- Email: [email protected]
Evaluation of Sequential and Simultaneously Integrated Boost IMRT Methods in Head-and-Neck Cancer
S812
International Journal of Radiation Oncology Biology Physics
Environment for Radiation Research (CERR) for computing DVHs. Volume differences and DVHs of the deformed structures were compared to plan structures. Results: Fractions were divided into 3 groups; initial, mid, and end of the course of treatments and the DVH results were summarized in Table for both non-deformed (ND) and deformed (D) structures with their differences indicated in parenthesis. For the PTVs, D95% reduced by at least 5% in 9/50 fractions while the V107% was increased by more than 5% in 28/50 fractions. Parotids consistently lost volume throughout the treatments, while PTVs saw the majority of the loss of volume from the middle to the final fractions with PTV70Gy and PTV56Gy decreasing by 22.4 cc and 33.5 cc, respectively. V26Gy and mean dose (MD) of the parotids were increased by at least 5% or 5cGy in 50% and 75% of the fractions, which was not seen in non-deformed dose reconstructions. Conclusion: Combining the reconstructed doses with deformed anatomy, gives more accurate dose distributions of the delivered treatments. Further study is needed to investigate the effects of dose deformations to achieve cumulative dose volumes. Author Disclosure: V. Rodriguez: None. J. Esthappan: None. P. Dyk: None. H. Gay: None. W. Thorstad: None. S. Goddu: None.
Conclusion: The generic PSFs are accurate and efficient radiation sources for MC dose calculations. The scattering dose from the metal implants is not of concern for H&N patients irradiated by 6MV IMRT beams. Author Disclosure: L. Wang: None. E. Mok: None. L. Xing: None. Q. Le: E. Research Grant; Varian Medical System.
3577 Monte Carlo Dose Calculations for IMRT Head-and-Neck Patients With Dental Fillings Using the Phase Space Files for LINAC L. Wang, E. Mok, L. Xing, and Q. Le; Stanford University, Stanford, CA Purpose/Objective(s): The merit of Monte Carlo (MC) dose calculation over others mainly resides in heterogeneous media, such as H&N cancer patients where small bone pieces and sinuses present. The possible dental fillings makes it more complex for dose calculations. MC method is also widely used to model a linac head. In a full simulation, all the linac head components are modeled which takes longer time than a partial simulation. Usually a phase-space file (PSF) is generated at a surface above the jaws representing the patient independent part of the linac head. One company created series of generic PSFs for linac. This study validates these PSFs with measured data and then uses the PSFs as the radiation source for the MC calculations for H&N patients treated by IMRT technique. Materials/Methods: First, the validation was done by varying jaw settings to various field sizes, using PSFs as the source and calculating the percentage depth-dose and lateral dose profiles at a few depths in water; and comparing them to measurement. Gamma analysis was performed for all comparisons. Next, two IMRT H&N patients with dental fillings were selected for the MC simulation of CT image based calculations, using the PSFs for the 6MV flattened beams as the radiation sources. The CT image artifacts from the dental fillings were replaced by tissue. To evaluate the impact of the metal on dose distributions, three sets of MC calculations were performed per patient: (1) treating implant as bone; (2) substituting implant with titanium, and (3) with gold. Mean doses in the PTV and nearby structures were compared for the three MC calculations. Results: For the gamma index analysis with a criterion of 2%/2mm, the minimum passing rate is 94.9% for the 6MV flattened beams. The computing time saved by using PSFs could be a factor of 5-10 compared to the full simulations. The patient calculation results (normalized to 1 at isocenter) are shown in Table I. With titanium, there is no significant difference in doses to PTV and other structures from that when treating implant as bone. With gold, the mean dose to PTV is slightly lowered by 1%; the mean dose to other structures is about the same, only for right tongue the maximum dose is increased by 5%.
Poster Viewing Abstract 3577; Table Mean (max) dose to PTV and nearby structures for different implant materials bone PTV R tongue R parotid L parotid
1.090 0.728 0.149 0.376
(1.263) (0.919) (0.353) (1.134)
Ti (Z Z 22)
Au (Z Z 79)
1.088 0.728 0.149 0.376
1.080 0.724 0.147 0.374
(1.258) (0.927) (0.348) (1.154)
(1.356) (0.978) (0.346) (1.158)
3578 Ventilation Imaging for Lung Radiation Therapy Planning: Free Breathing 4DCT Versus Breath-hold MRI K. Ding,1 K. Cao,2 K. Du,2 Q. Chen,1 D. Ennis,3 G. Christensen,2 J. Reinhardt,2 B. Libby,1 S. Benedict,1 and K. Sheng3; 1University of Virginia, Charlottesville, VA, 2University of Iowa, Iowa City, IA, 3 University of California, Los Angeles, Los Angeles, CA Purpose/Objective(s): Current Radiation Therapy (RT) planning for lung stereotactic body radiation therapy (SBRT) is based on dose volume histogram such as V20 and mean lung dose to avoid radiation induced lung injury based on the assumption that the pulmonary function is the same everywhere inside lung. However, the actual lung function is indeed region specific and it can be incorporated into RT planning for functional lung avoidance. We have previously developed ventilation imaging methods coupling either free breathing 4DCT or breath hold MRI with deformable image registration. In this study, we compare the ventilation measurements from two imaging modalities on a regional basis. Materials/Methods: The max inhale phase and the max exhale phase were extracted from the free breathing 4DCT for one lung cancer patient planned with SBRT with 55 Gy to PTVs in the right and the left lung respectively. A MRI scan pair near the end of inhale and near the end of exhale with breath hold were also acquired before RT. B-spline based vesselness preserving image registration algorithm was applied to register the breath hold MRI pair and the extreme phase pair from 4DCT separately for the calculation of regional ventilation measured as the Jacobian determinant of the displacement field from the registration result. The ventilation maps from the both breath hold MRI and free breathing 4DCT were then mapped to the planning CT using the same algorithm tuned for MRI-CT and CT-CT registrations. The ratio of the ventilation values from two methods were compared in a regional basis. Results: The ratio between the ventilation measured from free breathing 4DCT and the breath hold MRI has the mean value of 1.61. 91.8% of the lung voxels have the ratio large than 1 indicating the breath hold may recruit different breathing unit of the lung for breathing between the free breathing and the breath hold procedure. A strong heterogeneity was found inside the lung from the ratio map which also suggests different breathing pattern between the two procedures. Conclusion: We compared free breathing 4DCT based and static breathhold MRI based lung imaging to evaluate regional ventilation for RT planning which may take the underlying lung function into account. Further investigation is needed to study which method is more sensitive and robust for regional lung ventilation measurement. Author Disclosure: K. Ding: None. K. Cao: None. K. Du: None. Q. Chen: None. D. Ennis: None. G. Christensen: None. J. Reinhardt: None. B. Libby: None. S. Benedict: None. K. Sheng: None.
3579 Evaluation of Sequential and Simultaneously Integrated Boost IMRT Methods in Head-and-Neck Cancer S. Jang,1 A.P. Pyakuryal,2 O. Cahlon,1 L. Mao,1 D. Powell,1 A.S. Greenberg,1 H.K. Tsai,1 T.T. Sio,3 B.B. Mittal,4 and J. Hanley1; 1 Princeton Radiation Oncology, Jamesburg, NJ, 2University of Illinois at Chicago, Chicago, IL, 3Mayo Clinic, Rochester, MN, 4Northwestern Memorial Hospital, Chicago, IL Purpose/Objective(s): The objectives of this study were to evaluate and obtain the characteristics of the sequential and simultaneously integrated boost (SIB; also known as dose painting) IMRT methods in head and neck cancer using the HART (Histogram Analysis in Radiation Therapy) program.
Volume 84 Number 3S Supplement 2012 Materials/Methods: IMRT plans of 10 sequential and 12 SIB cases were studied retrospectively. For both methods 9 beam angles were used. A cumulative dose of 73.5 Gy was prescribed sequentially, and a range of 60 to 70 Gy for SIB method. The homogeneity (HI: Dmax/prescription dose (PD)) and radiation conformality indices (RCI: PTVPD/PTV0.95PD) were calculated from dose-volume histograms (DVHs). DVH statistics for the critical spots (normalized volume > normal tissue tolerance dose TD50,5 (>2/3 organ)) and hot spots (normalized volume > PD) were utilized to estimate the radiobiological outcomes of TCP and NTCP using the Poisson statistics and JT Lyman models in the HART. TD50,5 and other corresponding radiobiological parameters such as volume parameter (n) and slope parameter (m) were selected from Luxton et al. and Emami et al. TD50,5 used were 46, 80, and 47 Gy for bilateral parotids, larynx, and esophagus, respectively. Results: The HI and RCI were 1.100.1 and 0.980.01 in the sequential method, and 1.100.02 and 0.960.02 in SIB method, respectively. Critical spots for parotids, larynx, and esophagus were 0.750.03, 0.060.02, and 0.340.02 in the sequential method, and 0.290.07, <0.01, and 0.220.06 in SIB method, respectively. Hot spots for parotids, larynx, and esophagus were 0.340.03, 0.540.02, and <0.01, respectively in the sequential method whereas 0.100.05, <0.01, and 0.050.03 in SIB method, respectively. The estimated values of the TCP and NTCP correlated well with patient outcome in a 2-year follow up study with sequential boost; and the follow up study for SIB treatments is in process. NTCP estimates for parotids, larynx, and esophagus were 0.450.14, 0.030.01, and 0.170.09 in the sequential method, and 0.090.04, <0.01, and 0.180.04 in SIB method, respectively (95% confidence interval). Conclusion: Mean HIs were comparable for both techniques while mean RCI was relatively better with the sequential than SIB method (not statistically significant). The critical spots and hot spots were reduced in SIB method which may partially be related to smaller prescription doses. Both sequential and SIB methods yielded similar NTCP for larynx and esophagus. These findings are not in direct comparison due to the differences in tumors and stages; however, better parotid sparing with the SIB method than sequentially was observed. This novel methodology of radiobiological outcome-related analysis can be utilized to evaluate different treatment plan techniques. Author Disclosure: S. Jang: None. A.P. Pyakuryal: None. O. Cahlon: None. L. Mao: None. D. Powell: None. A.S. Greenberg: None. H.K. Tsai: None. T.T. Sio: None. B.B. Mittal: None. J. Hanley: None.
3580 Skin Flash in IMRT Plans J.K. Ha and A. Rashtian; LAC+USC, Los Angeles, CA Purpose/Objective(s): It is common practice to reduce PTV margin to about 3mm below the skin, if no subclinical disease is suspected to be in the skin. In these cases, bolus is generally not used to minimize skin dose. However, this compromises the patient setup margin, even for patients with head and neck cancer treated with thermoplastic masks that typically have a margin of uncertainty of a few millimeters. The objective of this dosimetric study is to include the appropriate PTV margin extended outside the patient body without having the plan dose algorithm depositing a lot of monitor units in the space distal to the skin and creating hot spots at and below the skin surface. Materials/Methods: Using planning software, GTV, CTV, and critical structures are contoured by the radiation oncologist. The two PTVs — PTV-skin and PTV-ext — are drawn from the CTV. The PTV-skin is an expansion from the CTV but with its distal surface being trimmed to 3 mm below the skin surface. The PTV-ext keeps the appropriate CTV expansion, even if it extends beyond the skin. The IMRT optimization and dose calculations are done in two steps: First, a one cm tissue-equivalent bolus is used for IMRT optimization. The dose is calculated using the generated leaf sequence but without bolus to simulate the actual dose delivery to the patient. At this first step, the calculated dose to the PTV-skin is expected to be underdosed because of the missing bolus. In the second step, this
Poster Viewing Abstracts S813 deficiency in dose coverage in the PTV-skin near the skin surface is corrected by creating an artificial PTV-cor structure which is used in the optimization to add more MUs to the underdose region. The dose is then recalculated with the new generated leaf sequence, again without bolus to simulate the actual dose delivery. The second step can be repeated by redesigning the PTV-cor or changing the dose constraint parameter until the desired dose coverage to the PTV-skin is achieved. The proposed method has been evaluated on treatment plans of patients with meningioma and cancers of the H&N. Results: Our study shows that if patient setup is off by 3mm in the radial direction perpendicular to the skin surface from that of the plan, PTV-skin coverage can drop from 95% of the prescribed dose, as planned, to only 90%. Clearly, depending on the volume of CTV and how close it is to the skin surface, dose to PTV-skin can drop even more significantly. By including the flash as proposed above, our study shows the PTV coverage can be made rather insensitive to the setup error. Conclusions: The proposed method is a good way to introduce flash in IMRT plans. The current practice by reducing PTV margin below the skin or adding a bolus is far from being ideal in treating disease regions near patient body surface. A future clinical study can confirm the importance of including flash in IMRT plans. Author Disclosure: J.K. Ha: None. A. Rashtian: None.
3581 Correlation of Delivery Reproducibility With Anatomical Modifications and Intrafraction Motion During Treatments Using Helical Tomotherapy (HT)-Integrated Detectors E. Cagni, A. Botti, E. Mezzenga, V. D’errico, P. Ciammella, and M. Iori; S. Maria Nuova Hospital, Reggio Emilia, Italy Purpose/Objective(s): To investigate the correlation of the dose reproducibility acquired during treatment delivered, using integrated HT MVCT detectors, with the patient anatomy modifications that can occur during fractionated treatments. To assess the dosimetric effect of intra-fraction motion on the signal acquired by MVCT detectors during the HT treatment delivery (sinogram). Materials/Methods: First, the MVCT detector data variations related only to the delivery component were quantified. Ten HT plans, with increased level of complexity, were ranked using an extension of the Webb’s Modulation Index (MI) for helical delivery. These plans were delivered several times with a phantom positioned between the radiation source and the detectors. Then, five H&N plans, acquired by MVCT detectors during clinical treatments delivery, were studied and correlated with patient anatomic variations evaluated by daily MVCT imaging. As a last step, an anthropomorphic phantom placed on a respiratory gating platform was used to simulate breathing movement during treatments. Setting different ranges of motion and respiratory amplitudes, a clinical HT plan was delivered several time and acquired with MVCT detectors. Two indices were introduced: the “Reproducibility Index” (RI) based on the definition of M.S. Padro (the RIDELIV was used to evaluate data acquired with the phantom while the RITREAT for patient data) and the SIN_DEV index, that quantify the differences on the acquired sinograms respect to the reference one. This indexes were related to the treatment complexity, the patient’s anatomy variation and the intra-fraction motions. Results: A dependence of the RIDELIV with the MI factor was observed (R2 Z 0.6367), pointing out higher RIDELIV values for the most complex plan. Our analysis confirm the existence of a correlation between the RITREAT and the patient anatomy reductions in the neck diameter (R2 Z 0.687) and parotids shrinkage (R2 Z 0.521). The analysis of SIN_DEV index related to the anatomical modifications was assessed by means of Spearman correlation (SC), indicate higher SIN_DEV indexes at the end of the treatment that correspond to a larger parotids shrinkage (SC Z -0.78, p<0.0001) and neck diameter reduction (SC Z -0.75, p<0.0001). The preliminary results concerning the sinogram variations related to the respiratory amplitude and frequency, have showed SIN_DEV indexes up to 5% respect to no movement MVCT data, with a 5,4% of the sinogram area over 5% of difference.
International Journal of Radiation Oncology Biology Physics
Environment for Radiation Research (CERR) for computing DVHs. Volume differences and DVHs of the deformed structures were compared to plan structures. Results: Fractions were divided into 3 groups; initial, mid, and end of the course of treatments and the DVH results were summarized in Table for both non-deformed (ND) and deformed (D) structures with their differences indicated in parenthesis. For the PTVs, D95% reduced by at least 5% in 9/50 fractions while the V107% was increased by more than 5% in 28/50 fractions. Parotids consistently lost volume throughout the treatments, while PTVs saw the majority of the loss of volume from the middle to the final fractions with PTV70Gy and PTV56Gy decreasing by 22.4 cc and 33.5 cc, respectively. V26Gy and mean dose (MD) of the parotids were increased by at least 5% or 5cGy in 50% and 75% of the fractions, which was not seen in non-deformed dose reconstructions. Conclusion: Combining the reconstructed doses with deformed anatomy, gives more accurate dose distributions of the delivered treatments. Further study is needed to investigate the effects of dose deformations to achieve cumulative dose volumes. Author Disclosure: V. Rodriguez: None. J. Esthappan: None. P. Dyk: None. H. Gay: None. W. Thorstad: None. S. Goddu: None.
Conclusion: The generic PSFs are accurate and efficient radiation sources for MC dose calculations. The scattering dose from the metal implants is not of concern for H&N patients irradiated by 6MV IMRT beams. Author Disclosure: L. Wang: None. E. Mok: None. L. Xing: None. Q. Le: E. Research Grant; Varian Medical System.
3577 Monte Carlo Dose Calculations for IMRT Head-and-Neck Patients With Dental Fillings Using the Phase Space Files for LINAC L. Wang, E. Mok, L. Xing, and Q. Le; Stanford University, Stanford, CA Purpose/Objective(s): The merit of Monte Carlo (MC) dose calculation over others mainly resides in heterogeneous media, such as H&N cancer patients where small bone pieces and sinuses present. The possible dental fillings makes it more complex for dose calculations. MC method is also widely used to model a linac head. In a full simulation, all the linac head components are modeled which takes longer time than a partial simulation. Usually a phase-space file (PSF) is generated at a surface above the jaws representing the patient independent part of the linac head. One company created series of generic PSFs for linac. This study validates these PSFs with measured data and then uses the PSFs as the radiation source for the MC calculations for H&N patients treated by IMRT technique. Materials/Methods: First, the validation was done by varying jaw settings to various field sizes, using PSFs as the source and calculating the percentage depth-dose and lateral dose profiles at a few depths in water; and comparing them to measurement. Gamma analysis was performed for all comparisons. Next, two IMRT H&N patients with dental fillings were selected for the MC simulation of CT image based calculations, using the PSFs for the 6MV flattened beams as the radiation sources. The CT image artifacts from the dental fillings were replaced by tissue. To evaluate the impact of the metal on dose distributions, three sets of MC calculations were performed per patient: (1) treating implant as bone; (2) substituting implant with titanium, and (3) with gold. Mean doses in the PTV and nearby structures were compared for the three MC calculations. Results: For the gamma index analysis with a criterion of 2%/2mm, the minimum passing rate is 94.9% for the 6MV flattened beams. The computing time saved by using PSFs could be a factor of 5-10 compared to the full simulations. The patient calculation results (normalized to 1 at isocenter) are shown in Table I. With titanium, there is no significant difference in doses to PTV and other structures from that when treating implant as bone. With gold, the mean dose to PTV is slightly lowered by 1%; the mean dose to other structures is about the same, only for right tongue the maximum dose is increased by 5%.
Poster Viewing Abstract 3577; Table Mean (max) dose to PTV and nearby structures for different implant materials bone PTV R tongue R parotid L parotid
1.090 0.728 0.149 0.376
(1.263) (0.919) (0.353) (1.134)
Ti (Z Z 22)
Au (Z Z 79)
1.088 0.728 0.149 0.376
1.080 0.724 0.147 0.374
(1.258) (0.927) (0.348) (1.154)
(1.356) (0.978) (0.346) (1.158)
3578 Ventilation Imaging for Lung Radiation Therapy Planning: Free Breathing 4DCT Versus Breath-hold MRI K. Ding,1 K. Cao,2 K. Du,2 Q. Chen,1 D. Ennis,3 G. Christensen,2 J. Reinhardt,2 B. Libby,1 S. Benedict,1 and K. Sheng3; 1University of Virginia, Charlottesville, VA, 2University of Iowa, Iowa City, IA, 3 University of California, Los Angeles, Los Angeles, CA Purpose/Objective(s): Current Radiation Therapy (RT) planning for lung stereotactic body radiation therapy (SBRT) is based on dose volume histogram such as V20 and mean lung dose to avoid radiation induced lung injury based on the assumption that the pulmonary function is the same everywhere inside lung. However, the actual lung function is indeed region specific and it can be incorporated into RT planning for functional lung avoidance. We have previously developed ventilation imaging methods coupling either free breathing 4DCT or breath hold MRI with deformable image registration. In this study, we compare the ventilation measurements from two imaging modalities on a regional basis. Materials/Methods: The max inhale phase and the max exhale phase were extracted from the free breathing 4DCT for one lung cancer patient planned with SBRT with 55 Gy to PTVs in the right and the left lung respectively. A MRI scan pair near the end of inhale and near the end of exhale with breath hold were also acquired before RT. B-spline based vesselness preserving image registration algorithm was applied to register the breath hold MRI pair and the extreme phase pair from 4DCT separately for the calculation of regional ventilation measured as the Jacobian determinant of the displacement field from the registration result. The ventilation maps from the both breath hold MRI and free breathing 4DCT were then mapped to the planning CT using the same algorithm tuned for MRI-CT and CT-CT registrations. The ratio of the ventilation values from two methods were compared in a regional basis. Results: The ratio between the ventilation measured from free breathing 4DCT and the breath hold MRI has the mean value of 1.61. 91.8% of the lung voxels have the ratio large than 1 indicating the breath hold may recruit different breathing unit of the lung for breathing between the free breathing and the breath hold procedure. A strong heterogeneity was found inside the lung from the ratio map which also suggests different breathing pattern between the two procedures. Conclusion: We compared free breathing 4DCT based and static breathhold MRI based lung imaging to evaluate regional ventilation for RT planning which may take the underlying lung function into account. Further investigation is needed to study which method is more sensitive and robust for regional lung ventilation measurement. Author Disclosure: K. Ding: None. K. Cao: None. K. Du: None. Q. Chen: None. D. Ennis: None. G. Christensen: None. J. Reinhardt: None. B. Libby: None. S. Benedict: None. K. Sheng: None.
3579 Evaluation of Sequential and Simultaneously Integrated Boost IMRT Methods in Head-and-Neck Cancer S. Jang,1 A.P. Pyakuryal,2 O. Cahlon,1 L. Mao,1 D. Powell,1 A.S. Greenberg,1 H.K. Tsai,1 T.T. Sio,3 B.B. Mittal,4 and J. Hanley1; 1 Princeton Radiation Oncology, Jamesburg, NJ, 2University of Illinois at Chicago, Chicago, IL, 3Mayo Clinic, Rochester, MN, 4Northwestern Memorial Hospital, Chicago, IL Purpose/Objective(s): The objectives of this study were to evaluate and obtain the characteristics of the sequential and simultaneously integrated boost (SIB; also known as dose painting) IMRT methods in head and neck cancer using the HART (Histogram Analysis in Radiation Therapy) program.
Volume 84 Number 3S Supplement 2012 Materials/Methods: IMRT plans of 10 sequential and 12 SIB cases were studied retrospectively. For both methods 9 beam angles were used. A cumulative dose of 73.5 Gy was prescribed sequentially, and a range of 60 to 70 Gy for SIB method. The homogeneity (HI: Dmax/prescription dose (PD)) and radiation conformality indices (RCI: PTVPD/PTV0.95PD) were calculated from dose-volume histograms (DVHs). DVH statistics for the critical spots (normalized volume > normal tissue tolerance dose TD50,5 (>2/3 organ)) and hot spots (normalized volume > PD) were utilized to estimate the radiobiological outcomes of TCP and NTCP using the Poisson statistics and JT Lyman models in the HART. TD50,5 and other corresponding radiobiological parameters such as volume parameter (n) and slope parameter (m) were selected from Luxton et al. and Emami et al. TD50,5 used were 46, 80, and 47 Gy for bilateral parotids, larynx, and esophagus, respectively. Results: The HI and RCI were 1.100.1 and 0.980.01 in the sequential method, and 1.100.02 and 0.960.02 in SIB method, respectively. Critical spots for parotids, larynx, and esophagus were 0.750.03, 0.060.02, and 0.340.02 in the sequential method, and 0.290.07, <0.01, and 0.220.06 in SIB method, respectively. Hot spots for parotids, larynx, and esophagus were 0.340.03, 0.540.02, and <0.01, respectively in the sequential method whereas 0.100.05, <0.01, and 0.050.03 in SIB method, respectively. The estimated values of the TCP and NTCP correlated well with patient outcome in a 2-year follow up study with sequential boost; and the follow up study for SIB treatments is in process. NTCP estimates for parotids, larynx, and esophagus were 0.450.14, 0.030.01, and 0.170.09 in the sequential method, and 0.090.04, <0.01, and 0.180.04 in SIB method, respectively (95% confidence interval). Conclusion: Mean HIs were comparable for both techniques while mean RCI was relatively better with the sequential than SIB method (not statistically significant). The critical spots and hot spots were reduced in SIB method which may partially be related to smaller prescription doses. Both sequential and SIB methods yielded similar NTCP for larynx and esophagus. These findings are not in direct comparison due to the differences in tumors and stages; however, better parotid sparing with the SIB method than sequentially was observed. This novel methodology of radiobiological outcome-related analysis can be utilized to evaluate different treatment plan techniques. Author Disclosure: S. Jang: None. A.P. Pyakuryal: None. O. Cahlon: None. L. Mao: None. D. Powell: None. A.S. Greenberg: None. H.K. Tsai: None. T.T. Sio: None. B.B. Mittal: None. J. Hanley: None.
3580 Skin Flash in IMRT Plans J.K. Ha and A. Rashtian; LAC+USC, Los Angeles, CA Purpose/Objective(s): It is common practice to reduce PTV margin to about 3mm below the skin, if no subclinical disease is suspected to be in the skin. In these cases, bolus is generally not used to minimize skin dose. However, this compromises the patient setup margin, even for patients with head and neck cancer treated with thermoplastic masks that typically have a margin of uncertainty of a few millimeters. The objective of this dosimetric study is to include the appropriate PTV margin extended outside the patient body without having the plan dose algorithm depositing a lot of monitor units in the space distal to the skin and creating hot spots at and below the skin surface. Materials/Methods: Using planning software, GTV, CTV, and critical structures are contoured by the radiation oncologist. The two PTVs — PTV-skin and PTV-ext — are drawn from the CTV. The PTV-skin is an expansion from the CTV but with its distal surface being trimmed to 3 mm below the skin surface. The PTV-ext keeps the appropriate CTV expansion, even if it extends beyond the skin. The IMRT optimization and dose calculations are done in two steps: First, a one cm tissue-equivalent bolus is used for IMRT optimization. The dose is calculated using the generated leaf sequence but without bolus to simulate the actual dose delivery to the patient. At this first step, the calculated dose to the PTV-skin is expected to be underdosed because of the missing bolus. In the second step, this
Poster Viewing Abstracts S813 deficiency in dose coverage in the PTV-skin near the skin surface is corrected by creating an artificial PTV-cor structure which is used in the optimization to add more MUs to the underdose region. The dose is then recalculated with the new generated leaf sequence, again without bolus to simulate the actual dose delivery. The second step can be repeated by redesigning the PTV-cor or changing the dose constraint parameter until the desired dose coverage to the PTV-skin is achieved. The proposed method has been evaluated on treatment plans of patients with meningioma and cancers of the H&N. Results: Our study shows that if patient setup is off by 3mm in the radial direction perpendicular to the skin surface from that of the plan, PTV-skin coverage can drop from 95% of the prescribed dose, as planned, to only 90%. Clearly, depending on the volume of CTV and how close it is to the skin surface, dose to PTV-skin can drop even more significantly. By including the flash as proposed above, our study shows the PTV coverage can be made rather insensitive to the setup error. Conclusions: The proposed method is a good way to introduce flash in IMRT plans. The current practice by reducing PTV margin below the skin or adding a bolus is far from being ideal in treating disease regions near patient body surface. A future clinical study can confirm the importance of including flash in IMRT plans. Author Disclosure: J.K. Ha: None. A. Rashtian: None.
3581 Correlation of Delivery Reproducibility With Anatomical Modifications and Intrafraction Motion During Treatments Using Helical Tomotherapy (HT)-Integrated Detectors E. Cagni, A. Botti, E. Mezzenga, V. D’errico, P. Ciammella, and M. Iori; S. Maria Nuova Hospital, Reggio Emilia, Italy Purpose/Objective(s): To investigate the correlation of the dose reproducibility acquired during treatment delivered, using integrated HT MVCT detectors, with the patient anatomy modifications that can occur during fractionated treatments. To assess the dosimetric effect of intra-fraction motion on the signal acquired by MVCT detectors during the HT treatment delivery (sinogram). Materials/Methods: First, the MVCT detector data variations related only to the delivery component were quantified. Ten HT plans, with increased level of complexity, were ranked using an extension of the Webb’s Modulation Index (MI) for helical delivery. These plans were delivered several times with a phantom positioned between the radiation source and the detectors. Then, five H&N plans, acquired by MVCT detectors during clinical treatments delivery, were studied and correlated with patient anatomic variations evaluated by daily MVCT imaging. As a last step, an anthropomorphic phantom placed on a respiratory gating platform was used to simulate breathing movement during treatments. Setting different ranges of motion and respiratory amplitudes, a clinical HT plan was delivered several time and acquired with MVCT detectors. Two indices were introduced: the “Reproducibility Index” (RI) based on the definition of M.S. Padro (the RIDELIV was used to evaluate data acquired with the phantom while the RITREAT for patient data) and the SIN_DEV index, that quantify the differences on the acquired sinograms respect to the reference one. This indexes were related to the treatment complexity, the patient’s anatomy variation and the intra-fraction motions. Results: A dependence of the RIDELIV with the MI factor was observed (R2 Z 0.6367), pointing out higher RIDELIV values for the most complex plan. Our analysis confirm the existence of a correlation between the RITREAT and the patient anatomy reductions in the neck diameter (R2 Z 0.687) and parotids shrinkage (R2 Z 0.521). The analysis of SIN_DEV index related to the anatomical modifications was assessed by means of Spearman correlation (SC), indicate higher SIN_DEV indexes at the end of the treatment that correspond to a larger parotids shrinkage (SC Z -0.78, p<0.0001) and neck diameter reduction (SC Z -0.75, p<0.0001). The preliminary results concerning the sinogram variations related to the respiratory amplitude and frequency, have showed SIN_DEV indexes up to 5% respect to no movement MVCT data, with a 5,4% of the sinogram area over 5% of difference.