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British Journal of Radiology (2005) 78, 742-745
© 2005 British Institute of Radiology
doi: 10.1259/bjr/11782857
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Short communication

Setup variations in locoregional radiotherapy for breast cancer: an electronic portal imaging study

P T Truong, MDCM, FRCPC 1,2 E Berthelet, MD, FRCPC 1,2 V Patenaude, RTT 1 J Bishop, RTT 1 B Sandwith, RTT 1 V Moravan, MSc 3 W Beckham, PhD 1,2 T Mitchell, DCR, RTT 1 and I A Olivotto, MD, FRCPC 1,2

1 Radiation Therapy Program, British Columbia Cancer Agency, Vancouver Island Centre, 2410 Lee Avenue, Victoria, BC, V8R 6V5, 2 University of British Columbia, BC and 3 Population and Preventive Oncology, British Columbia Cancer Agency, BC, Canada


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
 References
 
Recent trials demonstrating a survival benefit with locoregional radiotherapy (LRRT) to the chest wall and regional nodes in women with node-positive breast cancer have led to increased use of complex techniques to match three or more radiation fields, but information on setup reproducibility with LRRT for breast cancer is scarce. This study reports the magnitude and directions of random and systematic deviations in LRRT for breast cancer using an offline electronic portal imaging verification protocol. Electronic portal images (EPIs) of 46 consecutive women treated with LRRT for breast cancer from March 2001 to February 2002 with LRRT were analysed. Comparisons of EPIs to the corresponding digitally reconstructed radiographs were performed offline with anatomy matching. Displacements in mm were recorded in the superior–inferior (SI), medial–lateral (ML), and anterior–posterior (AP) directions. Random errors ranged from 2.0 mm to 2.5 mm for the breast/chest wall tangential treatments and 2.3 mm to 3.9 mm for the supraclavicular nodal treatments. Systematic errors occurred to a greater degree in the AP direction for the tangential fields and in the ML direction for the supraclavicular field. Displacements of ≥10 mm were found in 1.2% of breast/chest wall tangential treatments and in 6.2% of supraclavicular nodal treatments. These data demonstrate that EPI is a useful tool to verify setup reproducibility in LRRT for breast cancer.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusion
 References
 
Studies of setup verification in radiotherapy (RT) for breast cancer have largely focused on breast tangential treatments alone [1–4]. Recent randomized controlled trials demonstrating a survival benefit with locoregional radiotherapy (LRRT) in women with node-positive breast cancer [5–7] have led to increased use of techniques requiring matching of three or more fields to treat the breast/chest wall and regional lymphatics [8, 9]. Since a larger treatment volume is required to encompass these volumes, inaccuracies in treatment setup may increase risks of normal tissue toxicities and compromise disease control [10, 11]. There is a paucity of information on setup reproducibility with LRRT. This study determines the magnitude and directions of random and systematic errors in a cohort of women with node-positive breast cancer treated with LRRT using an offline electronic portal imaging (EPI) protocol.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusion
 References
 
46 consecutive women with node-positive breast cancer were treated at the British Columbia Cancer Agency, Vancouver Island Centre from March 2001 to February 2002 with LRRT. Treatment was administered to the breast or chest wall plus axilla and supraclavicular lymph nodes. All patients underwent CT planning in the supine position with the ipsilateral arm abducted. Immobilization was achieved with customized vacuum moulded devices to provide support under the neck, shoulders and arms. The breast/chest wall was treated with a pair of tangential photon fields and the upper axillary and supraclavicular nodal regions were treated with an anterior photon field. A monoisocentric technique was used to match the tangential fields with the anterior field at the isocentre (Figure 1Go). The isocentre is set at the level of the inferomedial aspect of the clavicular head. Two reference marks (an anterior midline and a lateral mid-axillary position) located 10 cm inferior to the isocentre are tattooed. These tattoos are the treatment reference centres and all moves and distance readings are made relative to these points. An anterior midline mark at 20 cm inferior to the isocentre is also tattooed and used to align the patient to parallel the long axis of the treatment couch and reproduce the CT setup position.



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  		Figure 1. Monoisocentric technique of locoregional radiotherapy to the breast/chest wall and regional nodes. The isocentre is at the level of the inferomedial aspect of the clavicular head. Two reference marks located 10 cm inferior to the isocentre (an anterior midline and a lateral mid-axillary mark) are tattooed. These tattoos are the treatment reference centres and all moves and distance readings are made relative to these points. An anterior midline mark at 20 cm inferior to the isocentre is also tattooed and used to align the patient to parallel the long axis of the treatment couch and reproduce the CT setup position.

 
Multileaf collimation was utilized for shielding of normal structures. The dose prescription was determined by the treating oncologist. All fields were treated once a day, five times a week over 3–5 weeks.

All 46 patients underwent portal imaging of each treatment beam during the first 3 fractions according to institutional policy. Comparisons of EPIs to the corresponding digitally reconstructed radiographs (DRRs) derived from the treatment planning CT scan data were performed offline with anatomy matching of bony structures in the axilla/supraclavicular fields and the chest wall/lung interface and soft tissue surface contours in the tangential breast fields. Since the use of mobile anatomical landmark structures in the upper thorax may introduce inaccuracies when studying setup variations, a review of the literature was conducted prior to the study to identify stable structures to use as landmarks for image comparisons. The thoracic structures demonstrated to be most stable include the clavicle (in the superior–inferior axis), paraspinal line (in the lateral direction) and the thoracic wall (in both directions) [12]. These structures were thus selected for use in this study's image analyses. Three experienced radiation therapists performed the anatomy matches. Displacements in millimetres were calculated in the superior–inferior (SI), medial–lateral (ML), and anterior–posterior (AP) directions.

Definitions of setup deviations
Patient setup deviations may have a random component related to patient motion or daily positioning and a systematic component related to equipment or protocol [13]. Individual setup deviations were characterized by their means and standard deviations (SDs). Random and systematic deviations were subsequently calculated for the group in accordance to previously published definitions [12–15]. Random deviations, {sigma}random, defined as variations between fractions during a treatment series, was determined by calculating the spread (1 SD) of differences around the corresponding mean in each patient and then calculating the average of these SDs for the whole group. Systematic deviations, {sigma}systematic, defined as deviations between the planned position and the average position over the treatment course, were obtained by calculating the spread (1 SD) in the individual means of differences between the planned DRRs and the portal images [12–15].

According to our institutional policy, deviations detected in patient setup were corrected offline based on the average of the deviations during the first three fractions with the tolerance limit set at 10 mm. The reported setup deviations in this study are thus the uncorrected random and systematic errors since patient setup was not adjusted during the first three fractions.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusion
 References
 
The median age of this study cohort was 55 years (range 32–85 years). All subjects had node-positive invasive breast cancer treated with breast conserving surgery (35/46) or mastectomy (11/46). Median total RT dose of 42.5 Gy (range 35–50 Gy) was delivered to the breast/chest wall and regional nodes. All patients were treated once a day with 34 patients receiving 16 daily fractions, 11 patients receiving 25 fractions and 1 patient receiving 10 fractions. A total of 216 tangential breast/chest wall and 108 anterior axilla/supraclavicular images were analysed.

Random and systematic errors
Data on random and systematic errors in three directions are presented in Table 1Go. Random errors ranged from 2.0 mm to 2.5 mm for the breast/chest wall tangential treatments and 2.3 mm to 3.9 mm for the supraclavicular nodal treatments. Systematic errors occurred to a greater degree in the AP direction for the tangential fields and in the ML direction for the supraclavicular fields.


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  		Table 1. Magnitude and directions of random and systematic errors in locoregional breast radiotherapy

 
Magnitudes and directions of all displacements
In the analysis of magnitudes and directions of all displacements (n=832) for the tangential breast/chest wall fields, 83.5 % were ≤5 mm, 8.5% were >5 mm and ≤7 mm, 6.7% were >7 mm and <10 mm, and 1.2% were ≥10 mm. In the AP direction, the corresponding values were 79.8%, 10.1%, 8.9%, and 1.2% and in the SI direction, the corresponding values were 87.3%, 7%, 4.6% and 1.2%, respectively (Table 2Go). For the anterior supraclavicular nodal field, 70.4% were ≤5 mm, 11.4% were >5 and ≤7 mm, 11.9% were >7 and <10 mm, and 6.2% were ≥10 mm. The corresponding values were 61.7%, 11.4%, 16.4%, and 10.5% in the ML direction and 79.1%, 11.4%, 7.5% and 1.2% in the SI direction (Table 2Go).


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  		Table 2. Magnitude and directions of all displacements of the electronic portal images compared with the digitally reconstructed radiographs

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusion
 References
 
The publication in 1997 of three randomized trials [5–7] demonstrating survival benefits with LRRT for high-risk women has led to increased utilization of techniques targeting the breast/chest wall and regional nodes. Recent advances in CT-based and intensity-modulated radiotherapy planning have the potential to improve the therapeutic ratio for breast cancer patients by minimizing exposure to the normal tissues, particularly the cardiac and pulmonary structures [8, 9]. The implementation of increasingly complex techniques warrant efforts to evaluate set up verification to ensure accuracy and reproducibility in delivering the planned therapy. This report is among the first to document the magnitude and directions of set up deviations in LRRT for breast cancer in the era of CT-based planning using defined protocols of patient immobilization and electronic portal imaging.

Previous studies have reported random or interfraction setup errors using measurements in three directions similar to our methods [1, 4, 16, 17], while others have reported the central lung distance (CLD) measured from the lung contour to the posterior field of the tangential field at the central axis [3, 18, 19]. The majority of these series comprised 20 patients or fewer, some with no immobilization [3, 16, 19, 20] and others with various fixation techniques including hemibody cradles [2, 18], foam cushions [17], cellulose casts [4], fixed arm support [16] and breast boards [21]. In these studies, wide ranges in random errors of 1.7–5.8 mm and systematic errors of 1.0–14.4 mm have been reported. Among the studies using electronic portal imaging [1, 3, 19–21], Lirette et al reported systematic errors of 3.9 mm and random errors of 3.4 mm in the SI direction among 20 patients receiving tangential breast irradiation with no immobilization [3]. Van Tienhoven et al reported systematic and random errors of 4.7 mm and 1.8 mm, respectively, in the SI direction among 12 patients who also received breast RT without immobilization [20]. Pradier et al reported that the variation in CLD of 3.9 mm was the largest setup error and that 90% of all setup errors were less than 10 mm [22].

The use of immobilization devices has been demonstrated to improve setup reproducibility in RT for breast cancer [1, 16, 17, 21]. Creutzberg et al, in a comparison of 17 breast cancer patients immobilized with plastic mask fixation and 14 patients without immobilization, demonstrated that immobilization reduced random errors from 4.4 mm to 2.1 mm standard deviations in the AP direction [1]. In a study comparing patients undergoing tangential breast RT positioned supine on a wedgeboard with and without a fixed arm support, Mitine et al reported no differences in setup errors in the AP directions but overall setup errors as large as 15.5 mm in the SI direction were reduced to 5.5 mm with the fixed arm support [16]. Thilmann et al similarly demonstrated that the use of immobilization with a cushion and arm handle decreased the mean population error from 4.2 mm to 2.6 mm and from 4.0 mm to 3.3 mm in the SI and AP directions, respectively. The stability of arm positioning provided by fixation devices was suggested to be the most likely reason for the improved setup reproducibility compared with a free setup [17]. In another study of 17 patients who underwent half of their tangential breast RT fractions using a breast board immobilization device and the other half using a vacuum moulded device, Nalder et al reported that 80% of random errors were <3 mm and 80% of systematic errors in the AP direction were <4 mm for both techniques. However, the 80% point in systematic errors in the SI direction was reduced from 5 mm using the breast board compared with 2.7 mm for the vacuum molded device [21]. In summary, since the available evidence supports the use of immobilization in improving breast RT set up, all breast cancer patients undergoing adjuvant radiotherapy at our centre were immobilized.

The random and systematic errors in the tangential portion of LRRT reported in our study are consistent with those in the literature. We postulate that the larger magnitude of random errors in the tangential treatments' AP direction is related to chest wall motion during normal respiration. However, 80% of all shifts in this direction were ≤5 mm and 99% of all shifts in this direction were within the 10 mm tolerance limit.

There are few available data reporting systematic and random errors in three-field locoregional treatment with which to compare our results. In a study of eight patients immobilized with a foam cast and 21 patients immobilized with an airtight plastic bag with polysterol microspheres (Vac-Fix device), Jakobsen et al, comparing the position of the anterior axillary/supraclavicular treatment centre position on portal films to that on simulation films, demonstrated that the use of Vac-Fix immobilization improved setup reproducibility [23]. These authors reported standard deviations of 3 mm and 2.9 mm in the transverse and longitudinal directions, respectively, with the Vac-Fix immobilization technique. Only 0.9% of treatments had displacements greater than 10 mm compared with 3.4% with the foam-cast system [23]. Although we employed a similar immobilization technique to the Vac-Fix device, several methodology considerations distinguish our study from this older series. The current study's matching methods used anatomical landmarks on EPI and DRR images as opposed to a reference point on portal and simulation films; portal images were acquired more frequently in a larger patient sample; and all quantitative measurements were computer-assisted with image enhancement rather than manual and visual hardcopy review. Although our study was not designed to test efficiency and effectiveness of EPIs in comparison with conventional film-based systems, it is noted that other studies have suggested that with less time required to process and interpret, EPIs may be a more accurate representation of patient setup reproducibility than film [24].


   Conclusion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
References
 
EPI is a useful tool to verify setup reproducibility in LRRT for breast cancer. The data demonstrated that, using the current planning and positioning protocol, the random and systematic deviations are within 5 mm for the majority of patients. However, with diverse immobilization and locoregional breast radiotherapy techniques used among different institutions, we suggest that centre-specific documentation of the magnitude and directions of setup errors be performed to guide clinical decisions, including the development of correction protocols to optimize accuracy in treatment delivery.


   Footnotes
 
Presented in part at the 7th International Conference on Electronic Portal Imaging, Vancouver, British Columbia, Canada, June 27–29, 2002 and the 44th American Society of Therapeutic Radiology and Oncology Annual Meeting, New Orleans, Louisiana, USA, October 8, 2002. Back

Received for publication May 5, 2004. Revision received January 13, 2005. Accepted for publication February 24, 2005.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
 References
 

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This Article
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