1976, British Journal of Radiology, 49, 572 Correspondence THE EDITOR—SIR,
THE 2,500 RET ENVELOPE
The results of treatment in cancer of the cervix vary widely around the world. Many reasons have been suggested to explain the discrepancies, including errors in staging and "personalization" of treatment. Unfortunately, it has not been possible to make useful comparisons between different centres. This has been partly due to inadequate methods of describing dosimetry patterns in the pelvis. Prescribed doses are often close to the limits of tolerance. Enormous doses are delivered to the tissues next to the intracavitary applicator, but, due to the inverse square law, these doses fall off extremely rapidly over small distances. Thus it has been difficult to agree on a point which is clinically meaningful for prescription purposes. The confusion is seen in several treatment situations. For instance, in endometrial carcinoma, some centres quote the uterine dose in milligram-hours, but the vaginal dose in rad. Where interstitial therapy is added, dosimetry by this means becomes very difficult if not impossible, even where sophisticated computer techniques are available. In describing a clinical example of such a case, the current edition of a well-known textbook (Fletcher, 1973) simply states "No attempt was made to calculate the implant". The problem is further complicated by the difficulty of combining radium dosimetry and external dosimetry (Joslin et al., 1972). It is sometimes felt that a radium rad is physically different from a megavoltage rad, and that adding the two units together is like adding "apples and pears". This may not be the case. The apparent clinical differences in dose tolerance are probably mostly due to the different volumes of tissue irradiated. A less important factor is the continuous low dose-rate effect of radium compared with intermittent fractionated external therapy. Thus in the treatment of pelvic cancer, the delicate balance between morbidity and mortality depends as much on the volume of tissue irradiated as on the dose at a fixed point. I have recently analysed this factor and noted that if the treatment volume is decreased by 50 per cent, the tolerated dosage can be increased by 25 per cent in the pelvis (Lowry, 1973). Bolande (1971) suggested incorporating this factor and proposed quoting the volume enclosed by an envelope whose surface dose is about that of a point A, namely 7,000 rad. In the final analysis, when we come to compare pelvic dosimetry in different centres, we will also need to know the fractionation used. The Manchester, Stockholm, Paris, Houston and other techniques all use different quantities of radium in different combinations with external therapy. It is now possible to compare these different fractionation schedules using the Ellis formula and ret system of dosimetry (Ellis, 1971). This concept can be extended to include intracavitary irradiation using an iso-effect link between continuous radium dosimetry and discontinuous external beam therapy. Applying this method, the nominal standard dose (NSD) at point A is about 2,500 rets (George, 1971). We now have an opportunity after many years of combining all the above concepts on the computer and, at the same time, incorporating current concepts in radiation therapy and radiobiology. In most cases of malignant disease, a dose of 2,500 rets is lethal. It is therefore recommended that the volume of tissue enclosed in the 2,500-ret envelope should be quoted. Tissue within this envelope would receive a supralethal dose. Tissue outside would have the potential for sublethal recovery. The envelope itself would still be a "flat pear" whose "waistline" would lie somewhere near point A. Calculations suggest that the volume of this envelope varies between 100 cm3 and 200 cm3. The exact volume can be determined on the computer by integrating slices of tissue across afixedplane. The shape of the envelope will vary in different treatment situations. But the outline can be determined on the
computer and the data displayed in the form of a computer print-out. It is then possible to sketch in the position of the tumour, bladder and rectum. Several examples of different treatment situations have been worked out in the United States and detailed results will be presented shortly. Lastly it is noted that the 2,500 ret envelope incorporates time and is essentially a four-dimensional concept. Yours, etc., W. S. LOWRY.
Department of Cancer Studies, The Queen's University of Belfast. REFERENCES BOLANDE, J., 1971. Afterloading in radiotherapy. Proceedings of Conference, panel discussion, U.S. Department of HEW, New York. ELLIS, F., 1971. Nominal standard dose and the ret. British Journal of Radiology, 44, 101. FLETCHER, G., 1973. Textbook of Radiotherapy, pp. 645 (Lea and Febiger, Philadelphia). GEORGE, F. W., 1971. Afterloading in radiotherapy. Proceedings of Conference, p. 292, U.S. Department of HEW, New York. JOSLIN, C. A. F., SMITH, C. W., and MALLIK, A., 1972.
The
treatment of cervix cancer using high activity 60Co sources. British Journal of Radiology, 45, 257-270. LOWRY, W. S., 1973. The volume factor in radiotherapy. Proceedings of the XIII International Congress of Radiology, Madrid.
THE EDITOR—SIR, RENAL OPACIFICATION—MISLEADING ILLUSTRATION
In the most recently published volume of Recent Advances in Radiology (1975), Dr. T. Sherwood discusses the role of volume change in renal opacification. Based on his own illustrated experiments, he concludes that an increase in the amount of contrast medium alone, without a change in concentration, will not readily lead to a discernible radiographic density effect. We believe that changes in the amount of contrast medium play an important role in renal opacification under all the many urographic situations (Dure-Smith et al, 1971; Dure-Smith et al, 1974; DureSmith et al, 1972; Dure-Smith, 1975; Dure-Smith et al, unpublished), but particularly in obstructive uropathy (Dure-Smith et al, 1972). On review of all the material, this difference of opinion seems to be more apparent than real, particularly as we have now had the privilege of seeing the original radiographs of Dr. Sherwood's experiments, and they bear little resemblance to the published illustrations. In fact these experiments confirm rather than conflict with our view. However, it is a pity that they are presented in this way in such a distinguished text-book since they appear to contravene several basic physical concepts and this may be misleading to those not conversant with the principles of X-ray imagery or the minutiae of this particular subject. The experiments consist of radiographing four balloons filled with increasing volumes (25, 5, 10 and 20 ml.) of contrast medium containing 15mg/ml. of iodine in a water-bath 7-2 cm deep. A repeat experiment used a concentration of 150 mg/ml. of iodine. The radiographs as published show no detectable difference between any of the balloons. This would imply that not only does volume change make no difference, but neither does a change in concentration from 15 to 150 mg/ ml. of iodine (virtually the whole range occurring during urography). The original radiographs do show the expected difference between the two concentrations for all volumes. The original radiographs do not, however, show a significant change in density with increasing volume. Nor is
572
THE 2,500 RET ENVELOPE
The results of treatment in cancer of the cervix vary widely around the world. Many reasons have been suggested to explain the discrepancies, including errors in staging and "personalization" of treatment. Unfortunately, it has not been possible to make useful comparisons between different centres. This has been partly due to inadequate methods of describing dosimetry patterns in the pelvis. Prescribed doses are often close to the limits of tolerance. Enormous doses are delivered to the tissues next to the intracavitary applicator, but, due to the inverse square law, these doses fall off extremely rapidly over small distances. Thus it has been difficult to agree on a point which is clinically meaningful for prescription purposes. The confusion is seen in several treatment situations. For instance, in endometrial carcinoma, some centres quote the uterine dose in milligram-hours, but the vaginal dose in rad. Where interstitial therapy is added, dosimetry by this means becomes very difficult if not impossible, even where sophisticated computer techniques are available. In describing a clinical example of such a case, the current edition of a well-known textbook (Fletcher, 1973) simply states "No attempt was made to calculate the implant". The problem is further complicated by the difficulty of combining radium dosimetry and external dosimetry (Joslin et al., 1972). It is sometimes felt that a radium rad is physically different from a megavoltage rad, and that adding the two units together is like adding "apples and pears". This may not be the case. The apparent clinical differences in dose tolerance are probably mostly due to the different volumes of tissue irradiated. A less important factor is the continuous low dose-rate effect of radium compared with intermittent fractionated external therapy. Thus in the treatment of pelvic cancer, the delicate balance between morbidity and mortality depends as much on the volume of tissue irradiated as on the dose at a fixed point. I have recently analysed this factor and noted that if the treatment volume is decreased by 50 per cent, the tolerated dosage can be increased by 25 per cent in the pelvis (Lowry, 1973). Bolande (1971) suggested incorporating this factor and proposed quoting the volume enclosed by an envelope whose surface dose is about that of a point A, namely 7,000 rad. In the final analysis, when we come to compare pelvic dosimetry in different centres, we will also need to know the fractionation used. The Manchester, Stockholm, Paris, Houston and other techniques all use different quantities of radium in different combinations with external therapy. It is now possible to compare these different fractionation schedules using the Ellis formula and ret system of dosimetry (Ellis, 1971). This concept can be extended to include intracavitary irradiation using an iso-effect link between continuous radium dosimetry and discontinuous external beam therapy. Applying this method, the nominal standard dose (NSD) at point A is about 2,500 rets (George, 1971). We now have an opportunity after many years of combining all the above concepts on the computer and, at the same time, incorporating current concepts in radiation therapy and radiobiology. In most cases of malignant disease, a dose of 2,500 rets is lethal. It is therefore recommended that the volume of tissue enclosed in the 2,500-ret envelope should be quoted. Tissue within this envelope would receive a supralethal dose. Tissue outside would have the potential for sublethal recovery. The envelope itself would still be a "flat pear" whose "waistline" would lie somewhere near point A. Calculations suggest that the volume of this envelope varies between 100 cm3 and 200 cm3. The exact volume can be determined on the computer by integrating slices of tissue across afixedplane. The shape of the envelope will vary in different treatment situations. But the outline can be determined on the
computer and the data displayed in the form of a computer print-out. It is then possible to sketch in the position of the tumour, bladder and rectum. Several examples of different treatment situations have been worked out in the United States and detailed results will be presented shortly. Lastly it is noted that the 2,500 ret envelope incorporates time and is essentially a four-dimensional concept. Yours, etc., W. S. LOWRY.
Department of Cancer Studies, The Queen's University of Belfast. REFERENCES BOLANDE, J., 1971. Afterloading in radiotherapy. Proceedings of Conference, panel discussion, U.S. Department of HEW, New York. ELLIS, F., 1971. Nominal standard dose and the ret. British Journal of Radiology, 44, 101. FLETCHER, G., 1973. Textbook of Radiotherapy, pp. 645 (Lea and Febiger, Philadelphia). GEORGE, F. W., 1971. Afterloading in radiotherapy. Proceedings of Conference, p. 292, U.S. Department of HEW, New York. JOSLIN, C. A. F., SMITH, C. W., and MALLIK, A., 1972.
The
treatment of cervix cancer using high activity 60Co sources. British Journal of Radiology, 45, 257-270. LOWRY, W. S., 1973. The volume factor in radiotherapy. Proceedings of the XIII International Congress of Radiology, Madrid.
THE EDITOR—SIR, RENAL OPACIFICATION—MISLEADING ILLUSTRATION
In the most recently published volume of Recent Advances in Radiology (1975), Dr. T. Sherwood discusses the role of volume change in renal opacification. Based on his own illustrated experiments, he concludes that an increase in the amount of contrast medium alone, without a change in concentration, will not readily lead to a discernible radiographic density effect. We believe that changes in the amount of contrast medium play an important role in renal opacification under all the many urographic situations (Dure-Smith et al, 1971; Dure-Smith et al, 1974; DureSmith et al, 1972; Dure-Smith, 1975; Dure-Smith et al, unpublished), but particularly in obstructive uropathy (Dure-Smith et al, 1972). On review of all the material, this difference of opinion seems to be more apparent than real, particularly as we have now had the privilege of seeing the original radiographs of Dr. Sherwood's experiments, and they bear little resemblance to the published illustrations. In fact these experiments confirm rather than conflict with our view. However, it is a pity that they are presented in this way in such a distinguished text-book since they appear to contravene several basic physical concepts and this may be misleading to those not conversant with the principles of X-ray imagery or the minutiae of this particular subject. The experiments consist of radiographing four balloons filled with increasing volumes (25, 5, 10 and 20 ml.) of contrast medium containing 15mg/ml. of iodine in a water-bath 7-2 cm deep. A repeat experiment used a concentration of 150 mg/ml. of iodine. The radiographs as published show no detectable difference between any of the balloons. This would imply that not only does volume change make no difference, but neither does a change in concentration from 15 to 150 mg/ ml. of iodine (virtually the whole range occurring during urography). The original radiographs do show the expected difference between the two concentrations for all volumes. The original radiographs do not, however, show a significant change in density with increasing volume. Nor is
572
