- Email: [email protected]
Impact of differences in ultrasound and computed tomography volumes on treatment planning of permanent prostate implants
Int. J. Radiation Oncology
PI1 SO360-3016(96)00618-9
ELSEVIER
l
Biol. Phys., Vol. 37, No. 5, pp. 1181-l 185, 1997 Copyright Q 1997 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/97 $17.00 + .OO
Physics Contribution IMPACT OF DIFFERENCES IN ULTRASOUND AND COMPUTED TOMOGRAPHY VOLUMES ON TREATMENT PLANNING OF PERMANENT PROSTATE IMPLANTS VRINDA
NARAYANA,
HOWARD *Providence
M. S.,* PETER L. ROBERSON, PH.D.,*,+ ANTHONY T. Pu, M.D.,’ SANDLER, M.D.,+ RAYMOND H. WINFIELD, M.D.* AND PATRICK W. MCLAUGHLIN, M.D.*,+
Cancer Center, Southfield, MI; and+Department of RadiationOncology, University of Michigan, Ann Arbor, MI
Purpose: Both ultrasound (US) and computerized tomography (CT) images have been used in the planning of prostate interstitial therapy. Ultrasound images more clearly define the apex and capsule of the prostate, while CT images deiine seed positions for postimplant dosimetry. Proper registration of the US volume with the CT volume is critical to the assessment of dosimetry. We therefore compared US and CT prostate volumes to determine if differences were significant. Methods and Materials: Ten consecutive patients entered in an interstitial implant program were studied by pretreatment US. In addition, pretreatment CT scans were obtained and three physicians independently outlined the dimensions of the prostate on these images. The patients subsequently underwent placement of radioactive ‘“I or lo3Pd. Posthnplant CT images were obtained the next day and the postimplant prostate volumes were outlined by the same three physicians. Seven of 10 patients underwent late CT scans 9-14 months postimplant for comparison of preimplant and immediate postimplant CT studies. Results: There were differences between US and CT volumes. Although the physician-to-physician variation wassignificant, the trends were consistent, with US prostate volume typically smaller (47%) than the preimplant CT volume and markedly smaller (120%) than the postimplant CT volume. Prostate volumes derived from late CT images did not consistently return to preimplant levels. Conclusions: Signiiicant differences in volume of the prostate structure were found between US and CT images. The data suggests that: (a) Implants planned on CT tend to overestimate the size of the prostate and may lead to unnecessary implantation of the urogenital diaphragm and penile urethra. (b) Registration of initial US and postimplant CT prostate volumes required for accurate dosimetry is diicult due to the increased volume of prostate secondary to trauma. (c) Further study to determine the optimal time for the postimplant CT is necessary. 0 1997 Elsevier Science Inc. Prostate implants,
Ultrasound,
Computed
tomography,
Dosimetry,
Three-dimensional
treatment
planning.
tremely low in favorable patients (prostatic specific antigen 4-lo), the mechanismof failure is unclear and may, in part, be due to underdosage. Critical to the success of any permanent implant is proper geometry (5,7,9). We have previously shown that while US guidance improves seed placement, it in no way guarantees adequate coverage of the gland and that significant error is still possible (10). Since local control in appropriately selected patients can be achieved when a “good implant” is accomplished, we have sought to evaluate postimplant dosimetry more accurately.
INTRODUCTION There has been a resurgenceof interest in the use of permanentprostateimplants with ‘=I and l”Pd for the treatment of early localized prostate carcinoma in the last decade (1,12,13,14). This appeal is largely due to improvement in seedplacement made possibleby either ultrasound (US) or computerized tomography (CT) guidance. These methods solve many of the technical and seedplacement problems which undermined the potential successof retropubic implants performed in the 1970s.Recently, Blasko and Wallner have reported excellent, early PSA and biopsy control employing such implants (2,3). Although the failure rate is ex-
ami, FL, Oct. 8-11, 1995. Acknowledgements-The authors wish to thank Mrs. Gloria Cowan for her assistance in the preparation of this manuscript. Accepted for publication 10 December 1996.
Reprint requests to: Vrinda Narayana, Providence Cancer Center, 22301 Foster Winter Drive, Southfield, MI 48075. E-Mail: [email protected]. This work was presented at the 37th Annual Meeting of the American Society for Therapeutic Radiology and Oncology, Mi1181
1182
I. J. Radiation Oncology l Biology l Physics
Postimplant dosimetry is difficult for several reasons. The sources are best localized on CT scans to relate the dose distribution to patient anatomy. Second, the CT and US prostate volumes differ and must be correlated to allow accurate registration of the sources with respect to the US volume. Finally, the implant itself can lead to marked distortions in prostate anatomy, further compounding the complexity of this procedure. In this article, we have studied the prostate volume as imaged using CT and US techniques on postimplant dosimetry.
METHODS
AND MATERIALS
Ten consecutive patients were entered in an interstitial implant program. Each patient underwent a pretreatment US study 2 weeks prior to implant. Axial US images at 0.5 cm intervals were obtained with the patient in the lithotomy position. The prostate volume on the US images was outlined in the presence of two physicians, a radiation oncologist and a urologist. Nonradioactive marker seeds were placed at the base and apex of the prostate. Three mm interval axial CT images with the patient lying supine were obtained within a week of the ultrasound procedure. Preimplant CT prostate volumes were each outlined by three physicians to overcome single physician bias in prostate volume definition. Customized treatments (preplan) were planned for each patient based on the US prostate volume plus a 0.5 cm inferior margin, for either “‘1 or lo3Pd radioisotopes using the University of Michigan treatment planning system (UMPLAN) (4). The preplan was designed to deliver 160 Gy using ‘25I or 120 Gy using ‘03Pd for definitive brachytherapy treatments. A brachytherapy boost dose of 80 Gy using rz51 or 60 Gy lo3Pd was planned for patients who had previously received external beam treatments of 55 Gy. Source activities were increased by 15% compared to preplan activities to compensate for source placement error. Preimplant CT prostate volumes, as outlined by all three physicians, were entered into UMPLAN. The marker positions were uniquely identified by interactively creating orthogonal reconstructed images of small cubes of CT data with dimensions 5 X 5 X 7 mm3, which is slightly larger than the source dimensions. The US and preimplant CT prostate volumes were registered using the rectal surface of the prostate. The position of the markers with respect to the ultrasound axial slices was determined. During the implant procedure, the patient was set up to reproduce the preimplant US prostate image. Radioactive sources were implanted both under US and fluoroscopic guidance. The axial US images were used to confirm the positioning of the sources along the left-right and the anterior-posterior dimensions of the prostate. The position of the sources along the superior-inferior dimensions was
Volume 37, Number 5, 1997
confirmed both on the lateral view of the US and relative to the marker positions on fluoroscopy. The day after the implant, a second CT with the patient lying supine was obtained for postimplant dosimetry. The same three physicians outlined the prostate volumes on all 10 patients. The radioactive source positions were uniquely identified on CT using UMPLAN. The US and postimplant CT prostate volumes were correlated by aligning the urethra (8). Dose volume histograms (DVH) were calculated for the prostate volumes (6,s). A follow-up CT with the patient lying supine was obtained 9- 14 months after implant. Follow-up CT prostate volumes were outlined by all three physicians.
RESULTS
Variation of US and CT prostate volumes The US prostate volumes were found to be smaller than the CT prostate volumes (Fig. 1). The preimplant CT prostate volumes as outlined by the three physicians averaged 13, 35 and 92% respectively, larger than US prostate volumes. Gn average, preimplant CT prostate volumes were 47% larger and 0.6 cm longer (Fig. 2) than the US prostate volumes. The percent difference between US and postimplant CT prostate volumes and lengths, as outlined by the three physicians, is shown in Tables 1 and 2. On average, the postimplant CT prostate volumes were 120% larger and 1.6 cm longer than the US prostate volumes. Time variation of CT prostate volumes Preimplant CT prostate volumes were found to be smaller than postimplant prostate volumes. Postimplant CT prostate volumes, as outlined by the three physicians, were found to be 52,47, and 60%, respectively, larger than the preimplant prostate volumes (Fig. 3). On average, postimplant prostate volumes were 53% larger and 1.O cm longer than the preimplant prostate volumes. Prostate volumes for follow-up CT studies, performed 9-14 months postimplant, did not consistently normalize to preimplant levels. None, in fact, fully normalized, and no consistent pattern emerged (Fig. 3). No conclusion regarding resolution of postimplant changes in relation to time can be drawn from the follow-up CT data. Although there were significant variations in physician-tophysician contour definitions, there was interval consistency as evidenced by the calculated percent difference between CT data volumes (Table 1). Postimplant dosimetry Figure 4 shows representative DVHs. Differences in the US and CT volumes are reflected in the DVHs. The postimplant dosimetry for the US prostate follows the planned US prostate DVH more closely than the postimplant CT prostate DVH.
Impact of differences in ultrasound and computed tomography 0 V. NARAYANA
er nl.
1183
Anterior
Ultrasoun Prostate Preimplant CT
Left
ol 0
Fig. 2. Prostatevolumesfrom ultrasound(squares)and preimplant computedtomography(CT) (circles)for the 10patientsin the study. The rangebarsrepresentthe physician-dependent differencesin the volume determinationon CT.
Posterior
a Superior
Right
Left
Inferior b
t Superior
Inferior
Posterior C
Fig. 1.Registration of ultrasound, preimplant CT,andpostimplant CT prostatevolumes:(a)axial plane(b) coronalplane(c) sagittalplane.
without fluoroscopy, or CT, template, and fluoroscopy. Postimplant dosimetry is based on either registration of initial prostate volumes with source localization, by orthogonal films or CT. Postimplant dosimetry by CT offers the advantage of correct registration with CT prostate volumes and normal tissue structures. Combining the convenience and accuracy of US-guided implants with the preferred postimplant CT dosimetry is the subject of the current paper. We encountered three discrepancies in attempting to correlate US and CT prostate volumes which impact on permanent prostate implant planning and dosimetry. The first discrepancy was that US prostate volumes were consistently smaller than CT volumes. This is mainly due to the poor definition of the lower limit of the prostate apical region on CT. This was previously noted by Sandler, et aE., (11) where apical markers were placed by US and visualized by CT. One must decide which (US or CT) volume representsthe target volume. Given the clarity of the apex on US, we have elected to plan the implant based on US volumes, and to register the US and CT volumes using techniques previously reported (8). This finding also has potential significant implications for treatment planning. If the area of urogenital diaphragm and penile urethra are implanted to high dose (based on CT volume), acute and possible chronic complications may be increasedcompared to US planned implants. Current follow-up does not suggestthat this is a problem for
Table 1. Apparentprostatepercentvolume changes Physician 1 Physician 2 Physician 3
DISCUSSION Permanent prostate implants are currently planned based on US (1,13) or CT (14) prostate volumes. Implantation may be carried out by US, template with or
(preCT-US)/US (postCT-US)/US (postCT-preCT)/preCT
12.6% 69.1% 52.4%
CT = Computed tomography. US = Ultrasound.
35.3% 94.7% 47.3%
91.9% 196.4% 59.5%
I. J. Radiation Oncologyl Biology0 Physics
1184
Volume37, Number5, 1997
Table2. Apparentprostatelength changes
(preCT-US)/,cm (postCT-US)/,cm (postCT-preCT)/,cm
Physician 1
Physician 2
Physician 3
0.2 1.1 0.9
0.8 1.7 0.9
0.9 2.1 1.2
CT = Computedtomography. US = Ultrasound.
CT based series(15), but this remains an important theoretical consideration. The second unexpected observation was the significant distortion and enlargement of the prostate following the implant. Although swelling was expected due to trauma, the degree and significance for postimplant dosimetry were unexpected. Early postimplant CT volumes were larger in all patients, further complicating registration with the intended target volume (US prostate). Early postimplant dosimetry was sought becauseprior studies(10) suggested possible error with US-guided implants, and early correction was desirable to insure a good outcome. This is especially true for short half-life isotopes, such aslo3Pd, where most dose is delivered in the early weeks postimplant. Enlargement during this critical period would have significant dosimetric consequences. The final unexpected observation was the unpredictable timing of the resolution of prostate swelling. The prostate volumes on the patients analyzed did not normalize to preimplant volumes as anticipated. The prostate either returned to near preimplant volume or remained expanded. It is possible that patients with persistent swelling will experience critical underdosage resulting in failure. Due to the dynamic nature of the changes, postimplant dosimetry may need to use changing volumes and source distributions in order to calculate the actual delivered dose. Finally, there may be implications for treatment planning. Although peripheral placement of sources may be advantageous with regard to homogeneity of dose in an unchanging target volume, this technique could result in greater underdosage centrally than a homogeneous source distribution with a planned central region of excess dose. Our results show a wide discrepancy in outlining the CT prostate volumes. This is expected because of the difficulty in visualizing the apical region of the prostate on CT. Our choice of three physicians, all radiation oncologists, to outline the CT prostate volume was to represent the broad range of potential volumes. Physician 1 was involved in the prostate brachytherapy program and was sensitized to the differences in US and CT prostate volumes. Physician 2 specialized in external beam treatment of the prostate while physician 3 did not specialize in the treatment of the prostate. The disparity amongst physicians in defining the prostate volume on
Fig. 3. Prostate volumes from preimplant computed tomography (circles), postimplant CT (triangles), and follow-up CT (squares) for 7 patients. The open, dark and gray symbols represent the prostate volumes drawn by three different physicians.
CT suggestsa need for defining a standard. Even though there exists a wide variation between physicians, each physician was consistent in that the preimplant CT prostate volume was larger than US prostate volume and smaller than postimplant CT prostate volume. In summary, correlation of CT and US volumes in permanent implants of the prostate uncovered several important discrepancies.The CT prostate volume is consistently greater than the US volume. Postimplant, the CT volume is significantly enlarged and distorted compared to the preimplant CT volume. Finally, the resolution of this distortion is variable in timing and degree. These findings have direct implications for planning and dosimetry of permanent prostate implants.
Fig. 4. Dose-volume histograms for preimplant ultrasound (US), postimplant computed tomography, and postimplant US. The preimplant plan was designed to deliver 160 Gy to the US prostate volume using 9.
Impact of differences in ultrasound and computed tomography 0 V. NARAYANA
et al.
118.5
REFERENCES 1. Blasko, J. C.; Grimm, P. D.; Ragde,H. Brachytherapy and organ preservationin the management of carcinomaof the prostate.Semin.in Radiat.Oncol. 3240-249; 1993. 2. Blasko, J. C.; Ragde,H.; Grimm, P. D.; Cavanagh,W.; Kenny, G. Transperinealultrasoundguided palladium-103 brachytherapy for prostate carcinoma. J. Urol. 153 (Suppl):385;1995. 3. Blasko,J. C.; Wallner, K.; Grimm, P. D.; Ragde,H. PSAbaseddiseasecontrol following ultrasoundguidedI-125 implantation for stage Tl/T2 prostate carcinoma.J. Urol. 154:1096-1099; 1995. 4. Fraass,B. A.; McShan, D. L. 3-D treatmentplanning: I. Overview of a clinical planningsystem.In: Bruinvis, I. A. D., ed. The Use of Computersin RadiationTherapy. Amsterdam:ElsevierSciencePublishers;1987:273-276. 5. Fuks, Z.; Liebel, S.; Wallner, K. E.; Begg, C. B.; Fair, W. R.; Anderson,L. A.; Hilaris, B. S.; Whitmore, W. F. The effect of local control on metastaticdisseminationin carcinomaof the prostate:Long-term resultsin patientstreated with lz51implantation. Int. J. Radiat. Oncol. Biol. Phys. 21:537-547; 1991. 6. Kessler,M. C.; Ten Haken,R. L.; Frass,B. A.; McShan,D. L. Expandingthe useandeffectivenessof dose-volumehistogramsfor 3-D treatmentplanning I: Integration of 3-D dose-display.Int. J. Radiat. Oncol. Biol. Phys. 29:11251131; 1994. 7. Morton, J. D.; Peschel,R. E. Iodine-125 implantsversus externalbeamtherapy for stagesA2, B, andC prostatecancer. Int. J. Radiat. Oncol. Biol. Phys. 14:1153-1157; 1988.
8. Narayana,V.; Roberson,P. L.; Winfield, R. J.; McLaughlin, P. W. Impact of ultrasoundandcomputedtomographyregistration on evaluation of permanentprostate implants. AmericanAssociationof Physicistsin Medicine, July 1995. Submittedto Int. J. Radiat.Oncol. Biol. Phys. 9. Peschel,R. E.; Fogel, T. D.; Kacinski, B. M.; Kelly, K.; Mate, T. Iodine-125implantsfor carcinomaof the prostate. Int. J. Radiat.Oncol. Biol. Phys. 11:1777-1781; 1985. 10. Roberson,P. L.; Narayana,V.; McShanD. L.; Winfield, R. J.; McLaughlin, P. W. Sourceplacementerror for permanent implantof the prostate.AmericanAssociationof Physicists in Medicine, July, 1995.Med. Phys. 24:251-257; 1997. 11. Sandler,H. M.; Bree,R. L.; McLaughlin, P. W.; Grossman, H. B.; Lichter A. S. Localization of the prostaticapex for radiation therapy using implantedmarkers.Int. J. Radiat. Oncol. Biol. Phys. 27:915-919; 1993. 12. Stock, R. G. Locoregionaltherapiesfor early stageprostate cancer.Oncology 9:803-811; 1995. 13. Stock, R. G.; Stone,N. N.; Wesson,M. F.; DeWyngaert,J. K. A modified techniqueallowing interactive ultrasound guided three dimensionaltransperinealprostateimplantation. Int. J. Radiat. Oncol. Biol. Phys. 32:219-225; 1995. 14. Wallner, K.; Chiu-Tsao,S.; Roy, J.; et al. An improved methodfor computerizedtomography-planned transperineal Iodine-125implants.J. Urol. 146:90-95; 1991. 15. Wallner, K.; Roy, J.; Zelefsky, M.; Fuks, Zvi.; Harrison,L. Short-term freedom from diseaseprogressionafter I-125 prostate implantation. Int. J. Radiat. Oncol. Biol. Phys. 30:405-409; 1994.
PI1 SO360-3016(96)00618-9
ELSEVIER
l
Biol. Phys., Vol. 37, No. 5, pp. 1181-l 185, 1997 Copyright Q 1997 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/97 $17.00 + .OO
Physics Contribution IMPACT OF DIFFERENCES IN ULTRASOUND AND COMPUTED TOMOGRAPHY VOLUMES ON TREATMENT PLANNING OF PERMANENT PROSTATE IMPLANTS VRINDA
NARAYANA,
HOWARD *Providence
M. S.,* PETER L. ROBERSON, PH.D.,*,+ ANTHONY T. Pu, M.D.,’ SANDLER, M.D.,+ RAYMOND H. WINFIELD, M.D.* AND PATRICK W. MCLAUGHLIN, M.D.*,+
Cancer Center, Southfield, MI; and+Department of RadiationOncology, University of Michigan, Ann Arbor, MI
Purpose: Both ultrasound (US) and computerized tomography (CT) images have been used in the planning of prostate interstitial therapy. Ultrasound images more clearly define the apex and capsule of the prostate, while CT images deiine seed positions for postimplant dosimetry. Proper registration of the US volume with the CT volume is critical to the assessment of dosimetry. We therefore compared US and CT prostate volumes to determine if differences were significant. Methods and Materials: Ten consecutive patients entered in an interstitial implant program were studied by pretreatment US. In addition, pretreatment CT scans were obtained and three physicians independently outlined the dimensions of the prostate on these images. The patients subsequently underwent placement of radioactive ‘“I or lo3Pd. Posthnplant CT images were obtained the next day and the postimplant prostate volumes were outlined by the same three physicians. Seven of 10 patients underwent late CT scans 9-14 months postimplant for comparison of preimplant and immediate postimplant CT studies. Results: There were differences between US and CT volumes. Although the physician-to-physician variation wassignificant, the trends were consistent, with US prostate volume typically smaller (47%) than the preimplant CT volume and markedly smaller (120%) than the postimplant CT volume. Prostate volumes derived from late CT images did not consistently return to preimplant levels. Conclusions: Signiiicant differences in volume of the prostate structure were found between US and CT images. The data suggests that: (a) Implants planned on CT tend to overestimate the size of the prostate and may lead to unnecessary implantation of the urogenital diaphragm and penile urethra. (b) Registration of initial US and postimplant CT prostate volumes required for accurate dosimetry is diicult due to the increased volume of prostate secondary to trauma. (c) Further study to determine the optimal time for the postimplant CT is necessary. 0 1997 Elsevier Science Inc. Prostate implants,
Ultrasound,
Computed
tomography,
Dosimetry,
Three-dimensional
treatment
planning.
tremely low in favorable patients (prostatic specific antigen 4-lo), the mechanismof failure is unclear and may, in part, be due to underdosage. Critical to the success of any permanent implant is proper geometry (5,7,9). We have previously shown that while US guidance improves seed placement, it in no way guarantees adequate coverage of the gland and that significant error is still possible (10). Since local control in appropriately selected patients can be achieved when a “good implant” is accomplished, we have sought to evaluate postimplant dosimetry more accurately.
INTRODUCTION There has been a resurgenceof interest in the use of permanentprostateimplants with ‘=I and l”Pd for the treatment of early localized prostate carcinoma in the last decade (1,12,13,14). This appeal is largely due to improvement in seedplacement made possibleby either ultrasound (US) or computerized tomography (CT) guidance. These methods solve many of the technical and seedplacement problems which undermined the potential successof retropubic implants performed in the 1970s.Recently, Blasko and Wallner have reported excellent, early PSA and biopsy control employing such implants (2,3). Although the failure rate is ex-
ami, FL, Oct. 8-11, 1995. Acknowledgements-The authors wish to thank Mrs. Gloria Cowan for her assistance in the preparation of this manuscript. Accepted for publication 10 December 1996.
Reprint requests to: Vrinda Narayana, Providence Cancer Center, 22301 Foster Winter Drive, Southfield, MI 48075. E-Mail: [email protected]. This work was presented at the 37th Annual Meeting of the American Society for Therapeutic Radiology and Oncology, Mi1181
1182
I. J. Radiation Oncology l Biology l Physics
Postimplant dosimetry is difficult for several reasons. The sources are best localized on CT scans to relate the dose distribution to patient anatomy. Second, the CT and US prostate volumes differ and must be correlated to allow accurate registration of the sources with respect to the US volume. Finally, the implant itself can lead to marked distortions in prostate anatomy, further compounding the complexity of this procedure. In this article, we have studied the prostate volume as imaged using CT and US techniques on postimplant dosimetry.
METHODS
AND MATERIALS
Ten consecutive patients were entered in an interstitial implant program. Each patient underwent a pretreatment US study 2 weeks prior to implant. Axial US images at 0.5 cm intervals were obtained with the patient in the lithotomy position. The prostate volume on the US images was outlined in the presence of two physicians, a radiation oncologist and a urologist. Nonradioactive marker seeds were placed at the base and apex of the prostate. Three mm interval axial CT images with the patient lying supine were obtained within a week of the ultrasound procedure. Preimplant CT prostate volumes were each outlined by three physicians to overcome single physician bias in prostate volume definition. Customized treatments (preplan) were planned for each patient based on the US prostate volume plus a 0.5 cm inferior margin, for either “‘1 or lo3Pd radioisotopes using the University of Michigan treatment planning system (UMPLAN) (4). The preplan was designed to deliver 160 Gy using ‘25I or 120 Gy using ‘03Pd for definitive brachytherapy treatments. A brachytherapy boost dose of 80 Gy using rz51 or 60 Gy lo3Pd was planned for patients who had previously received external beam treatments of 55 Gy. Source activities were increased by 15% compared to preplan activities to compensate for source placement error. Preimplant CT prostate volumes, as outlined by all three physicians, were entered into UMPLAN. The marker positions were uniquely identified by interactively creating orthogonal reconstructed images of small cubes of CT data with dimensions 5 X 5 X 7 mm3, which is slightly larger than the source dimensions. The US and preimplant CT prostate volumes were registered using the rectal surface of the prostate. The position of the markers with respect to the ultrasound axial slices was determined. During the implant procedure, the patient was set up to reproduce the preimplant US prostate image. Radioactive sources were implanted both under US and fluoroscopic guidance. The axial US images were used to confirm the positioning of the sources along the left-right and the anterior-posterior dimensions of the prostate. The position of the sources along the superior-inferior dimensions was
Volume 37, Number 5, 1997
confirmed both on the lateral view of the US and relative to the marker positions on fluoroscopy. The day after the implant, a second CT with the patient lying supine was obtained for postimplant dosimetry. The same three physicians outlined the prostate volumes on all 10 patients. The radioactive source positions were uniquely identified on CT using UMPLAN. The US and postimplant CT prostate volumes were correlated by aligning the urethra (8). Dose volume histograms (DVH) were calculated for the prostate volumes (6,s). A follow-up CT with the patient lying supine was obtained 9- 14 months after implant. Follow-up CT prostate volumes were outlined by all three physicians.
RESULTS
Variation of US and CT prostate volumes The US prostate volumes were found to be smaller than the CT prostate volumes (Fig. 1). The preimplant CT prostate volumes as outlined by the three physicians averaged 13, 35 and 92% respectively, larger than US prostate volumes. Gn average, preimplant CT prostate volumes were 47% larger and 0.6 cm longer (Fig. 2) than the US prostate volumes. The percent difference between US and postimplant CT prostate volumes and lengths, as outlined by the three physicians, is shown in Tables 1 and 2. On average, the postimplant CT prostate volumes were 120% larger and 1.6 cm longer than the US prostate volumes. Time variation of CT prostate volumes Preimplant CT prostate volumes were found to be smaller than postimplant prostate volumes. Postimplant CT prostate volumes, as outlined by the three physicians, were found to be 52,47, and 60%, respectively, larger than the preimplant prostate volumes (Fig. 3). On average, postimplant prostate volumes were 53% larger and 1.O cm longer than the preimplant prostate volumes. Prostate volumes for follow-up CT studies, performed 9-14 months postimplant, did not consistently normalize to preimplant levels. None, in fact, fully normalized, and no consistent pattern emerged (Fig. 3). No conclusion regarding resolution of postimplant changes in relation to time can be drawn from the follow-up CT data. Although there were significant variations in physician-tophysician contour definitions, there was interval consistency as evidenced by the calculated percent difference between CT data volumes (Table 1). Postimplant dosimetry Figure 4 shows representative DVHs. Differences in the US and CT volumes are reflected in the DVHs. The postimplant dosimetry for the US prostate follows the planned US prostate DVH more closely than the postimplant CT prostate DVH.
Impact of differences in ultrasound and computed tomography 0 V. NARAYANA
er nl.
1183
Anterior
Ultrasoun Prostate Preimplant CT
Left
ol 0
Fig. 2. Prostatevolumesfrom ultrasound(squares)and preimplant computedtomography(CT) (circles)for the 10patientsin the study. The rangebarsrepresentthe physician-dependent differencesin the volume determinationon CT.
Posterior
a Superior
Right
Left
Inferior b
t Superior
Inferior
Posterior C
Fig. 1.Registration of ultrasound, preimplant CT,andpostimplant CT prostatevolumes:(a)axial plane(b) coronalplane(c) sagittalplane.
without fluoroscopy, or CT, template, and fluoroscopy. Postimplant dosimetry is based on either registration of initial prostate volumes with source localization, by orthogonal films or CT. Postimplant dosimetry by CT offers the advantage of correct registration with CT prostate volumes and normal tissue structures. Combining the convenience and accuracy of US-guided implants with the preferred postimplant CT dosimetry is the subject of the current paper. We encountered three discrepancies in attempting to correlate US and CT prostate volumes which impact on permanent prostate implant planning and dosimetry. The first discrepancy was that US prostate volumes were consistently smaller than CT volumes. This is mainly due to the poor definition of the lower limit of the prostate apical region on CT. This was previously noted by Sandler, et aE., (11) where apical markers were placed by US and visualized by CT. One must decide which (US or CT) volume representsthe target volume. Given the clarity of the apex on US, we have elected to plan the implant based on US volumes, and to register the US and CT volumes using techniques previously reported (8). This finding also has potential significant implications for treatment planning. If the area of urogenital diaphragm and penile urethra are implanted to high dose (based on CT volume), acute and possible chronic complications may be increasedcompared to US planned implants. Current follow-up does not suggestthat this is a problem for
Table 1. Apparentprostatepercentvolume changes Physician 1 Physician 2 Physician 3
DISCUSSION Permanent prostate implants are currently planned based on US (1,13) or CT (14) prostate volumes. Implantation may be carried out by US, template with or
(preCT-US)/US (postCT-US)/US (postCT-preCT)/preCT
12.6% 69.1% 52.4%
CT = Computed tomography. US = Ultrasound.
35.3% 94.7% 47.3%
91.9% 196.4% 59.5%
I. J. Radiation Oncologyl Biology0 Physics
1184
Volume37, Number5, 1997
Table2. Apparentprostatelength changes
(preCT-US)/,cm (postCT-US)/,cm (postCT-preCT)/,cm
Physician 1
Physician 2
Physician 3
0.2 1.1 0.9
0.8 1.7 0.9
0.9 2.1 1.2
CT = Computedtomography. US = Ultrasound.
CT based series(15), but this remains an important theoretical consideration. The second unexpected observation was the significant distortion and enlargement of the prostate following the implant. Although swelling was expected due to trauma, the degree and significance for postimplant dosimetry were unexpected. Early postimplant CT volumes were larger in all patients, further complicating registration with the intended target volume (US prostate). Early postimplant dosimetry was sought becauseprior studies(10) suggested possible error with US-guided implants, and early correction was desirable to insure a good outcome. This is especially true for short half-life isotopes, such aslo3Pd, where most dose is delivered in the early weeks postimplant. Enlargement during this critical period would have significant dosimetric consequences. The final unexpected observation was the unpredictable timing of the resolution of prostate swelling. The prostate volumes on the patients analyzed did not normalize to preimplant volumes as anticipated. The prostate either returned to near preimplant volume or remained expanded. It is possible that patients with persistent swelling will experience critical underdosage resulting in failure. Due to the dynamic nature of the changes, postimplant dosimetry may need to use changing volumes and source distributions in order to calculate the actual delivered dose. Finally, there may be implications for treatment planning. Although peripheral placement of sources may be advantageous with regard to homogeneity of dose in an unchanging target volume, this technique could result in greater underdosage centrally than a homogeneous source distribution with a planned central region of excess dose. Our results show a wide discrepancy in outlining the CT prostate volumes. This is expected because of the difficulty in visualizing the apical region of the prostate on CT. Our choice of three physicians, all radiation oncologists, to outline the CT prostate volume was to represent the broad range of potential volumes. Physician 1 was involved in the prostate brachytherapy program and was sensitized to the differences in US and CT prostate volumes. Physician 2 specialized in external beam treatment of the prostate while physician 3 did not specialize in the treatment of the prostate. The disparity amongst physicians in defining the prostate volume on
Fig. 3. Prostate volumes from preimplant computed tomography (circles), postimplant CT (triangles), and follow-up CT (squares) for 7 patients. The open, dark and gray symbols represent the prostate volumes drawn by three different physicians.
CT suggestsa need for defining a standard. Even though there exists a wide variation between physicians, each physician was consistent in that the preimplant CT prostate volume was larger than US prostate volume and smaller than postimplant CT prostate volume. In summary, correlation of CT and US volumes in permanent implants of the prostate uncovered several important discrepancies.The CT prostate volume is consistently greater than the US volume. Postimplant, the CT volume is significantly enlarged and distorted compared to the preimplant CT volume. Finally, the resolution of this distortion is variable in timing and degree. These findings have direct implications for planning and dosimetry of permanent prostate implants.
Fig. 4. Dose-volume histograms for preimplant ultrasound (US), postimplant computed tomography, and postimplant US. The preimplant plan was designed to deliver 160 Gy to the US prostate volume using 9.
Impact of differences in ultrasound and computed tomography 0 V. NARAYANA
et al.
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