M. Sontag

The intrafraction motion induced dosimetric impacts in breast 3D radiation treatment: A 4DCT based study

The question remains regarding the dosimetric impact of intrafraction motion in 3D breast treatment. This study was conducted to investigate this issue utilizing the 4DCT scan. The 4D and helical CT scan sets were acquired for 12 breast cancer patients. For each of these patients, based on the helical CT scan, a conventional 3D conformal plan was generated. The breast treatment was then simulated based on the 4DCT scan. In each phase of the 4DCT scan, dose distribution was generated with the same beam parameters as the conventional plan. A software package was developed to compute the cumulative dose distribution from all the phases. Since the intrafraction organ motion is reflected by the 4DCT images, the cumulative dose computed based on the 4DCT images should be closer to what the patient received during treatment. Various dosimetric parameters were obtained from the plan and 4D cumulative dose distribution for the target volume and heart, and were compared to deduce the motion-induced impacts. The studies were performed for both whole breast and partial breast treatment. In the whole breast treatment, the average intrafraction motion induced changes in D95, D90, V100, V95, and V90 of the target volume were -5.4%, -3.1%, -13.4%, -5.1%, and -3.2%, respectively, with the largest values at -26.2%, -14.1%, -91.0%, -15.1%, and -9.0%, respectively. Motion had little impact on the Dmax of the target volume, but its impact on the Dmin of the target volume was significant. For left breast treatment, the motion-induced Dmax change to the heart could be negative or positive, with the largest increase at about 6 Gy. In partial breast treatment, the only non-insignificant impact was in the Dmin of the CTV (ranging from -15.2% to 11.7%). The results showed that the intrafraction motion may compromise target dose coverage in breast treatments and the degree of that compromise was correlated with motion magnitude. However, the dosimetric impact of the motion on the heart dose may be limited.

Initial outcomes analysis for large multicenter integrated cancer network implementation of intensity modulated radiation therapy for breast cancer.

Abstract:u2002 To analyze the initial clinical outcomes for breast cancer patients treated with intensity‐modulated radiation therapy (IMRT) in a large integrated cancer center network. A total of 495 patients with breast cancer received IMRT following breast conserving surgery among nine cancer centers. Seven community cancer centers span a 100‐mile radial distance from the two central academic sites. All nine cancer centers followed the same clinical pathway guidelines for the radiotherapeutic management of breast cancer. IMRT planning for all centers was performed at one central location, D3 Advanced Radiation Planning Service. The median IMRT prescription dose was 50u2003Gy followed by a boost with median dose of 10u2003Gy. The median breast volume was 918u2003cm3. The median Dose Homogeneity Index (DHI) was 93%. The median % of ipsilateral lung volume receiving >20u2003Gy was 4.6%. For left breast IMRT, the median % heart volume receiving more than 5% of prescription dose was 13.1. There was no statistical difference in the mean DHI, heart and lung dose between the academic and community sites. For all patients, NCI CTC Grades 0,1,2,3 for acute skin erythema was 16%, 55%, 28%, and 1%, respectively. The rates of Grade 0,1,2,3 acute skin desquamation were 75%, 20%, 4%, and 1%, respectively. There was no statistically significant difference in acute skin toxicities (>grade 2) among the academic and community cancer centers. With centralized processes, IMRT can be safely and effectively delivered in a large health system with an admixture of academic and community centers but long‐term follow‐up is necessary.

SU‐FF‐J‐65: Feasibility Study of Management of Respiration Induced Target Motion for the Radiotherapy Treatment of Lung Cancer Patients in the Absence of a 4DCT Simulator

Purpose: Varians RPM™ system for respiration induced tumor motion management allows acquisition of CTimages and gated treatments under free breathing. 4DCT may not be possible because of lack availability of appropriate CT hardware or software. This study evaluated whether a breath hold CT scanning technique can be used as a substitute for a 4DCT scan. Materials and Methods: A 4DCT scan is obtained on a 4 slice GE Lightspeed™ scanner with the patient breathing freely and the respiratory period regulated using audiovisual cues from RPM™. Additional helical scans are obtained using an end inhalation or exhalation breath hold modified gating method (MGM). The PTV is drawn on the MGM scan(s) and for each of phase of the 4DCT scan. Comparison of target volume, centroid and extent of target volume is made between the MGM scan and the corresponding phase of the 4DCT scan. A treatment plan is developed using the MGM scan. Dose is recalculated using the 4DCT scan with the beams isocenter and apertures obtained from the MGM scan. DVH comparison is made. Results: 20 patients had both a 4DCT scan and at least one MGM scan. 8 patients exhibited respiration induced target motion of >5 mm during free breathing. Maximum target motion observed was 25 mm. For 14 end inhalation scans, 9 passed, 3 passed marginally, and 2 failed the equivalency tests to the corresponding 4DCT scan. For 18 end exhalation scans, 14 passed, 4 passed marginally, and 0 failed the equivalency tests to the corresponding 4DCT scan. Conclusion: All end exhalation breath hold scans are suitable substitutes for the corresponding phase 50 4DCT scan. However only 6/18 patients exhibited sufficient (>5 mm) respiration induced target motion on which to base any conclusions about the suitability of MGM. Conflict of Interest: Research supported by Varian Medical Systems.

SU-FF-J-03: 4DCT-Based Study of Lung Tumor Motion Reproducibility

Purpose: To investigate the reproducibility of lungtumors motions and their characteristics during the course of the treatment. Materials and Methods: Two 4DCT scans were obtained at an interval of about three weeks with a GE‐scanner and Varians RPM System under free breathing for 13 patients. Each respiration cycle had 10 phases. 14 GTVs and 10 lungs in all phases were contoured. Geometrical characteristics of these structures were obtained with Eclipse TPS for the motion reproducibility analysis. Results: The GTVs of the initial scans ranged from 2.0cc to 280.5cc with a median of 50.3cc. Their median relative change in the second scan was a 28% decrease. The 3D extent of the respiration‐induced motion of the GTVs centroids in the first scan ranged from 0.34cm to 1.78cm with a median of 0.90cm. For the second scan it was 0.13cm, 1.99cm, and 0.89cm, respectively. The largest motion was in a projection on a sagittal plane. The overall displacements of the GTVs edges exhibited same trend. These motion tracks from two scans overlapped along DICOM directions. The median change of phase 50 lung volumes in both scans relative to the tidal volume at the first scan was −12% for ipsilateral and −11% for contralateral lungs. The median tidal volume change between scans relative to the first scan volume was 8% for ipsilateral and 4% for contralateral lungs.Conclusion: Most of the patients results show reproducible patterns of motion. The hysteresis of the motion varies between scans. This might be due to GTV changes during the treatment. A large and anisotropic shape change perturbs the initial motion trajectory, however general patterns appear repetitive. The respiratory changes of lung volumes were similar. They are not indicative of the GTVs motion reproducibility.

TH‐C‐ValB‐08: Motion Induced Dosimetric Impacts in Breast 3D Radiation Treatment — a 4D CT Based Study

Purpose: In radiotherapytreatments of breast patients, respirations may introduce uncertainties in target and heart locations. This study is to investigate the dosimetric impacts of these uncertainties in breast radiation treatments.Method and Materials: A 4D CT scan and a conventional helical CT scan set were acquired on each of 7 left breast patients and 5 right breast patients. Using the helical CT scan, a conventional 3D conformal plan, consisting of two tangential beams, was generated per physicians evaluation and decision. The 4D CT scan set was divided into 10 phases over the respiratory cycle. On each phase, treatment target and heart were contoured. Dose distributions were generated using the same beams as in the conventional plan. Software was developed to compute the cumulative dose distribution (4D doses) from all the phases. This 4D CT image based cumulative dose distribution would be closer to that in reality with motions taken into account. Various dosimetric parameters were obtained for treatment target and heart from the conventional plan and from the 4D cumulative dose distributions and compared to deduce the motion induced dosimetric impacts in breast radiation treatments. Studies were performed for both whole and partial breast treatments.Results: For whole breast treatment, the motion induced changes in D95, Dmax, and Dmin of PTV were 0.88% ± 20%, −0.28 ± 0.65%, and −10.17% ± 47%, respectively. For left breast, the motion induced Dmax change in heart was 22% ± 48%. For partial breast treatments, the motion induced changes in V90 and Dmin of CTV were 1.6% ± 2.7% and 3% ± 4%, respectively. Conclusions: Breathing motion may cause cold spots in the whole breast treatment, and may compromise treatment quality for some patients. It may also increase heart maximum dose. However, for the partial breast treatment, the motion impact may be insignificant with properly selected margin size. Supported in part by Varian Medical Systems.

SU‐FF‐T‐156: Multi‐Institutional Retrospective Analysis of IMRT QA Measurements

Purpose: To review IMRT QA measurements from several of the 50+ institutions for which we provide IMRT treatment plans and determine if institutional, anatomic site, or measurement biases exist. Method and Materials: For each patient receiving IMRT, the treatment plan is delivered to a solid water phantom and the dose measured using a small volume ion chamber and with a single EDR film placed 1 cm above the chamber plane. Of the almost 3000 IMRT treatment plans calculated and delivered in 2004, more than 1000 random, de‐identified plans were reviewed. Ratios of chamber/calculated and film‐center/calculated doses were tabulated for six anatomic sites (breast, prostate, pelvis, head & neck, brain, and other). Film dose distributions were compared to calculations using one of several commercially available QA packages. Results: The institutions with the best results had average errors of less than ±0.5% (i.e. randomly distributed about zero) with standard deviations of 1.25–1.50%. A few centers had average errors and standard deviations approaching 3%, indicating a bias in which a systematic dose measurement error was found. Agreement between chamber and film center dose was also institution specific with the best results found for those centers that had the lowest errors compared to calculation. One institution had excellent agreement between chamber and calculation (−0.2±1.7%), but 2–3% lower film dose. Although exceptions were found, little variation in the agreement between chamber measurement and calculation occurred as a function of anatomical site. Conclusion: Since all treatment plans were calculated in one central location and many centers had excellent agreement between measurement and calculation, it is likely that the higher errors were due to measurement technique rather than errors in the dose calculation. Error was not anatomic site dependent possibly due to the purposeful placement of the ion chamber in a region of relatively uniform dose.

SU‐FF‐T‐169: The Use of Diode in In‐Vivo Dosimetry Quality Assurance in IMRT

Purpose: In IMRTtreatments, the ultimate QA procedure is to carry out in‐vivo dosimetry measurement to ensure the accuracies of both patient setup and beam delivery. This study was designed to explore the use of in‐vivo diode dosimetry measurement for QA of IMRTtreatments. Method and Materials:IMRT plans were generated based on a set of CT scans of a head & neck anthropomorphic phantom. Corresponding IMRT QA verification plans were also generated. Diode calibration readings (Rc) were obtained for each beam during the routine dose verification QA process. During verification, a diode was placed along the beam central axis on the surface of a flat QA phantom at the SSD specified in the QA plan. Radiation was delivered dynamically using the same dynamic MLC files that were to be used for the patient treatment. For in‐vivo measurements, the anthropomorphic phantom was setup according to the treatment plan. For each beam, a diode was placed along the central axis at the beam surface entry point. Radiation was then delivered according to the plan and the diode reading (Ri) was recorded. If both the setup and the beam delivery were correct, Ri should be in agreement with quantity Rc*fSSD within certain uncertainty (fSSD is SSD correction factor); otherwise, it would be an indication of incorrect patient setup or incorrect beam delivery. Results: It was found that the calibration diode readings followed the SSD inverse square law within an uncertainty of 0.4%. f ssD = ( SSS c SSD i ) 2 . The derived in‐vivo diode readings (Rc*fSSD) were in agreement with those measured ones within 3.6% for three beams at different gantry angles, with an average difference of 1.8%. Conclusion: With a proper calibration method, diode verification can be used relatively accurately for in‐vivo measurements to check on the accuracies of patient setup and beam delivery for IMRTtreatments.