Igor J. Barani

SU-FF-J-85: Inter-Observer Variation In The Planning Of Head/Neck Radiotherapy

Purpose: Understanding inter‐observer variation is not only important for evaluating deformable image registration (DIR) which can be used to automatically transform the manual contours of a treatment plan into daily contours facilitating offline or online adaptive radiotherapy, but also helps physicians set reasonable margins on the regions of interest (ROI) when they make treatment plans. The purpose of this study is to determine the observer variation for Head/Neck patients using the implemented radial‐difference approach and the common‐area method to study the observer‐drawn contour pairs. Method and Materials: Two sets of images for each of seven head and neck patients were taken several weeks apart, and the original image sets were used for treatment plans. For each image set, the ROIs: Primary GTV, Nodal GTV, Cord, left Parotid, and right Parotid were contoured by three experienced physicians. The difference of two contours drawn by different physicians was measured using: (1) the radial difference of the paired contours where the center‐mass of a contour is used as the origin of the polar coordinate system, and (2) the ratio of the common area of two contours over the total area of those contours, R=2[C1∩C2/(C1∪C2)]. Results for all the ROI are plotted in histograms to quantify the observer variation. Results: The radial differences for primary GTV, nodal GTV, cord, left parotid, and right parotid are 2.7 ± 2.0 mm, 2.4 ± 1.9 mm, 1.8 ± 0.8 mm, 2.6 ± 2.1 mm, and 2.5 ± 1.9 mm, respectively, and the values of R are 0.75 ± 0.14, 0.82 ± 0.11, 0.82 ± 0.07, 0.80 ± 0.10, and 0.80 ± 0.12, respectively. Conclusion: The inter‐observer variations in the planning of Head/Neck patients are about 3 mm for Cord and 5 mm for the others. These values place an upper limit on the accuracy of DIR algorithm.

SU-GG-T-412: Modeling the Dose Rate Effect On High Dose Rate (HDR) Accelerated Partial Breast Irradiation (APBI) Treatment

Purpose: The objective of this study is to quantify the loss of radiobiological effect during a protracted Partial Breast Irradiation treatment using HDR interstitial brachytherapy and the MammoSite balloon applicator. Method and Materials: Given the relatively short half‐life of Ir‐192, a range of dose rates are employed clinically. The position and size of target relative to radiation source differs for MammoSite and an interstitial implant thereby affecting the treatment time. Two treatment plans (one for each modality) were used to simulate the treatment delivery with variable source strengths (3–9 Ci) and treatment times (250–1,000 s). The radiobiological effect was quantified using the Biologically Effective Dose (BED) formalism for each voxel of the target. These values were then aggregated, thereby removing the dose non‐uniformity contribution, into an Equivalent Uniform Dose (EUBED). Two models were employed: BED0 that simply takes into account the total dose, fraction size and α/β ratio, and BED1 which accounts for repair and the delivery time sequence. The PTV for the MammoSite applicator was 86.3 cc, and that of the interstitial implant was 120.5 cc (16 catheters). Results: While the EUBED0 for both modalities, assuming uniform dose distributions is 62.9Gy (α/β=4Gy, fraction size=3.4Gy), the effect of non‐uniform distributions raises the EUBED0 to 74.6Gy and 76.0 Gy for the interstitial case and the MammoSite applicator, respectively. When repair and the delivery time sequence is considered, EUBED1 drops dramatically to 54.5Gy, 39.5Gy and 36.4Gy, for treatment times one, two and three times longer than the actual treatment time. Similar results were noted with the MammoSite treatment.Conclusion: There appears to be a significant loss of radiobiological effect with protracted HDR treatments. We are advocating recording of treatment times along with other dosimetric parameters as these may impact the clinical outcome.

TH‐C‐M100F‐08: HDR Brachytherapy and Online Image‐Guided Adaptive IMRT for Dose Escalation in Prostate Cancer: Comparison of Brachytherapy and IMRT Boosts

Purpose: A course of one to three large fractions of HDR interstitial brachytherapy (HIB) is an attractive alternative to intensity modulated radiation therapy(IMRT) for delivering boost dose to the prostate in combination with additional external beam irradiation for intermediate risk disease. The purpose of this work is to quantitatively compare single‐fraction HIB boost to biologically equivalent fractionated IMRT, assuming idealized image‐guideddelivery (igIMRT) and conventional delivery (cIMRT). Materials & Methods: For 9 prostate patients, both 7‐field IMRT and HIB boosts were planned. The Linear‐Quadratic model (alpha/beta = 3Gy) was used to compute biologically equivalent dose (BED) prescriptions: (a) HIB boost delivered 9Gy in a single fraction, (b) igIMRT boost delivered 20.25Gy to the CTV (prostate gland) in 9 fractions, (c) cIMRT boost delivered 20.25Gy in 9 fractions to the PTV (10mm expansion, 6mm posteriorily). The cIMRT plan was evaluated as a static plan, and with a simulated random and setup errors. The plan evaluation endpoints were: (a) for Tumor Control, the CTV Equivalent Surviving Fraction (ESD index) and (b) for Bladder and Rectum toxicity the Generalized Equivalent Uniform Dose (gEUD) and BED VH parameters. Results: HIB delivery produces outcomes comparable to or better than the idealized igIMRT delivery. On average, the ESD is 16% higher in the HIB delivery than it is in the IMRTdelivery. For the HIB, the bladder/rectal gEUD values are strongly influenced by high dose DVH tails. A saturation BED, beyond which no further injury can occur, must be assumed. The gEUD values in HIB delivery are comparable, or better, than those for the IMRTdelivery. Modeling of organ motion uncertainties yields mean outcomes similar to static plan outcomes. Conclusion: HIB offers therapeutic gains which exceed even the most optimum igIMRT.