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The application of dynamic field shaping and dynamic dose rate control in conformal rotational treatment of the prostate
Medical Dosimetry, Vol. 27, No. 4, pp. 251–254, 2002 Copyright © 2002 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/02/$–see front matter
PII: S0958-3947(02)00148-6
THE APPLICATION OF DYNAMIC FIELD SHAPING AND DYNAMIC DOSE RATE CONTROL IN CONFORMAL ROTATIONAL TREATMENT OF THE PROSTATE MATT TOBLER, C.M.D., GORDON WATSON, M.D., PH.D., and DENNIS D. LEAVITT, PH.D. University of Utah Health Science Center, Salt Lake City, UT (Accepted 15 April 2001)
Abstract—The current philosophy of dose escalation in the treatment of prostate cancer has forced the treatment planner to re-evaluate his/her planning approach. Precise and accurate delivery of dose to the prostate while maintaining the required dose limits to the normal critical structures, such as the rectum, has become increasingly difficult in light of these escalated doses. Conformal treatment techniques allow the treatment planner to precisely shape each individual treatment field so that desired volume coverage and normal tissue sparing can be achieved. In addition to these beam-shaping advantages, adjustment of an individual beam’s weighting also helps to create the desired distribution and tissue sparing. Rotational therapy “simulates” treatment with multiple beams and angles, similar to the thought process behind conformal treatment technique. With rotational therapy, however, the treatment planner’s inability to provide adequate beam shaping and weighting adjustment has placed limits on its value as a viable planning option. The introduction of computer-controlled treatment machines, which allow dynamic adjustment of the field shape with the rotation of the beam, makes it possible to re-evaluate rotational therapy as a potential option. Similarly, the treatment planner’s ability to change field weighting can be accomplished by the application of dynamic dose rate control, allowing a rotational beam to deliver a weighting similar to that possible with conformal fixed-field techniques. Dose-volume histogram data will be used to evaluate doses delivered to the prostate, rectum, and bladder using rotational therapy with dynamic field shape and dynamic dose rate control as a treatment planning option. The dose delivery and normal tissue-sparing potential of this technique compared to coplanar and noncoplanar conformal fixed-field techniques will also be presented. © 2002 American Association of Medical Dosimetrists. Key Words:
Treatment planning, dynamic arc, dynamic dose rate, conformal field shaping.
these fixed fields to the prostate while maintaining better sparing of the rectum, bladder, and femoral heads. Current technology allows utilization of multileaf collimator systems (MLC) and applies computer control to allow adjustment of leaf positions and field shapes for each treatment field without the need for external blocking, such as cerrobend blocks. An extension of this computer control allows dynamic adjustment of individual leaf positions during treatment. For stationary fields, this allows precise delivery of varying intensities of dose across an individual beam trajectory to deliver a tightly conformal and uniform dose to the volume of interest (intensity-modulated radiotherapy). In a rotational setting, computer control allows the field shape to be dynamically adjusted as the machine rotates about the patient (dynamic arc therapy). While dynamic field shaping begins to reopen rotational therapy as a treatment option for the prostate, an added feature is required. Dynamic control and adjustment of the number of monitor units delivered per degree of machine rotation (dynamic dose rate control) extends the idea of adjustment of field weighting for arc therapy, allowing creation and adjustment of the dose distribution similar to the field-weighting option currently available when treating with fixed fields. This paper will employ dose-volume histogram (DVH) data to evaluate treatment planning for the pros-
INTRODUCTION In earlier works describing prostate treatment, one technique was the use of rotational fields,1 sometimes called the “butterfly arc” technique. This technique treated the prostate using bilateral rotational fields rotating from an angle of 45° above the lateral position to 45° below the lateral position (the exact starting and ending rotational position determined by the treatment planner). The absence of rotation in the anterior and posterior quadrants helped to limit delivery of excessive dose to the rectum and bladder. As focus moved toward tightening margins and escalating doses, the idea of using rotational fields to treat the prostate was abandoned because of the limited ability the treatment planner had to shape the field, and the resultant dose as the machine rotated. Fixed-field coplanar and noncoplanar techniques replaced consideration of rotational fields, primarily due to the planner’s ability to conform the shape of the field to the prostate volume. Similarly, the ability to change an individual beam’s weighting, something unachievable with rotational fields, helps the planner to conform dose from Reprint requests to: M. Tobler, University of Utah, Department of Radiation Oncology, Health Science Center, 50 North Medical Drive, Salt Lake City, UT 84132. E-mail: matthew.tobler@ hsc.utah.edu 251
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Medical Dosimetry
Volume 27, Number 4, 2002
tate utilizing rotational therapy that incorporates the use of dynamic field-shaping and dose-rate control. These data will be compared to doses delivered to the prostate, rectum, bladder, and femoral heads from a 6-field coplanar and a 4-field noncoplanar technique. METHODS AND MATERIALS The current technique used for treating prostate cancers at the University of Utah consists of treating the prostate and seminal vesicles to a dose of 4500 cGy with a 4-field box technique. Blocking is created that provides a 2-cm margin around this volume. At 4500 cGy, the prostate is either boosted with high-dose-rate brachytherapy (approximately 10% of patients) or boosted with conformal external-beam radiotherapy (EBRT) (approximately 90% of patients). Because treatment philosophies differ slightly from center to center concerning treatment of the initial prostate volume, treatment planning comparisons will be made for the boost portion of treatment only. If desired, however, these planning comparisons can be extended to the entire course of treatment of the prostate, provided that margins are set appropriately. When treated with EBRT, patients receive an additional 3060 cGy to the boost reduced volume. This reduced volume includes prostate but excludes the seminal vesicles. The boost volume is treated with a 6-field coplanar technique consisting of opposed lateral fields and opposed 45° anterior and posterior oblique fields. The beams are weighted slightly heavier toward the lateral fields to provide for better sparing of the rectum. While this technique has been previously described, slight modifications have been made to its application at our center.2 MLC boost blocks are created to provide a 1-cm anterior, superior, and inferior margin. The posterior block margin is reduced to 0.75 cm to reduce dose to the anterior rectal wall. The second technique evaluated for comparison is a 4-field noncoplanar technique described by Marsh et al.,3,4 consisting of 2 opposed lateral fields and 2 inferior, anterior oblique fields, 1 treating from the right side and 1 treating from the left. Forty-five degree wedges are placed on the lateral fields and weightings are set to provide a uniform dose distribution. As with the technique described above, blocking for all fields provides a 1-cm margin in the anterior, superior, and inferior directions, with a 0.75-cm margin posteriorly. The third technique is the dynamic conformal rotational technique with further incorporation of dynamic dose rate control. This technique utilizes the treatment machine’s ability to dynamically shape the treatment field as it rotates around the patient. Secondly, this technique proposes dynamic adjustment of the machine’s dose rate during rotation. In effect, the machine will start out at a posterior oblique angle and begin by delivering a small number of monitor units as it begins rotating
Fig. 1. Comparative DVH data for the bladder, shown for the 6-field technique, the 4-field noncoplanar technique, and the dynamic conformal arc technique.
anteriorly. As it rotates toward the direct lateral position, the dose rate will progressively increase until the highest number of monitor units are delivered at the direct lateral position. As the machine continues on its rotation path toward the anterior, the dose rate (effective monitor units per degree) will again decrease until it reaches a minimum dose rate at the anterior oblique stop angle. The opposite side would then be treated in a similar fashion. Currently, treatment planning systems are unable to represent the dynamic dose rate control option that will be required for this dynamic conformal rotational treatment technique. To simulate this technique, multiple rotational fields were created at intervals of 12° of rotation. Each of these smaller rotational fields were designed to represent the desired adjustment of the field shape, as well as adjustment of the relative beam weighting with every 12° of machine rotation. For use in the comparison, a representative patient CT data set was chosen. It was important to choose a data set that provided enough extension superiorly and inferiorly to allow evaluation of a noncoplanar technique. The CT scan was taken with a 1-cm spacing from above and below the prostate volume and a 0.5-cm spacing through the prostate volume. The CT extended superiorly to the level of the iliac crests and inferiorly to the level of the mid-femur. Structures of concern were outlined, including the rectum, bladder, and both femoral heads. Initial and boost tumor volumes were drawn by the physician. All treatment plans were evaluated using the RAHD
Dynamic field shaping and dose rate control ● M. TOBLER et al.
Fig. 2. Comparative DVH data for the rectum, shown for the 6-field technique, the 4-field noncoplanar technique, and the dynamic conformal arc technique.
treatment planning system (RAHD Oncology Products, St. Louis, MO). Blocking was designed for use with the Varian 2100 CD treatment machine (Varian Associates, Palo Alto, CA) and an 80-leaf MLC blocking system. RESULTS DVH data for the bladder shows that 6-field coplanar treatment provided the worst results of the 3 techniques. Comparison of doses shows slightly lower but comparable results delivered by the 4-field noncoplanar technique compared to treatment with dynamic arcs (Fig. 1). Comparison of the rectal volume shows comparable doses delivered between the both the 4-field noncoplanar and the dynamic arc technique in the high-dose region, with slight improvement in the lower dose areas for the dynamic rotational technique. Again, the 6-field technique produced the least desirable results. The 6-field technique produces the best results when evaluating doses delivered to the femoral heads. Histogram data crosses at different points between the noncoplanar and dynamic arc techniques; however, the dynamic technique appears to produce more favorable results. DISCUSSION While dramatic improvements were not seen for the dynamic rotational technique compared to the noncoplanar technique, there are still advantages to be gained by
253
Fig. 3. Comparative DVH data for the femoral heads, shown for the 6-field technique, the 4-field noncoplanar technique, and the dynamic conformal arc technique.
using this technique. First, there is a reduction in the number of CT slices required for evaluation of the technique, resulting in the potential for savings, both in cost and time. Evaluation of the noncoplanar technique was not even possible with the limited number of CT slices required to plan either of the coplanar techniques. Only when an extended CT patient data set was obtained were we even able to evaluate the use of this technique. Secondly, the eventual simplicity of application of this technique will reduce not only the treatment time but also the complexity, also resulting in potential for cost savings, as well as a reduction in the potential for errors in setup. As previously mentioned, the evaluation of these techniques was done considering the use of a multileaf collimator system with a leaf size of 1 cm. The availability of multileaf systems with leaf sizes of 0.5 cm or less will allow better field definition and tissue sparing. While the execution of this technique is possible with currently available technologies, the need for new treatment planning tools was identified. While weights for dynamic arc were manually set at the discretion of the treatment planner, the addition of an optimization routine could potentially produce better results. An optimized blocking routine that applies differing leaf positions, similar to those used in planning for IMRT treatments, may also produce further gains in dose delivery or tissue sparing by further adjusting the relative weightings within an individual beam position.
254
Medical Dosimetry
Volume 27, Number 4, 2002
CONCLUSIONS
REFERENCES
This technique shows promise as a treatment planning option, particularly when combined with treatment planning optimization tools to aid in the adjustment of the relative dose delivery and field shape. While evaluation for this technique was done for treatment of the prostate, the idea of dynamic collimation and dose rate control can be applied to treatment of other areas, such as the lung or mediastinum.
1. Luka, S.; Kurup, R. Comparison of treatment plans for irradiating adenocarcinoma of the prostate. Med. Dosim. 20:117–22; 1995. 2. Ten Haken, R.K.; Perez-Tamayo, C.; Tesser, R.J.; McShane, D.L.; Fraass, B.A.; Lichter, A.S. Boost treatment of the prostate using shaped, fixed fields. Int. J. Radiat. Biol. Phys. 16:193–200; 1989. 3. Marsh, L.H.; Ten Haken, R.K.; Sandler, H.M. A customized nonaxial external beam technique for treatment of prostate carcinomas. Med. Dosim. 17:123–7; 1992. 4. Mesina, C.F.; Sharma, R.; Rissman, L.S.; Geering, L.; He, T.; Forman, J.D. Comparison of a conformal nonaxial boost with a four-field boost technique in the treatment of adenocarcinoma of the prostate. Int. J. Radiat. Oncol. Biol. Phys. 30:427–30; 1994.
PII: S0958-3947(02)00148-6
THE APPLICATION OF DYNAMIC FIELD SHAPING AND DYNAMIC DOSE RATE CONTROL IN CONFORMAL ROTATIONAL TREATMENT OF THE PROSTATE MATT TOBLER, C.M.D., GORDON WATSON, M.D., PH.D., and DENNIS D. LEAVITT, PH.D. University of Utah Health Science Center, Salt Lake City, UT (Accepted 15 April 2001)
Abstract—The current philosophy of dose escalation in the treatment of prostate cancer has forced the treatment planner to re-evaluate his/her planning approach. Precise and accurate delivery of dose to the prostate while maintaining the required dose limits to the normal critical structures, such as the rectum, has become increasingly difficult in light of these escalated doses. Conformal treatment techniques allow the treatment planner to precisely shape each individual treatment field so that desired volume coverage and normal tissue sparing can be achieved. In addition to these beam-shaping advantages, adjustment of an individual beam’s weighting also helps to create the desired distribution and tissue sparing. Rotational therapy “simulates” treatment with multiple beams and angles, similar to the thought process behind conformal treatment technique. With rotational therapy, however, the treatment planner’s inability to provide adequate beam shaping and weighting adjustment has placed limits on its value as a viable planning option. The introduction of computer-controlled treatment machines, which allow dynamic adjustment of the field shape with the rotation of the beam, makes it possible to re-evaluate rotational therapy as a potential option. Similarly, the treatment planner’s ability to change field weighting can be accomplished by the application of dynamic dose rate control, allowing a rotational beam to deliver a weighting similar to that possible with conformal fixed-field techniques. Dose-volume histogram data will be used to evaluate doses delivered to the prostate, rectum, and bladder using rotational therapy with dynamic field shape and dynamic dose rate control as a treatment planning option. The dose delivery and normal tissue-sparing potential of this technique compared to coplanar and noncoplanar conformal fixed-field techniques will also be presented. © 2002 American Association of Medical Dosimetrists. Key Words:
Treatment planning, dynamic arc, dynamic dose rate, conformal field shaping.
these fixed fields to the prostate while maintaining better sparing of the rectum, bladder, and femoral heads. Current technology allows utilization of multileaf collimator systems (MLC) and applies computer control to allow adjustment of leaf positions and field shapes for each treatment field without the need for external blocking, such as cerrobend blocks. An extension of this computer control allows dynamic adjustment of individual leaf positions during treatment. For stationary fields, this allows precise delivery of varying intensities of dose across an individual beam trajectory to deliver a tightly conformal and uniform dose to the volume of interest (intensity-modulated radiotherapy). In a rotational setting, computer control allows the field shape to be dynamically adjusted as the machine rotates about the patient (dynamic arc therapy). While dynamic field shaping begins to reopen rotational therapy as a treatment option for the prostate, an added feature is required. Dynamic control and adjustment of the number of monitor units delivered per degree of machine rotation (dynamic dose rate control) extends the idea of adjustment of field weighting for arc therapy, allowing creation and adjustment of the dose distribution similar to the field-weighting option currently available when treating with fixed fields. This paper will employ dose-volume histogram (DVH) data to evaluate treatment planning for the pros-
INTRODUCTION In earlier works describing prostate treatment, one technique was the use of rotational fields,1 sometimes called the “butterfly arc” technique. This technique treated the prostate using bilateral rotational fields rotating from an angle of 45° above the lateral position to 45° below the lateral position (the exact starting and ending rotational position determined by the treatment planner). The absence of rotation in the anterior and posterior quadrants helped to limit delivery of excessive dose to the rectum and bladder. As focus moved toward tightening margins and escalating doses, the idea of using rotational fields to treat the prostate was abandoned because of the limited ability the treatment planner had to shape the field, and the resultant dose as the machine rotated. Fixed-field coplanar and noncoplanar techniques replaced consideration of rotational fields, primarily due to the planner’s ability to conform the shape of the field to the prostate volume. Similarly, the ability to change an individual beam’s weighting, something unachievable with rotational fields, helps the planner to conform dose from Reprint requests to: M. Tobler, University of Utah, Department of Radiation Oncology, Health Science Center, 50 North Medical Drive, Salt Lake City, UT 84132. E-mail: matthew.tobler@ hsc.utah.edu 251
252
Medical Dosimetry
Volume 27, Number 4, 2002
tate utilizing rotational therapy that incorporates the use of dynamic field-shaping and dose-rate control. These data will be compared to doses delivered to the prostate, rectum, bladder, and femoral heads from a 6-field coplanar and a 4-field noncoplanar technique. METHODS AND MATERIALS The current technique used for treating prostate cancers at the University of Utah consists of treating the prostate and seminal vesicles to a dose of 4500 cGy with a 4-field box technique. Blocking is created that provides a 2-cm margin around this volume. At 4500 cGy, the prostate is either boosted with high-dose-rate brachytherapy (approximately 10% of patients) or boosted with conformal external-beam radiotherapy (EBRT) (approximately 90% of patients). Because treatment philosophies differ slightly from center to center concerning treatment of the initial prostate volume, treatment planning comparisons will be made for the boost portion of treatment only. If desired, however, these planning comparisons can be extended to the entire course of treatment of the prostate, provided that margins are set appropriately. When treated with EBRT, patients receive an additional 3060 cGy to the boost reduced volume. This reduced volume includes prostate but excludes the seminal vesicles. The boost volume is treated with a 6-field coplanar technique consisting of opposed lateral fields and opposed 45° anterior and posterior oblique fields. The beams are weighted slightly heavier toward the lateral fields to provide for better sparing of the rectum. While this technique has been previously described, slight modifications have been made to its application at our center.2 MLC boost blocks are created to provide a 1-cm anterior, superior, and inferior margin. The posterior block margin is reduced to 0.75 cm to reduce dose to the anterior rectal wall. The second technique evaluated for comparison is a 4-field noncoplanar technique described by Marsh et al.,3,4 consisting of 2 opposed lateral fields and 2 inferior, anterior oblique fields, 1 treating from the right side and 1 treating from the left. Forty-five degree wedges are placed on the lateral fields and weightings are set to provide a uniform dose distribution. As with the technique described above, blocking for all fields provides a 1-cm margin in the anterior, superior, and inferior directions, with a 0.75-cm margin posteriorly. The third technique is the dynamic conformal rotational technique with further incorporation of dynamic dose rate control. This technique utilizes the treatment machine’s ability to dynamically shape the treatment field as it rotates around the patient. Secondly, this technique proposes dynamic adjustment of the machine’s dose rate during rotation. In effect, the machine will start out at a posterior oblique angle and begin by delivering a small number of monitor units as it begins rotating
Fig. 1. Comparative DVH data for the bladder, shown for the 6-field technique, the 4-field noncoplanar technique, and the dynamic conformal arc technique.
anteriorly. As it rotates toward the direct lateral position, the dose rate will progressively increase until the highest number of monitor units are delivered at the direct lateral position. As the machine continues on its rotation path toward the anterior, the dose rate (effective monitor units per degree) will again decrease until it reaches a minimum dose rate at the anterior oblique stop angle. The opposite side would then be treated in a similar fashion. Currently, treatment planning systems are unable to represent the dynamic dose rate control option that will be required for this dynamic conformal rotational treatment technique. To simulate this technique, multiple rotational fields were created at intervals of 12° of rotation. Each of these smaller rotational fields were designed to represent the desired adjustment of the field shape, as well as adjustment of the relative beam weighting with every 12° of machine rotation. For use in the comparison, a representative patient CT data set was chosen. It was important to choose a data set that provided enough extension superiorly and inferiorly to allow evaluation of a noncoplanar technique. The CT scan was taken with a 1-cm spacing from above and below the prostate volume and a 0.5-cm spacing through the prostate volume. The CT extended superiorly to the level of the iliac crests and inferiorly to the level of the mid-femur. Structures of concern were outlined, including the rectum, bladder, and both femoral heads. Initial and boost tumor volumes were drawn by the physician. All treatment plans were evaluated using the RAHD
Dynamic field shaping and dose rate control ● M. TOBLER et al.
Fig. 2. Comparative DVH data for the rectum, shown for the 6-field technique, the 4-field noncoplanar technique, and the dynamic conformal arc technique.
treatment planning system (RAHD Oncology Products, St. Louis, MO). Blocking was designed for use with the Varian 2100 CD treatment machine (Varian Associates, Palo Alto, CA) and an 80-leaf MLC blocking system. RESULTS DVH data for the bladder shows that 6-field coplanar treatment provided the worst results of the 3 techniques. Comparison of doses shows slightly lower but comparable results delivered by the 4-field noncoplanar technique compared to treatment with dynamic arcs (Fig. 1). Comparison of the rectal volume shows comparable doses delivered between the both the 4-field noncoplanar and the dynamic arc technique in the high-dose region, with slight improvement in the lower dose areas for the dynamic rotational technique. Again, the 6-field technique produced the least desirable results. The 6-field technique produces the best results when evaluating doses delivered to the femoral heads. Histogram data crosses at different points between the noncoplanar and dynamic arc techniques; however, the dynamic technique appears to produce more favorable results. DISCUSSION While dramatic improvements were not seen for the dynamic rotational technique compared to the noncoplanar technique, there are still advantages to be gained by
253
Fig. 3. Comparative DVH data for the femoral heads, shown for the 6-field technique, the 4-field noncoplanar technique, and the dynamic conformal arc technique.
using this technique. First, there is a reduction in the number of CT slices required for evaluation of the technique, resulting in the potential for savings, both in cost and time. Evaluation of the noncoplanar technique was not even possible with the limited number of CT slices required to plan either of the coplanar techniques. Only when an extended CT patient data set was obtained were we even able to evaluate the use of this technique. Secondly, the eventual simplicity of application of this technique will reduce not only the treatment time but also the complexity, also resulting in potential for cost savings, as well as a reduction in the potential for errors in setup. As previously mentioned, the evaluation of these techniques was done considering the use of a multileaf collimator system with a leaf size of 1 cm. The availability of multileaf systems with leaf sizes of 0.5 cm or less will allow better field definition and tissue sparing. While the execution of this technique is possible with currently available technologies, the need for new treatment planning tools was identified. While weights for dynamic arc were manually set at the discretion of the treatment planner, the addition of an optimization routine could potentially produce better results. An optimized blocking routine that applies differing leaf positions, similar to those used in planning for IMRT treatments, may also produce further gains in dose delivery or tissue sparing by further adjusting the relative weightings within an individual beam position.
254
Medical Dosimetry
Volume 27, Number 4, 2002
CONCLUSIONS
REFERENCES
This technique shows promise as a treatment planning option, particularly when combined with treatment planning optimization tools to aid in the adjustment of the relative dose delivery and field shape. While evaluation for this technique was done for treatment of the prostate, the idea of dynamic collimation and dose rate control can be applied to treatment of other areas, such as the lung or mediastinum.
1. Luka, S.; Kurup, R. Comparison of treatment plans for irradiating adenocarcinoma of the prostate. Med. Dosim. 20:117–22; 1995. 2. Ten Haken, R.K.; Perez-Tamayo, C.; Tesser, R.J.; McShane, D.L.; Fraass, B.A.; Lichter, A.S. Boost treatment of the prostate using shaped, fixed fields. Int. J. Radiat. Biol. Phys. 16:193–200; 1989. 3. Marsh, L.H.; Ten Haken, R.K.; Sandler, H.M. A customized nonaxial external beam technique for treatment of prostate carcinomas. Med. Dosim. 17:123–7; 1992. 4. Mesina, C.F.; Sharma, R.; Rissman, L.S.; Geering, L.; He, T.; Forman, J.D. Comparison of a conformal nonaxial boost with a four-field boost technique in the treatment of adenocarcinoma of the prostate. Int. J. Radiat. Oncol. Biol. Phys. 30:427–30; 1994.