Int. 1. Radiation
0
Oncology
Biol. Phys.,
1977, Vol. 2. p. 1233.
Pergamon Press.
Printed in the U.S.A
Letter to the Editor
DOSIMETRY
FOR IRREGULAR
ROBERT
L.
DIXON,
FIELDS
Ph.D.
Department of Radiology, Bowman Gray School of Medicine of Wake Forest University, Winston-Salem, NC 27103, U.S.A. In their recent article, “A Method of Dosimetry for Irregular Fields,” Agarwal et al.’ describe a method for deducing the equival’ent square field representative of a radiation field which has been shaped by shielding blocks. It is easy to show by the following simple example (which requires no experimental verification) that the proposed method has no logical, theoretical basis. To wit, consider an 8 x 24 cm field which has had an 8 x 8 cm area on each end blocked out by shielding blocks, thereby leaving an 8 x 8 cm open field in the center. The surface field size appropriate for per cent depth-dose computation in this case clearly is 8 x 8 cm. Using the method proposed by Agarwal, we would compute the equivalent square field to be 2 x 2 cm-an answer which clearly is incorrect and which leads to a 15% error in depth dose at a depth of 10 cm for a 4 MV linear accelerator. Other empirical methods that have been proposed such as using the ratio of field area to field perimeter2 also give erroneous answers in many cases, i.e. one can easily envision methods of doubling or tripling the perimeter of a radiation field without affecting the overall area (or the depth-dose) significantly.
In short, one should be extremely wary of applying such empirical methods as a matter of routine to a wide variety of irregular field shapes, i.e., in many cases an intelligent guess of the eflective field size can be better than application of such a rote method. Finally, it should be noted that the proper method for computation of absorbed dose rate at the depth of maximum dose buildup (d,,,) for irregular fields is to calibrate the treatment unit such that one has determined independently both the relative variation of dose rate in free space and the relative variation at d,,, in a water phantom. The reason for this is that the output in air is determined by the collimator setting alone, whereas the backscatter is determined by the surface field size. In the previous example, to obtain the dose at d,,, one would look up the calibration data in-phantom for an 8 x 8 cm surface field (obtained with an 8 x 8 cm collimator setting) and multiply this dose by a factor which represents relative increase of in-air output of the unit resulting from the 8 x 24 cm collimator opening over an 8 x 8 cm collimator opening. This method is applicable to both linear accelerators and cobalt units.
AUTHOR’S SURESH
K.
REPLY
AGARWAL,
Ph.D.
Director, Radiological Physics, Department of Radiology, School of Medicine, Charlottesville, VA 22901, U.S.A. We note Dr. Dixon’s comments and appreciate his interest in our work. It is dificult to envision when his example might be used in practice, since one would generally set and 8 x 8 cm field if that were the required size, but our method, if applied, would indeed give an erroneous answer in such a case. It was pointed out in the paper,’ however, that the blocks should be “scattered all over the field” in order to achieve the stated accuracy. Zf, instead of the pos-
University of Virginia,
tulated 8 X 24 cm field, a 24 x 24 cm field is taken, and an 8 cm strip is removed from all sides to leave an 8 x 8 cm square in the center, then our method correctly predicts this equivalent square, It should be noted that the proposed method was never intended to be rigorously proved; it is simply a useful device which simplifies calculation when applied intelligently.
REFERENCES 1. Agarwal, S.K.,
Wakley, J., Scheele, R.V., Normansell, A.: A method of dosimetry for irregularly shaped fields. Int. J. Radiol. Oncol. Biol. Phys. 2: 199-203, 1977.
2. Wrede, D.E.: Central axis tissue-air ratios as a function of area/perimeter at depth and their applicability to irregularly shaped fields. Phys. Med. Biol. 17: 548-554, 1972. 1233
0
Oncology
Biol. Phys.,
1977, Vol. 2. p. 1233.
Pergamon Press.
Printed in the U.S.A
Letter to the Editor
DOSIMETRY
FOR IRREGULAR
ROBERT
L.
DIXON,
FIELDS
Ph.D.
Department of Radiology, Bowman Gray School of Medicine of Wake Forest University, Winston-Salem, NC 27103, U.S.A. In their recent article, “A Method of Dosimetry for Irregular Fields,” Agarwal et al.’ describe a method for deducing the equival’ent square field representative of a radiation field which has been shaped by shielding blocks. It is easy to show by the following simple example (which requires no experimental verification) that the proposed method has no logical, theoretical basis. To wit, consider an 8 x 24 cm field which has had an 8 x 8 cm area on each end blocked out by shielding blocks, thereby leaving an 8 x 8 cm open field in the center. The surface field size appropriate for per cent depth-dose computation in this case clearly is 8 x 8 cm. Using the method proposed by Agarwal, we would compute the equivalent square field to be 2 x 2 cm-an answer which clearly is incorrect and which leads to a 15% error in depth dose at a depth of 10 cm for a 4 MV linear accelerator. Other empirical methods that have been proposed such as using the ratio of field area to field perimeter2 also give erroneous answers in many cases, i.e. one can easily envision methods of doubling or tripling the perimeter of a radiation field without affecting the overall area (or the depth-dose) significantly.
In short, one should be extremely wary of applying such empirical methods as a matter of routine to a wide variety of irregular field shapes, i.e., in many cases an intelligent guess of the eflective field size can be better than application of such a rote method. Finally, it should be noted that the proper method for computation of absorbed dose rate at the depth of maximum dose buildup (d,,,) for irregular fields is to calibrate the treatment unit such that one has determined independently both the relative variation of dose rate in free space and the relative variation at d,,, in a water phantom. The reason for this is that the output in air is determined by the collimator setting alone, whereas the backscatter is determined by the surface field size. In the previous example, to obtain the dose at d,,, one would look up the calibration data in-phantom for an 8 x 8 cm surface field (obtained with an 8 x 8 cm collimator setting) and multiply this dose by a factor which represents relative increase of in-air output of the unit resulting from the 8 x 24 cm collimator opening over an 8 x 8 cm collimator opening. This method is applicable to both linear accelerators and cobalt units.
AUTHOR’S SURESH
K.
REPLY
AGARWAL,
Ph.D.
Director, Radiological Physics, Department of Radiology, School of Medicine, Charlottesville, VA 22901, U.S.A. We note Dr. Dixon’s comments and appreciate his interest in our work. It is dificult to envision when his example might be used in practice, since one would generally set and 8 x 8 cm field if that were the required size, but our method, if applied, would indeed give an erroneous answer in such a case. It was pointed out in the paper,’ however, that the blocks should be “scattered all over the field” in order to achieve the stated accuracy. Zf, instead of the pos-
University of Virginia,
tulated 8 X 24 cm field, a 24 x 24 cm field is taken, and an 8 cm strip is removed from all sides to leave an 8 x 8 cm square in the center, then our method correctly predicts this equivalent square, It should be noted that the proposed method was never intended to be rigorously proved; it is simply a useful device which simplifies calculation when applied intelligently.
REFERENCES 1. Agarwal, S.K.,
Wakley, J., Scheele, R.V., Normansell, A.: A method of dosimetry for irregularly shaped fields. Int. J. Radiol. Oncol. Biol. Phys. 2: 199-203, 1977.
2. Wrede, D.E.: Central axis tissue-air ratios as a function of area/perimeter at depth and their applicability to irregularly shaped fields. Phys. Med. Biol. 17: 548-554, 1972. 1233
