Clinical Evaluation of an Immbolization System for Stereotactic Body Radiotherapy Using Helical Tomotherapy

Medical Dosimetry, Vol. 36, No. 2, pp. 126-129, 2011 Copyright © 2011 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/11/$–see front matter

doi:10.1016/j.meddos.2010.02.003

CLINICAL EVALUATION OF AN IMMBOLIZATION SYSTEM FOR STEREOTACTIC BODY RADIOTHERAPY USING HELICAL TOMOTHERAPY RICHARD

ALONSO N. GUTIÉRREZ, PH.D., SOTIRIOS STATHAKIS, PH.D., CROWNOVER, M.D., PH.D., CARLOS ESQUIVEL, PH.D., CHENGYU SHI, PH.D., and NIKO PAPANIKOLAOU, PH.D.

Department of Radiation Oncology, School of Medicine, Cancer Therapy and Research Center, The University of Texas Health Science Center–San Antonio, San Antonio, TX (Received 28 October 2009; accepted 1 February 2010)

Abstract—In this study, a clinical evaluation of the Body Pro-Lok™ System combined with the TomoTherapy megavoltage computed tomography (MVCT) was performed for lung and liver stereotactic body radiotherapy (SBRT) to reduce interfractional setup uncertainty. Twenty patients treated with 3–5 fractions of SBRT were analyzed retrospectively. The Body Pro-Lok™ system was used in both CT simulation and during patient treatment setup. Patients were immobilized with a vacuum cushion placed posteriorly over the thoracic region, an abdominal compression plate, and a knee and foot sponge. Pretreatment MVCT scans of the TomoTherapy HI-ART II unit were fused with the planning kVCT before delivery of each fraction to determine the interfractional setup error. A total of 84 shifts were analyzed to assess the interfractional setup accuracy. Results showed that the mean interfractional setup errors and standard deviations were – 0.9 ⴞ 3.1 mm, 1.2 ⴞ 5.5 mm, and 6.5 ⴞ 2.6 mm for lateral (IEC-X), longitudinal (IEC-Y), and vertical (IEC-Z) variations, respectively. The maximum motion was 17.1 mm in the longitudinal direction. When all 3 translational coordinates were analyzed, a mean composite displacement vector of 8.2 ⴞ 2.0 mm (range 4.1–11.7 mm) was obtained for all patients. Based on the findings, image-guided SBRT using the Body Pro-Lok™ system in conjunction with the MVCT of TomoTherapy is capable of minimizing interfractional setup error and improving treatment accuracy. © 2011 American Association of Medical Dosimetrists. Key Words: Stereotactic radiotherapy, Tomotherapy, Immobilization.

volumetric megavoltage computed tomography (MVCT) imaging enables the generation of highly precise 3D dose distributions and allows for tumor localization on a fraction-by-fraction basis. The fusion registration software provides tools to detect the tumor/organ shift coordinates relative to the original planning kVCT and enables the machine to apply these shifts automatically—with the exception of the lateral (IEC-X) shift on later model HI-ART units, which must be applied manually. Details regarding the use of helical tomotherapy for SBRT have been thoroughly reviewed in the literature.8 In this study, a clinical evaluation of the interfractional setup accuracy of the SBRT Body ProLok™ system (CIVCO, Orange City, IA)—a new immobilization and localization system—in conjunction with the TomoTherapy HI-ART II system was performed. The purpose of the study was to quantify the setup accuracy for SBRT patient treatments of the paired systems in a clinical setting.

INTRODUCTION Stereotactic body radiotherapy (SBRT) has been gaining a large interest over the past few years partly because of the promising results for early-stage cancers of the lung and liver and partly because of highly advanced radiation delivery equipment.1,2 Because of the large doses per fraction delivered during SBRT (i.e., typically ⬎10 Gy), SBRT necessitates certain dosimetric requirements such as reduced tumor margins, high prescription dose conformality, and sharp dose fall-off away from the target, as well as mechanical requirements such as rigid immobilization and accurate patient positioning. Rigid fixation devices have been used to reduce daily set-up uncertainties with much success.3,4 More complex fixation devices have also incorporated components to reduce intrafraction motion by means of abdominal compression and breath-hold techniques.4,5 When paired with pretreatment volumetric imaging (i.e., conebeam CT or MVCT), immobilization devices offer superior delivery accuracy of precise dose distributions.6,7 Helical tomotherapy (TomoTherapy Inc., Madison, WI) has been used previously to deliver image-guided SBRT.6 The unique delivery geometry and incorporated

MATERIALS AND METHODS Patients Twenty patients (n ⫽ 20) with either lung or liver cancers were treated with a SBRT technique in our clinic using helical tomotherapy. Table 1 shows the tumor location and total number of treatment fractions per

Reprint requests to: Alonso N. Gutiérrez, Ph.D., University of Texas Health Science Center San Antonio, School of Medicine, Department of Radiation Oncology, 7979 Wurzbach Rd., MC 7889, G237, San Antonio, TX 78229. E-mail: [email protected] 126

SBRT and helical tomotherapy ● A. N. GUTIÉRREZ et al.

Table 1. Summary of patient disease location and total treated fractions Patient 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Total Fractions

Target Location

No. of Treatment Fractions

Lung Liver Liver Liver Liver Liver Lung Lung Liver Liver Liver Lung Liver Liver Liver Liver Liver Liver Liver Liver

3 3 5 5 5 5 3 3 5 3 3 3 5 5 5 5 5 3 5 5 84

patient. In all, the majority of patients treated (80%; 16/20) presented with liver cancer. A total of 84 recorded daily tumor shifts from 20 patients were collected for interfraction setup variation analysis. For each patient, the gross tumor volume (GTV) was obtained from a helical CT scan using a LightSpeed™ 16-slice scanner (GE Medical, Waukesha, WI). A patientspecific setup margin, typically 5.0 mm in the anteriorposterior and left-right directions and 15.0 mm in the superior-inferior direction, was added to generate the planning target volume (PTV). Treatment plans were optimized to deliver a prescribed dose of 45.0 – 60.0 Gy in 3–5 fractions to at least 98% of the PTV. Tomotherapy treatment parameters used for most cases were: pitch ⫽ 0.123, field width ⫽ 2.5 cm, modulation factor ⫽ 2.0, and fine dose grid resolution. Body Pro-Lok™ system A Body Pro-Lok™ system was used to immobilize and localize patients during the CT simulation and for each treatment fraction. The Body Pro-Lok™ system consists of a patient custom-made vacuum cushion placed posteriorly to the torso of the patient, an abdominal compression bridge, knee and foot sponges, and a wing board (Fig. 1). Additional components such as a respiratory belt, forehead brace bridge, patient handgrips, and shoulder restraint bridge are also available. The Pro-Lok™ system is entirely indexable, and components attach to the Pro-Lok™ platform via multiple T-pin Lok-Bar™. CT simulation and MVCT localization imaging For simulation, patients were first placed in the Pro-Lok™ system for vacuum cushion formation and customization. Patients were coached to use shallow

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breaths while the abdominal compression was applied. All patients underwent a free-breathing CT scan for treatment planning. Helical CT scanning was performed using 2.5-mm slices. For MVCT localization, all patients underwent a pretreatment MVCT scan of the region-ofinterest using a 2.0-mm-slice thickness. Images were acquired and fused with the planning kVCT using mutual information largely based on bony structure correlation. The fused images were independently reviewed by the prescribing radiation oncologist and a radiation oncology physicist. The TomoTherapy software calculated 3D translational, pitch, yaw, and roll corrections. Pitch and yaw were set to zero because of the lack of correction capability currently available with the HI-ART II unit. Roll corrections were accomplished by the automatic gantry rotation. The TomoTherapy HI-ART II unit performs automatic vertical, longitudinal, and roll movements according to the registration results. Lateral shifts must be adjusted manually. For each patient, the lateral shift and final positioning was verified by a radiation oncologist and a radiation oncology physicist before radiation delivery. RESULTS The mean and single standard deviation of the interfractional setup error calculated for each translational dimension and roll from all of the fractions evaluated are summarized in Table 2. The mean setup deviations for all patients in the lateral, longitudinal, and vertical directions were – 0.9 ⫾ 3.1 mm, 1.2 ⫾ 5.5 mm, and 6.5 ⫾ 2.6 mm, respectively. The mean rotational variation was 0.4 ⫾ 0.7°. Figure 2 shows a scatter plot of the lateral, longitudinal, and vertical shifts for all 84 treatments. With regards to each dimension, significant shifts were present in both the longitudinal (17.1 mm) and

Fig. 1. Typical patient SBRT setup using the SBRT Body Pro-Lok™ system (CIVCO, Orange City, IA). The system is composed of a customizable vacuum cushion, knee and feet sponges, and compression plate—additional accessories (not shown) such as hand grips are also part of the system. All of the patient immobilization devices can be completely indexed to the baseboard.

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Table 2. Average error, standard deviation, and maximum shifts from interfractional setup uncertainties with the Body ProLok™ system

Lateral (IEC-X) Long. (IEC-Y) Vert. (IEC-Z) Roll*

Mean

SD

Max Shifts

⫺0.9 1.2 6.5 0.4

3.1 5.5 2.6 0.7

9.9 17.1 12.8 3.5

All values in millimeters. *Values in degrees.

vertical direction (12.8 mm). When all 3 translational coordinates were analyzed, a mean composite displacement vector of 8.2 ⫾ 2.0 mm (range 4.1–11.7 mm) was obtained for all patients. Figure 3 shows a scatter plot of the mean displacement vector of each patient from all fractions. DISCUSSION The study was undertaken to quantify the interfractional setup accuracy of the new SBRT Body Pro-Lok™ system when used with the volumetric MVCT imaging of the TomoTherapy HI-ART II unit. The significance of this quantification is of much importance, for accurate target positioning along with rigid immobilization is essential for SBRT treatments. Several studies have reported on the accuracy of stereotactic frames and patient setup uncertainties with various forms of image guidance.4,6,9,10 In our institution, we have previously reported our SBRT experience using a BodyFIX (Elekta, Stockholm, Sweden) immobilization system for a study of 36 patients and showed a mean displacement vector of 5.7 ⫾ 3.7 mm.4 Specifically with TomoTherapy, Hodge et al. reported using the BodyFIX immobilization system in conjunction with the MVCT image guidance of TomoTherapy in 9 SBRT patients.6 They reported max-

Fig. 2. The scatter plot shows the mean displacement vector of each patient over all fractions. The average mean displacement vector for all patients in the study was 8.2⫹/- 2.0 mm (solid line).

Fig. 3. Scatter plot shows the 3D shifts for all 84 treatment fractions analyzed. (Top) Vertical and lateral shifts plotted; (bottom) longitudinal and lateral shifts plotted.

imum lateral, longitudinal, and vertical shifts of 6.8 ⫾ 2.9 mm, –9.9 ⫾ 2.6 mm, and –9.2 ⫾ 3.8 mm, respectively. Our data fall well within these published results, with a mean displacement vector of 8.2 ⫾ 2.0 mm. Relatively large deviations (⬎7.0 mm) were noted for the longitudinal and vertical directions in the patients evaluated in this study. For the longitudinal direction, part of the reason for the large uncertainty could be attributed to the image acquisition and registration technique. Because of the slow gantry rotation of the MVCT scanner of TomoTherapy—5 sec per revolution—the motion pattern of the target was included into the pretreatment MVCT scan.11 Thus, differences in respiratory phases between the planning CT and MVCT scans may be a factor contributing to setup uncertainty. More significantly, large deviations in the longitudinal direction are induced when large deviations in the vertical direction occur. This is attributed to the fact that both dimensions are coupled together because of the cobra couch design of the TomoTherapy couch. When the couch moves in the vertical direction, offsets are induced in the longitudinal direction strictly because of the system behavior and not because of the initial patient setup. In this

SBRT and helical tomotherapy ● A. N. GUTIÉRREZ et al.

study, these offsets were decoupled for the results shown. With regards to the vertical direction, significant displacements arise because of the sag that occurs between the virtual isocenter (location where the patient is set up) and actual isocenter (location where the patient is imaged and treated) of tomotherapy. This is a wellunderstood issue with the tomotherapy couch, and Gutiérrez et al. have reported that the absolute displacement caused by sag on a tomotherapy couch can be as large as 8.0 mm.12 Because sag shows dependency on patient weight, weight distribution, and length of couch cantilevered, decoupling the sag from true setup error because of the Body Pro-Lok™ system is not trivial. If we make the reasonable assumption that the vertical shift on the first treatment fraction is strictly a result of couch sag, we can then attribute the setup uncertainty solely as a result of the Body Pro-Lok™ system. By doing so, the mean vertical setup uncertainty reduces to 0.1 ⫾ 1.8 mm, and the mean displacement vector becomes 4.7 ⫾ 2.0 mm—a setup uncertainty reduction of 1.0 mm when compared with the BodyFIX system, as quantified by Fuss et al. Given our collective institutional experience with both the BodyFIX and Body Pro-Lok systems, we have found the latter to be more user-friendly and faster to position and immobilize the patient. The precision indexing of all accessories makes the system quite reproducible and provides both the radiation therapist and radiation oncology physicist a robust and systematic method of verifying the patient positioning both accurately and efficiently. CONCLUSIONS This study reported initial clinical results of the interfractional setup errors associated with the SBRT Body Pro-Lok immobilization system when used in conjunction with the TomoTherapy MVCT imaging system for lung and liver SBRT treatments. Results showed that rigid immobilization through the Pro-Lok immobilization system could achieve a mean displacement vector of 8.2 ⫾ 2.0 mm. Ultimately, successful implementation of

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image-guided SBRT techniques requires rigid immobilization and vigilant volumetric image guidance.

Acknowledgments—The authors would like to thank CIVCO for the opportunity to alpha and beta test the SBRT Body Pro-Lok immobilization system. In particular, we especially thank Marc Mlyn and Dayna Bodensteiner for providing product information and technical support.

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