Dynamic radiotherapy: Interactive movement of patient couch for treatment of craniospinal axis

Int. J. Radiation

Oncology

Biol.

Phys., Vol. 35. No. 4. pp. 771-777, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/96 $15.00 + .OO

PII: SO360-3016(96)00173-3

l

Technical fnnovations and Notes DYNAMIC

RADIOTHERAPY: INTERACTIVE MOVEMENT COUCH FOR TREATMENT OF CRANIOSPINAL

OF PATIENT AXIS

M.Sc.,* FRANKVAN DEN HEUVE%, PH.D.,* WILPRIED DE NEVE, M.D., PH.D.:

DIRKVERELLEN,

MARCDE

BE UKELEER,

B.Sc.*

AND GUY STORME, M.D.,

PH.D.*

*Departmentof Radiotherapy,AcademicHospital,Free University Brussels(AZ-VUB), Belgium; ‘Departmentof RadiotherapyandNuclearMedicine, University HospitalGhent,Belgium Purpose:The various techniquesthat have beendescribedfor treatmentof the craniospinalaxisshowthe common challengeof edgematching between adjacent orthogonal and parallel photon beams.Such edge matching is neededbecausethe maximum field length provided by modern treatment machinesis generally insufllcient to treat adults with lessthan three matching fields. Using the commontechniques,field edgematching becomes difficult, if for medicalreasons,the patient cannot be treated in the prone position. Methods and Materials: A scanningcouchtechniqueis proposed,with the patient lying in supineposition.After treating the cerebral and upper neck regionsby two lateral opposedhalf beam fields defined by asymmetric collimators(split beam),the patient is being movedalongthe spinalaxis through an 8.0 cm wide by 15.0cm long posterior split beam (allowing edgematching with the lateral fields at the neck region) by meansof remote controlled couch movement.Stopping and starting of the scanningfield resultedin a linear decreaseof doseon both sidesof the scan.Two ways of resolvingthis problemwere investigated. Results:The administereddosevaried lessthan 8.5% through the craniospinalaxis. Flatnessof the rectangular scannedfield was0.76%. Apart from dosehomogeneity,patient comfort and decreasedsimulationtime are major advantages. Conclusions:The proposedtechnique representsa suitablealternative using a commonlinear accelerator, requiring a remotecouch controller asan additional component. Dynamic radiotherapy, Cranio-spinalaxis, Treatment techniques.

INTRODUCTION The conventional techniques that have been described for treatment of the craniospinal axis show the common challenge of edge matching between adjacent orthogonal and parallel photon beams (5-8, 11). Various techniques have been suggested, avoiding this problem of edge matching (4,9) in which a prone treatment position remains a common factor. Yet, a different approach is needed if, for medical reasons,the patient cannot be treated in the prone position. The proposed technique is basedon the work of the late Dr. Davy et al. (9) on dynamic therapy. In particular, a simplified approach is presented here, using a common linear accelerator, avoiding the need of a computer controlled tracking unit. The only requirement is the introduction of a remote couch controller outside the treatment room. After treating the cerebral and upper neck regions by two lateral opposed half-beam fields defined by asymmetric collimators (referred to as “split beam” in the remains of the article), the patient (lying in supine

position) is being moved along the spinal axis through an 8.0 cm wide by 15.0 cm long posterior split beam (allowing edge matching with the lateral fields at the neck region) by means of remote controlled couch movement. The resulting field can be treated as though it has a rectangular profile, so that dose is accumulated uniformly between the 50% beam edges.In addition to an improvement in patient comfort, the scanning couch technique introduces a simplification of the simulation procedure as well as a significant reduction of simulation time. The latter becomes obvious in a comparison with the common techniques, which had been described in detail elsewhere (1). The emphasisin this article is on the dosimetry aspectsof this dynamic therapy. The linear decreasein dose at the ends of the scanning region, caused by starting and stopping of the couch is analyzed. An elegant solution could be presented by the use of a dynamic wedge. The absence of this device in our department forced us to look for alternatives. Both small compensating fields or a compensating block techniques will be discussedin detail.

Reprint requeststo: D. Verellen, Dept. of Radiotherapy,AcademicHospital,Free University Brussels(AZ-VUB), Belgium.

L. Coppensand I. Van De Vondel for technical assistance and modifications.

Acknowledgements-Financial fered by the “Sportvereniging

support to this work has been oftegen kanker.” Special thanks to

Accepted for publication 771

8 March

1996.

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METHODS

Volume 35. Number 4, 1996

AND MATERIALS

Scanning technique Patient setup and immobilization is performed by using a thermoplast cast’ in a supine position as in the conventional treatment of the head and neck region. After treating the cerebral and upper neck regions by two lateral split beam fields, the patients were moved along the spinal axis through an 8.0 cm wide by 15.0 cm long posterior field, in supine position. In view of a study on on-line portal imaging performed at our department (2, 3, 10, 12), we modified an accelerator* with a remote couch controller outside the treatment room. In conjunction with a digital display of couch parameters, this feature enabled to move the patient table in a controlled manner during irradiation. Tabletop carriage is driven by a DC current motor, of which the speed is adjustable with jumpers on the processing board of the table. While the accelerator operates at a dose rate of 300 MU/min (3.00 Gy/min at depth of maximum dose) in the 18 MV photon mode and the output factor for a 15 X 8 cm2 asymmetric field being 1.04, we choose to set the speed for longitudinal table top drive at 1.5 cm/s. With respect to the longitudinal field size, the jaws were set asymmetrical ([B, + B2] X [A, + A21 = [4.0 + 4.01 X [15.0 + 0.01 cm2), allowing edge matching with the lateral fields (Fig. 1). Neglecting start and stop areas, a point at depth of maximum dose d, (3.3 cm) will be inside the beam for 10 s when scanned through a 15.0 cm long radiation field for treatment at a fixed source surface distance @SD) of 100.0 cm. This results in a dose of 0.50 Gy per scan. Total scan length of the treatment table is limited to 55.0 cm in longitudinal direction. To obtain a total treatment dose of 30 Gy in 10 fractions, six scans were needed per fraction. Edges of the scanning region Because, near the edges of the scan, points will not be inside the radiation beam the full 10 s, a linear decrease of dose is to be observed in these regions. Two ways of resolving this problem are proposed in this communication. Scan in conjunction with small compensating fields At the initial part of the scanning region, several static fields were applied with increasing longitudinal jaw Al from 0.0 to 15.0 cm and A2 = 0.0 cm, transverse jaws B, and B2 were fixed at 4.0 cm. At the end the same procedure was followed, this time decreasing jaw A2 from 0.0 to -9.0 cm, keeping jaw A, at 15.0 cm. The accelerator* offers the possibility of a central line over travel at the ‘ORFIT RAYCAST, ORFIT Industries N.V., Vosveld 9a, B2110 Wijnegem, Belgium. *KDS-2 MEVATRON, Siemens Medical Systems-Oncology Care Systems, Concord, CA. 3Lipowitz block: cerrobenda, Cerro M.P., USA. ‘Par-Scientific, Byghovej 25, 5250 Odense, Denmark.

Fig. 1. Schematic view of lateral cranial fields and posterior scanning field.

isocenter of 2 cm with the outer collimators (B) and 10 cm with the inner collimators (A), indicated by a negative sign. The principle of adding small consecutive fields is explained in Fig. 2. Scan in conjunction with a compensating block Knowing the slope at both ends of the scanning region, a block3 (a low melting point alloy) was made compensating the linear decrease in absorbed dose at d,, using a milling device. 4 At the top and bottom of the moving field, a static field was given as shown in Figs. 3a and b. Dosimetry Four different methods of dose measurements were used and compared to determine the absolute dose and the relative dose distribution of the moving field technique. The methods were: cylindrical ionization chamber5 measurements, plane parallel ionization chamber6 measurements, thermoluminescent dosimetry (TLD), a beam measuring system’ (BMS) consisting of an array of 88 diodes and film dosimetry. Flatness was defined at 80% of the field size or in this case at 80% of the plateau phase, using the following formula:

I.44 - -L x II&4 + L I

100

5NE 2571 0.6 cc, Nuclear Enterprises, Bath Road, Beenham, Reading RG7 5PR, England. 6MARKUS, PTW-Freiburg, Loerracher Str. 7, D-7800 Freiburg i.Br., Germany. ‘BMS 96, Schuster, Klosterstrasse 9, 8550 Forchheim, Germany.

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Fig. 2. Expected scanningprofile and addition of consecutivefields with increasing,respectivelydecreasing jaws A, and AZ. The top panel visualizesthe doseprofile of the scanningfield with the plateauphasein the middleand two regionsof lineardosefalloff the first andlast 15cm at the edges.The underlyingpanelsshowthe superposition of the consecutivesmall static fields (with asymmetricfield sizesAl and AZ) in this region of linear dosefalioff. The lower panelshowsthe final result, being the doseoriginatingfrom the smallstatic fieldssuperimposed on the scannedfield.

with IlvI and I, dose maximum and minimum within 80%

of the field size, which is defined at 50% of the maximum dose at depth of maximum dose. A flatness value equal to or <6% was considered to be acceptable for 40 X 40 cm2 field size. Follow-up A final check on the absorbed dose delivered to the patient was carried out by in vivo dosimetry, using an array of TLDs along the spinal axis with suitable buildup for obtaining entrance dose.

RESULTS Scanning profile The total field length was set at 38.0 cm for reasons of compatibility of the different detection techniques. Longitudinal field size being 15.0 cm, the scanning length was defined at 23.0 cm. Combining couch velocity (1.5 cm/s) and dose rate (300 MU/mm), 77 MU were needed for one scan yielding an absorbed dose of 0.50 Gy at depth of maximum dose in the plateau phase.

Figure 4 shows the expected scanning profile as well as the measured profiles by BMS, TLD, and film. The expected slope at both edges of the scan is 20.0333 Gy/cm. Linear regression on data obtained at the start area yielded -0.0326 Gy/cm using BMS, -0.0321 Gy/cm by TLD measurements, and -0.0338 Gy/cm from film densitometry. At the stop area 0.0333, 0.0304, and 0.0331 Gy/cm were obtained from BMS, TLD, and film, respectively. For reasons of comparison, we determined flatness of a 40.0 X 40.0 cm2 static field under the same experimental conditions, yielding a flatness of 2.76%. Flatness of the scanning profile was calculated at 0.76% using film. In addition, we measured a scanning profile with a 37.0-cm plateau phase, yielding a flatness of 1.54%. Mean dose at this plateau was 0.501 Gy with a standard deviation (SD) of 0.007 Gy. Scan in conjunction with small compensating fields Adding consecutive small fields to the scanned profile results in a sawtooth profile at the edges (Fig. 5). Within each additional field, dose was raised with 0.10 Gy, taking into account the total scatter correction for small field sizes. Flatness was defined at 5.77%.

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Follow-up The TLDs being mounted on the patients skin (with suitable bolus) give little data with regard to the junction. The results confirm the predicted dosewithin the precision of the TLD, being within 25%. DISCUSSION

A f

SFD

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(b)

Fig. 3. (a) (left) Expected scanningprofile (top) and the result from the compensatingblocked fields (center)superimposed on the lower panel. (b) (right) Positioningof compensatingblock and resultingdosedistribution.

Scan in conjunction with compensating block Adding a compensating field (Fig. 6), yields a field of dimensions 38.0 X 8.0 cm2, and flatness 3.03%. Junction of scanning field and lateral cerebral fields The dose distribution along the spinal axis resulting from both the scannedfield with additional compensating field (method with block) and the lateral cranial fields is shown in Fig. 7. Assuming an average target depth of 3.3 cm, an overall flatness of 4.08% is achieved. The administered dose varies lessthan 8.5% through the spinal axis.

‘Siemens

cord, CA.

Medical Systems-Oncology Care Systems,Con-

The proposed technique representsa suitable alternative for treatment of the craniospinal axis, using a common linear accelerator. The only necessary assetis the introduction of a remote couch controller outside the treatment room. A linear accelerator equipped with a dynamic wedge would be of more value. Davy et al. (9) developed a similar technique introducing a computer controlled tracking unit to keep the spinal cord constantly positioned at the beam isocenter and adjusting the speedof the longitudinal couch travel. The latter enables the delivery of the prescribed dose to predefined segmentsof the target. The tracking technique allows generation of a homogeneous dose distribution along the entire length of the spinal cord. The production of the machine control data, however, is time consuming. Assuming the approach of delivering the prescribed dose to an average tumor depth. will result in a considerable simplification, which is the basis of this presentation. The remote couch controller being introduced for a study on on-line portal imaging, implied the use of a Siemens Z IV@ treatment table. ’ Scanning length is limited to 40 + 15 cm for this type of patient couch. For this reason we were not able to treat the full target in the plateau phase of the scan, with a complete blocking of stop and start regions. This limitation, however, introduces no restrictions with respect to patient length. Due to the absence of any framework needed to fix the immobilization mask for treatment in prone position, field size of the lateral fields could be increased from the mandibula (for static couch treatment) up to vertebra C5. This implied a considerable decreasewith respect to the remaining spinal column to be treated. In addition to patient comfort, the supine treatment position allows for reduction of lordoses by lifting of the legs. Hence, an increase in dose homogeneity is obtained, by reducing variations in source-target distance. There is a small deviation from linearity at the beginning of the scanning profile, which may be explained by a nonlinear offset at the start caused by the acceleration from 0.0 to 1.5 cm/s and dose rate stabilization. Once a constant couch velocity and output stabilization are reached, this effect disappears. Although the slope of the curve is approximately 20.0333 Gy/cm, there is a small shift to be noticed caused by a delay of couch movement with respect to irradiation. For the time be-

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Expected TLD Film BMS

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Fig. 4. Expected vs. measured scanning profile, expected slope at both edges of the scan is +- 0.0333 Gy/cm.

ing, couch movement is performed manually by an operator, and starts at the time irradiation begins. However, a small error in the reaction of the operator is inevitable; hence, a static field of 8.0 X 15.0 cm2 is added to this area during a fraction of a second. This phenomenon is comparable to the effect achieved by adding consecutive static fields to the scanning profile (Fig. 2). The experimental results confirm the predicted 0.50 Gy, with a flatness exceeding results obtained with static fields, with regard to the plateau area of the scanned field. Comparing both solutions with respect to

start and stop areas, the compensating block technique is preferred. However, inaccurate positioning of this block will affect flatness. The problem of edge matching for these particular treatments can be solved in different ways (4, 8, 9). In this communication we showed one solution to the problem. However, some pitfalls remain. Edge matching with the two lateral fields remains critical, as is positioning of the compensating block, absorbed dose is limited to 0.50 Gy per scan, and couch movement and irradiation need to be coupled more accurately. The longitudinal speed of the

dose GY)

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Length Fig. 5. Saw tooth profile resulting from scan together with small consecutive

compensating

(cm) fields.

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treatment couch should be slowed down to enable treatment by using one scan only. Velocity needs to be reduced to 0.25 cm/s. In addition, an automation of couch movement linked to the linear accelerator, will be introduced in cooperation with the electronic portal imaging project also performed at our department.

CONCLUSION An alternative technique for craniospinal irradiation is described, especially relevant for patients who cannot be treated in the prone position. The proposed technique for irradiation of the spine consists of two components:

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(a) a linear scan along the length of the spine, and (b) a boost at the top and the bottom of the spine either in the form of multiple fields or single compensating fields. The procedure for a boost with multiple fields turned out to be complex and resulted in a substantial

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variation in the dose delivered to the spine. The compensating block technique is preferred. However, accurate positioning of this block remains critical. Dosimetry aspects and limitations due to the equipment are presented in this article.

REFERENCES 1. De Beukeleer, M.; Verellen, D.; Van den Heuvel, F.; De Neve, W.; Storme, G. Interactive movement of patient couch for treatment of the craniospinal axis In: The 8th Intemational Conference on Cancer Nursing. Vancouver, British Columbia, Canada: Canadian Association of Nurses in Oncology: 1994:7-12. 2. De Neve, W.; Van den Heuvel, F.; De Beukeleer, M.; Coghe, M.; Thon, L.; De Roover, P.; Van Lancker, M.; Storme, G. Routine clinical on-line portal imaging followed by immediate field adjustment using a tele-controlled patient couch. Radiother. Oncol. 24:45-54; 1992. 3. De Neve, W.; Van den Heuvel, F.; De Beukeleer, M.; Coghe, M.; Verellen, D.; Thon, L.; De Roover, P.; Roelstraete, A.; Storme, G. Interactive use of on-line portal imaging in pelvic radiation. Int. J. Radiat. Oncol. Biol. Phys. 25517-524; 1992. 4. Glasgow, G. P.; Marks, J. E. The dosimetry of a single “hockey stick” portal for treatment of tumors of the craniospinal axis. Med. Phys. 10:672; 1983. 5. Glasgow, G. P.; Purdy, J. A. External beam dosimetry and treatment planning. In: Perez, C. A.; Brady, L. W., eds. Principles and practice of radiation oncology. Philadelphia: Lippincott Co.; 1992:221-224, 254. 6. Lim, M. L. F. A study of four methods of junction change

7. 8.

9.

10.

11. 12.

in the treatment of medulloblastoma. Am. Assoc. Med. Dossim. J. 10:17; 1985. Lim, M. L. F. Evolution of medulloblastoma treatment techniques. Am. Assoc. Med. Dossim. J. 11:25; 1986. Sohn, J. W.; Schell, M. C.; Dass, K. K.; Suh, J. H.; Tefft, M. Uniform irradiation of the craniospinal axis with a penumbra modifier and an asymmetric collimator. Int. J. Radiat. Oncol. Biol. Phys. 29:187-190; 1994. Tate, T.; Brace, J. A.; Davy, T. J.; Morgan, H. M.; Skeggs, D. B. L.; Tookman, A. J. Case report: Treatment of medulloblastoma using a computer-controlled tracking cobalt unit. Clin. Radiol. 36:209-212; 1985. Van den Heuvel, F.; De Neve, W.; Coghe, M.; Verellen, D.; Storme, G. Relations of image quality in on-line portal images and patient individual parameters for pelvic field radiotherapy. Eur. Radiol. 2:433-438; 1992. Van Dyk, J.; Jenkins, R. D. T.; Leung, P. M. K. Medulloblastoma: Treatment techniques and irradiation dosimetry. Int. J. Radiat. Oncol. Biol. Phys. 2:993; 1977. Verellen, D.; De Neve, W.; Van den Heuvel, F.; Coghe, M.; Louis, 0.; Stonne, G. On-line portal imaging: Image quality defining parameters for pelvic fields-A clinical evaluation. Int. J. Radiat. Oncol. Biol. Phys. 27:945-952; 1993.