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Assessment of IMRT Plans Optimized With Deliverable Step and Shoot (SS) and Dynamic MLC (DMLC) Techniques
Proceedings of the 49th Annual ASTRO Meeting
2916
Impact of Rectal Distension in IGRT for Prostate Cancer
P. J. Kim, A. Parthasarathy, J. Ho, B. Tam, H. Gee, J. W. Lee California Pacific Medical Center, San Francisco, CA Purpose/Objective(s): In obtaining CT scans for external beam radiotherapy planning for prostate cancer, various degrees of rectal distension are observed. We investigated if the degree of daily isocenter shifts guided by intraprostatic fiducial gold seeds correlated with the amount of rectal distension present at the time of CT simulation prior to treatment. Materials/Methods: 41 consecutive patients with prostatic adenocarcinoma had intraprostatic gold seed-guided IGRT treatments in a community-hospital setting. Patients were instructed to empty bladder and rectum prior to simulation and daily treatments. Based on gold seed markers, daily shifts from the isocenter were performed with an onboard kV imager and recorded in X, Y, and Z dimensions. Standard deviations of shifts in all 3 dimensions were calculated. Patients with the five highest and five lowest standard deviations of shifts in each of the 3 dimensions were further analyzed by calculating individual rectal and bladder volumes at the time of CT simulation. In an effort to characterize rectal distension, rectal volumes were further divided into 3 parts: a) superior rectum, defined as located superior to the prostate up to the level of the recto-sigmoid junction, b) middle rectum, defined as located at the level of the prostate, and c) inferior rectum, defined as located inferior to the prostate up to 2 cm and may include anal canal. A single physician contoured all bladder, rectal, and prostate volumes for reproducibility. The correlation between degree of daily shifts throughout the treatment and contoured volumes were analyzed. Results: Range of standard deviations of isocenter shifts in the X, Y, and Z directions were 0.4–1.28 cm, 0.24–0.82 cm, and 0.16– 0.62 cm, respectively. Overall rectal volume did not correlate with standard deviation of shifts among all 3 dimensions (r = 0.148, p = 0.435, df = 28). Superior, middle, and inferior rectal volumes each did not correlate with standard deviation of shifts in all 3 dimensions, nor did the ratio of combined superior and middle rectal volumes to inferior rectal volumes (r = 0.081, p = 0.672). Similarly, there was no correlation between standard deviation of shifts and greatest rectal width on axial images (r = 0.245, p = 0.192). Finally, other parameters such as overall bladder (r = 0.176, p = 0.352) and prostate (r = 0.259, p = 0.167) volumes did not correlate with standard deviation of shifts among all 3 dimensions. Conclusions: There was no correlation between parameters such as rectal or bladder distension at the time of simulation and IGRT shifts guided by gold seed fiducial markers. This lack of correlation may suggest that other factors are also influencing the degree of isocenter shifts. Rectal or bladder distension at the time of CT simulation may reflect patient-specific properties that can be consistent throughout an external radiation treatment course. With the spectrum of isocenter shifts observed in this study, our data underscores the importance of daily localization of the prostate for accurate delivery of radiotherapy. Author Disclosure: P.J. Kim, None; A. Parthasarathy, None; J. Ho, None; B. Tam, None; H. Gee, None; J.W. Lee, None.
2917
Assessment of IMRT Plans Optimized With Deliverable Step and Shoot (SS) and Dynamic MLC (DMLC) Techniques
R. George, N. Dogan Virginia Commonwealth University, Richmond, VA Introduction: Traditional IMRT process consists of two steps (1) Estimation of ideal intensity distribution (2) Obtaining deliverable intensity distribution using a leaf sequencing algorithm. An alternative technique consists of a single step in which multileaf collimator (MLC) delivery constraints are incorporated into the optimization process. This technique is called deliverable optimization. The goal of this work was to determine if the IMRT plans obtained with dynamic MLC (DMLC) based and step and shoot (SS) based deliverable optimization techniques were equivalent in terms of target coverage and critical structure sparing. Materials/Methods: Four head and neck (HN) and four prostate patients were selected for this study. IMRT plans were created using (1) in-house DMLC based deliverable optimization software and (2) SS based deliverable software called direct machine parameter optimization (DMPO) available in Pinnacle3 system. While HN IMRT plans utilized nine coplanar beams, all prostate plans involved seven coplanar beams. DMLC plans are composed of large number segments. For DMPO plans, the maximum number of segments per beam was initially set to ten (e.g., for 9 split HN fields, 180 segments). To evaluate the effect of fewer segments the number of segments (DMPOless) were reduced to 90 for HN and 70 for prostate and the plans were optimized with the same constraints. The dose-volume criteria evaluated for the target and critical structures are shown in Table 1. Results: The results of this analysis are shown in Table 1. IMRT plans using DMLC were comparable to DMPO. The maximum % volume difference between the DMLC and DMPO plans were 0.71% in terms of target volume coverage and 2.33% in terms of sparing of the critical structures over all cases. Similarly the maximum % volume difference between the DMLC and DMPOless plans were 2.44% in terms of target volume coverage and 2.94% in terms of sparing of the critical structures. In addition the monitor units for DMPO were seen to decrease by 29% for HN and 35% for prostate cases. Conclusions: DMPO provides plans equivalent in quality to deliverable DMLC. Reducing the number of segments by half (DMPOless) did not significantly affect the quality of the plans. The number of monitor units significantly decreased when using the step and shoot deliverable optimization i.e. DMPO. Future Work: Besides dose volume criteria evaluated in this study, the plan quality can be evaluated based on the hotspots in the target, dose volume histograms and isodose lines. Monte Carlo based analysis and film measurements will be used to confirm the results.
S715
I. J. Radiation Oncology d Biology d Physics
S716
Volume 69, Number 3, Supplement, 2007
Dose (second column)–volume (third column) constraints used in the optimization and evaluation of DMLC, DMPO and DMPOless plans. A T test with 5% significance level indicated that the ‘‘means were equal’’ for all structures. The % of volume that each of these structures receives a certain dose (second column) based on using three optimization techniques is shown here. The number of monitor units for all three techniques are also listed here Dose (Gy)
Volume (%)
DMLC (%)
DMPO (%)
DMPOless (%)
50 45 25 25 68.1 60 54
0 0 50 50 98 98 95
0.00 ± 0.00 0.00 ± 0.00 60.54 ± 10.71 59.34 ± 16.28 97.82 ± 1.68 97.58 ± 1.47 95.17 ± 1.65
0.00 ± 0.00 0.00 ± 0.00 59.58 ± 9.89 57.01 ± 13.88 97.97 ± 1.66 98.29 ± 1.38 96.73 ± 1.12
0.00 ± 0.00 0.90 ± 0.00 59.21 ± 8.86 56.40 ± 13.90 95.38 ± 3.53 97.74 ± 1.85 95.90 ± 1.74
Head and Neck (Average of 4 patients) Brainstem Cord Lt Parotid Rt Parotid PTV1 PTV2 PTV3
Number of Monitor Units
Prostate (Average of 4 patients) Small Bowel Bladder Rectum PTV PTV nodes
Number of Monitor Units
DMLC (±s)
DMPO (±s)
DMPOless (±s)
1482 ± 191
1059 ± 137
1046 ± 88
Dose (Gy)
Volume (%)
DMLC (%)
DMPO (%)
DMPOless (%)
50 60 60 63 50.4
0 20 10 97 95
1.70 ± 1.36 23.42 ± 6.08 10.93 ± 9.54 97.87 ± 1.07 96.13 ± 1.48
1.71 ± 0.93 23.76 ± 5.81 11.24 ± 7.85 97.32 ± 1.44 95.69 ± 2.39
2.11 ± 1.47 22.91 ± 5.52 10.74 ± 7.53 95.57 ± 2.82 94.74 ± 4.08
DMLC (±s)
DMPO (±s)
DMPOless (±s)
1357 ± 295
890 ± 435
859 ± 355
Author Disclosure: R. George, None; N. Dogan, None.
2918
On the Clinical and Dosimetric Accuracy of Anisotropic Analytical Algorithm Photon Dose Calculation in the Case of Lung Inhomogeneities for IMRT Treatment
M. Khodri, D. Plattard, H. Martin, I. Teˆte, N. Remini, T. Schmitt, G. Delaroche Institut de Cance´rologie de La Loire, Saint-Etienne, France Background: Advanced radiotherapy techniques, as IMRT, require accurate dose calculation in any relevant clinical situation. One of these situations is the treatment of thoracic tumours where the dose to the lung may compromise the coverage of PTV. In this study, we evaluate the errors in dose values calculated by Analytical Anisotropic Algorithm (AAA) and Pencil Beam Convolution (PBC): two-photon dose calculation engines available on the Varian-Eclipse (version 7.5.43) commercial radiotherapy treatment planning system. For IMRT treatment of lung cancer. Materials/Methods: AAA and PBC were used to calculate doses treatment plans using photons beams of 6 and 20 MV from 210O CD linac (Varian, USA) under a range of clinically relevant irradiations geometries, and thoracic wall-lung-thoracic wall (lung phantom of CIRS incorporation). The calculated data were compared with measurements performed with ionization chamber and radiographic films. Results: Both algorithms agree with measurement to acceptable tolerance levels in most cases in homogeneous water-equivalent media irradiated under full scatter conditions. The dose in media irradiated under missing tissue geometry, is modelled best with the AAA algorithm; in low-density media, PBC algorithm leads to an average deviation from dose measurements of over 12%.for high energy.(20 MV). For the AAA algorithm, deviations between ionometric and films measurements were within 3%; and a maximum of 2.5 mm deviation in the beam fringe for any depth was found. Conclusions: Analytical Anisotropic Algorithm (AAA) assesses with a reasonable accuracy dose distributions in inhomogeneous regions; this is of particular clinical interest in treatment planning of the thorax. Therefore and the respecting the inhomogeneity dose calculation, the AAA algorithm could be used in routine clinical practice for modern conformal radiotherapy whereas this is not true in the case of PBC algorithms which leads to errors greater than 12% in dose calculation. Author Disclosure: M. Khodri, None; D. Plattard, None; H. Martin, None; I. Teˆte, None; N. Remini, None; T. Schmitt, None; G. Delaroche, None.
2919
Multi-Sensor Measurement of External Movement to Improve Correlation Between External Marker and Internal Motion for Respiratory Motion Management During Radiotherapy
J. D. Christensen, A. Tai, X. A. Li Medical College of Wisconsin, Milwaukee, WI Purpose/Objective(s): Current methods using a single external sensor as a surrogate marker of internal motion are not optimal, since good correlation does not always exist between the external marker and internal motion. This study sought to determine
2916
Impact of Rectal Distension in IGRT for Prostate Cancer
P. J. Kim, A. Parthasarathy, J. Ho, B. Tam, H. Gee, J. W. Lee California Pacific Medical Center, San Francisco, CA Purpose/Objective(s): In obtaining CT scans for external beam radiotherapy planning for prostate cancer, various degrees of rectal distension are observed. We investigated if the degree of daily isocenter shifts guided by intraprostatic fiducial gold seeds correlated with the amount of rectal distension present at the time of CT simulation prior to treatment. Materials/Methods: 41 consecutive patients with prostatic adenocarcinoma had intraprostatic gold seed-guided IGRT treatments in a community-hospital setting. Patients were instructed to empty bladder and rectum prior to simulation and daily treatments. Based on gold seed markers, daily shifts from the isocenter were performed with an onboard kV imager and recorded in X, Y, and Z dimensions. Standard deviations of shifts in all 3 dimensions were calculated. Patients with the five highest and five lowest standard deviations of shifts in each of the 3 dimensions were further analyzed by calculating individual rectal and bladder volumes at the time of CT simulation. In an effort to characterize rectal distension, rectal volumes were further divided into 3 parts: a) superior rectum, defined as located superior to the prostate up to the level of the recto-sigmoid junction, b) middle rectum, defined as located at the level of the prostate, and c) inferior rectum, defined as located inferior to the prostate up to 2 cm and may include anal canal. A single physician contoured all bladder, rectal, and prostate volumes for reproducibility. The correlation between degree of daily shifts throughout the treatment and contoured volumes were analyzed. Results: Range of standard deviations of isocenter shifts in the X, Y, and Z directions were 0.4–1.28 cm, 0.24–0.82 cm, and 0.16– 0.62 cm, respectively. Overall rectal volume did not correlate with standard deviation of shifts among all 3 dimensions (r = 0.148, p = 0.435, df = 28). Superior, middle, and inferior rectal volumes each did not correlate with standard deviation of shifts in all 3 dimensions, nor did the ratio of combined superior and middle rectal volumes to inferior rectal volumes (r = 0.081, p = 0.672). Similarly, there was no correlation between standard deviation of shifts and greatest rectal width on axial images (r = 0.245, p = 0.192). Finally, other parameters such as overall bladder (r = 0.176, p = 0.352) and prostate (r = 0.259, p = 0.167) volumes did not correlate with standard deviation of shifts among all 3 dimensions. Conclusions: There was no correlation between parameters such as rectal or bladder distension at the time of simulation and IGRT shifts guided by gold seed fiducial markers. This lack of correlation may suggest that other factors are also influencing the degree of isocenter shifts. Rectal or bladder distension at the time of CT simulation may reflect patient-specific properties that can be consistent throughout an external radiation treatment course. With the spectrum of isocenter shifts observed in this study, our data underscores the importance of daily localization of the prostate for accurate delivery of radiotherapy. Author Disclosure: P.J. Kim, None; A. Parthasarathy, None; J. Ho, None; B. Tam, None; H. Gee, None; J.W. Lee, None.
2917
Assessment of IMRT Plans Optimized With Deliverable Step and Shoot (SS) and Dynamic MLC (DMLC) Techniques
R. George, N. Dogan Virginia Commonwealth University, Richmond, VA Introduction: Traditional IMRT process consists of two steps (1) Estimation of ideal intensity distribution (2) Obtaining deliverable intensity distribution using a leaf sequencing algorithm. An alternative technique consists of a single step in which multileaf collimator (MLC) delivery constraints are incorporated into the optimization process. This technique is called deliverable optimization. The goal of this work was to determine if the IMRT plans obtained with dynamic MLC (DMLC) based and step and shoot (SS) based deliverable optimization techniques were equivalent in terms of target coverage and critical structure sparing. Materials/Methods: Four head and neck (HN) and four prostate patients were selected for this study. IMRT plans were created using (1) in-house DMLC based deliverable optimization software and (2) SS based deliverable software called direct machine parameter optimization (DMPO) available in Pinnacle3 system. While HN IMRT plans utilized nine coplanar beams, all prostate plans involved seven coplanar beams. DMLC plans are composed of large number segments. For DMPO plans, the maximum number of segments per beam was initially set to ten (e.g., for 9 split HN fields, 180 segments). To evaluate the effect of fewer segments the number of segments (DMPOless) were reduced to 90 for HN and 70 for prostate and the plans were optimized with the same constraints. The dose-volume criteria evaluated for the target and critical structures are shown in Table 1. Results: The results of this analysis are shown in Table 1. IMRT plans using DMLC were comparable to DMPO. The maximum % volume difference between the DMLC and DMPO plans were 0.71% in terms of target volume coverage and 2.33% in terms of sparing of the critical structures over all cases. Similarly the maximum % volume difference between the DMLC and DMPOless plans were 2.44% in terms of target volume coverage and 2.94% in terms of sparing of the critical structures. In addition the monitor units for DMPO were seen to decrease by 29% for HN and 35% for prostate cases. Conclusions: DMPO provides plans equivalent in quality to deliverable DMLC. Reducing the number of segments by half (DMPOless) did not significantly affect the quality of the plans. The number of monitor units significantly decreased when using the step and shoot deliverable optimization i.e. DMPO. Future Work: Besides dose volume criteria evaluated in this study, the plan quality can be evaluated based on the hotspots in the target, dose volume histograms and isodose lines. Monte Carlo based analysis and film measurements will be used to confirm the results.
S715
I. J. Radiation Oncology d Biology d Physics
S716
Volume 69, Number 3, Supplement, 2007
Dose (second column)–volume (third column) constraints used in the optimization and evaluation of DMLC, DMPO and DMPOless plans. A T test with 5% significance level indicated that the ‘‘means were equal’’ for all structures. The % of volume that each of these structures receives a certain dose (second column) based on using three optimization techniques is shown here. The number of monitor units for all three techniques are also listed here Dose (Gy)
Volume (%)
DMLC (%)
DMPO (%)
DMPOless (%)
50 45 25 25 68.1 60 54
0 0 50 50 98 98 95
0.00 ± 0.00 0.00 ± 0.00 60.54 ± 10.71 59.34 ± 16.28 97.82 ± 1.68 97.58 ± 1.47 95.17 ± 1.65
0.00 ± 0.00 0.00 ± 0.00 59.58 ± 9.89 57.01 ± 13.88 97.97 ± 1.66 98.29 ± 1.38 96.73 ± 1.12
0.00 ± 0.00 0.90 ± 0.00 59.21 ± 8.86 56.40 ± 13.90 95.38 ± 3.53 97.74 ± 1.85 95.90 ± 1.74
Head and Neck (Average of 4 patients) Brainstem Cord Lt Parotid Rt Parotid PTV1 PTV2 PTV3
Number of Monitor Units
Prostate (Average of 4 patients) Small Bowel Bladder Rectum PTV PTV nodes
Number of Monitor Units
DMLC (±s)
DMPO (±s)
DMPOless (±s)
1482 ± 191
1059 ± 137
1046 ± 88
Dose (Gy)
Volume (%)
DMLC (%)
DMPO (%)
DMPOless (%)
50 60 60 63 50.4
0 20 10 97 95
1.70 ± 1.36 23.42 ± 6.08 10.93 ± 9.54 97.87 ± 1.07 96.13 ± 1.48
1.71 ± 0.93 23.76 ± 5.81 11.24 ± 7.85 97.32 ± 1.44 95.69 ± 2.39
2.11 ± 1.47 22.91 ± 5.52 10.74 ± 7.53 95.57 ± 2.82 94.74 ± 4.08
DMLC (±s)
DMPO (±s)
DMPOless (±s)
1357 ± 295
890 ± 435
859 ± 355
Author Disclosure: R. George, None; N. Dogan, None.
2918
On the Clinical and Dosimetric Accuracy of Anisotropic Analytical Algorithm Photon Dose Calculation in the Case of Lung Inhomogeneities for IMRT Treatment
M. Khodri, D. Plattard, H. Martin, I. Teˆte, N. Remini, T. Schmitt, G. Delaroche Institut de Cance´rologie de La Loire, Saint-Etienne, France Background: Advanced radiotherapy techniques, as IMRT, require accurate dose calculation in any relevant clinical situation. One of these situations is the treatment of thoracic tumours where the dose to the lung may compromise the coverage of PTV. In this study, we evaluate the errors in dose values calculated by Analytical Anisotropic Algorithm (AAA) and Pencil Beam Convolution (PBC): two-photon dose calculation engines available on the Varian-Eclipse (version 7.5.43) commercial radiotherapy treatment planning system. For IMRT treatment of lung cancer. Materials/Methods: AAA and PBC were used to calculate doses treatment plans using photons beams of 6 and 20 MV from 210O CD linac (Varian, USA) under a range of clinically relevant irradiations geometries, and thoracic wall-lung-thoracic wall (lung phantom of CIRS incorporation). The calculated data were compared with measurements performed with ionization chamber and radiographic films. Results: Both algorithms agree with measurement to acceptable tolerance levels in most cases in homogeneous water-equivalent media irradiated under full scatter conditions. The dose in media irradiated under missing tissue geometry, is modelled best with the AAA algorithm; in low-density media, PBC algorithm leads to an average deviation from dose measurements of over 12%.for high energy.(20 MV). For the AAA algorithm, deviations between ionometric and films measurements were within 3%; and a maximum of 2.5 mm deviation in the beam fringe for any depth was found. Conclusions: Analytical Anisotropic Algorithm (AAA) assesses with a reasonable accuracy dose distributions in inhomogeneous regions; this is of particular clinical interest in treatment planning of the thorax. Therefore and the respecting the inhomogeneity dose calculation, the AAA algorithm could be used in routine clinical practice for modern conformal radiotherapy whereas this is not true in the case of PBC algorithms which leads to errors greater than 12% in dose calculation. Author Disclosure: M. Khodri, None; D. Plattard, None; H. Martin, None; I. Teˆte, None; N. Remini, None; T. Schmitt, None; G. Delaroche, None.
2919
Multi-Sensor Measurement of External Movement to Improve Correlation Between External Marker and Internal Motion for Respiratory Motion Management During Radiotherapy
J. D. Christensen, A. Tai, X. A. Li Medical College of Wisconsin, Milwaukee, WI Purpose/Objective(s): Current methods using a single external sensor as a surrogate marker of internal motion are not optimal, since good correlation does not always exist between the external marker and internal motion. This study sought to determine