A prediction model for defining a proactive planning target volume in external beam treatment of...
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216 I. J. Radiation Oncology * Biology • Physics Volume 42, Number 1 Supplement, 1998 1015 A PREDICTION MODEL FOR DEFINING A PROACTIVE PLANNING TARGET VOLUME IN EXTERNAL BEAM TREATMENT OF PROSTATE CANCER Di Yan, David Lockman and D. Brabbins Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan, USA Purpose: To maximize the effective delivery of radiation dose to the clinical target volume (CTV) and minimize the irradiated volume of the organ at risk (OAR), accumulated knowledge of the individual patient's setup error and internal organ motion should be applied to modify the treatment plan during the course of external beam radiotherapy. One of the major issues in plan modification is construction of a new planning target volume (PTV) proactively according to the feedback information. In this study, a prediction model to form such PTV for the treatment of prostate adenocarcinoma will be introduced and evaluated retrospectively using portal imaging measurements and CT scans acquired from daily treatment. Materials & Methods: We hypothesize that clinical target displacements due to patient setup and internal motion in prostate treatment are confined inside a region which can be proactively built up from daily measurements of portal images and CT scans acquired within the first k days of treat- ment. We name this region the proaetive PTV (p-PTV) and use it to modify the treatment plan for the remaining treatment. Let V k be a semi-convex hull of Ui=0..... k CTV(ti), where CTV(ti), i = 0,..., k, represent the CTV delineated from the initial planning CT scan and the CT scans over the first k days of treatment. The p-PTV is then formed by enlarging V k in 3 dimensions (l.5a) based upon the random setup error, ~ = (cj, ~2, a3), which is predicted from the daily portal imaging measurements. In the prediction model, k is the minimum number of measurements required to ensure that the maximum dose reduction in the CTV, due to patient setup error and internal target motion, is less than 5% of the prescribed dose. The prediction model is tested against clinical prostate treatment data. Each patient is scanned, without rectal or urethral contrast, daily in the first week of treatment and biweekly for the remainder of treatment. There is a total of 18 daily CT scans per patient. In addition, a portal image is acquired daily for each treatment field. The maximum dose reduction in the clinical target is evaluated assuming that the p-PTV is covered by the prescribed dose, and the dose outside of the p-PTV has a gradient of-5% per mm. Finally, the total number of patients required to achieve statistical confidence is determined by the Null Hypothesis Test. Results: To date, 12 patients have been entered into the study and 7 of tham were analyzed using the prediction model. Including the initial planning CT scan, 4 _+ 1 days of CT measurements (p < 0.01) acquired during the first week of treatment (k = 5) were needed to achieve the predefined criteria. Current results have not demonstrated significant correlation between the required number of CT measurements and the magnitude of internal mo- tion. The initial manifestation CTV(t0) of the CTV, delineated from the treatment planning CT, was randomly located in the union of the CTV(ti), delineated from the daily treatment CT scans. In 25% of the patients, CTV(t0) lies closer to the anterior border of the Vk, while for 40% of the pa- tients, CTV(t0) is closer to the posterior border. This indicates that a conventional margin based on patient population data could either over or under compensate internal target motion. Furthermore, for the 7 patients who have completed treatment, a uniform margin of lcm is required to achieve the same dosimetrie criteria that was specified for the V k. Conclusion: In external beam treatment of prostate cancer, the p-PTV can be formed in the first week of treatment using the daily portal images and CT measurements. This suggests that a single plan modification can be performed to the individual patient treatment after the first week of dose de- livery. This modification process will greatly improve the efficacy of dose delivery to the clinical target. Supported in part by NCI-CA 71785 1016 PORTAL FILM ANALYSIS OF AN ESCALATED DOSE CONFORMAL PROSTATIC IRRADIATION PROTOCOL USING FIDUCIAL MARKERS AND PORTAL IMAGES TO CONFIRM TARGET ORGAN AND ISOCENTRE POSITION C. Catton, T. Haycocks, H. Alasti, *A. Toi, G. Ottewell, N. Middlemas, J. Mui, P. Warde. Dept of Radiation Oncology, The Princess Margaret Hospital, *Dept of Diagnostic Imaging, The Toronto Hospital. The University of Toronto, Toronto. Purpose A technique was developed for delivery of escalated dose conformal prostatic irradiation that used fiducial markers to assess target organ position and daily portal imaging to confirm the accuracy of isocentre position. The overall accuracy of treatment delivery was evaluated retrospectively with an analysis of alternate daily portal films. Methods. Patients with localized prostate cancer undergoing escalated dose conformal prostatic irradiation had 3 gold fiducial prostatic markers inserted under transrectal ultrasound guidance into the apex, base and midportion of the prostate prior to planning, to identify target organ position, and to assist with target volume delineation. Patients were treated supine with rigid immobilization to 75.6 Gy in 42 fractions over 8.5 weeks using a six field ¢onformal technique. Posteriorly, the clinical target volume was 5 mm beyond the prostate (gross tumor volume) throughout treatment. Anteriorly and laterally the clinical target volume was 15 mm beyond the prostate up to 54 Gy, and then reduced to 5 mm. Superiorly and inferiorly the margin was 10 mm throughout treatment. Isocentre position was screened and misalignments were corrected for daily by the therapists using a portal imaging device to match bony landmarks to those on the simulator film. Patients were instructed on a bowel emptying and bladder filling protocol, and reminded daily in an attempt to minimize target organ motion. For 10 patients, lateral portal films of the treated volume were taken alternate days for a total of 258 films which were compared to the simulator film. Isocentre and fiducial marker deviations were measured, and the isocentre positioning error, target organ positioning error and combined error calculated. The resolution of the portal imaging device did not permit reliable visualization of the fiducial markers, and consequently real time screening of the target organ position. The information from the portal images will be analyzed at another time. Results. Negative numbers denote a deviation in the inferior or posterior direction, positive numbers a deviation in the anterior or superior direction. The mean target organ deviation in the supero-inferior direction was 0.2 mm + 2.1 mm SD, range -4.0 mm to +4.4 mm. In the antero-posterior direction it was +0.6 mm + 1.8 mm SD, range 3.0 mm to + 4.3 mm. The mean isocentre deviation measured after portal image correction, in the superior-inferior direction was 0.2 mm + 1.2 mm SD, range -2.2 mm to +2.6 mm. In the antero-posterior direction the mean deviatio n was -1.1 mm + 2.5 mm SD, range -6.2 mm to +4.0 mm. The mean combined targeting error of marker position relative to isoeentre position, due to target organ and isocentre deviation was in the supero-inferior direction M).I mm _+3.7 mm SD, range -7.4 to 7.3 mm. In the antero-posterior direction it was 1.9 _ o o mm + 4 mm SD, range -6.1 mm to 9.9 mm. There were 21/258 portals (8Fo) that did not fully encompass the target organ within the 95~ isodose o line. All misses were in the antero-posterior direction, and 50Fo were less than 2 mm outside the 95% isodose line. Conclusion. A portal film analysis showed that daily portal image screening produced a mean random error in isoeentre placement of 1.1 mm in the antero-posterior and 0.2 mm the supero-inferior directions. Patient instruction on bowel and bladder preparation produced a mean target organ deviation of less than 1 mm in both the antero-posterior direction and the supero-inferior direction. The overall accuracy in encompassing the entire target organ within the 95% isodose line was 92% for patients undergoing escalated dose eonformal prostatic irradiation. Increasing the minimum clinical target volume from 5 mm to 7 mm beyond the prostate in the antero-posterior direction will increase this accuracy to 96%. Further improvements in accuracy will require improvements in the resolution of portal imaging devices to permit the real-time screening of fiducial marker position prior to therapy.
Transcript of A prediction model for defining a proactive planning target volume in external beam treatment of...
216 I. J. Rad ia t ion O n c o l o g y * B io logy • Phys ics V o l u m e 42, N u m b e r 1 S u p p l e m e n t , 1998
1015 A PREDICTION MODEL F O R DEFINING A PROACTIVE PLANNING TARGET VOLUME IN EXTERNAL BEAM TREATMENT OF PROSTATE CANCER
Di Yan, David Lockman and D. Brabbins Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan, USA
Purpose: To maximize the effective delivery of radiation dose to the clinical target volume (CTV) and minimize the irradiated volume of the organ at risk (OAR), accumulated knowledge o f the individual patient's setup error and internal organ motion should be applied to modify the treatment plan during the course of external beam radiotherapy. One of the major issues in plan modification is construction of a new planning target volume (PTV) proactively according to the feedback information. In this study, a prediction model to form such PTV for the treatment o f prostate adenocarcinoma will be introduced and evaluated retrospectively using portal imaging measurements and CT scans acquired from daily treatment. Materials & Methods: We hypothesize that clinical target displacements due to patient setup and internal motion in prostate treatment are confined inside a region which can be proactively built up from daily measurements of portal images and CT scans acquired within the first k days of treat- ment. We name this region the proaetive PTV (p-PTV) and use it to modify the treatment plan for the remaining treatment. Let V k be a semi-convex
hull of Ui=0..... k CTV(ti), where CTV(ti), i = 0,..., k, represent the CTV delineated from the initial planning CT scan and the CT scans over the first k
days of treatment. The p-PTV is then formed by enlarging V k in 3 dimensions ( l .5a) based upon the random setup error, ~ = (cj, ~2, a3), which is
predicted from the daily portal imaging measurements. In the prediction model, k is the minimum number of measurements required to ensure that the maximum dose reduction in the CTV, due to patient setup error and internal target motion, is less than 5% of the prescribed dose. The prediction model is tested against clinical prostate treatment data. Each patient is scanned, without rectal or urethral contrast, daily in the first week of treatment and biweekly for the remainder of treatment. There is a total o f 18 daily CT scans per patient. In addition, a portal image is acquired daily for each treatment field. The maximum dose reduction in the clinical target is evaluated assuming that the p-PTV is covered by the prescribed dose, and the dose outside o f the p-PTV has a gradient o f - 5 % per mm. Finally, the total number of patients required to achieve statistical confidence is determined by the Null Hypothesis Test. Results: To date, 12 patients have been entered into the study and 7 of tham were analyzed using the prediction model. Including the initial planning CT scan, 4 _+ 1 days of CT measurements (p < 0.01) acquired during the first week o f treatment (k = 5) were needed to achieve the predefined criteria. Current results have not demonstrated significant correlation between the required number o f CT measurements and the magnitude of internal mo- tion. The initial manifestation CTV(t0) o f the CTV, delineated from the treatment planning CT, was randomly located in the union o f the CTV(ti),
delineated from the daily treatment CT scans. In 25% of the patients, CTV(t0) lies closer to the anterior border of the Vk, while for 40% of the pa-
tients, CTV(t0) is closer to the posterior border. This indicates that a conventional margin based on patient population data could either over or under
compensate internal target motion. Furthermore, for the 7 patients who have completed treatment, a uniform margin of lcm is required to achieve the same dosimetrie criteria that was specified for the V k.
Conclusion: In external beam treatment o f prostate cancer, the p-PTV can be formed in the first week of treatment using the daily portal images and CT measurements. This suggests that a single plan modification can be performed to the individual patient treatment after the first week of dose de- livery. This modification process will greatly improve the efficacy of dose delivery to the clinical target. Supported in part by NCI-CA 71785
1016 PORTAL FILM ANALYSIS OF AN ESCALATED DOSE CONFORMAL PROSTATIC IRRADIATION PROTOCOL USING FIDUCIAL MARKERS AND PORTAL IMAGES TO CONFIRM TARGET ORGAN AND ISOCENTRE POSITION C. Catton, T. Haycocks, H. Alasti, *A. Toi, G. Ottewell, N. Middlemas, J. Mui, P. Warde. Dept of Radiation Oncology, The Princess Margaret Hospital, *Dept of Diagnostic Imaging, The Toronto Hospital. The University of Toronto, Toronto.
Purpose A technique was developed for delivery of escalated dose conformal prostatic irradiation that used fiducial markers to assess target organ position and daily portal imaging to confirm the accuracy of isocentre position. The overall accuracy of treatment delivery was evaluated retrospectively with an analysis of alternate daily portal films. Methods. Patients with localized prostate cancer undergoing escalated dose conformal prostatic irradiation had 3 gold fiducial prostatic markers inserted under transrectal ultrasound guidance into the apex, base and midportion of the prostate prior to planning, to identify target organ position, and to assist with target volume delineation. Patients were treated supine with rigid immobilization to 75.6 Gy in 42 fractions over 8.5 weeks using a six field ¢onformal technique. Posteriorly, the clinical target volume was 5 mm beyond the prostate (gross tumor volume) throughout treatment. Anteriorly and laterally the clinical target volume was 15 mm beyond the prostate up to 54 Gy, and then reduced to 5 mm. Superiorly and inferiorly the margin was 10 mm throughout treatment. Isocentre position was screened and misalignments were corrected for daily by the therapists using a portal imaging device to match bony landmarks to those on the simulator film. Patients were instructed on a bowel emptying and bladder filling protocol, and reminded daily in an attempt to minimize target organ motion. For 10 patients, lateral portal films of the treated volume were taken alternate days for a total of 258 films which were compared to the simulator film. Isocentre and fiducial marker deviations were measured, and the isocentre positioning error, target organ positioning error and combined error calculated. The resolution of the portal imaging device did not permit reliable visualization of the fiducial markers, and consequently real time screening of the target organ position. The information from the portal images will be analyzed at another time. Results. Negative numbers denote a deviation in the inferior or posterior direction, positive numbers a deviation in the anterior or superior direction. The mean target organ deviation in the supero-inferior direction was 0.2 mm + 2.1 mm SD, range -4 .0 mm to +4.4 mm. In the antero-posterior direction it was +0.6 mm + 1.8 mm SD, range 3.0 mm to + 4.3 mm. The mean isocentre deviation measured after portal image correction, in the superior-inferior direction was 0.2 mm + 1.2 mm SD, range -2.2 mm to +2.6 mm. In the antero-posterior direction the mean deviatio n was -1.1 mm + 2.5 mm SD, range -6.2 mm to +4.0 mm. The mean combined targeting error of marker position relative to isoeentre position, due to target organ and isocentre deviation was in the supero-inferior direction M).I mm _+ 3.7 mm SD, range -7 .4 to 7.3 mm. In the antero-posterior direction it was 1.9
_ o o mm + 4 mm SD, range -6.1 mm to 9.9 mm. There were 21/258 portals (8Fo) that did not fully encompass the target organ within the 9 5 ~ isodose o line. All misses were in the antero-posterior direction, and 50Fo were less than 2 mm outside the 95% isodose line.
Conclusion. A portal film analysis showed that daily portal image screening produced a mean random error in isoeentre placement of 1.1 mm in the antero-posterior and 0.2 mm the supero-inferior directions. Patient instruction on bowel and bladder preparation produced a mean target organ deviation of less than 1 mm in both the antero-posterior direction and the supero-inferior direction. The overall accuracy in encompassing the entire target organ within the 95% isodose line was 92% for patients undergoing escalated dose eonformal prostatic irradiation. Increasing the minimum clinical target volume from 5 mm to 7 mm beyond the prostate in the antero-posterior direction will increase this accuracy to 96%. Further improvements in accuracy will require improvements in the resolution of portal imaging devices to permit the real-time screening of fiducial marker position prior to therapy.
