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A Technique to Improve the Calculated Skin Dose Accuracy by a Commercial Treatment Planning System
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Volume 96 Number 2S Supplement 2016 that is consistent with the more complex equations in the literature. The second objective was to use this equation to generate a biological model to predict toxicity and influence treatment planning in patients treated with proton beam therapy for cancer of the head and neck. Materials/Methods: The equation we generated is RBE Z (1.1) [0.08(LET) +0.88]. This equation was plotted against equations proposed previously reported in the literature. We then evaluated head and neck radiation plans for 5 patients treated with proton beam therapy at our institution. Our biological model based on our RBE equation was compared with our physical (Monte Carlo) model. GTV, CTV, and selected organs at risk were contoured. Isodose lines were contoured at 105%, 110%, 115%, and 120% of the target dose. The volume of organs at risk receiving a dose greater than the target dose was calculated. Results: Our equation aligns well with RBE equations in the literature for a/b Z 3 and LET less than 5 keV/mm. Thus it is suitable for evaluating radiation plans. Tumors treated included squamous cell carcinoma of the tonsil (2), external auditory canal, nasopharynx, and skin. Compared to our physical model, our biological model revealed several localized regions of increased dose. Implicated organs at risk include the oral cavity, mandible, temporomandibular joint, pharyngeal constrictor muscles, and brain. The biological model predicted substantially higher doses to these structures than the physical model (Table 1). Conclusion: Our biological model identified areas of potential toxicity that were not identifiable in the physical model and thus could influence the treatment planning process. Such areas will need to be monitored closely for signs of toxicity during treatment and in follow-up appointments. The alignment of clinical features with toxicity predicted by our biological model would highlight the importance of using an equation to predict RBE instead of assuming a constant RBE value of 1.1. Author Disclosure: C. Fossum: None. C. Beltran: None. D.J. Ma: None. R.L. Foote: None.
3514 A Technique to Improve the Calculated Skin Dose Accuracy by a Commercial Treatment Planning System L. Wang,1 A.J. Cmelak,2 and G.X. Ding2; 1Vanderbilt Ingram Cancer Center at Franklin, Franklin, TN, 2Vanderbilt University Medical Center, Nashville, TN Purpose/Objective(s): Patient skin dose toxicity significantly affects the patient radiotherapy treatment especially for head and neck (H&N) cancer patient. The accuracy of predicted skin dose from a radiotherapy treatment planning system is important in patient treatment planning. The objective of this study is to present technique to improve the accuracy of skin dose calculations by a commercial treatment planning system (TPS). Materials/Methods: The accuracy of skin dose calculations is evaluated for the anisotropic analytical algorithm (AAA) implemented in Varian Eclipse (V.11) TPS. Photon beams of 6 MV, 10 MV, and 15 MV from a Varian TrueBeam linear accelerator are investigated. Validated Monte Carlo (MC) simulations are utilized for the skin dose evaluation of CT based realistic patient dose calculations with VMAT technique for various cancer sites including H&N. Skin dose is calculated as mean dose to a contoured structure of 0.5 cm thickness from the skin surface. Three scenarios of external body contours in Eclipse for skin dose calculations are compared to the MC calculation: (1) default patient external body contour; (2) patient body contour including supporting/immobilization devices; (3) patient body contour extended 2 cm air layer in addition to including the couch table. The same incident beams are used for all scenarios so that the calculated dose differences are due to the difference of external body contours. Results: Results show that the AAA in Eclipse TPS underestimates skin doses by up to 20% for the H&N patient when external body contour starts at the patient skin. When the external patient body contour is extended by 2 cm air, the calculated skin dose differences between Eclipse AAA algorithm and the Monte Carlo simulation are generally within 5%. Conclusion: Although the AAA algorithm in Eclipse treatment planning system has its limitations in predicting patient skin dose, this study shows the calculation accuracy can be significantly improved to within 5% by simply
extending the external body contour by 2cm without affecting the dose calculation accuracy to the treatment target and internal organs at risk. Author Disclosure: L. Wang: None. A. Cmelak: None. G.X. Ding: None.
3515 Early-Stage Breast Radiation Therapy Assessment Using Transcytolemmal Water Exchange Analysis of DCE-Magnetic Resonance Imaging: Initial Results C. Wang, J.K. Horton, Sr, F.F. Yin, E. Subashi, and Z. Chang; Duke University Medical Center, Durham, NC Purpose/Objective(s): To investigate the use of transcytolemmal water exchange analysis in the treatment assessment of early stage breast cancer single-dose preoperative radiotherapy with paired dynamic contrastenhanced MRI (DCE-MRI). Materials/Methods: 15 female patients with biopsy-proven early stage breast cancer received one of three prescription doses (15Gy, 18Gy, or 21Gy) in a single-fraction using IMRT. Each patient had paired pre- and post-treatment DCE-MRI scans acquired sagittally in the prone position. Gd-DTPA2 contrast agent (CA) was administered at a dose of 0.1mmol/kg bodyweight and 2ml/s flow rate after the calibration scans. One preenhanced volume and six post-enhanced volumes were acquired, and the temporal resolution was approximately 1 minute. For quantitative pharmacokinetic analysis, the intracellular and the extracellular-extravascular water molecules were modelled separately with different longitudinal relaxation rates due to the potentially inhomogeneous CA distribution in extracellular-extravascular space (EES). The mean residence time of intracellular water molecules si, which is the inverse of the transcytolemmal water exchange rate describing the limited water molecule movement from intracellular space to EES, was modeled as a novel physiological biomarker. The Bloch equations were corrected with the presence of si, and the calculation of microvessel permeability rate Ktrans in the classic Tofts model was modified as a nonlinear least-squares problem of integral equation. For each scan, both Ktrans and si were calculated voxel-by-voxel in CTV, and the CTV mean values were reported as primary biomarkers. A biological subvolume (BSV) was identified based on si distribution in CTV using a histogram-based automatic thresholding method. The volume ratio of BSV to CTV was calculated, and Ktrans and si mean values in BSV were also recorded. Signed-rank tests were performed to assess the differences of the recorded parameters before and after radiotherapy. Rank-sum tests were conducted to find parameters that may reflect response difference between different dose groups. Results: After radiotherapy, both CTV mean Ktrans (p < .007) and si (p < .002) significantly decreased. The volume ratio of BSV to CTV showed a decrease after radiotherapy (p < .003), and the decrease of BSV mean Ktrans were also found to be statistically significant (p < .001). For the patients in the 21Gy dose group, the relative BSV Ktrans values (post-RT value/pre-RT value) were significantly smaller (p < .045) than the values of the patients who received less than 21 Gy treatment dose. Conclusion: The initial results suggest that physiological biomarkers from transcytolemmal water exchange analysis significantly changed after radiotherapy. Changes in these biomarkers within the identified biological subvolume may have a role in predicting the clinical impact of radiation dose. Author Disclosure: C. Wang: None. J.K. Horton: None. F. Yin: Board member-at-large; AAPM. E. Subashi: None. Z. Chang: None.
3516 The Effect of Dose Rate Decrease and Speed Variation on the Delivery Accuracy of a Volumetric Modulated Arc Therapy Plan K. Wang,1 Y. Feng,2 J. Jin,3 and Y. Cao1; 1Tianjin Medical University General Hospital, Tianjin, China, 2East Carolina University, Greenville, NC, 3Elekta China, Beijing, China Purpose/Objective(s): To evaluate the delivery accuracy of VMAT plan for Elekta-Synergy accelerator with MLCi2 controlled by the treatment control system IntegrityR1.1 which supports continuously variable dose rate (CVDR), and investigate the effect of dose rate decrease, gantry speed variation and MLC speed variation on the delivery errors.
Volume 96 Number 2S Supplement 2016 that is consistent with the more complex equations in the literature. The second objective was to use this equation to generate a biological model to predict toxicity and influence treatment planning in patients treated with proton beam therapy for cancer of the head and neck. Materials/Methods: The equation we generated is RBE Z (1.1) [0.08(LET) +0.88]. This equation was plotted against equations proposed previously reported in the literature. We then evaluated head and neck radiation plans for 5 patients treated with proton beam therapy at our institution. Our biological model based on our RBE equation was compared with our physical (Monte Carlo) model. GTV, CTV, and selected organs at risk were contoured. Isodose lines were contoured at 105%, 110%, 115%, and 120% of the target dose. The volume of organs at risk receiving a dose greater than the target dose was calculated. Results: Our equation aligns well with RBE equations in the literature for a/b Z 3 and LET less than 5 keV/mm. Thus it is suitable for evaluating radiation plans. Tumors treated included squamous cell carcinoma of the tonsil (2), external auditory canal, nasopharynx, and skin. Compared to our physical model, our biological model revealed several localized regions of increased dose. Implicated organs at risk include the oral cavity, mandible, temporomandibular joint, pharyngeal constrictor muscles, and brain. The biological model predicted substantially higher doses to these structures than the physical model (Table 1). Conclusion: Our biological model identified areas of potential toxicity that were not identifiable in the physical model and thus could influence the treatment planning process. Such areas will need to be monitored closely for signs of toxicity during treatment and in follow-up appointments. The alignment of clinical features with toxicity predicted by our biological model would highlight the importance of using an equation to predict RBE instead of assuming a constant RBE value of 1.1. Author Disclosure: C. Fossum: None. C. Beltran: None. D.J. Ma: None. R.L. Foote: None.
3514 A Technique to Improve the Calculated Skin Dose Accuracy by a Commercial Treatment Planning System L. Wang,1 A.J. Cmelak,2 and G.X. Ding2; 1Vanderbilt Ingram Cancer Center at Franklin, Franklin, TN, 2Vanderbilt University Medical Center, Nashville, TN Purpose/Objective(s): Patient skin dose toxicity significantly affects the patient radiotherapy treatment especially for head and neck (H&N) cancer patient. The accuracy of predicted skin dose from a radiotherapy treatment planning system is important in patient treatment planning. The objective of this study is to present technique to improve the accuracy of skin dose calculations by a commercial treatment planning system (TPS). Materials/Methods: The accuracy of skin dose calculations is evaluated for the anisotropic analytical algorithm (AAA) implemented in Varian Eclipse (V.11) TPS. Photon beams of 6 MV, 10 MV, and 15 MV from a Varian TrueBeam linear accelerator are investigated. Validated Monte Carlo (MC) simulations are utilized for the skin dose evaluation of CT based realistic patient dose calculations with VMAT technique for various cancer sites including H&N. Skin dose is calculated as mean dose to a contoured structure of 0.5 cm thickness from the skin surface. Three scenarios of external body contours in Eclipse for skin dose calculations are compared to the MC calculation: (1) default patient external body contour; (2) patient body contour including supporting/immobilization devices; (3) patient body contour extended 2 cm air layer in addition to including the couch table. The same incident beams are used for all scenarios so that the calculated dose differences are due to the difference of external body contours. Results: Results show that the AAA in Eclipse TPS underestimates skin doses by up to 20% for the H&N patient when external body contour starts at the patient skin. When the external patient body contour is extended by 2 cm air, the calculated skin dose differences between Eclipse AAA algorithm and the Monte Carlo simulation are generally within 5%. Conclusion: Although the AAA algorithm in Eclipse treatment planning system has its limitations in predicting patient skin dose, this study shows the calculation accuracy can be significantly improved to within 5% by simply
extending the external body contour by 2cm without affecting the dose calculation accuracy to the treatment target and internal organs at risk. Author Disclosure: L. Wang: None. A. Cmelak: None. G.X. Ding: None.
3515 Early-Stage Breast Radiation Therapy Assessment Using Transcytolemmal Water Exchange Analysis of DCE-Magnetic Resonance Imaging: Initial Results C. Wang, J.K. Horton, Sr, F.F. Yin, E. Subashi, and Z. Chang; Duke University Medical Center, Durham, NC Purpose/Objective(s): To investigate the use of transcytolemmal water exchange analysis in the treatment assessment of early stage breast cancer single-dose preoperative radiotherapy with paired dynamic contrastenhanced MRI (DCE-MRI). Materials/Methods: 15 female patients with biopsy-proven early stage breast cancer received one of three prescription doses (15Gy, 18Gy, or 21Gy) in a single-fraction using IMRT. Each patient had paired pre- and post-treatment DCE-MRI scans acquired sagittally in the prone position. Gd-DTPA2 contrast agent (CA) was administered at a dose of 0.1mmol/kg bodyweight and 2ml/s flow rate after the calibration scans. One preenhanced volume and six post-enhanced volumes were acquired, and the temporal resolution was approximately 1 minute. For quantitative pharmacokinetic analysis, the intracellular and the extracellular-extravascular water molecules were modelled separately with different longitudinal relaxation rates due to the potentially inhomogeneous CA distribution in extracellular-extravascular space (EES). The mean residence time of intracellular water molecules si, which is the inverse of the transcytolemmal water exchange rate describing the limited water molecule movement from intracellular space to EES, was modeled as a novel physiological biomarker. The Bloch equations were corrected with the presence of si, and the calculation of microvessel permeability rate Ktrans in the classic Tofts model was modified as a nonlinear least-squares problem of integral equation. For each scan, both Ktrans and si were calculated voxel-by-voxel in CTV, and the CTV mean values were reported as primary biomarkers. A biological subvolume (BSV) was identified based on si distribution in CTV using a histogram-based automatic thresholding method. The volume ratio of BSV to CTV was calculated, and Ktrans and si mean values in BSV were also recorded. Signed-rank tests were performed to assess the differences of the recorded parameters before and after radiotherapy. Rank-sum tests were conducted to find parameters that may reflect response difference between different dose groups. Results: After radiotherapy, both CTV mean Ktrans (p < .007) and si (p < .002) significantly decreased. The volume ratio of BSV to CTV showed a decrease after radiotherapy (p < .003), and the decrease of BSV mean Ktrans were also found to be statistically significant (p < .001). For the patients in the 21Gy dose group, the relative BSV Ktrans values (post-RT value/pre-RT value) were significantly smaller (p < .045) than the values of the patients who received less than 21 Gy treatment dose. Conclusion: The initial results suggest that physiological biomarkers from transcytolemmal water exchange analysis significantly changed after radiotherapy. Changes in these biomarkers within the identified biological subvolume may have a role in predicting the clinical impact of radiation dose. Author Disclosure: C. Wang: None. J.K. Horton: None. F. Yin: Board member-at-large; AAPM. E. Subashi: None. Z. Chang: None.
3516 The Effect of Dose Rate Decrease and Speed Variation on the Delivery Accuracy of a Volumetric Modulated Arc Therapy Plan K. Wang,1 Y. Feng,2 J. Jin,3 and Y. Cao1; 1Tianjin Medical University General Hospital, Tianjin, China, 2East Carolina University, Greenville, NC, 3Elekta China, Beijing, China Purpose/Objective(s): To evaluate the delivery accuracy of VMAT plan for Elekta-Synergy accelerator with MLCi2 controlled by the treatment control system IntegrityR1.1 which supports continuously variable dose rate (CVDR), and investigate the effect of dose rate decrease, gantry speed variation and MLC speed variation on the delivery errors.