L Yin

Biological consequences of MLC calibration errors in IMRT delivery and QA

PURPOSEnThe purpose of this work is threefold: (1) to explore biological consequences of the multileaf collimator (MLC) calibration errors in intensity modulated radiotherapy (IMRT) of prostate and head and neck cancers, (2) to determine levels of planning target volume (PTV) and normal tissue under- or overdose flagged with clinically used QA action limits, and (3) to provide biologically based input for MLC QA and IMRT QA action limits.nnnMETHODSnTen consecutive prostate IMRT cases and ten consecutive head and neck IMRT cases were used. Systematic MLC offsets (i.e., calibration error) were introduced for each control point of the plan separately for X1 and X2 leaf banks. Offsets were fromu2009-u20092 to 2 mm with a 0.5 mm increment. The modified files were imported into the planning system for forward dose recalculation. The original plan served as the reference. The generalized equivalent uniform dose (gEUD) was used as the biological index for the targets, rectum, parotid glands, brainstem, and spinal cord. Each plan was recalculated on a CT scan of a 27 cm diameter cylindrical phantom with a contoured 0.6 cc ion chamber. Dose to ion chamber and 3D gamma analysis were compared to the reference plan. QA pass criteria: (1) at least 95% of voxels with a dose cutoff of 50% of maximum dose have to pass at 3 mm/3% and (2) dose to chamber within 2% of the reference dose.nnnRESULTSnFor prostate cases, differences in PTV and rectum gEUD greater than 2% were identified. However, a larger proportion of plans leading to greater than 2% difference in prostate PTV gEUD passed the ion chamber QA but not 3D gamma QA. A similar trend was found for the rectum gEUD. For head and neck IMRT, the QA pass criteria flagged plans leading to greater than 4% differences in PTV gEUD and greater than 5% differences in the maximum dose to brainstem. If pass criteria were relaxed to 90% for gamma and 3% for ion chamber QA, plans leading to a 5% difference in PTV gEUD and a 5%-8% difference in brainstem maximum dose would likely pass IMRT QA. A larger proportion of head and neck plans with greater than 2% PTV gEUD difference passed 3D gamma QA compared to ion chamber QA.nnnCONCLUSIONSnFor low modulation plans, there is a better chance to catch MLC calibration errors with 3D gamma QA rather than ion chamber QA. Conversely, for high modulation plans, there is a better chance to catch MLC calibration errors with ion chamber QA rather than with 3D gamma QA. Ion chamber and 3D gamma analysis IMRT QA can detect greater than 2% change in gEUD for PTVs and critical structures for low modulation treatment plans. For high modulation treatment plans, ion chamber and 3D gamma analysis can detect greater than 2% change in gEUD for PTVs and a 5% change in critical structure gEUD since either QA methods passes the QA criteria. For gEUD changes less than those listed above, either QA method has the same proportion of passing rate.

Complexity and accuracy of image registration methods in SPECT-guided radiation therapy.

The use of functional imaging in radiotherapy treatment (RT) planning requires accurate co-registration of functional imaging scans to CT scans. We evaluated six methods of image registration for use in SPECT-guided radiotherapy treatment planning. Methods varied in complexity from 3D affine transform based on control points to diffeomorphic demons and level set non-rigid registration. Ten lung cancer patients underwent perfusion SPECT-scans prior to their radiotherapy. CT images from a hybrid SPECT/CT scanner were registered to a planning CT, and then the same transformation was applied to the SPECT images. According to registration evaluation measures computed based on the intensity difference between the registered CT images or based on target registration error, non-rigid registrations provided a higher degree of accuracy than rigid methods. However, due to the irregularities in some of the obtained deformation fields, warping the SPECT using these fields may result in unacceptable changes to the SPECT intensity distribution that would preclude use in RT planning. Moreover, the differences between intensity histograms in the original and registered SPECT image sets were the largest for diffeomorphic demons and level set methods. In conclusion, the use of intensity-based validation measures alone is not sufficient for SPECT/CT registration for RTTP. It was also found that the proper evaluation of image registration requires the use of several accuracy metrics.

Incorporating Quantitative Single Photon Emission Computed Tomography into Radiation Therapy Treatment Planning for Lung Cancer: Impact of Attenuation and Scatter Correction on the Single Photon Emission Computed Tomography–Weighted Mean Dose and Functional Lung Segmentation

PURPOSEnTo assess the impact of attenuation and scatter corrections on the calculation of single photon emission computed tomography (SPECT)-weighted mean dose (SWMD) and functional volume segmentation as applied to radiation therapy treatment planning for lung cancer.nnnMETHODS AND MATERIALSnNine patients with lung cancer underwent a SPECT lung perfusion scan. For each scan, four image sets were reconstructed using the ordered subsets expectation maximization method with attenuation and scatter corrections ranging from none to a most comprehensive combination of attenuation corrections and direct scatter modeling. Functional volumes were segmented in each reconstructed image using 10%, 20%, …, 90% of maximum SPECT intensity as a threshold. Systematic effects of SPECT reconstruction methods on treatment planning using functional volume were studied by calculating size and spatial agreements of functional volumes, and V(20) for functional volume from actual treatment plans. The SWMD was calculated for radiation beams with a variety of possible gantry angles and field sizes.nnnRESULTSnFunctional volume segmentation is sensitive to the particular method of SPECT reconstruction used. Large variations in functional volumes, as high as >50%, were observed in SPECT images reconstructed with different attenuation/scatter corrections. However, SWMD was less sensitive to the type of scatter corrections. SWMD was consistent within 2% for all reconstructions as long as computed tomography-based attenuation correction was used.nnnCONCLUSIONnWhen using perfusion SPECT images during treatment planning optimization/evaluation, the SWMD may be the preferred figure of merit, as it is less affected by reconstruction technique, compared with threshold-based functional volume segmentation.

Changes in Lung Perfusion Distribution Due to Radiation in Lung Cancer Patients Treated with 3D Conformal Radiation Therapy and Stereotactic Body Radiation Therapy

Purpose: To compare correlations between radiation and perfusion damage in lung cancer patients treated with 3D conformal radiation therapy (CRT) (both radical and palliative intent) and stereotactic body radiation therapy (SBRT).

SU‐E‐T‐412: Biological Consequences of Errors in MLC Calibration in the Context of IMRT Delivery and QA

Purpose: To quantify biological consequences of MLCcalibration errors in prostate IMRT and relate those biological consequences to IMRT QA action levels. Methods: Ten consecutive prostate cancer patients treated with a 5‐field IMRT (Varian iX with Millenium 120 leaf MLC) were selected. All cases were treated with a prescription dose of 74 Gy. An in‐house program was used to introduce systematic MLC offsets. Combinations of X1 and X2 offsets in the range from −2 to +2mm were explored. Modified files were imported into the treatment planning system (TPS) to recalculate dose for each modified plan. DVHs were exported from the TPS and generalized equivalent uniform doses (gEUD) were calculated for the rectum (n=0.09) and PTV (a=−10). Each plan was also recalculated on a CT scan of a cylindrical phantom with a contoured ion camber. Dose distributions were exported for 3D gamma analysis with dose and distance to agreement set to 3% and 3 mm, respectively. Pass criteria were: at least 95% of voxels have to pass and dose to chamber has to be within 2% of the reference dose. Results: Plans with offsets of 2 mm, single leaf bank or combined, have not passed gamma QA, however 30% of plans passed chamber dose QA. For 1.5 mm offsets 20% and 70% of plans passed gamma and chamber dose QA, respectively. Change in gEUD for PTV and rectum was of the order of 2% for plans passing both QA methods and larger than 3% for those which failed QA. Conclusions: With currently used methods and criteria for prostate IMRT QA, we can only detect changes in gEUD greater than 3% that are resultant from MLCcalibration errors. This is important because it provides a lower limit on the ability of current IMRT QA to estimate a biological impact for machine errors.

SU‐E‐J‐86: Comparing Regional Perfusion Loss in Lung Cancer Patients Treated with 3D Conformal Radiation Therapy and Stereotactic Body Radiation Therapy

Purpose: To compare correlations between radiation and perfusion damage in lungcancer patients treated with 3D‐CRT (both radical and palliative intent) and SBRT. Methods: Eleven (3 palliative 3D‐CRT, 5 radical 3D‐CRT, 3 SBRT) non‐small cell lungcancer patients receiving radiation therapy were included in this study. 99mTc SPECT scans were performed before RT and 2∼3 months after RT. Perfusion images were reconstructed with quantitative attenuation and scatter corrections. These reconstructions were used to assess the change in lung perfusion, i.e. blood flow, as an effect of radiation on the normal lung during RT. Dose and corresponding percentage reduction in the SPECT intensity were used to establish dose‐response curve. Results: 1): No dose‐response was observed in the palliative patient group possibly due to tumor regression and reperfusion. The threshold dose of compensation (increased perfusion in low dose region) could not be found, although increased perfusion in either low dose or high dose volume was common. 2) A consistent pattern of compensation and reduction of perfusion was observed in four patients. D50, dose causing 50% reduction in perfusion ranged from 39 to 54Gy. Reperfusion was observed in one of the five patients in the radical patient group. 3) In the SBRT patient group, a consistent pattern of decreased perfusion was observed in two patients. Compared to patients treated with radical 3D‐CRT, the reduction in perfusion caused by high grade dose (>30Gy) is much smaller in SBRT patients (25% reduction at 45Gy compared to 35% in radical 3D‐CRT group). Conclusions: A strong correlation of radiationdose and perfusion change was found in patients treated with 3D‐CRT with a radical intent, but not palliative intent. Interestingly, perfusion loss in SBRT is smaller than 3D‐CRT, suggesting that normal lung in SBRT eligible patients is more capable of preserving lung function.

SU‐GG‐T‐486: Sparing of Lung Function Using Perfusion SPECT Guided IMRT Treatment Planning for Lung Cancer Patients

Purpose: To develop a method for improving functional lung sparing utilizing perfusion SPECT information and Monte Carlo(MC)dose calculation in IMRToptimization of lungcancerradiotherapy.Methods and Materials: 99mTc‐macroaggregated albumin (MAA) SPECT scans were acquired before radiotherapy (RT) for five non‐small cell lungcancer patients treated with RT (60Gy / 30 fx). Perfusion SPECTimages were reconstructed with attenuation, scatter correction and rigidly registered with planning CT afterwards. Beamlets generated from MC simulation were incorporated into direct aperture optimization (DAO) of static gantry step and shoot IMRTtreatment planning. For each patient, three RT plans were generated. 1. Clinical plan: 3D‐CRT plan generated in vendors software which is clinically delivered. 2. DVH driven plan: IMRT plan generated using DAO and MCdose calculation without SPECT guidance. Conventional DVH constraints for targets and OARs were used in the treatment planning, 3. SPECT driven plan: IMRT plan generated using DAO and MCdose calculation with SPECT guidance. SPECT weighted mean dose (SWMD) and Equivalent uniform dose (EUD) constraints were incorporated into the objective function. SWMD constraints were applied to ipsilateral and contralateral lungs respectively as the metric of lung function sparing. EUD was adopted to optimize planning target volume (PTV) dose coverage. Results: 1. Compared to clinical plan, same target dose coverage was achieved in both DVH driven and SPECT driven plans. 95% iso‐dose line covered more than 97% of PTV in both plans. 2. Compared to DVH driven plans, in SPECT driven plans, V5 and V20 were reduced by ∼8% and ∼3% respectively, SWMD were reduced by ∼1Gy. Thus superior sparing of both lung function and volume was achieved. Conclusion: Comparing to conventional DVH driven IMRT plans, superior lung sparing of both anatomical and functional volumes can be achieved in SPECT driven IMRT planning based on EUD and SWMD.

Poster — Wed Eve—46: Quantitative Evaluation on the Accuracy of Image Registration Methods in SPECT Guided Radiation Therapy for Lung Cancer Patients

Purpose: To quantitatively evaluate the accuracy of several SPECT/CT image registration methods in recent studies and its impact on the functional lung volume segmentation in SPECT guided radiation therapy (RT) treatment planning.Methods and Materials: Five lungcancer patients were consented to have a perfusion SPECT scan with 99mTc‐macroaggregated albumin. During the scan, a low resolution CTimage was acquired using the SPECT/CT scanner. This CT scan was co‐registered to the patients planning CT scan through four rigid and deformable image registration programs (rigid registration, skin/lung control points based registration and B‐spline deformable registration). After the CT to CT co‐registration, original SPECT reconstructions were warped and co‐registered to the planning CT scan. The functional lung volumes were segmented from each deformed SPECT using 10, 20, …, 90% of maximum pixel value as a threshold. The differences in the size and contours of each functional volume were calculated. Results: Based on the evaluation of registered CTimages, the result from B‐spline registration demonstrated the smallest intensity difference. Using the warped SPECTimages obtained from this registration method as a reference, the smallest difference in the size and contour of functional volumes was found using rigid registration. In the point‐based registrations, a better result was found when the control points were placed on lung volume instead of body contour. Conclusion: Apply B‐spline based image registration method in SPECT‐guided RT studies was shown to be accurate. Point‐based image registration using skin markers with a standalone SPECTscanner was found least accurate.

Sci—Thurs PM: Planning—07: Impact of Quantitative SPECT Corrections on SPECT‐Weighted Mean Dose and Functional Lung Volume Segmentation as Applied in Functional Sparing RT Planning

Purpose: To compare functional lung volume segmentation and Single Photon Emission Computed Tomography(SPECT) weighted mean dose (SWMD) approaches used in SPECT guided treatment planning studies for patients with lungcancer.Methods and Materials: Nine lungcancer patients were consented to have a perfusion SPECT scan with 99mTc‐macroaggregated albumin. Four image sets were reconstructed from each scan: one using a vendor provided ordered subsets expectation maximization (OSEM) algorithm and three quantitative SPECTreconstructions using OSEM methods with different types of attenuation and scatter corrections. SWMDs were calculated for open field with different sizes and gantry angles. Functional lung volumes were segmented in each reconstructed image using 10, 20, …, 90% of maximum SPECT intensity as a threshold. Results:Image reconstruction accuracy and thus functional lung volume segmentation are affected by several factors, such as attenuation and scatter correction, resolution recovery method and number of iterations used in reconstruction. Large differences in functional volumes (more than 50%) were found between images obtained from the four reconstructions. In contrast, the SWMD calculation produced consistent results for all SPECTreconstructions which included attenuation correction, regardless of whether or not scatter correction was used and the number of iterations.Conclusion: Functional volume segmentation is sensitive to the type of attenuation and scatter correction and number of iterations. In contrast, for imagesreconstructed with attenuation correction, the SWMD calculation produces consistent results and appears to be a more robust choice to be used in future studies incorporating SPECT into treatment planning.

SU‐GG‐T‐427: Dose Painting to Combat Tumor Hypoxia While Sparing Urethra in Prostate IMRT: A Biologically‐Based Adaptive Approach Accounting for Setup Uncertainties and Organ Motion

Purpose: To explore means of incorporating prostate motion and setup uncertainties into biologically‐based adaptive IMRT optimization for Volumetric Modulated Arc Therapy (VMAT). Method and Materials:IMRT planning objectives were set to produce a dose distribution, which, after accounting for setup uncertainties and prostate motion, meets the following requirements. 1) The CTV is covered with the desired dose; 2) Hypoxia volumes arbitrarily drawn inside CTV are boosted; 3) Desired urethra sparing is achieved. The objective function was based on equivalent uniform dose (EUD). Repositioning data from five prostate patients with implanted fiducial markers were used. Systematic and random uncertainties for each patient were incorporated into an in‐house treatment planning platform for VMAT. During the optimization, the dose matrix was shifted based on the systematic error, then convolved with a pre‐calculated Gaussian kernel to account for random errors. For each of these patients seven plans were generated using uncertainty data accumulated in 5 fraction increments. Results: The urethra EUD increased by >5Gy when the dose distribution from the static plan was propagated to account for geometrical uncertainties. In contrast, if uncertainties were accounted for in planning, urethra EUD was reduced without compromising CTV coverage (EUD decreased <1.5Gy). For four of five patients, the repositioning data from the first five fractions was sufficient to account for uncertainty in planning. In the proposed biologically‐based optimization inclusive of uncertainties, the PTV is not required. Appropriate dose coverage of the CTV is intrinsically produced by the optimization procedure. Because this coverage is typically tight and patient specific, improved normal tissue sparing was achieved. The rectal EUD was lowered by up to 23Gy compared to static plans that utilize the generic PTV concept. Conclusion: Our approach allows the inclusion of geometrical uncertainties into biologically‐based optimization of IMRT. Partial uncertainty data are sufficient to produce an optimal plan.