Effect of intrafractional prostate motion on simultaneous boost intensity-modulated radiotherapy to the prostate: A simulation study based on intrafractional motion in the prone position

Effect of intrafractional prostate motion on simultaneous boost intensity-modulated radiotherapy to the prostate: A simulation study based on intrafractional motion in the prone position

Medical Dosimetry ] (2015) ]]]–]]] Medical Dosimetry journal homepage: www.meddos.org Effect of intrafractional prostate motion on simultaneous boos...

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Medical Dosimetry ] (2015) ]]]–]]]

Medical Dosimetry journal homepage: www.meddos.org

Effect of intrafractional prostate motion on simultaneous boost intensity-modulated radiotherapy to the prostate: A simulation study based on intrafractional motion in the prone position Itaru Ikeda, M.D., Ph.D.,* Takashi Mizowaki, M.D., Ph.D.,* Tomohiro Ono, M.S.,* Masahiro Yamada, Ph.D.,* Mitsuhiro Nakamura, Ph.D.,* Hajime Monzen, Ph.D.,* Shinsuke Yano, R.T.T.,† and Masahiro Hiraoka, M.D., Ph.D.* Department of Radiation Oncology and Image-applied Therapy, Graduate School of Medicine, Kyoto University Kyoto, Japan; and †Division of Clinical Radiology Service, Kyoto University Hospital, Kyoto, Japan

*

A R T I C L E I N F O

A B S T R A C T

Article history: Received 29 January 2015 Received in revised form 24 March 2015 Accepted 10 April 2015

Although the prostate displacement of patients in the prone position is affected by respiration-induced motion, the effect of intrafractional prostate motion in the prone position during “simultaneous integrated boost intensity-modulated radiotherapy” (SIB-IMRT) is unclear. The purpose of this study was to evaluate the dosimetric effects of intrafractional motion on SIB-IMRT to a dominant intraprostatic lesion (IPL) using measured motion data of patients in a prone position, fixed with a thermoplastic shell. We obtained 2 orthogonal x-ray fluoroscopic images at the same moment every 0.2 seconds for 30 seconds before and after treatment, once weekly, from 7 patients with localized prostate cancer with detectable prostatic calcification. Prostate displacements in the left-right (LR), anteroposterior (AP), and superoinferior (SI) directions were calculated using the prostatic calcification as a fiducial marker. We defined the displacement between pretreatment and posttreatment as baseline drift (BD). An SIB-IMRT plan was generated in which each IPL þ 3 mm received a dose of 94.5 Gy, whereas the remainder of the prostate þ 7 mm received a dose of 75.6 Gy in 9 fields. A simulated plan of dose blurring was generated by the convolution of isocenter-shifted plans using measured motion data in 30 seconds and motion in 30 seconds þ distance between pretreatment and posttreatment position (BD) for each of the 7 patients. The motion in 30 seconds mainly reflected respiration-induced motion. The mean displacements of BD were 1.4 mm ( 3.1 to 8.2 mm),  2.2 mm ( 9.1 to 1.5 mm), and  0.3 mm ( 5.0 to 1.8 mm) in the AP, SI, and LR directions, respectively. The differences in the target coverage with V90% of the IPL and V100% of the prostate between the simulated plan and original plan were  3.9% to  0.3% and  0.6% to 1.1% for respiration-induced motion and 3.1% to  67.8% and 3.6% to  13.3% for BD with respiration-induced motion, respectively. The large motion of BD resulted in an inadequate coverage by the prescribed dose of the SIB-IMRT to the IPL. A 7-mm margin is recommended when real-time tracking techniques are not applied. The effect of respiration-induced motion was small, so long as a 3-mm margin was added. & 2015 American Association of Medical Dosimetrists.

Keywords: Intensity-modulated radiotherapy Intrafractional motion Prone position Prostate cancer Simulation

Introduction Advances in radiotherapy delivery techniques, such as intensity-modulated radiotherapy (IMRT), allows delivery of higher radiation doses to the prostate while minimizing the

Reprint requests to Takashi Mizowaki, M.D., Ph.D., Department of Radiation Oncology and Image-applied Therapy, Kyoto University Graduate School of Medicine, 54 Shogoin Kawahara-cho, Sakyo-ku, Kyoto 606-8507, Japan. E-mails: [email protected], [email protected] http://dx.doi.org/10.1016/j.meddos.2015.04.003 0958-3947/Copyright Ó 2015 American Association of Medical Dosimetrists

dose to normal tissues. 1 It has been demonstrated that biochemical disease-free survival improves with dose escalation to the prostate. 2 Therefore, curative radiotherapy using IMRT for the treatment of prostate cancer is now performed widely. However, because of the vicinity of the bladder and the rectum to the prostate, dose escalation to the entire prostate also results in increased toxicity.3 One option to address this problem of increased complications with increased doses is to better focus the dose escalation onto a dominant intraprostatic lesion (IPL).

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Several authors have recently reported the results of implementing simultaneous boost in patients with IMRT.4-7 In our institution, patients with prostate cancer are treated with IMRT in the prone position, fixed with a thermoplastic shell. Although Bittner et al.8 and Wilder et al.9 reported that both the prone and the supine positions resulted in similar magnitudes of intrafractional prostate motion, Butler et al.10 found significantly greater net intrafractional prostate displacement of patients in the prone position than that in the supine position. Although there is no consensus for optimal treatment position, respiration-induced motion in the prone position has been reported widely.11-14 Dawson et al.12 reported prostate motion of up to 5 mm in normal respiration and up to 10 mm in deep breathing in the superoinferior (SI) direction. A thermoplastic shell can also contribute to large respiratory-induced prostate motion.15 Because a treatment plan with a complex dose distribution, such as the simultaneous integrated boost (SIB) to an IPL, tends to be created by intricate movements of multileaf collimators, the SIB-IMRT plan can theoretically be affected more strongly by intrafractional organ motion than by regular prostate IMRT. To our knowledge, no reported study has examined the effect of intrafractional prostate motion in the prone position with a thermoplastic shell on the SIB-IMRT plan. Thus, the aim of this study was to evaluate the dosimetric effects of intraprostatic motion on SIB-IMRT to a dominant IPL using measured motion data from patients in the prone position.

Methods and Materials Patients’ characteristics We enrolled 7 patients with localized prostate cancer with prostatic calcification who underwent definitive external beam radiation therapy with a Vero4DRT (marketed as the MHI-TM2000; Mitsubishi Heavy Industries, Ltd., Japan, and Brainlab AG, Feldkirchen, Germany)16 in this study. The patients were participants in a clinical trial that evaluated intrafractional prostate motion. The clinical trial was approved by the institutional review board (registration number C594) and registered at the University Hospital Medical Information Network Center (clinical trial registration number UMIN000006906). All patients provided written informed consent.

Patient setup Details of our setup protocol were reported previously.17,18 Briefly, patients were immobilized in the prone position with a thermoplastic shell (Hip Fix system; CIVCO Medical Solutions, Kalona, IA) that extended from the midthigh to the upper third of the leg in combination with a vacuum pillow (Vac-Lok system; CIVCO Medical Solutions) and a leg support. Planning computed tomography (CT) scans (LightSpeed RT; GE Healthcare, Waukesha, WI) were obtained at 2.5-mm slice thickness. Patients were instructed to empty their bladder and rectum  1 hour before the CT simulation. At each treatment, patients were required to void the bladder and the rectum with timing identical to that established in the CT simulation. Evaluation of intrafractional prostate motion The Vero4DRT is equipped with 2 pairs of kilovolt (kV) x-ray imaging systems that can obtain x-ray fluoroscopic images and cone-beam CT (CBCT) images. For clinical practice, patients first underwent manual skin marking, and position correction was then performed using CBCT at each treatment. In this study, 2 orthogonal x-ray fluoroscopic images with a spatial resolution of 0.2 mm were taken simultaneously every 0.2 seconds for 30 seconds before and after treatment once weekly. In total,  1800 pairs were acquired per patient. The treatment positioning was setup to calculate the intrafractional prostate motion. Using in-house software, prostate displacements in the left-right (LR), anteroposterior (AP), and SI directions were calculated using these orthogonal images with the prostatic calcification as a marker.19 If more than 1 calcification was identified within the prostate, the largest was used (Fig. 1). It has been reported that the displacements of intraprostatic calcification were equivalent to implanted markers in the 3 directions.20 Treatment planning for simultaneous boost to a dominant IPL Pretreatment magnetic resonance imaging (MRI) was performed to delineate the region of the biopsy-proven IPL. MRI and CT fusion with treatment planning CT was performed using iPlan RT image (ver. 4.1.1; Brainlab AG) and verified manually by checking the center of the prostate in both the scans. The IPL was then contoured using an empirical algorithm.21 An IMRT plan with simultaneous boost was generated with the IPL in the left peripheral zone beside the rectum using iPlan RT Dose (ver. 4.5.1; Brainlab AG). A 3-mm margin was established around this IPL. The IPL þ 3 mm received a dose of 94.5 Gy in 2.25-Gy daily fractions, whereas the remainder of the prostate þ 7 mm received a dose of 75.6 Gy in 1.8-Gy daily fractions. This prescription was determined by reference to a protocol reported previously.6,7 Figure 2 shows a treatment planning image in the axial plane. Target coverage was satisfied because the V90% of the IPL and the V100% of the prostate were 96.9% and 96.4%, respectively. The doses to the at-risk organs were within the limits, e.g., the V70 Gy of the rectum and the V65 Gy of the bladder were 10.2% and 25.0%, respectively. The maximum doses to the rectum, bladder, and urethra were 80.4, 78.8, and 79.6 Gy, respectively. An IMRT plan consisted of 9 beams at gantry

Fig. 1. An example of an in-house software image used to calculate the 3-dimensional positions of prostatic calcification. The centroid of the largest prostatic calcification is marked within the red square on the 2 orthogonal x-ray fluoroscopic images. (Color version of figure is available online.)

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Results Measured motion data

Fig. 2. An axial plane image of an SIB-MRT plan with the IPL in the left peripheral zone. (Color version of figure is available online.) angles of 01, 351, 601, 1001, 1601, 2001, 2601, 3001, and 3301. IMRT was delivered with the step-and-shoot method in 25 segments at all beam angles.

Simulation of dose blurring by intrafractional motion First, the 150 pretreatment data points for 30 seconds were classified into position coordinates in 1-mm units for each axis, in the LR, AP, and SI directions. The mean position of the prostatic calcification for 30 seconds was defined as point 0. A histogram of the prostate position was then generated (Fig. 3). Using the original IMRT plan of simultaneous boost to the IPL, a shifted dose distribution was created by moving the isocenter contrary to the prostatic motion at each position coordinate for each field. Thereafter, a blurred-dose distribution, which incorporated the effect of the intrafractional motion of the prostate, was generated by summing up the shifted dose distributions weighted by each corresponding existence probability calculated from the histogram of the prostate position (Fig. 3). Second, the distance was calculated by subtraction of the mean prostatic position from the pretreatment and posttreatment data (defined as baseline drift [BD]). The BD within the treatment time was assumed to shift linearly (Fig. 4). Although there were several patterns of intrafractional prostate motion,14,22-24 the reason for using this method was that the slow, continuous drift pattern was considered to best express the influence of intrafractional motion between the 2 points, on average. The effect of dose blurring with BD was calculated by adding the BD to the pretreatment histogram in each field. To reproduce the actual medical treatment, the data of the calculated BD from 5 to 14 minutes were used because 1 minute passed between the irradiation of each field. In fact,  5 minutes passed between the start of CBCT and the irradiation of the first field during clinical practice at our institution, and  1 minute is required to treat and move to the next field in the simulated IMRT plan. All shifted dose distributions with or without BD were then summed using MIM Maestro (ver. 6.1.2; MIM Software Inc., Cleveland, OH). Dose weights were calculated by the rate of the shifted position. A dose-volume histogram (DVH) was also evaluated using MIM Maestro (Fig. 5).

In total, 41 data sets from 7 patients were examined for intrafractional motion in the prone position. An example of actual respiration-induced motion and BD for patient 6 is shown in Fig. 6. The mean peak-to-peak distances of respiration-induced motion were 2.7, 4.6, and 1.6 mm in the AP, SI, and LR directions, respectively. The mean displacements of BD were 1.4,  2.2, and  0.3 mm in the AP, SI, and LR directions, respectively, with a 3dimensional displacement of 3.8 mm. The mean time between pretreatment and posttreatment monitoring was 15.9 minutes (range: 12 to 23). These results are summarized in Table 1. The average prostate displacement every 4 seconds along each directional axis was calculated for the 30 seconds of pretreatment as a function of time (Fig. 7). In all directions, BD was negligible and the mean prostate position was stable for the full 30 seconds. Thus, the data for these 30 seconds reflected primary respirationinduced motion, and this dose distribution was termed the respiration-induced dose distribution. The data of the largest BD in each patient were selected to simulate the dose blurring. The results of the maximum prostate motion for each patient are summarized in Table 2. The 3dimensional baseline displacement was 2.3 to 11.3 mm. Simulation of dose blurring Simulated plans blurred by motion in 30 seconds (respirationinduced motion [R]) and R þ BD were generated for each patient. The dose-volume parameter and DVH of respiratory motion only are shown in Table 3 and Fig. 8. Target coverage was almost satisfied; the differences with V90% of the IPL and the V100% of the prostate were  3.9% to  0.3% and  0.6% to 1.1%, respectively, when compared with those of the standard plan. The rates of increase in the maximum dose to normal tissue such as that to the rectum, bladder, and urethra were 1.7%, 0.5%, and 0.8%, respectively. The DVH of the bladder indicated that the medium dose was increased slightly (V40 Gy increased by 4.5%). The dose-volume parameter and DVH of R þ BD are shown in Table 4 and Fig. 9. The V90% of the IPL and the V100% of the prostate with the R þ BD plan were 3.1% to  67.8% and 3.6% to  13.3%, respectively. The increases in the maximum dose to the rectum, bladder, and urethra were 11.2%, 7.0%, and 5.5%, respectively.

Discussion We simulated dose blurring of irradiation with IMRT using measured data for intrafractional prostate motion from patients in the prone position. Intrafractional prostate motion in the prone

Fig. 3. Small red dots on the far left indicate prostatic displacement data in the 30 seconds. Using these data, a histogram of the prostate position at 1-mm intervals was generated. The isocenter was shifted by displacement, and the irradiated dose was calculated for each position. Each shifted plan was summed as a function of the existence probability calculated from the histogram of the prostate position. The blurred-dose distribution was then generated. (Color version of figure is available online.)

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Ikeda et al. / Medical Dosimetry ] (2015) ]]]–]]] Table 1 Intrafractional prostate motion

Fig. 4. The baseline drift was calculated by the mean position of the prostate between pretreatment and posttreatment. The baseline drift within the treatment time was assumed to shift linearly. To reproduce the actual medical treatment, 5 minutes pass from the start of irradiation and 1 minute passes between irradiation of each field. F1 to F9 indicate fields 1 to 9. (Color version of figure is available online.)

Patients

N¼7

Pretreatment and posttreatment data Treatment time (min) Motion in 30 s AP (mm) SI (mm) LR (mm)

n ¼ 41 12 to 23 (mean ¼ 15.9)

Baseline drift AP (mm) SI (mm) LR (mm) 3D (mm)

1.0 to 5.1 (mean ¼ 2.7) 2.0 to 8.8 (mean ¼ 4.6) 0.9 to 3.4 (mean ¼ 1.6) 3.1 to 8.2 (mean ¼ 1.4) 9.1 to 1.5 (mean ¼  2.2) 5.0 to 1.8 (mean ¼ – 0.3) 0.3 to 11.6 (mean ¼ 3.8)

3D ¼ 3 dimensional ¼ √(AP2 þ SI2 þ LR2) þ ¼ motion to the posterior/superior/left directions about the isocenter.

position is influenced by respiration-induced motion.11-14 Although several articles reported the influence of intrafractional prostate motion in the supine position,22,23,25 no previous study has examined the effects of intrafractional motion on prostate SIBIMRT in the prone position, including respiration-induced motion and BD. We sought to clarify this issue by conducting an analysis of real prostate motion in patients. In the present study, respiration-induced prostate motion was detected in the SI and the AP directions, consistent with other reports,11,12 with a range of 1 cm or less. Our results showed that the effect of dose blurring of respiration-induced motion was small. Chen et al.26 investigated the interplay effect in respiratory-gated IMRT with a moving phantom. They suggested

Fig. 5. All shifted dose distributions of every field with or without baseline drift were summed. The DVHs of the IPL, prostate, rectum, bladder, and urethra were then evaluated for each distribution of respiration-induced motion both alone and with baseline drift. Dose comparison between these simulated plans and the original plan was then performed. (Color version of figure is available online.)

Fig. 6. Prostate displacement in the LR, SI, and AP vectors is shown as a function of time in seconds for each pretreatment and posttreatment of patient 5. Respirationinduced motion in the SI and the AP directions was detected as approximately 1 movement every 4 seconds for this patient. The mean pretreatment position was defined as point 0. For the 30 seconds, prostate displacement was affected mainly by respiration-induced motion. Baseline drift occurred within the treatment time in the AP and SI directions. þ ¼ displacement to the left/superior/posterior directions about the isocenter. (Color version of figure is available online.)

Fig. 7. Average prostate displacement in mm as a function of time in seconds. Displacement was measured in the AP (A), SI (B), and LR (C) directions. (Color version of figure is available online.)

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Table 2 Maximum prostate motion for each patient Patient 1 Motion in 30 seconds (respiration-induced motion) AP (mm) 1.2 SI (mm) 3.1 LR (mm) 1.7

Patient 2

Patient 3

Patient 4

Patient 5

Patient 6

Patient 7

2.0 4.2 1.4

4.2 7.1 1.3

1.0 4.3 1.1

2.2 5.4 1.6

1.5 3.1 1.1

3.7 4.4 2.1

16  3.1 1.5 1.8 3.9

15 2.2  5.2 0.1 5.6

13 3.3  4.9  0.8 5.9

12 3.4  6.6  0.5 7.4

14 7.0  9.1  1.6 11.6

16 5.0 – 5.1 1.8 7.3

Baseline drift Treatment time (min) AP (mm) SI (mm) LR (mm) 3D (mm)

18 0.2  2.0  0.5 2.1

that maximum motion up to 1.1 cm was acceptable by the gamma method and the dose distribution was almost unchanged if the motion was less than 5 mm. Because the maximum and mean respiration-induced motions in the SI direction were 8.8 and 4.6 mm, respectively, respiration-induced prostate motion in the prone position should not be a problem for the dosimetric effect. However, a large BD resulted in insufficient dose coverage for the IPL in this study. When compared with other reports of pretreatment and posttreatment intrafractional prostate motion, our data agree with those of Wilder et al.9 They used cine MRI in 46 patients with localized prostate cancer to measure intrafractional prostate motion. The maximum displacement was 1.2 cm. The pretreatment and posttreatment displacement data could not clarify the real intrafractional motion, and we assumed that BD occurred linearly in this study. However, continuous evaluation of the intrafractional prostate motion using real-time monitoring of implanted radiofrequency transponders has been reported recently.8,14,24 Although no predictable trend was observed in the motion, 2 general types of motion have been reported to occur: one was a slow drift and the other was sudden and transient. Abdellatif et al.22 assessed each of these types of intrafractional prostate motion and found that the random motion of the prostate during treatment was less motion sensitive than was the continuous drift pattern. Thus, our results seem reasonable for assessing the effect of intrafractional prostate motion, although only the slow drift pattern was examined.

In our study, the prostate tracks drifted toward the patients’ posterior and inferior. This tendency was also reported by Shah et al.,14 Bittner et al.,8 and Langen et al.24 They theorized that the movements might be related to relaxation of the pelvic muscles, movement of the rectum, and bladder volume. No correlation between these movements and the bladder volume or the rectum or prostate motion was observed in our assessment using CBCT data from the patients in this study. We consider that relaxation of the pelvic muscles is the most important factor associated with BD. There were several limitations to this study. We used only 1 patient plan and did not investigate other patients with different locations of the IPL. Additionally, we calculated the motion effect using only 1 fraction of the largest intrafractional prostate motion, although the simulated plan was set for 42 fractionations. We also did not account for additional factors, such as changes in the source-to-skin distances or treatment depths due to movements of the prostate, the organ deformation, and rotation, and the differences in the width of the multileaf collimator. However, dosimetric effects arising from those factors were reported as negligible in other studies.27-31 Because Bittner et al. also reported that BD typically started  5 minutes after initiation of tracking,8 rapid setup and rapid irradiation, such as that achieved in arc therapy, may reduce the probability of a motion effect. However, intrafractional prostate motion is complex, and the movements are largely unpredictable.24 Additionally, if hypofractionation, which requires a smaller

Table 3 Differences between dose values for the simulated plan with respiration-induced motion and those for the treatment plan Plan

Patient 1 (%)

Patient 2 (%)

Patient 3 (%)

Patient 4 (%)

Patient 5 (%)

Patient 6 (%)

Patient 7 (%)

IPL D50% V90% Mean Dmax Dmin

95.6 Gy 96.9% 94.9 Gy 101.5 Gy 78.5 Gy

   

0.0 3.9 0.6 0.4 2.5

0.0  0.3  0.1  0.3 0.2

4.7  0.5  0.9  1.5 1.4

   

    

    

   

Prostate V100% D95% Mean Dmin

96.4% 100.5% 79.9 Gy 71.4 Gy

 0.6  0.1 0.4  0.4

0.8 0.3 0.5 0.6

1.1 0.5 0.4 1.9

0.4 0.2 0.5 0.5

0.5 0.2 0.4 0.9

0.2 0.1 0.6 0.2

0.8 0.4 0.3 1.2

Rectum Max V70 Gy

80.4 Gy 10.2%

 2.7  1.2

0.4 0.3

1.7  0.7

0.4 0.2

0.2  0.2

 0.4  0.2

0.5  0.2

Bladder Max V65 Gy

78.8 Gy 25.0%

0.3 2.6

0.2 1.9

0.1 1.2

0.2 1.8

0.1 1.7

0.5 2.3

0.0 1.4

Urethra Max

79.6 Gy

0.8

0.0

 0.9

0.0

 0.1

0.1

 0.2

0.2 0.4 0.1 0.2 0.1

0.5 1.1 0.5 0.5 0.1

0.2 1.1 0.1 0.2 0.5

0.5 0.8 0.6 0.8 0.3

V90% ¼ volume of 90% of prescription dose; D95% ¼ dose to 95% of the volume; Mean ¼ mean dose; Dmax ¼ maximum dose; Dmin ¼ minimum dose; V100% ¼ volume of 100% of prescription dose; V70 Gy ¼ volume of receiving more than 70 Gy; V65 Gy ¼ volume of receiving more than 65 Gy.

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Fig. 8. A dose-volume histogram for simulation of motion in 30 seconds (respiration-induced motion). (Color version of figure is available online.)

planning target volume (PTV) margin and longer irradiation time, is considered, accidental large motion over the PTV margins may result in an inadequate dose distribution. Real-time monitoring or tracking methods may be required in hypofractionated IMRT. Intrafractional motion of the prostate has recently been a concern for hypofractionation therapies, such as stereotactic radiotherapy. The current study indicates that sporadic prostate movement can result in a significant dosimetric effect, and caution is required regarding intraprostatic motion, especially during hypofractionation. Because the dose coverage of the prostate was maintained against the intrafractional movements of the prostate, a 7-mm

margin to the IPL would be appropriate to ensure the planned dose delivery to the IPL without using real-time monitoring or tracking techniques.

Conclusions The effect of the respiration-induced motion on actually delivered dose was small. However, the BD of the intrafractional prostate motion of BD resulted in an insufficient dose coverage of the IPL in the simultaneous boost plan with a 3-mm margin.

Table 4 Difference between dose values for the simulated plan of baseline drift with respiration-induced motion and those for the treatment plan Plan

Patient 1

IPL D50% V90% Mean Dmax Dmin

95.6 Gy 96.9% 94.9Gy 101.5 Gy 78.5 Gy

– – – – –

Prostate V100% D95% Mean Dmin

Patient 2

Patient 3

Patient 4

Patient 5

Patient 6

Patient 7

1.5 4.4 1.3 0.4 2.2

0.6 3.1 0.9 0.0 9.8

– 1.4 3.1 – 0.7 – 1.1 7.7

– 7.3 – 32.9 – 7.2 – 0.5 – 8.6

– 10.8 – 45.7 – 10.2 – 2.0 – 9.4

– 16.8 – 67.8 – 15.1 – 3.8 – 12.8

– 6.4 – 27.3 – 6.6 – 1.1 – 7.8

96.4% 100.5% 79.9 Gy 71.4 Gy

0.3 0.4 0.7 0.3

3.6 2.2 1.4 4.8

3.6 1.7 0.5 5.9

– 5.2 – 2.2 0.5 – 0.9

– 6.9 – 3.1 0.3 – 2.1

– 13.3 – 5.2 – 0.7 – 6.2

– 5.2 – 2.2 0.7 – 1.5

Rectum Max V70 Gy

80.4 Gy 10.2%

– 2.5 0.0

11.2 5.6

10.1 5.1

– 5.1 – 5.2

– 5.1 – 6.5

– 7.3 – 8.8

– 6.2 – 5.9

Bladder Max V65 Gy

78.8 Gy 25.0%

1.1 3.7

0.6 – 1.3

– 0.1 – 5.5

1.8 9.0

4.1 10.9

7.0 14.9

2.0 8.4

Urethra Max

79.6 Gy

0.4

1.0

0.0

2.5

2.9

2.4

5.5

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Fig. 9. A dose-volume histogram for simulation of baseline drift with respiration-induced motion (R þ BD). (Color version of figure is available online.)

Compensation for BD should be taken into consideration, especially in SIB-IMRT to the IPL or a small PTV margin, as in hypofractionation. A 7-mm margin around the IPL is recommended to compensate the BD of the prostate, if a real-time tracking technique is not combined.

Acknowledgments This work was supported by grants-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (24591838), and New Energy and Industrial Technology Development Organization (NEDO), Japan (P12009). The sponsor played no role in the study design; in the collection, analysis, and interpretation of the data; the writing of this manuscript; or in the decision to submit this article for publication.

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