Planar IGRT dose reduction: A practical approach

Practical Radiation Oncology (2014) xx, xxx–xxx

www.practicalradonc.org

Original Report

Planar IGRT dose reduction: A practical approach Alec M. Block MD, Jason Luce BS, Jeffrey Y. Lin MS, Mark A. Hoggarth MS, John C. Roeske PhD ⁎ Loyola University Medical Center, Department of Radiation Oncology, Maywood, Illinois Received 18 April 2014; revised 19 September 2014; accepted 22 September 2014

Abstract Purpose: To retrospectively estimate the cumulative absorbed dose (at the skin) from kilovoltage planar x-rays received by 90 patients treated on a Varian iX and to determine if that dose could be reduced without sacrificing image quality. Methods and materials: To estimate surface dose, measurements were obtained using the “in-air” method by varying the source-to-detector distance from 80 to 100 cm in steps of 5 cm. Energy was varied from 70 to 120 kVp. Using these data, a global equation was developed to estimate the cumulative skin dose by applying the imaging settings (kVp, mAs), patient-specific source-to-skin distance, and total number of images. To reduce the imaging dose, anterior and lateral images of RANDO phantoms were obtained using the same kVp; however, the mAs settings were systematically reduced. Contrast-to-noise ratios (CNRs) were calculated for both the standard phantom images and reduced mAs images. The mAs values were chosen to minimize skin dose while maintaining a similar CNR. Last, daily kV anterior and lateral images were obtained using these reduced mAs settings for 7 patients currently being treated with image guided radiation therapy. CNR was determined and compared with the values obtained on images taken 1 day before this change. Results: Average cumulative kV imaging dose was as large as 162.2 cGy for pelvic cases with standard kVp, mAs. Other doses varied by site and technique. By lowering mAs, this dose could be reduced by 49% with only a 0.9% decrease in CNR. For the 7 patients currently being treated with image guided radiation therapy, CNR values were not statistically different (P = .79), whereas the skin dose was reduced by an average of approximately 50%. Conclusions: kV planar imaging dose reduction should be considered, given the large cumulative skin dose for certain disease sites. When mAs are reduced, planar dose reduction is clinically feasible without sacrificing image quality. © 2014 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved.

Introduction Conflicts of interest: None. ⁎ Corresponding author. Department of Radiation Oncology, Loyola University Medical Center, Maguire Center - Room 2944, 2160 S. 1st Ave, Maywood, IL 60153. E-mail address: [email protected] (J.C. Roeske).

Since the advent of modern radiation therapy techniques, such as intensity modulated radiation therapy and stereotactic body radiation therapy, accurate and precise positioning of the target volume on a daily basis is required to ensure that the

http://dx.doi.org/10.1016/j.prro.2014.09.008 1879-8500/© 2014 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved.

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goals of the treatment plan can be realized. Image guided radiation therapy (IGRT) is a technique in which images of the patient are acquired before treatment and are used to ensure that the patient/target volume is positioned as intended. 1-9 The imaging dose from IGRT procedures is often neglected because it is small in comparison to the high level of therapeutic radiation to which it is added. 10 The risks from radiation exposure have been well studied. In addition to normal tissue toxicity associated with radiation therapy, these reported risks include skin erythema and the induction of secondary malignancies. 11-15 In terms of IGRT, a summary of dose levels from several different imaging techniques has been compiled by the American Association of Physicists in Medicine (AAPM) Task Group 75, allowing physicians to estimate imaging dose given a specific clinical scenario. 10 However, the imaging dose is variable depending on the equipment and protocols that are unique to each treatment facility. The aims of this study are thus: (1) to retrospectively estimate the cumulative absorbed dose (at the skin) from kV planar x-rays received by 90 patients previously treated with IGRT, (2) to determine if that dose could be reduced without sacrificing image quality, and (3) to apply dose reduction techniques clinically to daily images of patients undergoing IGRT. By outlining a method to reduce imaging dose, this study offers a practical approach to IGRT dose reduction that may be used in any clinical setting.

Methods and materials Study population In this Institutional Review Board–approved retrospective study, data were used from the treatment of 90 patients who underwent IGRT in our clinic between April 2009 and September 2011. These patients were grouped together by anatomic body site, and the treatment sites included: male pelvis (19 patients), female pelvis (20 patients), thorax (18 patients), head and neck (H&N)/brain (14 patients), and abdomen (19 patients). Patients ranged in age from 3 to 92 years, with an average age of 64 and median age of 67. The population was 52% male and 48% female.

Cumulative absorbed skin dose assessment At our institution, IGRT is delivered on a Varian iX (Varian Medical Systems, Palo Alto, CA) linear accelerator equipped with an On-Board Imager (OBI) Version 1.5. The system consists of a kV x-ray tube and flat panel detector. Image parameters (kVp, mA, and time) are based on standardized protocols (provided by the manufacturer) and designed for each disease site and body habitus. Given the size of patients being treated and the subjective judgment of the therapists, in our clinic, the “large” protocol was routinely

Practical Radiation Oncology: Month 2014 Table 1 Standardized manufacturer-recommended parameters by body site and image view using “large” protocol Site

Anterior (kVp, mAs)

Lateral (kVp, mAs)

Pelvis Thorax Head and neck/brain Abdomen

75, 16 75, 5 100, 8 80, 32

120, 95, 70, 85,

126 40 5 40

used for all patients and all body sites. Table 1 demonstrates standardized manufacturer-provided parameters by body site and image view using the “large” protocol. IGRT patients typically have anterior and lateral images taken before each treatment fraction. The images are then aligned with the corresponding digitally reconstructed radiograph, and repositioning adjustments are executed. If these adjustments exceed a threshold (eg, 5 mm), a second set of images may also be obtained. To estimate the dose to the skin, the AAPM protocol for 40-300 kV x-ray dosimetry was used. 16 Measurements were performed using the “in-air” method where the dose to the surface is given by: h i DW;z¼0 ¼ M Nk BW PStem;air ðμen =ρÞW air air

where M is the charge corrected for temperature, pressure, recombination, polarity, and electrometer accuracy; Nk is the air-kerma calibration factor; BW is the backscatter factor; PStem,air is the chamber stem effect; and [(μen/ρ) W air]air is the ratio of water to air of the mass energy-absorption coefficients. 16 Using this approach, measurements were obtained by varying the source-to-detector distance from 80 to 100 cm in steps of 5 cm, and the energy was varied from 70 to 120 kVp. Subsequently, a global equation was developed to estimate the cumulative skin dose by applying the imaging settings (kVp, mAs), patient specific source-to-skin distance (SSD), and the total number of anterior or lateral images: h i Dose ðmGyÞ ¼ 0:115  mAs x ðkVpÞ2:047  ðSSDÞ‐2:023 ð# imagesÞ

To estimate the cumulative skin dose, the anterior and lateral SSDs at isocenter were obtained from the treatment planning system (XiO version 4.60, Elekta, Maryland Heights, MO). The number of images acquired in each position was obtained from the Mosaiq database (Elekta, Sunnyvale, CA).

Imaging dose reduction and quantitative comparison of image quality Based on the patient dose assessment, the goal was to determine if imaging dose could be reduced without

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mAs values were selected that were slightly above the point that yielded the initial decrease in CNR. To evaluate these new imaging parameters, daily kV anterior and lateral images were obtained using these reduced mAs settings for 7 patients being treated with IGRT (3 thoracic, 2 abdominal, 2 pelvic). For these 7 patients, CNRs were calculated for both the reduced mAs settings and the standard settings used to image the patients 1 day prior. Using a paired t test, the CNR values for the reduced images and standard images from 1 day prior were compared. The cumulative absorbed skin doses using the reduced mAs were subsequently calculated for the same 90 patients. The CNR values and corresponding absorbed skin doses were compared for both the standard and reduced mAs images. Figure 1 Example of a high-intensity region (bone, in red) and low-intensity region (background, in yellow) used to calculate contrast-to-noise ratio (CNR). (For color version, see online at www.practicalradonc.org).

sacrificing image quality. To obtain this endpoint, anterior and lateral images of a RANDO phantom (Radiation Support Devices, Long Beach, CA) (at each treatment site) were obtained using the same kVp, but systematically reducing the manufacturer-recommended mAs settings. Reducing the mAs was chosen as a dose reduction strategy because it is simple and would not alter image resolution. To quantitatively compare the images taken with the standard versus reduced parameters, a program was written in Matlab (MathWorks, Natick, MA) to calculate a metric known as the contrast-to-noise ratio (CNR) for each image: CNR ¼ ðIH –IL Þ=σ where IH represents the pixel value of a high intensity region (such as bone or fiducial markers) and IL is the pixel value of a low-intensity region (ie, the background; Fig 1). The noise (σ) is estimated as the average of the standard deviations of these 2 regions. Thus, for the purpose of patient positioning, we wish to maintain the same level of contrast, without introducing additional noise into the image. For each site, the CNR from the phantom measurements was plotted versus mAs, and the mAs at the point where CNR begins to decrease was determined. From each plot, Table 2 Site

Results Cumulative absorbed skin dose assessment In this study, we estimated the dose at the skin for 90 patients who were treated using planar IGRT. A total of 3612 anterior and 3687 lateral images were considered. The treatment site with the largest number of images per patient was male pelvis, with an average number of approximately 45 anterior and 46 lateral images per patient. The treatment site with the least number of images per patient was H&N/brain, with an average number of approximately 32 anterior and 33 lateral images per patient. Based on this analysis, it was observed that the kV planar imaging dose was most significant for lateral imaging in pelvic cases (Table 2). The kV planar imaging dose was the least in H&N/brain cases. Using the standard kVp and mAs, patients being treated for pelvic malignancies received an average kV imaging skin dose of 162.2 cGy (range, 141.8206.1 cGy) from lateral imaging fields. The average cumulative absorbed skin doses for lateral imaging of thoracic, abdominal, and H&N/brain patients were 30.0 cGy (range, 20.5-65.1 cGy), 20.6 cGy (range, 14.7-30.4 cGy), and 1.2 cGy (range, 0.5-1.7 cGy), respectively, based on standard imaging parameters. In general, average cumulative skin doses for anterior imaging were less than that for lateral

Differences in absorbed skin dose by body site using standard versus reduced mAs Standard lateral cumulative dose, cGy (no. of images)

Reduced lateral cumulative dose, cGy (no. of images)

Standard anterior cumulative dose, cGy (no. of images)

Pelvis 162.2 ± 15.3 (46) 83.1 ± 7.8 (46) 6.4 ± 0.6 (45) Thorax 30.0 ± 10.0 (44) 7.5 ± 2.5 (44) 2.0 ± 0.6 (44) Abdomen 20.6 ± 5.6 (41) 16.5 ± 4.5 (41) 12.8 ± 3.5 (41) Head and 1.2 ± 0.3 (33) 0.8 ± 0.2 (33) 4.1 ± 1.0 (32) neck/brain

Reduced anterior cumulative dose, cGy (no. of images)

Standard lateral dose per image, cGy

Reduced lateral dose per image, cGy

Standard anterior dose per image, cGy

Reduced anterior dose per image, cGy

5.0 ± 0.5 (45) 3.5 ± 0.3 1.8 ± 0.2 0.1 ± 0.01 0.1 ± 0.01 1.0 ± 0.3 (44) 0.7 ± 0.2 0.2 ± 0.06 0.05 ± 0.01 0.02 ± 0.01 6.4 ± 1.8 (41) 0.5 ± 0.1 0.4 ± 0.1 0.3 ± 0.09 0.2 ± 0.04 2.0 ± 0.5 (32) 0.04 ± 0.01 0.02 ± 0.01 0.1 ± 0.03 0.06 ± 0.02

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imaging, except in H&N/brain cases. The average anterior doses for pelvic, thoracic, abdominal, and H&N/brain patients were 6.4 cGy (range, 5.4-8.3 cGy), 2.0 cGy (range, 1.5-3.6 cGy), 12.8 cGy (range, 8.7-19.3 cGy), and 4.1 cGy (range, 2.1-5.6 cGy), respectively.

Imaging dose reduction and quantitative comparison of image quality Figure 2 illustrates a plot of the CNR versus the mAs for anterior and lateral radiographs obtained from a pelvic RANDO phantom. The phantom was positioned so that the isocenter was placed at mid-plane. The anterior and lateral SSDs were 88.5and 80.5 cm, respectively. Anterior radiographs were obtained using 75 kVp, whereas the lateral radiographs were obtained using 120 kVp. As shown in the figure, for lower value mAs, the CNR is decreased because of the increased noise in the image. For the anterior image, the CNR significantly decreases for values b 8 mAs, whereas for the lateral image, the decrease is observed at approximately 40 mAs. Thus for the reduced settings, values of 12.6 mAs and 64 mAs were chosen for the anterior and lateral views, respectively. These values were chosen based on the available settings on the Varian OBI. This process was repeated for the other body sites considered in this study. The reduced mAs settings are summarized in Table 3.

Practical Radiation Oncology: Month 2014 Table 3

Reduced parameters by body site and image view

Site

Anterior (kVp, mAs)

Lateral (kVp, mAs)

Pelvis Thorax Head and neck/brain Abdomen

75, 12.6 75, 2.5 100, 4 80, 16

120, 95, 70, 85,

64 10 3.2 32

In the case of the lateral pelvic fields, the mAs could be reduced by 49% (from 126 to 64 mAs), thus reducing the skin dose by the same proportion. Based on the phantom study, this reduction will result in only a 0.9% decrease in CNR (from 11.7 to 11.6). Using the lower mAs settings for thoracic, abdominal, and H&N/brain cases also showed that doses could be reduced by 75%, 50%, and 51%, respectively, with decreases in CNR of only 1.6%, 7.5%, and 1.8%, respectively. Table 2 shows the absolute differences in absorbed skin doses using standard versus reduced mAs for each body site. Figure 3 compares pelvic, thoracic, and abdominal images of 3 of the 7 patients who were currently undergoing IGRT at the time of this study. The images in the figure were obtained with standard and reduced mAs on 2 separate days. By comparing the 2 sets of images in the figure, it is evident that there is no reduction in image quality with regard to the viewer’s ability to use bony alignment for daily setup verification. This is consistent with CNR values that showed no statistical difference between the standard and reduced sets of images. Quantitatively, the average CNR using the standard parameters was 5.18 ± 3.22; with the reduced parameters, CNR was 5.24 ± 3.12. A paired t test comparing these CNR values showed no statistical difference in CNR using standard versus reduced mAs for the 7 patients currently being treated with IGRT (P = .79).

Discussion

Figure 2 Contrast-to-noise ratio (CNR) versus mAs for anterior (A) and lateral (B) pelvic RANDO phantom. As mAs increase, there is a point at which CNR levels off, indicating that there is no need to use higher mAs values after a given CNR is reached.

To our knowledge, this is the first study to systematically quantify the cumulative skin dose of patients receiving kV planar IGRT to several different body sites on the Varian OBI. Moreover, this study proposes a practical method of reducing imaging dose without compromising image quality. Because the absorbed cumulative skin dose from kV planar IGRT may be as large as ~ 2 Gy depending on disease site, imaging dose reduction should be considered. Using actual patient images, this study shows that, by altering mAs, the absorbed imaging dose can be greatly reduced without sacrificing image quality (CNR). These results imply that planar dose reduction is clinically feasible. During an age in which IGRT is used more frequently to ensure precise tumor localization, the measurement and reduction of absorbed dose secondary to IGRT is an important

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Figure 3 kV planar x-ray images comparing standardized (A, C, E) and reduced (B, D, F) mAs settings. Images B, D, and F were acquired with reduced mAs settings 1 day after images A, C, and E were acquired with standard mAs. Although there are slight variations in positioning, the location of the crosshair before shifts made by the therapists, and for 1 patient, the amount of bowel gas, visual comparisons show no difference in image quality with regard to bony anatomy used for daily setup verification.

topic of ongoing research. Previously, AAPM Task Group 75 reported dose levels from several different imaging techniques, thus allowing the radiation oncologist to predict the imaging dose given a specific scenario. However, this report did not apply dose calculation techniques to actual patient data, nor did it apply dose reduction techniques to patients currently undergoing treatment. 10 There are several studies that have investigated kV planar and cone beam computed tomography imaging dose, along with methods to decrease the imaging dose. 3,10,17-25 Alvarado et al investigated the skin dose from kV planar imaging. Using radiochromic film on a RANDO phantom and a thoracic protocol with the same kVp as in the current study, they measured doses at the skin of 0.2-0.9 cGy/image. 20 With the exception of the lateral pelvic fields, these values are consistent with the

dose estimates reported in our study. However, they did not investigate skin dose estimates for other body sites and did not provide a quantitative method to evaluate image quality. Stock et al estimated skin and organ doses from cone beam computed tomography, kV planar, and MV planar imaging using thermoluminescent dosimeters on an Alderson phantom. 24 Measurements were performed using the Elekta XVI (Elekta Ltd., Crawley, UK). Standardized imaging parameters for this device are not referenced in the study, but they report kV planar pelvic imaging doses at the skin ranging from 0.3 to 1.3 mGy, depending on position. These doses are significantly lower than data in the present study, likely because of variations in mAs settings between the Elekta XVI and the Varian OBI. Walter et al also used the Elekta XVI and reported planar kV skin doses of 0.8 ± 0.1 mGy per anterior image

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and 1.1 ± 0.2 mGy per lateral image, measured by an ionization chamber affixed to the patient’s skin. 25 These doses are similar to those reported by Stock et al and much lower than those reported in the present study. In reporting imaging parameters used on the Elekta XVI, Walter et al used 120 kVp and 5 mAs for each anterior image, in comparison to 16 mAs, which was the default mAs setting used in this present study. Similarly, Walter et al reports 120 kVp and 6.4 mAs for each lateral imaging procedure, whereas the provided setting on the Varian OBI used in the present study was 126 mAs for each lateral image. These differences in imaging parameter settings account for the relatively higher skin doses obtained for the lateral pelvic fields in this study. The present study is relevant because we have not only measured the impact of imaging dose reduction by applying dose calculations to actual patient data, but also have shown that imaging dose reduction is clinically feasible in the setting of reviewing daily patient images. Furthermore, we have quantified the cumulative impact of kV planar x-ray IGRT doses on the skin. Finally, this study outlines a method by which IGRT dose reduction could be practically applied to any clinical setting. There are some limitations to this study. First, the retrospective portion of the study that estimates standard imaging dose included only 90 patients. Had a larger patient population been available, a broader range of cumulative dose values may have been obtained. Additionally, we adopted the approach of reducing the skin dose while maintaining nearly the same CNR as was obtained using the original imaging parameters. The skin dose could have been decreased much more if we had accepted a lower CNR value. However, it is not clear what minimum CNR value would be required for the images to be clinically acceptable. Future goals involve designing an observer study in which radiation oncologists are asked to evaluate both standard and reduced-dose images. In this manner, further dose reduction may be possible. Despite these limitations, this study maintains its value in that it quantifies and reduces the kV planar IGRT dose and outlines a method by which planar IGRT dose reduction could be practically applied to any clinical setting without sacrificing image quality.

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