio of the central axis dose rate measured at a refer39
40
Medical Dosimetry
Table 1. The average wedge factor and range for each of four Virtual Wedges Virtual wedge angle degree (“I
Wedge factor
Range
15 30 45 60
0.9950 0.9882 0.9937 1.0024
.9942-.995? .9854-.9904 .9864- 1.000 .9912-1.014
ence depth (in our case 5 cm) for a wedged field to that for the equivalent open field. The linear diode array was used to measure lateral profiles of fields modified by the Virtual Wedge and each of the four physical wedges available ( 15”, 30”, 45” and 60”). Profiles were determined for 20 x 20 cm2 fields and at depths of 1.5, 5.0, 10.0 and 15.0 cm. Central axis profiles were not measured directly for the Virtual Wedge. Possible alteration of the central axis percentage depth dose due to the Virtual Wedge was studied by comparing the wedged and open field dose/MU on the central axis at the four depths at which profiles were determined. The monitor unit dependence of the Virtual Wedge profiles was examined by generating 60” wedged fields for 9, 50 and 200 MU. To expedite the introduction of the Virtual Wedge without compromising clinical utility it was decided to commission only lVW60 (a 60” Virtual Wedge generated by motion of the Al jaw). Theraplan version 5.OB is not designed to simulate the process by which a virtual or dynamically wedged field is generated. Therefore, profiles for lVW60 were fitted in Theraplan in the usual manner for a physical wedge, i.e., by defining a geometry for an attenuator with a specified linear attenuation coefficient. Final validation of the Virtual Wedge dosimetry consisted of generating a plan in which a wedged 60’ field was combined with an open field in the monitor unit ratio of 0.34 to 0.66. The field size was 12 (wedged direction) x 21 cm’. The relative beam weights and field size were selected as they had been approved by an oncologist for use with the first patient to receive a Virtual Wedge treatment in this centre. Using a 0.6 cc ion chamber in a water tank, point doses were measured at ten locations within the irradiated volume, and these were compared with predicted doses based on Theraplan calculations and associated dosimetry.
Volume 22, Number 1, 1997
ited by a similar device supplied by another manufacturer.4 In view of the very small field size dependence, our clinical dosimetry is based on a wedge factor of 1 for the lVW60 wedge. At the time of writing, the Virtual Wedge has been in clinical use for three months and the wedge factor for lVW60 has been constant to within 1.5%. It has been shown that the wedge factor is independent of gantry angle.3 Percentage depth dose Comparing the relative central axis doses measured during the acquisition of beam profiles at 1.5, 5.0, 10.0 and 15.0 cm with the open field depth dose measured in the conventional manner by a scanning single diode in water we conclude that the percentage depth doses along the central axis for 20 X 20 cm2 open and virtually wedged (60”) fields are identical to within 0.7%. Beam projile It is important to compare the Virtual Wedge profile with that generated by the corresponding physical wedge. If sufficient similarity exists it will be possible to substitute a physical wedge for a Virtual Wedge in the event of equipment failure without having to recompute the dose distribution. Figure 1 shows such a
RESULTS
Wedge factors Table 1 shows the average wedge factor and its range for field sizes of 6 x 6, 10 X 10, 15 X 15 and 20 x 20 cm2. Smaller wedge factors were associated with smaller fields although the field size dependence is 2% or less. This behaviour contrasts with that exhib-
Distance
from
Central
Axis
(mm)
Fig. 1. Profiles for 60” wedged fields generated by a physical ( -) and virtual wedge (w). The field size was 20 X 20 cm2 and the depth 5 cm. Profiles have been normalized to 100% on the central axis.
The Siemens
Virtual
Wedge
Table 2. Comparison of calculated and measured doses in water for combined open and 60 degree virtual wedged fields Measured
Position (mm) Depth 1.5 1.5 1.5 5 5 5 7.5 10 10 10
Offset -3 0 3 -3 0 3 0 -3 0 3
Fields are 12 X
Calculated
Ratio
Cc@)
(CGY)
(CM
146.4 128.9 120.6 125.8 112.7 104.7 100.0 96.7 87.9 81.4
149 130 123 127 113 106 101 98 89 83
1.018 1.008 1.020 1.009 1.002 1.012 1.010 1.014 1.013 1.019
21 (A X B).
are those used for our first clinical application of the Virtual Wedge (0.34 wedged; 0.66 open) Field
weightings
comparison between profiles at 5 cm deep for lVW60 and the 60” physical wedge. The physical wedge profile has been divided by the wedge factor to give a common normalization on the central axis. The largest discrepancy between the profiles occurs at the hot end of the distribution and is less than 5% of the local dose. As a result of the similarity of the profiles shown in Fig. 1 our policy permits substitution of the 60” physical wedge for the 60” Virtual Wedge without replanning should this, for technical reasons, be necessary. The only change in the dosimetry calculations is that the different wedge factors for the two wedges must be taken into account. A comparison was also made of lVW60 profiles taken for 9, 50 and 200 MU. The three profiles, measured in water at a depth of 5 cm agreed to within 1% over the distance examined (40 cm) except at the toe end where discrepancies of up to t 2% were observed. It is apparent from Fig. 1 that with the same dose delivered to a given reference point the peripheral dose in the virtually wedged field is lower than that measured when a physical wedge is present. A similar observation has been made using a comparable device of different design2 Validation With an open/wedged field combination based on that to be used as part of the first clinical Virtual Wedge treatment, experimental confirmation of the computed dose distribution was carried out. Table 2 shows computed and measured point doses together with the locations of each of the ten points relative to the central axis and the surface. All points are in the central axis plane in the wedged dimension. The average and maximum discrepancies observed are 1.3% and 2% respcc-
0 P. MCCHEE
41
et al.
tively. These are well within Van Dyk et al5
the criteria suggested by
DISCUSSION The Siemens Mevatron MX2 accelerator on which the Virtual Wedge has been implemented is also equipped with a Beamview Plus portal imager. The radiation therapists operating the machine rely on the imager to confirm that the patient is correctly positioned with respect to the radiation beam. An early concern was that the portal imager could not be used effectively for virtual wedged fields as the full field periphery is not visible before a significant dose has been delivered as the dynamic jaw starts from the closed position. It was this potential difficulty that was the primary motivation for planning clinical irradiations for combined open and wedged fields. The open field is delivered first and Beamview Plus is used to confirm field location and shape. This is followed by the virtually wedged (lVW60) field. There is no impairment of the clinical utility of the Virtual Wedge facility resulting from the use of only one wedge. In addition to facilitating the use of Beamview Plus such an approach has two further advantages. Firstly, commissioning only one wedge, in our case lVW60, minimizes the workload involved. Secondly, with the wedge in a fixed orientation with respect to the accessory holder the possibility of virtually wedging in the opposite direction to that intended is reduced. A visual check of the collimator rotation is sufficient to confirm wedge orientation. The Virtual Wedge described in this article has been enthusiastically accepted by all those involved in its use.
Acknowledgements-The purchase of the Virtual Wedge was made possible through the generosity of The Royal Canadian Legion Ontario Provincial Command Branches and Lgdies AuxiliariesCharitable Foundation. The assistance of Mark McCarthy of Siemens Medical Systems, Inc. is gratefully acknowledged.
REFERENCES 1. Kijewski, P.K.; Chin, L.M.; Bjamgard, B.E. Wedge-shaped dose distributions by computer controlled collimator motion. Med. Phys. 5426-429; 1978. 2. Weides, CD.; Mok, E.C.; Chang, W.C.; Findlay, D.O.; Shostak, CA. Evaluating the dose to the contralateral breast when using a dynamic wedge versus a regular wedge. Med. Dosim. 20:287293; 1995. 3. van Santvoort, J.P.C.; Huizenga, H.; van Battum, L.J. Dosimetric evaluation of the Siemens virtual wedge. (Abstract) Med. Phys. 22:976; 1995. 4. Klein, E.E.; Low, D.A.; Meigooni, A.S.; Purdy, J.A. Dosimetry and clinical implementation of dynamic wedge. Int. J. Radial. Oncol. Biol. Phys. 31:583-592; 1995. 5. Van Dyk, J.; Bamet, R.B.; Cygler, J.E.; Shragge, P.C. Commissioning and quality assurance of treatment planning computers. Int. J. Radiat. Oncol. Biol. Phys. 26:261-273; 1993.