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 increas-
`ingly 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.
`
`INTRODUCTION
`
`In earlier works describing prostate treatment, one tech-
`nique 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 ab-
`sence 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 consider-
`ation 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 rota-
`tional 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
`
`these fixed fields to the prostate while maintaining better
`sparing of the rectum, bladder, and femoral heads. Cur-
`rent 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 set-
`ting, computer control allows the field shape to be dy-
`namically adjusted as the machine rotates about the pa-
`tient (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 adjust-
`ment of the dose distribution similar to the field-weight-
`ing option currently available when treating with fixed
`fields. This paper will employ dose-volume histogram
`(DVH) data to evaluate treatment planning for the pros-
`
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`Medical Dosimetry
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`Volume 27, Number 4, 2002
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`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 copla-
`nar 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 brachyther-
`apy (approximately 10% of patients) or boosted with
`conformal external-beam radiotherapy (EBRT) (approx-
`imately 90% of patients). Because treatment philoso-
`phies differ slightly from center to center concerning
`treatment of the initial prostate volume, treatment plan-
`ning comparisons will be made for the boost portion of
`treatment only. If desired, however, these planning com-
`parisons can be extended to the entire course of treatment
`of the prostate, provided that margins are set appropri-
`ately.
`When treated with EBRT, patients receive an addi-
`tional 3060 cGy to the boost reduced volume. This
`reduced volume includes prostate but excludes the sem-
`inal 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 poste-
`rior 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 tech-
`nique described above, blocking for all fields provides a
`1-cm margin in the anterior, superior, and inferior direc-
`tions, with a 0.75-cm margin posteriorly.
`The third technique is the dynamic conformal rota-
`tional 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 tech-
`nique 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 mini-
`mum 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 rep-
`resent 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 infe-
`riorly 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
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`Dynamic field shaping and dose rate control G M. TOBLER et al.
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`253
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`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.
`
`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.
`
`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 copla-
`nar treatment provided the worst results of the 3 tech-
`niques. 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 tech-
`nique 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 noncopla-
`nar technique, there are still advantages to be gained by
`
`using this technique. First, there is a reduction in the
`number of CT slices required for evaluation of the tech-
`nique, 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 avail-
`ability 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.
`
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`Medical Dosimetry
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`Volume 27, Number 4, 2002
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`CONCLUSIONS
`
`This technique shows promise as a treatment plan-
`ning option, particularly when combined with treatment
`planning optimization tools to aid in the adjustment of
`the relative dose delivery and field shape. While evalu-
`ation 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.
`
`REFERENCES
`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 non-
`axial 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.
`
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