- Email: [email protected]
Daily variations in the position of the prostate bed in patients with prostate cancer receiving postoperative external beam radiation therapy
Int. J. Radiation Oncology Biol. Phys., Vol. 66, No. 2, pp. 593–596, 2006 Copyright © 2006 Elsevier Inc. Printed in the USA. All rights reserved 0360-3016/06/$–see front matter
doi:10.1016/j.ijrobp.2006.05.071
PHYSICS CONTRIBUTION
DAILY VARIATIONS IN THE POSITION OF THE PROSTATE BED IN PATIENTS WITH PROSTATE CANCER RECEIVING POSTOPERATIVE EXTERNAL BEAM RADIATION THERAPY PATRICK A. KUPELIAN, M.D., KATJA M. LANGEN, PH.D., TWYLA R. WILLOUGHBY, M.S., THOMAS H. WAGNER, PH.D., OMAR A. ZEIDAN, PH.D., AND SANFORD L. MEEKS, PH.D. Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, Orlando, FL Purpose: The aim of this study was to evaluate the extent of the variation in the position of the prostate bed with respect to the bony anatomy. Methods and Materials: Four patients were treated to 70 Gy in 35 fractions. Before each fraction, a megavoltage computed tomography (CT) of the prostate bed was obtained, resulting in a total of 140 CT studies. Retrospectively, each CT scan was aligned to the simulation kilovoltage scan based on bony anatomy and the prostate bed. The difference between the 2 alignments was calculated for each scan. Results: The average differences (ⴞ1 SD) between the two alignments were 0.06 ⴞ 0.37, 0.10 ⴞ 0.86, and 0.39 ⴞ 1.27 mm in the lateral, longitudinal (SI), and vertical (AP) directions, respectively. Laterally, there was no difference >3 mm. The cumulative frequency of SI differences were as follows; >3 mm: 3%, >4 mm: 1%, and >5 mm: 1% (maximum: 5 mm). The cumulative frequency of AP differences were as follows; >3 mm: 7%, and >4 mm: 3% (maximum: 4 mm). Conclusion: In patients with prostate cancer receiving postoperative radiotherapy, the prostate bed motion relative to the pelvic bony anatomy is of a relatively small magnitude. Significant motion (>3 mm) is infrequent. However, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed. © 2006 Elsevier Inc. Prostate bed, Motion, Postoperative radiotherapy.
The use of salvage or adjuvant radiotherapy to the prostate bed after radical prostatectomy for localized prostate cancer is frequently performed and outcomes are regularly reported (1, 2). However, the treatment techniques for prostate bed irradiation have varied throughout the years and still vary from institution to institution. One technical aspect of postoperative radiotherapy for localized prostate cancer that has received little attention is prostate bed motion. Significant work has been done in the description of daily position variations of the prostate gland during the course of external beam radiotherapy. However, there is limited information about the variation of the position of the prostate bed during the course of postoperative radiation therapy. With the advent of image-guided radiotherapy, specifically with techniques that provide daily soft tissue imaging in the treatment room, it is now possible to document position changes of target areas during entire external radiotherapy courses. There is no question that the prostate bed would vary in
position with respect to external skin marks, as those are notoriously unreliable. With the availability of portal imagers, in-room X-rays, and in-room computed tomography (CT) scans, the evaluation of discrepancies between alignment on the bony anatomy and the prostate bed proper can be performed. If daily alignment is performed on the bony anatomy, treatment margins can consequently be adjusted according to the expected motion of the prostate bed itself. A related issue that needs to be addressed is the definition of the prostate bed and planning target volume for postoperative radiation therapy. This issue is beyond the scope of this study, but would affect the observations and conclusions made about the motion of the target areas during a course of external beam radiation. The current study looks at the prostate bed as the small area beneath the bladder, corresponding to where the apex of the intact prostate would have been located. Therefore, the conclusions of this study cannot be extrapolated to target volumes extending significantly above the level of the bladder base.
Reprint requests to: Patrick Kupelian, M.D., Department of Radiation Oncology, M.D. Anderson Cancer Center Orlando, 1400 South Orange Avenue, Orlando FL 32806. Tel: (407) 841-8666; Fax: (407) 649-6895; E-mail: [email protected] The M. D. Anderson Cancer Center Orlando has received research funding from TomoTherapy, Inc.
Acknowledgments—This work was supported by a grant from Women Playing For T.I.M.E. We are grateful for all the encouragement and inspiration from Mrs. Elaine Lustig and her colleagues at Women Playing For T.I.M.E. Received March 30, and in revised form April 25, 2006. Accepted for publication May 19, 2006.
INTRODUCTION
593
594
I. J. Radiation Oncology
● Biology ● Physics
METHODS AND MATERIALS Four patients receiving postoperative radiotherapy to the prostatic fossa with a helical tomotherapy unit (TomoTherapy Hi*ART II; TomoTherapy Inc., Madison, WI) constituted the study sample. The patients were treated as per routine clinical care. Local IRB approval was obtained for the retrospective analysis of the scans. All patients were treated to 70 Gy at 2 Gy per fraction, in 35 fractions. Before each treatment, megavoltage CT (MVCT) images of the prostate bed were acquired using the on-board imaging capabilities of the TomoTherapy Hi*ART II unit. Positional adjustments were made on the basis of the pelvic bony anatomy for the actual treatments. Subsequently, the 140 CT image sets were used retrospectively to study the position of the prostate bed with respect to the bony anatomy. Each megavoltage scan was aligned to the simulation kilovotlage CT (kVCT) scan based on either of two methods: (1) the bony anatomy, and (2) the prostate bed itself, defined by surgical clips in or around the prostate bed. The definition of the “prostate bed” is somewhat broad. The area that is of most relevance to the definition of postoperative radiation therapy target areas is ultimately the urethral anastomotic site. This is probably only part of what is generally defined as the postoperative radiation therapy target area. In the current study, the surgical clips were mostly present superior to the actual anastomosis location, and defined a broader anatomic location. Although imperfect, the location of these surgical clips was considered to be representative of the general location of a typical postoperative radiation therapy target area. Ultimately, the ideal approach should be the placement of metallic fiducials near the anastomosis site under transrectal ultrasound guidance immediately before the design and delivery of any postoperative radiation therapy. Target localization during therapy would be subsequently performed by the determination of the location of those metallic fiducials. Figure 1 shows corresponding scans obtained at simulation (kVCT) and at treatment (MVCT) clearly showing a surgical clip. The bony anatomy was aligned automaticaly on the treatment console using an extracted feature fusion algorithm supplied by the manufacturer (TomoTherapy Inc.) (3). The automatic fusion was visualy checked for accuracy by the physician (P.A.K.). The retrospective alignement of the prostate bed as defined by surgical clips was done manually by the physician (P.A.K.). For each of the 140 fractions, the difference in position between the prostate bed alignment and bony anatomy was calculated. The differences were then analyzed with respect to the average positional difference in the anterior/posterior (AP), superior/inferior (SI), and lateral dimensions. In addition, the frequencies of differences were also analyzed. To assess the uncertainity of the marker to bony movement measurement, three markers were implanted into a anthrophormor-
Fig. 1. Corresponding computed tomography (CT) images of the prostate bed with a surgical clip. Megavoltage CT obtained during treatment (a) and kilovoltage CT obtained during simulation (b).
Volume 66, Number 2, 2006
pic phantom. This phantom was then treated like a patient (i.e., a kVCT scan was performed and MVCT scans of the phantom were registered with this kVCT phantom). Since the markers are rigidly implanted into the phantom an alignment of the phantom based on markers should be identical to a bony anatomy based alignment (i.e., the movement of the markers relative to the bony anatomy is zero). The 35 MVCT scans were obtained and these scans were visually aligned to the kVCT scan based on the marker locations. The same MVCT scans were then registered with the kVCT scans using the automatic bone based fusion tool provided by the manufacturer. The difference between these 2 alignments were calculated and averaged over the 35 studies the mean and standard devitions were 0.6 ⫾ 0.4 mm, 0.5 ⫾ 1.0 mm, 0.2 ⫾ 0.7 mm in the lateral, SI, and AP directions, respectively.
RESULTS The average differences (⫾1 SD) between the bony anatomy alignments and the prostate bed alignments were 0.06 ⫾ 0.37 mm, 0.1 ⫾ 0.86 mm, and 0.39 ⫾ 1.27 mm in the lateral, SI, and AP directions, respectively. The average directional differences (⫾1 SD) between the bony anatomy alignments and the prostate bed alignments were 0.11 ⫾ 0.36 mm, 0.39 ⫾ 0.77 mm, and 0.89 ⫾ 0.98 mm in the lateral, SI and AP directions, respectively. The directional displacements are similar in size to the uncertainties of the measurement and consequently no evidence of large and frequent prostate bed motion relative to the bony anatomy was found. However, a few displacements that were ⬎3 SD of the measurement uncertanty were observed. In the SI dimension, 3% of all differences were ⱖ3 mm, 1% were ⱖ4 mm, and 1% were ⱖ5 mm. The largest SI difference was 5 mm. In the AP dimension, 7% of all differences were ⱖ3 mm, and 3% ⱖ4 mm. The largest AP difference was 4 mm. DISCUSSION Daily image guidance leads to the understanding of position variations of various targets in different clinical situations. For the alignment of the prostate bed in patients treated with external radiotherapy after radical prostatectomy, it is still not clear how to best localize the target area. Previous techniques relied on relatively large areas to be treated in the pelvis. However, modern conformal techniques have been implemented to specifically treat the prostate bed only. This leads to the implementation of relatively small fields. If the prostate bed moves as significantly as the prostate gland during definitive external radiotherapy, then daily targeting becomes crucial for the implementation of such techniques. There are numerous reports on the significant daily position variations of the intact prostate gland mostly because of rectal and bladder filling (4 –9). Therefore, one would assume that the prostate bed would be subject to similar motions. However, there is a paucity of reports in the literature studying this issue (10, 11). Historically, patient alignments had been done on the basis of skin marks, which are considered unreliable if tight
Prostate bed motion and external beam radiotherapy
conformal fields are planned to be delivered. In a unique report trying to address this issue, Chinnaiyan et al. reported on the daily transabdominal ultrasound for targeting the prostate bed (10). In the 6 patients that constituted the study sample, the authors reported average (⫾SD) ultrasoundbased shifts from skin marks in the AP, lateral, and SI dimensions of 5 ⫾ 4 mm, 3 ⫾ 3 mm, and 3 ⫾ 4 mm, respectively The authors considered these shifts to be important and justifying the implementation of daily targeting during conformal radiotherapy to the prostate bed (10). However, because of the nature of imaging and targeting with transabdominal ultrasound, the accuracy of such ultrasound based alignment has been questioned for targeting the intact prostate (11). Similar inaccuracies can lead to misalignment of the prostate bed with transabdominal ultrasound. It is also important to note that the authors reported the shifts from set-up skin marks, and not from the patients’ bony anatomy as in the current study. Pelvic bony anatomy has been demonstrated to be an unreliable proxy for the position of the intact prostate gland. Because of bladder filling but mostly because of rectal filling, the position of the prostate varies significantly from day to day, independently from the pelvic bony anatomy. This motion has been shown to be less prominent at the apex, and most significant toward the base, as the prostate gland is relatively fixed inferiorly and more subject to the pitching from rectal distention superiorly. After radical prostatectomy, the prostate bed occupies mostly the area corresponding to the prostatic apex, with the bladder filling the space where the prostate was, and the rectum distending mostly superior to the apical area where the prostate bed is. This relatively small area might not move as significantly as the intact prostate gland. Therefore, alignment on pelvic bony anatomy could be an accurate proxy for alignment of the prostate bed, whereas it has been demonstrated to be unreliable for the intact prostate (4, 6). The data from our current study indicate that prostate bed motion seems to be of smaller magnitude and frequency to what has been reported for the intact prostate. However, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed. The differences between the position of the prostate bed and the pelvic bony anatomy were small; the largest differences were 2 mm in the lateral dimension, 4 mm in the AP dimension, and 5 mm in the SI dimension. Overall, the average differences were remarkably small, indicating infrequent discrepancy in the difference between prostate bed and pelvic anatomy positions. These data indicate that rectal and bladder distention has a smaller effect on the position of the prostate bed compared with the effect on an intact prostate gland. Further work needs to be done to quantify the impact of rectal and bladder filling on the rectal and bladder doses, rather than the position of the prostate bed itself. Fiorino et al. describe trends in bladder and rectal filling in the postoperative setting, and concluded that there is significant variation in the dosimetry because of
● P. A. KUPELIAN et al.
595
such motion. The authors note that the rectal volume variation was mostly in the superior part of the rectum, similar to our observations (12). However, they inferred that this rectal distention can cause deformation of the posterior part of the target area, compared with what was observed at the time of simulation. It is clear that the more superior (cephalad) the planning target structures are, the more variation there will be in the target position and structure (i.e., deformation) because of rectal distention. This begs the question of how the prostate bed should be defined. The definition of what should be considered the prostate bed and the appropriate planning target volume for postoperative radiation therapy after radical prostatectomy is beyond the scope of this study. However, as the current study concentrated strictly on the prostate bed, the findings cannot be extrapolated to situations where planning target volumes are extended superiorly beyond the prostate bed. Figure 2 shows two sagittal CT reconstructions in two different patients demonstrating the low position of the prostate bed within the pelvis. The only report in the literature that has a similar methodology to the present study has been reported by Schiffner et al. (13). A total of 163 treatment fractions were analyzed from 10 patients who had markers placed in the prostate bed and localized using portal imaging devices. They reported mean (⫾SD) differences between the bony anatomy and the markers of 0.2 ⫾ 4.5 mm, 0.4 ⫾ 2.4 mm, and 1.1 ⫾ 2.1 mm in the lateral, SI, and AP dimensions respectively. There are slightly larger variations observed compared with the ones observed in the current study. However, these are clearly of smaller magnitude compared with what is expected in the setting of intact prostates. Although the study sample only included 4 patients and the current findings might be consequently skewed, the anatomies from these 4 patients were typical of what is expected in a postoperative situation. Pelvic anatomy varies significantly with the shape and size of an intact prostate gland, but the variation in the postoperative anatomy of the prostate bed is apparently much less prominent. Therefore,
Fig. 2. Sagittal kilovoltage computed tomography (CT) scans obtained during simulation in two separate patients, in both cases showing the low position of the prostate bed within the pelvis as demonstrated by the relative position of the prostate bed with respect to the pubis. The part of the rectum that distends is typically more cephalad than the prostate bed.
596
I. J. Radiation Oncology
● Biology ● Physics
it is reasonable to assume that the findings for the current 4 patients are representative of other patients that receive postoperative radiotherapy. Finally, it is important to repeat the current analysis in a larger number of patients, to make these observations more relevant to the radiation oncology community in general. However, it is difficult to find a large number of postprostatectomy prostate cancer patients undergoing radiation therapy with surgical clips within the prostate bed proper. Our goal is to repeat this study pro-
Volume 66, Number 2, 2006
spectively with the placement of metallic fiducials within the prostate bed before radiation therapy. CONCLUSION In conclusion, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed.
REFERENCES 1. Pollack A, Hanlon A, Pisansky T, et al. A multi-institutional analysis of adjuvant and salvage radiotherapy after radical prostatectomy. Int J Radiat Oncol Biol Phys 2004;60:S186. 2. Stephenson AJ, Shariat SF, Zelefsky MJ, et al. Salvage radiotherapy for recurrent prostate cancer after radical prostatectomy. JAMA 2004;291:1325–1332. 3. Ruchala KJ, Olivera GH, Kapatoes JM. Limited-data image registration for radiotherapy positioning and verification. Int J Radiat Oncol Biol Phys 2002;54:592– 605. 4. Crook JM, Raymond Y, Salhani D, et al. Prostate motion during standard radiotherapy as assessed by fiducial markers. Radiother Oncol 1995;37:35– 42. 5. Huang E, Dong L, Chandra A, et al. Intrafraction prostate motion during IMRT for prostate cancer. Int J Radiat Oncol Biol Phys 2002;53:261–268. 6. Langen KM, Jones DT. Organ motion and its management. Int J Radiat Oncol Biol Phys 2001;50:265–278. 7. Nederveen AJ, van der Heide UA, Dehnad H, et al. Measurements and clinical consequences of prostate motion during a radiotherapy fraction. Int J Radiat Oncol Biol Phys 2002;53: 206 –214.
8. van Herk M, Bruce A, Kroes AP, et al. Quantification of organ motion during conformal radiotherapy of the prostate by three dimensional image registration. Int J Radiat Oncol Biol Phys 1995;33:1311–1320. 9. Zellars RC, Roberson PL, Strawderman M, et al. Prostate position late in the course of external beam therapy: Patterns and predictors. Int J Radiat Oncol Biol Phys 2000;47:655– 660. 10. Chinnaiyan P, Tomee W, Patel R, et al. 3D-ultrasound guided radiation therapy in the post-prostatectomy setting. Technol Cancer Res Treat 2003;2:455– 458. 11. Langen KM, Pouliot J, Anezinos C, et al. Evaluation of ultrasound-based prostate localization for image-guided radiotherapy. Int J Radiat Oncol Biol Phys 2003;57:635– 644. 12. Fiorino C, Foppiano F, Franzone P, et al. Rectal and bladder motion during conformal radiotherapy after radical prostatectomy. Radiother Oncol 2005;74:187–195. 13. Schiffner D, Gottschalk A, Lometti M, et al. Daily electronic portal imaging of implanted gold seed fiducials in patients undergoing salvage. Int J Radiat Oncol Biol Phys 2005;63: S304.
doi:10.1016/j.ijrobp.2006.05.071
PHYSICS CONTRIBUTION
DAILY VARIATIONS IN THE POSITION OF THE PROSTATE BED IN PATIENTS WITH PROSTATE CANCER RECEIVING POSTOPERATIVE EXTERNAL BEAM RADIATION THERAPY PATRICK A. KUPELIAN, M.D., KATJA M. LANGEN, PH.D., TWYLA R. WILLOUGHBY, M.S., THOMAS H. WAGNER, PH.D., OMAR A. ZEIDAN, PH.D., AND SANFORD L. MEEKS, PH.D. Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, Orlando, FL Purpose: The aim of this study was to evaluate the extent of the variation in the position of the prostate bed with respect to the bony anatomy. Methods and Materials: Four patients were treated to 70 Gy in 35 fractions. Before each fraction, a megavoltage computed tomography (CT) of the prostate bed was obtained, resulting in a total of 140 CT studies. Retrospectively, each CT scan was aligned to the simulation kilovoltage scan based on bony anatomy and the prostate bed. The difference between the 2 alignments was calculated for each scan. Results: The average differences (ⴞ1 SD) between the two alignments were 0.06 ⴞ 0.37, 0.10 ⴞ 0.86, and 0.39 ⴞ 1.27 mm in the lateral, longitudinal (SI), and vertical (AP) directions, respectively. Laterally, there was no difference >3 mm. The cumulative frequency of SI differences were as follows; >3 mm: 3%, >4 mm: 1%, and >5 mm: 1% (maximum: 5 mm). The cumulative frequency of AP differences were as follows; >3 mm: 7%, and >4 mm: 3% (maximum: 4 mm). Conclusion: In patients with prostate cancer receiving postoperative radiotherapy, the prostate bed motion relative to the pelvic bony anatomy is of a relatively small magnitude. Significant motion (>3 mm) is infrequent. However, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed. © 2006 Elsevier Inc. Prostate bed, Motion, Postoperative radiotherapy.
The use of salvage or adjuvant radiotherapy to the prostate bed after radical prostatectomy for localized prostate cancer is frequently performed and outcomes are regularly reported (1, 2). However, the treatment techniques for prostate bed irradiation have varied throughout the years and still vary from institution to institution. One technical aspect of postoperative radiotherapy for localized prostate cancer that has received little attention is prostate bed motion. Significant work has been done in the description of daily position variations of the prostate gland during the course of external beam radiotherapy. However, there is limited information about the variation of the position of the prostate bed during the course of postoperative radiation therapy. With the advent of image-guided radiotherapy, specifically with techniques that provide daily soft tissue imaging in the treatment room, it is now possible to document position changes of target areas during entire external radiotherapy courses. There is no question that the prostate bed would vary in
position with respect to external skin marks, as those are notoriously unreliable. With the availability of portal imagers, in-room X-rays, and in-room computed tomography (CT) scans, the evaluation of discrepancies between alignment on the bony anatomy and the prostate bed proper can be performed. If daily alignment is performed on the bony anatomy, treatment margins can consequently be adjusted according to the expected motion of the prostate bed itself. A related issue that needs to be addressed is the definition of the prostate bed and planning target volume for postoperative radiation therapy. This issue is beyond the scope of this study, but would affect the observations and conclusions made about the motion of the target areas during a course of external beam radiation. The current study looks at the prostate bed as the small area beneath the bladder, corresponding to where the apex of the intact prostate would have been located. Therefore, the conclusions of this study cannot be extrapolated to target volumes extending significantly above the level of the bladder base.
Reprint requests to: Patrick Kupelian, M.D., Department of Radiation Oncology, M.D. Anderson Cancer Center Orlando, 1400 South Orange Avenue, Orlando FL 32806. Tel: (407) 841-8666; Fax: (407) 649-6895; E-mail: [email protected] The M. D. Anderson Cancer Center Orlando has received research funding from TomoTherapy, Inc.
Acknowledgments—This work was supported by a grant from Women Playing For T.I.M.E. We are grateful for all the encouragement and inspiration from Mrs. Elaine Lustig and her colleagues at Women Playing For T.I.M.E. Received March 30, and in revised form April 25, 2006. Accepted for publication May 19, 2006.
INTRODUCTION
593
594
I. J. Radiation Oncology
● Biology ● Physics
METHODS AND MATERIALS Four patients receiving postoperative radiotherapy to the prostatic fossa with a helical tomotherapy unit (TomoTherapy Hi*ART II; TomoTherapy Inc., Madison, WI) constituted the study sample. The patients were treated as per routine clinical care. Local IRB approval was obtained for the retrospective analysis of the scans. All patients were treated to 70 Gy at 2 Gy per fraction, in 35 fractions. Before each treatment, megavoltage CT (MVCT) images of the prostate bed were acquired using the on-board imaging capabilities of the TomoTherapy Hi*ART II unit. Positional adjustments were made on the basis of the pelvic bony anatomy for the actual treatments. Subsequently, the 140 CT image sets were used retrospectively to study the position of the prostate bed with respect to the bony anatomy. Each megavoltage scan was aligned to the simulation kilovotlage CT (kVCT) scan based on either of two methods: (1) the bony anatomy, and (2) the prostate bed itself, defined by surgical clips in or around the prostate bed. The definition of the “prostate bed” is somewhat broad. The area that is of most relevance to the definition of postoperative radiation therapy target areas is ultimately the urethral anastomotic site. This is probably only part of what is generally defined as the postoperative radiation therapy target area. In the current study, the surgical clips were mostly present superior to the actual anastomosis location, and defined a broader anatomic location. Although imperfect, the location of these surgical clips was considered to be representative of the general location of a typical postoperative radiation therapy target area. Ultimately, the ideal approach should be the placement of metallic fiducials near the anastomosis site under transrectal ultrasound guidance immediately before the design and delivery of any postoperative radiation therapy. Target localization during therapy would be subsequently performed by the determination of the location of those metallic fiducials. Figure 1 shows corresponding scans obtained at simulation (kVCT) and at treatment (MVCT) clearly showing a surgical clip. The bony anatomy was aligned automaticaly on the treatment console using an extracted feature fusion algorithm supplied by the manufacturer (TomoTherapy Inc.) (3). The automatic fusion was visualy checked for accuracy by the physician (P.A.K.). The retrospective alignement of the prostate bed as defined by surgical clips was done manually by the physician (P.A.K.). For each of the 140 fractions, the difference in position between the prostate bed alignment and bony anatomy was calculated. The differences were then analyzed with respect to the average positional difference in the anterior/posterior (AP), superior/inferior (SI), and lateral dimensions. In addition, the frequencies of differences were also analyzed. To assess the uncertainity of the marker to bony movement measurement, three markers were implanted into a anthrophormor-
Fig. 1. Corresponding computed tomography (CT) images of the prostate bed with a surgical clip. Megavoltage CT obtained during treatment (a) and kilovoltage CT obtained during simulation (b).
Volume 66, Number 2, 2006
pic phantom. This phantom was then treated like a patient (i.e., a kVCT scan was performed and MVCT scans of the phantom were registered with this kVCT phantom). Since the markers are rigidly implanted into the phantom an alignment of the phantom based on markers should be identical to a bony anatomy based alignment (i.e., the movement of the markers relative to the bony anatomy is zero). The 35 MVCT scans were obtained and these scans were visually aligned to the kVCT scan based on the marker locations. The same MVCT scans were then registered with the kVCT scans using the automatic bone based fusion tool provided by the manufacturer. The difference between these 2 alignments were calculated and averaged over the 35 studies the mean and standard devitions were 0.6 ⫾ 0.4 mm, 0.5 ⫾ 1.0 mm, 0.2 ⫾ 0.7 mm in the lateral, SI, and AP directions, respectively.
RESULTS The average differences (⫾1 SD) between the bony anatomy alignments and the prostate bed alignments were 0.06 ⫾ 0.37 mm, 0.1 ⫾ 0.86 mm, and 0.39 ⫾ 1.27 mm in the lateral, SI, and AP directions, respectively. The average directional differences (⫾1 SD) between the bony anatomy alignments and the prostate bed alignments were 0.11 ⫾ 0.36 mm, 0.39 ⫾ 0.77 mm, and 0.89 ⫾ 0.98 mm in the lateral, SI and AP directions, respectively. The directional displacements are similar in size to the uncertainties of the measurement and consequently no evidence of large and frequent prostate bed motion relative to the bony anatomy was found. However, a few displacements that were ⬎3 SD of the measurement uncertanty were observed. In the SI dimension, 3% of all differences were ⱖ3 mm, 1% were ⱖ4 mm, and 1% were ⱖ5 mm. The largest SI difference was 5 mm. In the AP dimension, 7% of all differences were ⱖ3 mm, and 3% ⱖ4 mm. The largest AP difference was 4 mm. DISCUSSION Daily image guidance leads to the understanding of position variations of various targets in different clinical situations. For the alignment of the prostate bed in patients treated with external radiotherapy after radical prostatectomy, it is still not clear how to best localize the target area. Previous techniques relied on relatively large areas to be treated in the pelvis. However, modern conformal techniques have been implemented to specifically treat the prostate bed only. This leads to the implementation of relatively small fields. If the prostate bed moves as significantly as the prostate gland during definitive external radiotherapy, then daily targeting becomes crucial for the implementation of such techniques. There are numerous reports on the significant daily position variations of the intact prostate gland mostly because of rectal and bladder filling (4 –9). Therefore, one would assume that the prostate bed would be subject to similar motions. However, there is a paucity of reports in the literature studying this issue (10, 11). Historically, patient alignments had been done on the basis of skin marks, which are considered unreliable if tight
Prostate bed motion and external beam radiotherapy
conformal fields are planned to be delivered. In a unique report trying to address this issue, Chinnaiyan et al. reported on the daily transabdominal ultrasound for targeting the prostate bed (10). In the 6 patients that constituted the study sample, the authors reported average (⫾SD) ultrasoundbased shifts from skin marks in the AP, lateral, and SI dimensions of 5 ⫾ 4 mm, 3 ⫾ 3 mm, and 3 ⫾ 4 mm, respectively The authors considered these shifts to be important and justifying the implementation of daily targeting during conformal radiotherapy to the prostate bed (10). However, because of the nature of imaging and targeting with transabdominal ultrasound, the accuracy of such ultrasound based alignment has been questioned for targeting the intact prostate (11). Similar inaccuracies can lead to misalignment of the prostate bed with transabdominal ultrasound. It is also important to note that the authors reported the shifts from set-up skin marks, and not from the patients’ bony anatomy as in the current study. Pelvic bony anatomy has been demonstrated to be an unreliable proxy for the position of the intact prostate gland. Because of bladder filling but mostly because of rectal filling, the position of the prostate varies significantly from day to day, independently from the pelvic bony anatomy. This motion has been shown to be less prominent at the apex, and most significant toward the base, as the prostate gland is relatively fixed inferiorly and more subject to the pitching from rectal distention superiorly. After radical prostatectomy, the prostate bed occupies mostly the area corresponding to the prostatic apex, with the bladder filling the space where the prostate was, and the rectum distending mostly superior to the apical area where the prostate bed is. This relatively small area might not move as significantly as the intact prostate gland. Therefore, alignment on pelvic bony anatomy could be an accurate proxy for alignment of the prostate bed, whereas it has been demonstrated to be unreliable for the intact prostate (4, 6). The data from our current study indicate that prostate bed motion seems to be of smaller magnitude and frequency to what has been reported for the intact prostate. However, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed. The differences between the position of the prostate bed and the pelvic bony anatomy were small; the largest differences were 2 mm in the lateral dimension, 4 mm in the AP dimension, and 5 mm in the SI dimension. Overall, the average differences were remarkably small, indicating infrequent discrepancy in the difference between prostate bed and pelvic anatomy positions. These data indicate that rectal and bladder distention has a smaller effect on the position of the prostate bed compared with the effect on an intact prostate gland. Further work needs to be done to quantify the impact of rectal and bladder filling on the rectal and bladder doses, rather than the position of the prostate bed itself. Fiorino et al. describe trends in bladder and rectal filling in the postoperative setting, and concluded that there is significant variation in the dosimetry because of
● P. A. KUPELIAN et al.
595
such motion. The authors note that the rectal volume variation was mostly in the superior part of the rectum, similar to our observations (12). However, they inferred that this rectal distention can cause deformation of the posterior part of the target area, compared with what was observed at the time of simulation. It is clear that the more superior (cephalad) the planning target structures are, the more variation there will be in the target position and structure (i.e., deformation) because of rectal distention. This begs the question of how the prostate bed should be defined. The definition of what should be considered the prostate bed and the appropriate planning target volume for postoperative radiation therapy after radical prostatectomy is beyond the scope of this study. However, as the current study concentrated strictly on the prostate bed, the findings cannot be extrapolated to situations where planning target volumes are extended superiorly beyond the prostate bed. Figure 2 shows two sagittal CT reconstructions in two different patients demonstrating the low position of the prostate bed within the pelvis. The only report in the literature that has a similar methodology to the present study has been reported by Schiffner et al. (13). A total of 163 treatment fractions were analyzed from 10 patients who had markers placed in the prostate bed and localized using portal imaging devices. They reported mean (⫾SD) differences between the bony anatomy and the markers of 0.2 ⫾ 4.5 mm, 0.4 ⫾ 2.4 mm, and 1.1 ⫾ 2.1 mm in the lateral, SI, and AP dimensions respectively. There are slightly larger variations observed compared with the ones observed in the current study. However, these are clearly of smaller magnitude compared with what is expected in the setting of intact prostates. Although the study sample only included 4 patients and the current findings might be consequently skewed, the anatomies from these 4 patients were typical of what is expected in a postoperative situation. Pelvic anatomy varies significantly with the shape and size of an intact prostate gland, but the variation in the postoperative anatomy of the prostate bed is apparently much less prominent. Therefore,
Fig. 2. Sagittal kilovoltage computed tomography (CT) scans obtained during simulation in two separate patients, in both cases showing the low position of the prostate bed within the pelvis as demonstrated by the relative position of the prostate bed with respect to the pubis. The part of the rectum that distends is typically more cephalad than the prostate bed.
596
I. J. Radiation Oncology
● Biology ● Physics
it is reasonable to assume that the findings for the current 4 patients are representative of other patients that receive postoperative radiotherapy. Finally, it is important to repeat the current analysis in a larger number of patients, to make these observations more relevant to the radiation oncology community in general. However, it is difficult to find a large number of postprostatectomy prostate cancer patients undergoing radiation therapy with surgical clips within the prostate bed proper. Our goal is to repeat this study pro-
Volume 66, Number 2, 2006
spectively with the placement of metallic fiducials within the prostate bed before radiation therapy. CONCLUSION In conclusion, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed.
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