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Improved dose distribution homogeneity in conservative breast cancer irradiation
Radiotherapy and Oncology, 22 (1991) 245-247 © 1991 Elsevier Science Publishers B.V. All rights reserved. 0167-8140/91/$03.50
245
RADION00903
Improved dose distribution homogeneity in conservative breast cancer irradiation G u i d o Garavaglia, Christel P o r e p p and M a r i a n J o z e f o w s k y Department of Radiation Oncology and Nuclear Medicine, Ospedale San Giovanni, Bellinzona, Switzerland (Received 4 February 1991, revision received 21 July 1991, accepted 20 August 1991)
Key words: Breast conserving therapy; Dose distribution homogeneity; Cranio-caudal wedging
Summary At the occasion of recent meetings of the radiation oncology community, the description of a whole breast irradiation technique making use of cranio-caudal oriented wedge filters to compensate for dose distribution inhomogeneity in this direction has given rise to some discussions and misunderstandings. It is the scope of this presentation to describe the technique and its possible use for other localizations.
Introduction In patients undergoing conservative treatment of breast carcinoma, the usual radiotherapy technique is to irradiate the whole breast with two tangential opposing fields as described for example in the EORTC Phase III Study, Protocol 22881/10882. It is also usual to treat with wedged beams in order to compensate for the varying amount of breast tissue in the transverse direction. The wedge angle depends primarily on the shape of the breast but also on the amount of lung tissue at the base of the fields. This can, of course, only be seen if lung density corrections are carried out: for the same dose to a reference point at the center of the breast tissue, the lung dose is reduced by using smaller wedge filter angles [2-4]. Figure 1, a picture of a CT-scan through the center of the breast, shows the typical beam arrangement: two tangential fields with their central axis slightly more than 180 degrees apart in order to align the dorsal beam edges and thus resulting in a lower lung dose. The figure also shows that only one field is wedged, as indicated by the triangular symbol: indeed, rather than using two
wedges, one per field, it is possible, and in this case desirable as will be seen below, to combine in one field all the wedging necessary to compensate for the shape of the breast in the transverse direction. The wedge
Fig. 1. CT-scan through the center of the breast showing the typical beam arrangement. Transverse wedging for the left lateral field is indicated by the triangular symbol outside the body contour.
Address for correspondence: Guido Garavaglia, Department of Radiation Oncology and Nuclear Medicine, Ospedale San Giovanni, CH-6500 Bellinzona, Switzerland.
246 Methods and results
This dose gradient can be reduced by taking advantage of the above mentioned fact that all the wedging necessary for the transverse direction can be applied through one field, thus allowing the use in the second field of a wedge oriented in the cranio-caudal direction: with the thin end of the wedge oriented cranially the dose is raised there and lowered caudally. Table I summarizes the results obtained for the typical case of Figs. 1 and 2:
Fig. 2. Oblique cranio-caudal section perpendicular to the transverse sections as indicated in Fig. 1 by the line 12. The cranial end is to the left.
angle needed is just the sum of the two individual wedge angles. The amount of irradiated lung volume is also reduced by positioning the patient on a table wedge [4] or, more simply, rotating the collimator as can be seen in the oblique cranio-caudal section (Fig. 2) perpendicular to the transverse sections as indicated in Fig. 1 by the line 12. Figure 2 shows the reconstructed outlines of the breast and of the lung as well as the rotation of the collimator used to reduce the amount of irradiated lung. This has, however, a disturbing consequence: as pointed out in the literature [1,3], even without table wedge or collimator rotation, the dose in the inferior part of the breast is usually higher because of the shorter beam entrance-exit distance there. Rotation of the collimator as shown in Fig. 2 moves the field out of the breast caudally and into the breast cranially, so that an even larger amount of tissue is irradiated in cranial sections as compared to caudal sections. Depending on the patient anatomy this can result in a dose gradient of more than 20~o.
- Calculations are usually performed in several transverse CT slices (9 or 11, sometimes more). The table presents data for only three transversal planes: the central plane shown in Fig. 1, one caudal and one cranial plane, 6 cm below and above the central plane. - For each calculation plane, the first half of the table gives the very different traversed tissue thicknesses measured along the dorsal beam edge of the fields and also at half the depth of breast tissue from the skin to the anterior outline of the lung measured at midseparation. - The second half of the table gives the range of the calculated dose distribution in the target area of each plane, first with a 30 degree transverse wedge only, then with the addition of a 15 degree longitudinal wedge. The numbers represent dose values normalized to 100 ~o at the isocenter in the central plane. The calculations have been performed on the Philips O S S Treatment Planning System. As can be seen, the dose inhomogeneity of 21~o (90-111) observed in this case is reduced to 16~o (94-110) by applying the longitudinal wedge. More important, the minimum target dose is raised from 90 to 94~o of the prescription dose (100 ~o). The improvement is often even better than this. It is worth mentioning that, as in the case of transverse wedging, the choice
TABLE I
Dose distribution range with and without longitudinal wedging. Cranial plane
Central plane
Caudal plane
Position of calculation plane (cm) Tissue thickness at the dorsal beam edge (cm) Tissue thickness at half breast tissue depth (cm)
+6 20 14
0 19 10
-6 14 7
Dose range without cranio-caudal wedging (%) Dose range with cranio-caudal wedging (%)
90-105 94-110
100-105 100-105
105-111 102-106
Note the large dose heterogeneity without longitudinal wedging, 90~ (cranial plane) to 111% (caudal plane) and the relative improvement with longitudinal wedging: 94-110% (cranial plane).
247 of the optimal wedge angle is facilitated by using the universal wedge approach [1] (i.e. for each treatment session the field in question is irradiated with a combination of a wedged and an open beam); this, in turn, is conveniently implemented with a linear accelerator having a motor-wedge system. The small differences in dose distribution observed when exchanging the placement of the wedges in the medial and lateral fields are easily compensated by adjusting the respective beam weights. Discussion
The purpose of this presentation is to introduce and describe a modification of the usual technique for the conservative irradiation of early breast cancer and to encourage its implementation. The proposed modification should help reduce the observed dose distribution inhomogeneity down to 95-110~o of the prescription dose. No detailed dosimetric verification of the calculated dose distribution has been carried out for the breast irradiation. The technique has been in routine clinical use for several years, is implemented for the large majority of breast conserving therapy patients, and gives full satisfaction. At the time of the introduction of the technique, in vivo dosimetry has been carried out confirming the presence of the dose gradient without and the improved dose distribution with longitudinal wedging. In vivo dosimetry is still performed for all breast and head and neck cases. Usually, five semiconductor detectors are placed cranially, centrally, and caudally on the irradiated portion of the patient's surface. No corrections for field size, source distance or other parameters are applied so that the results are of a relative nature, to be compared among themselves for a given irradiation session, or with the results of previous sessions (for example with and without longitudinal wedging), or with those of other patients.
In addition, the validity of the method is further warranted by the fact that one important source of inaccuracy in the dose distribution calculation for tangential breast and other irradiations like head and neck cases, namely the reduced scattering due to the large "in-air" portion of the tangential beams, is accounted for in the treatment planning system by using depth dose data and output factors for field sizes reduced to the actual tissue cross-section intercepted by the beams. Concerning the implementation of the technique by other centers, possibly on other commercially available treatment planning systems, a quick survey has shown that most systems currently used in Switzerland allow dose distribution calculation in several parallel transverse planes taking into account both transversal and longitudinal wedging. Full 3-D systems are beginning to be available and it is expected that the validity of this technique will be confirmed. The technique has been well accepted by the X-ray technicians since it does not require longer set-up time or, except at the beginning, planning time. It is obviously not restricted to breast treatments, but can also be used advantageously in head and neck and in chest irradiations. In the first case, the typical application is with parallel opposed lateral beams: one field may or may not be wedged in the transverse direction, depending on whether only the anterior portion of the neck is to be irradiated (sparing of the spinal cord) or not, whilst the opposite field is wedged longitudinally with the thin wedge end oriented cranially to compensate for the increasing tissue thickness in that direction. As for chest irradiations, the advantage of using cranio-caudal wedging for AP/PA parallel opposed fields is immediately apparent from the sloping shape of the thorax and the quite different tissue thicknesses of the upper and lower chest, depending of course on the specific tumor localization and patient anatomy.
References 1 Chin L. M., Cheng C. W., Siddon R. L., Rice R. K., Mijnheer B. J., and Harris J.R. Three dimensional photon dose distribution with and without lung corrections for tangential breast intact treatments. Int. J. Radiat. Oncol. Biol. Phys. 17: 1327-1335, 1989. 2 Fraass B. A., Lichter A. S., McShan D. L., Yanke B. R., Diaz R. F., Yeakel K. S. and Van de Geijn J. The influence of lung density corrections on treatment planning for primary breast cancer. Int. J. Radiat. Oncol. Biol. Phys. 14: 179-190, 1988. 3 Van Tienhoven G., van Bree, N. A. M. and Mijnheer, B.J. Qual-
ity assurance of the EORTC trial 22881/10882 "Assessment of the role of the booster dose in breast conserving therapy": the dummy run. Radiother. Oncol. (this issue). 4 Webb S., Leach M. O., Bentley R. E., Maureemootoo K., Yarnold J. R., Toms M. A., Gardiner J. and Parton D. Clinical dosimetry for radiotherapy to the breast based on imaging with the prototype Royal Marsden Hospital CT simulator. Phys. Med. Biol. 32: 835-845, 1987.
245
RADION00903
Improved dose distribution homogeneity in conservative breast cancer irradiation G u i d o Garavaglia, Christel P o r e p p and M a r i a n J o z e f o w s k y Department of Radiation Oncology and Nuclear Medicine, Ospedale San Giovanni, Bellinzona, Switzerland (Received 4 February 1991, revision received 21 July 1991, accepted 20 August 1991)
Key words: Breast conserving therapy; Dose distribution homogeneity; Cranio-caudal wedging
Summary At the occasion of recent meetings of the radiation oncology community, the description of a whole breast irradiation technique making use of cranio-caudal oriented wedge filters to compensate for dose distribution inhomogeneity in this direction has given rise to some discussions and misunderstandings. It is the scope of this presentation to describe the technique and its possible use for other localizations.
Introduction In patients undergoing conservative treatment of breast carcinoma, the usual radiotherapy technique is to irradiate the whole breast with two tangential opposing fields as described for example in the EORTC Phase III Study, Protocol 22881/10882. It is also usual to treat with wedged beams in order to compensate for the varying amount of breast tissue in the transverse direction. The wedge angle depends primarily on the shape of the breast but also on the amount of lung tissue at the base of the fields. This can, of course, only be seen if lung density corrections are carried out: for the same dose to a reference point at the center of the breast tissue, the lung dose is reduced by using smaller wedge filter angles [2-4]. Figure 1, a picture of a CT-scan through the center of the breast, shows the typical beam arrangement: two tangential fields with their central axis slightly more than 180 degrees apart in order to align the dorsal beam edges and thus resulting in a lower lung dose. The figure also shows that only one field is wedged, as indicated by the triangular symbol: indeed, rather than using two
wedges, one per field, it is possible, and in this case desirable as will be seen below, to combine in one field all the wedging necessary to compensate for the shape of the breast in the transverse direction. The wedge
Fig. 1. CT-scan through the center of the breast showing the typical beam arrangement. Transverse wedging for the left lateral field is indicated by the triangular symbol outside the body contour.
Address for correspondence: Guido Garavaglia, Department of Radiation Oncology and Nuclear Medicine, Ospedale San Giovanni, CH-6500 Bellinzona, Switzerland.
246 Methods and results
This dose gradient can be reduced by taking advantage of the above mentioned fact that all the wedging necessary for the transverse direction can be applied through one field, thus allowing the use in the second field of a wedge oriented in the cranio-caudal direction: with the thin end of the wedge oriented cranially the dose is raised there and lowered caudally. Table I summarizes the results obtained for the typical case of Figs. 1 and 2:
Fig. 2. Oblique cranio-caudal section perpendicular to the transverse sections as indicated in Fig. 1 by the line 12. The cranial end is to the left.
angle needed is just the sum of the two individual wedge angles. The amount of irradiated lung volume is also reduced by positioning the patient on a table wedge [4] or, more simply, rotating the collimator as can be seen in the oblique cranio-caudal section (Fig. 2) perpendicular to the transverse sections as indicated in Fig. 1 by the line 12. Figure 2 shows the reconstructed outlines of the breast and of the lung as well as the rotation of the collimator used to reduce the amount of irradiated lung. This has, however, a disturbing consequence: as pointed out in the literature [1,3], even without table wedge or collimator rotation, the dose in the inferior part of the breast is usually higher because of the shorter beam entrance-exit distance there. Rotation of the collimator as shown in Fig. 2 moves the field out of the breast caudally and into the breast cranially, so that an even larger amount of tissue is irradiated in cranial sections as compared to caudal sections. Depending on the patient anatomy this can result in a dose gradient of more than 20~o.
- Calculations are usually performed in several transverse CT slices (9 or 11, sometimes more). The table presents data for only three transversal planes: the central plane shown in Fig. 1, one caudal and one cranial plane, 6 cm below and above the central plane. - For each calculation plane, the first half of the table gives the very different traversed tissue thicknesses measured along the dorsal beam edge of the fields and also at half the depth of breast tissue from the skin to the anterior outline of the lung measured at midseparation. - The second half of the table gives the range of the calculated dose distribution in the target area of each plane, first with a 30 degree transverse wedge only, then with the addition of a 15 degree longitudinal wedge. The numbers represent dose values normalized to 100 ~o at the isocenter in the central plane. The calculations have been performed on the Philips O S S Treatment Planning System. As can be seen, the dose inhomogeneity of 21~o (90-111) observed in this case is reduced to 16~o (94-110) by applying the longitudinal wedge. More important, the minimum target dose is raised from 90 to 94~o of the prescription dose (100 ~o). The improvement is often even better than this. It is worth mentioning that, as in the case of transverse wedging, the choice
TABLE I
Dose distribution range with and without longitudinal wedging. Cranial plane
Central plane
Caudal plane
Position of calculation plane (cm) Tissue thickness at the dorsal beam edge (cm) Tissue thickness at half breast tissue depth (cm)
+6 20 14
0 19 10
-6 14 7
Dose range without cranio-caudal wedging (%) Dose range with cranio-caudal wedging (%)
90-105 94-110
100-105 100-105
105-111 102-106
Note the large dose heterogeneity without longitudinal wedging, 90~ (cranial plane) to 111% (caudal plane) and the relative improvement with longitudinal wedging: 94-110% (cranial plane).
247 of the optimal wedge angle is facilitated by using the universal wedge approach [1] (i.e. for each treatment session the field in question is irradiated with a combination of a wedged and an open beam); this, in turn, is conveniently implemented with a linear accelerator having a motor-wedge system. The small differences in dose distribution observed when exchanging the placement of the wedges in the medial and lateral fields are easily compensated by adjusting the respective beam weights. Discussion
The purpose of this presentation is to introduce and describe a modification of the usual technique for the conservative irradiation of early breast cancer and to encourage its implementation. The proposed modification should help reduce the observed dose distribution inhomogeneity down to 95-110~o of the prescription dose. No detailed dosimetric verification of the calculated dose distribution has been carried out for the breast irradiation. The technique has been in routine clinical use for several years, is implemented for the large majority of breast conserving therapy patients, and gives full satisfaction. At the time of the introduction of the technique, in vivo dosimetry has been carried out confirming the presence of the dose gradient without and the improved dose distribution with longitudinal wedging. In vivo dosimetry is still performed for all breast and head and neck cases. Usually, five semiconductor detectors are placed cranially, centrally, and caudally on the irradiated portion of the patient's surface. No corrections for field size, source distance or other parameters are applied so that the results are of a relative nature, to be compared among themselves for a given irradiation session, or with the results of previous sessions (for example with and without longitudinal wedging), or with those of other patients.
In addition, the validity of the method is further warranted by the fact that one important source of inaccuracy in the dose distribution calculation for tangential breast and other irradiations like head and neck cases, namely the reduced scattering due to the large "in-air" portion of the tangential beams, is accounted for in the treatment planning system by using depth dose data and output factors for field sizes reduced to the actual tissue cross-section intercepted by the beams. Concerning the implementation of the technique by other centers, possibly on other commercially available treatment planning systems, a quick survey has shown that most systems currently used in Switzerland allow dose distribution calculation in several parallel transverse planes taking into account both transversal and longitudinal wedging. Full 3-D systems are beginning to be available and it is expected that the validity of this technique will be confirmed. The technique has been well accepted by the X-ray technicians since it does not require longer set-up time or, except at the beginning, planning time. It is obviously not restricted to breast treatments, but can also be used advantageously in head and neck and in chest irradiations. In the first case, the typical application is with parallel opposed lateral beams: one field may or may not be wedged in the transverse direction, depending on whether only the anterior portion of the neck is to be irradiated (sparing of the spinal cord) or not, whilst the opposite field is wedged longitudinally with the thin wedge end oriented cranially to compensate for the increasing tissue thickness in that direction. As for chest irradiations, the advantage of using cranio-caudal wedging for AP/PA parallel opposed fields is immediately apparent from the sloping shape of the thorax and the quite different tissue thicknesses of the upper and lower chest, depending of course on the specific tumor localization and patient anatomy.
References 1 Chin L. M., Cheng C. W., Siddon R. L., Rice R. K., Mijnheer B. J., and Harris J.R. Three dimensional photon dose distribution with and without lung corrections for tangential breast intact treatments. Int. J. Radiat. Oncol. Biol. Phys. 17: 1327-1335, 1989. 2 Fraass B. A., Lichter A. S., McShan D. L., Yanke B. R., Diaz R. F., Yeakel K. S. and Van de Geijn J. The influence of lung density corrections on treatment planning for primary breast cancer. Int. J. Radiat. Oncol. Biol. Phys. 14: 179-190, 1988. 3 Van Tienhoven G., van Bree, N. A. M. and Mijnheer, B.J. Qual-
ity assurance of the EORTC trial 22881/10882 "Assessment of the role of the booster dose in breast conserving therapy": the dummy run. Radiother. Oncol. (this issue). 4 Webb S., Leach M. O., Bentley R. E., Maureemootoo K., Yarnold J. R., Toms M. A., Gardiner J. and Parton D. Clinical dosimetry for radiotherapy to the breast based on imaging with the prototype Royal Marsden Hospital CT simulator. Phys. Med. Biol. 32: 835-845, 1987.