Interfractional Anatomic Variations Research Articles

Investigation of interfractional range variation owing to anatomical changes with beam directions based on water equivalent thickness in proton therapy for pancreatic cancer.

To assess the interfractional anatomical range variations (ARVs) with beam directions and their impact on dose distribution in intensity modulated proton therapy, we analyzed water equivalent thickness (WET) from 10 patients with pancreatic cancer. The distributions of the interfractional WET difference ($\Delta{\mathrm{WET}}^{\theta }$) across 360° were visualized using polar histograms. Interfractional ARVs were evaluated using the mean absolute error and ΔWET pass rate, indicating the percentage of $\Delta \mathrm{WE}{\mathrm{T}}^{\theta }$ < thresholds. The impact on dose distribution in proton therapy was evaluated based on two treatment plans for 40Gy(RBE)/5 fractions: 'Plan A', using two beam angles, in which the target was closest to the body surface among four perpendicular directions; and 'Plan B', using two beam angles with small ARVs. Analysis revealed individual variations in angular trends of interfractional ARVs. Three distinct trends were identified: Group 1 exhibited small ARVs around posterior directions; Group 2 exhibited small ARVs except ~60°; Group 3 demonstrated minimal ARVs only ~90°. In dose evaluation, while 150° and 210° were selected in Plan B for 9 out of 10 patients, for the remaining patient, 60° and 90° were chosen. Comparing dose volume histogram parameters for all patients, Plan B significantly reduced target coverage loss while maintaining organ-at-risk sparing comparable to Plan A. These results demonstrated that selecting beam angles with small interfractional ARVs for each patient enhances the robustness of dose distribution, reducing target coverage loss.

Comparing Efficacy Between Robust and PTV Margin-based Optimizations for Interfractional Anatomical Variations in Prostate Tomotherapy.

Interfractional anatomical variations cause considerable differences between planned and actual radiotherapy doses. This study aimed to investigate the efficacy of robust and planning target volume (PTV) margin-based optimizations for the anatomical variations in helical tomotherapy for prostate cancer. Ten patients underwent treatment-planning kilovolt computed tomography (kVCT) and daily megavolt computed tomography (MVCT). Two types of nominal plans, with a prescription of 60 Gy/20 fractions, were created using robust and PTV margin-based optimizations on kVCT for each patient. Subsequently, the daily estimated doses were recalculated using nominal plans, and all available MVCTs modified the daily patient-setup errors. Due to the difference in dose calculation accuracy between kVCT and MVCT, three scenarios with dose corrections of 1, 2, and 3% were considered in the recalculation process. The dosimetric metrics, including target coverage with the prescription dose, Paddick's conformity index, homogeneity index, and mean dose to the rectum, were analyzed. A dosimetric comparison of the nominal plans demonstrated that the robust plans had better dose conformity, lower target coverage, and dose homogeneity than the PTV plans. In the daily estimated doses of any dose-corrected scenario, the target coverage and dose sparing to the rectum in the robust plans were significantly higher than those in the PTV plans, whereas dose conformity and homogeneity were identical to those of the nominal case. Robust optimization is recommended as it accounts for anatomical variations during treatment regarding target coverage in helical tomotherapy plans for prostate cancer.

Image guidance and interfractional anatomical variation in paediatric abdominal radiotherapy.

To identify variables predicting interfractional anatomical variations measured with cone-beam CT (CBCT) throughout abdominal paediatric radiotherapy, and to assess the potential of surface-guided radiotherapy (SGRT) to monitor these changes. Metrics of variation in gastrointestinal (GI) gas volume and separation of the body contour and abdominal wall were calculated from 21 planning CTs and 77 weekly CBCTs for 21 abdominal neuroblastoma patients (median 4 years, range: 2 - 19 years). Age, sex, feeding tubes, and general anaesthesia (GA) were explored as predictive variables for anatomical variation. Furthermore, GI gas variation was correlated with changes in body and abdominal wall separation, as well as simulated SGRT metrics of translational and rotational corrections between CT/CBCT. GI gas volumes varied 74 ± 54 ml across all scans, while body and abdominal wall separation varied 2.0 ± 0.7 mm and 4.1 ± 1.5 mm from planning, respectively. Patients < 3.5 years (p = 0.04) and treated under GA (p < 0.01) experienced greater GI gas variation; GA was the strongest predictor in multivariate analysis (p < 0.01). Absence of feeding tubes was linked to greater body contour variation (p = 0.03). GI gas variation correlated with body (R = 0.53) and abdominal wall (R = 0.63) changes. The strongest correlations with SGRT metrics were found for anterior-posterior translation (R = 0.65) and rotation of the left-right axis (R = -0.36). Young age, GA, and absence of feeding tubes were linked to stronger interfractional anatomical variation and are likely indicative of patients benefiting from adaptive/robust planning pathways. Our data suggest a role for SGRT to inform the need for CBCT at each treatment fraction in this patient group. This is the first study to suggest the potential role of SGRT for the management of internal interfractional anatomical variation in paediatric abdominal radiotherapy.

A novel TPS toolkit to assess correlation between transit fluence dosimetry and DVH metrics for adaptive head and neck radiotherapy.

Inter-fractional anatomical variations in head and neck (H&N) cancer patients can lead to clinically significant dosimetric changes. Adaptive re-planning should thus commence to negate any potential over-dosage to organs-at-risk (OAR), as well as potential under-dosage to target lesions. The aim of this study is to explore the correlation between transit fluence, as measured at an electronic portal imaging device (EPID), and dose volume histogram (DVH) metrics to target and OAR structures in a simulated environment. Planning data of eight patients that have previously undergone adaptive radiotherapy for H&N cancer using volumetric modulated arc therapy (VMAT) at the Royal Adelaide Hospital were selected for this study. Through delivering the original treatment plan to both the planning and rescan CTs of these eight patients, predicted electronic portal images (EPIs) and DVH metrics corresponding to each data set were extracted using a novel RayStation script. A weighted projection mask was developed for target and OAR structures through considering the intra-angle overlap between fluence and structure contours projected onto the EPIs. The correlation between change in transit fluence and planning target volume (PTV) D98 and spinal cord D0.03cc with and without the weighting mask applied was investigated. PTV D98 was strongly correlated with mean fluence percentage difference both with and without the weighting mask applied (RMask = 0.69, RNo Mask = 0.79, N = 14, p < 0.05), where spinal cord D0.03cc exhibited a weak correlation (RMask = 0.35, RNo Mask = 0.53, N = 7, p > 0.05) however this result was not statistically significant. The simulation toolkit developed in this work provided a useful means to investigate the relationship between change in transit fluence and change in key dosimetric parameters for H&N cancer patients.

Dosimetric Uncertainties Resulting From Interfractional Anatomic Variations for Patients Receiving Pancreas Stereotactic Body Radiation Therapy and Cone Beam Computed Tomography Image Guidance

PurposeTo estimate the effects of interfractional anatomic changes on dose to organs at risk (OARs) and tumors, as measured with cone beam computed tomography (CBCT) image guidance for pancreatic stereotactic body radiation therapy. Methods and MaterialsWe evaluated 11 patients with pancreatic cancer whom were treated with stereotactic body radiation therapy (33-40 Gy in 5 fractions) using daily CT-on-rails (CTOR) image guidance immediately before treatment with breath-hold motion management. CBCT alignment was simulated in the treatment planning software by aligning the original planning CT to each fractional CTOR image set via fiducial markers. CTOR data sets were used to calculate fractional doses after alignment by applying the rigid shift of the planning CT and CTOR image sets to the planning treatment isocenter and recalculating the fractional dose. Accumulated dose to the gross tumor volume (GTV), tumor vessel interface, duodenum, small bowel, and stomach were calculated by summing the 5 fractional absolute dose-volume histograms into a single dose-volume histogram for comparison with the original planned dose. ResultsFour patients had a GTV D100% of at least 1.5 Gy less than the fractional planned value in several fractions; 4 patients had fractional underestimation of duodenum dose by 1.0 Gy per fraction. The D1.0 cm3 <35 Gy constraint was violated for at least 1 OAR in 3 patients, with either the duodenum (n = 2) or small bowel (n = 1) D1.0 cm3 being higher on the accumulated dose distribution (P = .01). D100% was significantly lower according to accumulated dose GTV (P = .01) and tumor vessel interface (P = .02), with 4 and 2 patients having accumulated D100% ≥4 Gy lower than the planned value for the GTV and tumor vessel interface, respectively. ConclusionsFor some patients, CBCT image guidance based on fiducial alignment may cause large dosimetric uncertainties for OARs and target structures, according to accumulated dose.

4D strategies for lung tumors treated with hypofractionated scanning proton beam therapy: Dosimetric impact and robustness to interplay effects

PurposeTo investigate the impact of four-dimensional robust optimization (4DRO) on dose delivered to lung cancer patients in pencil beam scanning proton therapy. Methods and materials2 strategies were compared for 20 lung cancer patients, using a different number of breathing phases of the reconstructed 4D computed tomography (CT) included in the plan optimization problem. In the restricted approach combined with gating, only 3 phases close to reference end-exhale were considered instead of the whole breathing cycle. The prescribed dose was 60 Gy(RBE) in 10 fractions. Target coverage (D98%) and dose to healthy tissues were evaluated using Wilcoxon signed-rank test.To assess the robustness against interfractional anatomical and respiratory variations, the optimized plans were recalculated on re-evaluation 4DCTs.To compare the sensitivity of both strategies to interplay effects, we implemented an end-to-end test with a home-made heterogeneous moving phantom and ionization chambers measurements. Robustly optimized plans with prescription doses of 6 Gy(RBE) were delivered in different dynamic conditions. ResultsBoth 4D robustly optimized plans reached the same target coverage (p = 0.56), while a statistically significant decrease of the homolateral lung dose was observed using the restricted approach (p < 0.0001).Plan recalculations within 15 days from the treatment simulation showed the same robustness of target D98% against interfractional variations (p = 0.48), with an average decrease of approximately 3 Gy(RBE).Phantom measurements confirmed the delivery accuracy of the restricted approach (mean dose deviations <5%). Higher deviations were found for ungated full 4DRO and larger motion amplitude. ConclusionThe restricted approach combined with gating improved normal tissue sparing and was shown to be more robust to single fraction deliveries and large motion amplitude.

Dosimetric Impact of Interfractional Variations for Post-prostatectomy Radiotherapy to the Prostatic Fossa-Relevance for the Frequency of Position Verification Imaging and Treatment Adaptation.

Background and purpose: To analyze divergences between the planned and applied treatment doses for post-prostatectomy radiotherapy to the prostatic fossa on a voxel-by-voxel basis based on interfractional anatomic variations and imaging frequency.Materials and methods: For 10 patients receiving intensity-modulated postoperative radiotherapy to the prostatic fossa, position verification was carried out by daily in-room CT imaging in treatment position (340 fraction CTs). Applied fraction doses were recalculated on daily CT scans, and treatment doses were accumulated on a voxel-by-voxel basis after deformable image registration. To simulate weekly imaging, derived weekly position correction vectors were used to rigidly register all daily scans of the respective treatment week onto the planning CT before dose accumulation. Detailed dose statistics of the prescribed and applied treatment doses were compared in relation to the frequency of position verification imaging. Derived NTCP and Pinjury values were calculated for the rectum and bladder.Results: Despite a large variability in the pelvic anatomy, daily CT-based patient repositioning resulted in largely negligible deviations of the analyzed dose-volume, conformity, and uniformity parameters from the planned doses for post-prostatectomy radiotherapy, and only the bladder exhibited significant increases in the accumulated mean and median doses. Derived NTCP for the applied doses to the rectum and bladder and Pinjury values did not significantly deviate from the treatment plan. In contrast, weekly CT-based repositioning resulted in significant decreases of the PTV coverage and dose conformity as well as large deviations of the applied doses to the rectum and bladder from the planned doses. Consecutively, NTCP for the rectum and Pinjury were found falsely reduced for weekly patient repositioning.Conclusions: Our data indicate for the first time in a voxel-by-voxel analysis that daily imaging is required for reliable adaptive delivery of intensity-modulated radiotherapy to the prostatic fossa. This work will help guiding adaptive treatment strategies for post-prostatectomy radiotherapy.

End-to-end test of an online adaptive treatment procedure in MR-guided radiotherapy using a phantom with anthropomorphic structures

Online adaptive treatment procedures in magnetic resonance (MR)-guided radiotherapy (MRgRT) allow compensating for inter-fractional anatomical variations in the patient. Clinical implementation of these procedures, however, requires specific end-to-end tests to validate the treatment chain including imaging, treatment planning, positioning, treatment plan adaption and accurate dose delivery. For this purpose, a new phantom with reproducibly adjustable anthropomorphic structures has been developed. These structures can be filled either with contrast materials providing anthropomorphic image contrast in MR and CT or with polymer dosimetry gel (PG) allowing for 3D dose measurements. To test an adaptive workflow at a 0.35 T MR-Linac, the phantom was employed in two settings simulating inter-fractional anatomical variations within the patient. The settings included two PG-filled structures representing a tumour and an adjacent organ at risk (OAR) as well as five additional structures. After generating a treatment plan, three irradiation experiments were performed: (i) delivering the treatment plan to the phantom in reference setting, (ii) delivering the treatment plan after changing the phantom to a displaced setting without adaption, and (iii) adapting the treatment plan online to the new setting and delivering it to the phantom. PG measurements revealed a homogeneous tumour coverage and OAR sparing for experiment (i) and a significant under-dosage in the PTV (down to 45% of the prescribed dose) and over-dosage in the OAR (up to 180% relative to the planned dose) in experiment (ii). In experiment (iii), a uniform dose in the PTV and a significantly reduced dose in the OAR was obtained, well-comparable to that of experiment (i) where no adaption of the treatment plan was necessary. PG measurements were well comparable with the corresponding treatment plan in all irradiation experiments. The developed phantom can be used to perform end-to-end tests of online adaptive treatment procedures at MR-Linac devices before introducing them to patients.

Locally Advanced Pancreatic Cancer Treated with SBRT: Interfractional Bowel Movements, Measured Dose, and Acute Toxicity

To analyze dosimetric impact of inter-fractional anatomic variations in patients with locally advanced pancreatic cancer (PC) treated with stereotactic radiotherapy (SBRT) using a diagnostic in-room CT with a linear accelerator and to correlate OAR (Organ at Risk) volumes with acute toxicity in a prospective phase 2 study. Thirty-five PC patients treated with SBRT (40 Gy/5 fr) underwent a CT scan in treatment position before each of the first three fractions of the treatment. OAR (stomach, duodenum, and bowel) and a summation organ (Gastro-Intestinal Organ, GIO) were manually delineated. Image-based matching with planning CT and dose distribution overlay was performed. The planned dose or volume of the OAR was compared to the mean dose or volume of the OAR of the 3 daily fractions (fraction dose or volume). Acute toxicity was scored with CTCAE v4.03. Median delivered fraction D2 and Dmax of GIO increased in median of 5% and 13 % compared to planned values, respectively. Median planned V35 of GIO was 0.2 cc and increased to a median fraction volume of 1.6 cc. The largest dose increase was seen in the stomach and duodenum. No grade 3-5 acute toxicity was reported. Acute toxicity consisted of grade ≤2 nausea, pain and diarrhea in 11 (31%), 15 (43%) and 4 (11%) patients, respectively. Nausea was related to increased daily V35 (mean 1.2 vs 0.6 cc, p=0.025) and D10 (mean 26.8 vs 19.8 Gy, p=0.01) in duodenum, and to D10 in GIO (30.9 vs 26.3 Gy, p=0.01). Higher D10 in GIO (23.4 vs 15.3 Gy, p=0.034) was found in patients with diarrhea. Interfractional anatomic variations led to a significant increase in dose to the OAR. No grade 3-5 acute toxicity was found. Acute toxicity was significantly associated with V35, and D10 of the Organs at Risk. Daily imaging using integrated CT/Linac should be warranted future evaluation for clinical application.Abstract SU_18_2179; Table 1V35 (in cc)D2 (in Gy)D10 (in Gy)Dmax (in Gy)Duodenummedian planned (IQR)0.0 (0.0-1.7)30.7 (24.3-32.8)24.0 (14.2-27.1)36.1 (34.2-37.2)median daily (IQR)0.4 (0.0-3.2)32.3 (24.7-34.0)23.1 (15.8-28.2)39.7 (37.0-43.0)median difference (IQR)+0.3 (-0.2-3.9)+1.8% (0.0-9.4)+2.3% (-1.4-10.0)+11.2% (3.6-16.0)Stomachmedian planned (IQR)0.0 (0.0-0.3)27.6 (21.1-32.7)20.6 (14.9-27.2)36,6 (32.7-37-7)median daily (IQR)0.5 (0.0-1.0)29.2 (24.9-33.6)21.6 (17.1-27.1)38.9 (35.7-40.8)median difference (IQR)+0.2 (-0.1-9.6)+2.6% (-2.4-12.7)0.0% (-7.0-11.2)+5.9% (1,1-11,6)Bowelmedian planned (IQR)0.0 (0.0-0.1)19.6 (14.0-28.7)15.6 (10.1-22.5)31.7 (21.2-36.3)median daily (IQR)0.0 (0.0-0.2)20.9 (14.9-27.6)16.1 (9.9-21.0)32.3 (23.4-38.8)median difference (IQR)0.0 (0.0-0.2)+1.0% (-10.8-10.6)-2.7% (-16.0-28.8)+2.6% (-8.7-21.0)GIOmedian planned (IQR)0.2 (0.0-0.9)33.3 (30.7-34.0)29.1 (20.8-30.8)37.2 (36.4-40.4)median daily (IQR)1.6 (0.6-3.0)34.6 (31.1-36.2)29.8 (22.6-31.7)42.2 (41.0-44.7)median difference (IQR)1.0 (0.2-2.6)+4.4% (0.4-10.8)+3.3% (-0.4-7.1)+12.0% (5.0-18.9) Open table in a new tab

EP-2042: Use of on-treatment EPID images to detect inter-fractional anatomical variations

EP-2042: Use of on-treatment EPID images to detect inter-fractional anatomical variations

SU‐F‐J‐81: Evaluation of Automated Deformable Registration Between Planning Computed Tomography (CT) and Daily Cone Beam CT Images Over the Course of Prostate Cancer Radiotherapy

Purpose:To compute daily dose delivered during radiotherapy, deformable registration needs to be relatively fast, automated, and accurate. The aim of this study was to evaluate the performance of commercial deformable registration software for deforming between two modalities: planning computed tomography (pCT) images acquired for treatment planning and cone beam (CB) CT images acquired prior to each fraction of prostate cancer radiotherapy.Methods:A workflow was designed using MIM Software™ that aligned and deformed pCT into daily CBCT images in two steps: (1) rigid shifts applied after daily CBCT imaging to align patient anatomy to the pCT and (2) normalized intensity‐based deformable registration to account for interfractional anatomical variations. The physician‐approved CTV and organ and risk (OAR) contours were deformed from the pCT to daily CBCT over the course of treatment. The same structures were delineated on each daily CBCT by a radiation oncologist. Dice similarity coefficient (DSC) mean and standard deviations were calculated to quantify the deformable registration quality for prostate, bladder, rectum and femoral heads.Results:To date, contour comparisons have been analyzed for 31 daily fractions of 2 of 10 of the cohort. Interim analysis shows that right and left femoral head contours demonstrate the highest agreement (DSC: 0.96±0.02) with physician contours. Additionally, deformed bladder (DSC: 0.81±0.09) and prostate (DSC: 0.80±0.07) have good agreement with physician‐defined daily contours. Rectum contours have the highest variations (DSC: 0.66±0.10) between the deformed and physician‐defined contours on daily CBCT imaging.Conclusion:For structures with relatively high contrast boundaries on CBCT, the MIM automated deformable registration provided accurate representations of the daily contours during treatment delivery. These findings will permit subsequent investigations to automate daily dose computation from CBCT. However, improved methods need to be investigated to improve deformable results for rectum contours.

A Prospective Comparison of the Effects of Interfractional Variations on Proton Therapy and Intensity Modulated Radiation Therapy for Prostate Cancer

To quantify and compare the impact of interfractional setup and anatomic variations on proton therapy (PT) and intensity modulated radiation therapy (IMRT) for prostate cancer. Twenty patients with low-risk or intermediate-risk prostate cancer randomized to receive passive-scattering PT (n=10) and IMRT (n=10) were selected. For both modalities, clinical treatment plans included 50.4 Gy(RBE) to prostate and proximal seminal vesicles, and prostate-only boost to 79.2 Gy(RBE) in 1.8 Gy(RBE) per fraction. Implanted fiducials were used for prostate localization and endorectal balloons were used for immobilization. Patients in PT and IMRT arms received weekly computed tomography (CT) and cone beam CT (CBCT) scans, respectively. The planned dose was recalculated on each weekly image, scaled, and mapped onto the planning CT using deformable registration. The resulting accumulated dose distribution over the entire treatment course was compared with the planned dose using dose-volume histogram (DVH) and γ analysis. The target conformity index remained acceptable after accumulation. The largest decrease in the average prostate D98 was 2.2 and 0.7 Gy for PT and IMRT, respectively. On average, the mean dose to bladder increased by 3.26 ± 7.51 Gy and 1.97 ± 6.84 Gy for PT and IMRT, respectively. These values were 0.74 ± 2.37 and 0.56 ± 1.90 for rectum. Differences between changes in DVH indices were not statistically significant between modalities. All volume indices remained within the protocol tolerances after accumulation. The average pass rate for the γ analysis, assuming tolerances of 3 mm and 3%, for clinical target volume, bladder, rectum, and whole patient for PT/IMRT were 100/100, 92.6/99, 99.2/100, and 97.2/99.4, respectively. The differences in target coverage and organs at risk dose deviations for PT and IMRT were not statistically significant under the guidelines of this protocol.

Assessment and management of interfractional variations in daily diagnostic‐quality‐CT guided prostate‐bed irradiation after prostatectomy

To quantify interfractional anatomic variations and limitations of the current practice of image-guided radiation therapy (IGRT) for prostate-bed patients and to study dosimetric benefits of an online adaptive replanning scheme that addresses the interfractional variations. Contours for the targets and organs at risk (OARs) from daily diagnostic-quality CTs acquired with in-room CT (CTVision, Siemens) were generated by populating the planning contours using an autosegmentation tool based on deformable registration (ABAS, Elekta) with manual editing for ten prostate-bed patients treated with postoperative daily CT-guided IMRT. Dice similarity coefficient (DSC) obtained by maximizing the overlap of contours for a structure between the daily and plan contours was used to quantify the organ deformation between the plan and daily CTs. Three interfractional-variation-correction schemes, the current standard practice of IGRT repositioning, a previously developed online adaptive RT (ART), and the full reoptimization, were applied to these daily CTs and a number of dose-volume quantities for the targets and organs at risk were compared for their effectiveness to account for the interfractional variations. Large interfractional organ deformations in prostate-bed irradiation were seen. The mean DSCs for CTV, rectum, and bladder were 86.6 ± 5.1% (range from 61% to 97%), 77.3% ± 7.4% (range from 55% to 90%), and 75.4% ± 11.2% (range from 46% to 96%), respectively. The fractional and cumulative dose-volume quantities for CTV and PTV: V100 (volume received at least 100% prescription dose), and rectum and bladder: V45Gy and V60Gy (volume received at least 45 or 60 Gy), were compared for the repositioning, adaptive, reoptimization, and original plans. The fractional and cumulative dosimetric results were nearly the same. The average cumulative CTV V100 were 88.0%, 98.4%, 99.2%, and 99.3% for the IGRT, ART, reoptimization, and original plans, respectively. The corresponding rectal V45Gy (V60Gy) were 58.7% (27.3%), 48.1% (20.7%), 43.8% (16.1%), and 44.9% (16.8%). The results for bladder were comparable among three schemes. Paired two-tailed Wilcoxon signed-rank tests were performed and it was found that ART and reoptimization provide better target coverage and better OAR sparing, especially rectum sparing. The interfractional organ motions and deformations during prostate-bed irradiation are significant. The online adaptive replanning scheme is capable of effectively addressing the large organ deformation, resulting in cumulative doses equivalent to those originally planned.

SU-E-J-11: Characterization of Interfractioanal Anatomical Variations in Post-Operative Radiation Therapy for Prostate Bed.

To quantitatively characterize the interfractional anatomic variations in post-operative radiation therapy (RT) for prostate bed, so that appropriate strategy that can fully address these variations can be developed. A total of 102 daily pre-treatment CT acquired using an in-room CT (CTVision, Siemens) for 10 patients treated with post-operative IG-IMRT of prostate bed. Prior to each fraction, patients were repositioned to correct for interfractional translational shifts based on the alignment of both anterior rectal wall and surgical clips between the daily CT and the planning CT. The PTV was assumed to be the volume enclosed by the 95% prescription isodose surface generated for the conventional four field box. Contours of the PTV, rectum and bladder on each daily CT were generated by populating the planning contours using an auto-segmentation tool based on deformable registration (ABAS, Elekta) with manual editing. Interfrcational variations in the volumes, shapes and positions of these contours were obtained. The displacement of the center of mass (DCOM) with respect to the isocenter was used to measure interfractional organ motion, and the maximum overlap rate (MOR) was used to measure organ deformation. Interfractional variations in the volumes of rectum and bladder were in the range of 50-270% (average 116±41%) and 30-180% (average 67±26%), respectively. The averages of DCOM for rectum are: - 0.35±0.46cm (lateral, varying from -1.58 to 1.23cm), -0.33±0.99cm (longitudinal, varying from -2.4 to 1.8cm), and -3.41±1.14cm (vertical, varying from -5.56 to -0.99cm). These values for bladder are: -0.20±0.50cm (lateral, varying from -1.48 to 0.61cm), 3.63±1.10cm (longitudinal, varying from -5.7 to 3.1cm), and -0.31±0.97cm (vertical, varying from -1.68 to 2.77cm). Large interfractional changes in organ volumes, shapes and positions are seen in post-operative RT for prostate bed. These changes cannot be accounted for by the current standard practice of IGRT repositioning. This work is supported partially by MCW Cancer Center Fotsch Foundation.

Characterization and Management of Interfractional Anatomic Changes for Pancreatic Cancer Radiotherapy

To quantitatively characterize interfractional anatomic variations in pancreatic cancer radiotherapy (RT) and to study dosimetric advantages for using an online adaptive replanning scheme to account for these variations. Targets and organs at risk (OAR) were delineated by autosegmentation based on daily computed tomography (CT) images acquired using a respiration-gated in-room CT during daily image-guided RT (IGRT) for 10 pancreatic cancer patients. Various parameters, including the maximum overlap ratio (MOR) between the volumes based on planning and daily CTs for a structure, while the overlapping volumes were maximized, were used to quantify the interfractional organ deformation with the intrafractional variations largely excluded. An online adaptive RT (ART) was applied to these daily CTs. To evaluate the dosimetric benefits of ART, the dose distributions from the online ART were compared to those from the repositioning in the current standard IGRT practice. The interfractional anatomic variations, particularly the organ deformation, are significant during pancreas irradiation. For the patients studied, the average MORs of all daily CTs were 80.2%, 61.7%, and 72.2% for pancreatic head, duodenum, and stomach, respectively. The online ART leads to improved dosimetric plan with better target coverage and/or OAR sparing than IGRT repositioning. For the patients studied, the mean V(50.4 Gy) (volume covered by 50.4 Gy) for the duodenum was reduced from 43.4% for IGRT to 15.6% for the online ART scheme. The online adaptive RT scheme can effectively account for the significant interfractional anatomic variations observed in pancreas irradiation. The dosimetric advantages with the online ART may enable safe dose escalation in radiation therapy for pancreatic cancer.

Evaluation of the dosimetric impact of interfractional anatomical variations on prostate proton therapy using daily in‐room CT images

To quantify interfractional anatomical variations and their dosimetric impact during the course of fractionated proton therapy (PT) of prostate cancer and to assess the robustness of the current treatment planning techniques. Simulation and daily in-room CT scans from ten prostate carcinoma patients were analyzed. PT treatment plans (78 Gy in 39 fractions of 2 Gy) were created on the simulation CT, delivering 25 fractions to PTV1 (expanded from prostate and seminal vesicles), followed by 14 boost fractions to PTV2 (expanded from prostate). Plans were subsequently applied to daily CT, with beams aligned to the prostate center in the sagittal plane. For five patients having a sufficiently large daily imaging volume, structure contours were manually drawn, and plans were evaluated for all CT sets. For the other five patients, the plans were evaluated for six selected fractions. The daily CT was matched to the simulation CT through deformable registration. The registration accuracy was validated for each fraction, and the three patients with a large number of accurately registered fractions were used for dose accumulation. In individual fractions, the coverage of the prostate, seminal vesicles, and PTV1 was generally maintained at the corresponding prescription dose. For PTV2, the volume covered by the fractional prescription dose of 2 Gy (i.e., V2) was, on average, reduced by less than 3% compared to the simulation plan. Among the 225 (39 x 5 + 6 x 5) fractions examined, 15 showed a V2 reduction larger than 5%, of which ten were caused by a large variation in rectal gas, and five were due to a prostate shift in the craniocaudal direction. The fractional dose to the anterior rectal wall was found to increase for one patient who had large rectal gas volume in 25 of the 39 fractions, and another who experienced significant prostate volume reduction during the treatment. The fractional bladder dose generally increased with decreasing fullness. In the total accumulated dose for the three patients after excluding a few fractions with inaccurate registration due to a large amount of rectal gas (a condition inconsistent with RTOG protocol), 98.5%, 96.6%, and 98.2% of the PTV2 received the prescription dose of 78 Gy. The V75 and V70 of the anterior rectal wall and bladder both remained within tolerance. The results confirm that the PT planning techniques and dose constraints used at our institution ensure that target coverage to the prescription dose is maintained in the presence of interfractional anatomical variations. Dose coverage in individual fractions can be compromised, and normal tissue dose increased, due to deviations in the bladder and rectal volume compared to the simulation plans or progressive changes in the prostate volume during the treatment. Deviations from the plan can be reduced with efforts aimed at maintaining consistent daily patient anatomy.

SU-E-T-599: Distal and Proximal Margin Requirements for Intensity Modulated Proton Therapy of Nodal Boost during Cervical Cancer Treatment Using Repeat In-Room CT Imaging

Purpose: Women with intact cervical cancer with involved pelvic lymph nodes require additional boost radiation therapy to involved lymph nodes following pelvic irradiation. These boosts are particularly challenging to plan due to the need to avoid central structures, which are treated to organ tolerance with intracavitary brachytherapy. A single posterior (PA) proton beam may be an ideal treatment technique for nodal boost, which can spare the rectum, bladder and small bowel. However, appropriate margins to account for setup error and range uncertainty are unknown in this setting. The purpose of this study was to determine the margins required to account for interfractional anatomical variations and setup uncertainties during intensity modulated proton treatment (IMPT). Methods: Five patients treated with definitive radiation therapy for cervical cancer were used retrospectively for this study. All patients received 4–5 weekly CTs during treatment. External iliac nodes were contoured by a radiation oncologist on each planning image. Rigid alignments using bony structures near the external iliac nodes are used. A single IMPT PA-beam was simulated. Ray- trace methods were used to determine the water equivalent thickness (WET) of each ray parallel to the beam path. The resulting 2D-WET matrixes were then mapped back to the planning CT image and converted back to physical depth to determine the actual margin required to cover the target. Results: Distal margins are represented by a positive number while the opposite is true for proximal margins. The average margin mean for all patients is - 0.7mm (Range −2 to 5mm). The average variation for all patients is 6mm (Range 2 to 12mm). Maximum distal and proximal margins needed to cover all fractions were 8 and 9mm, respectively Conclusions: We determined that a ∼9mm margin would be sufficient for nodal boost IMPT, which produced excellent dosimetric results, compared to corresponding photon plans. Supported in part by NCI P01 CA021239

Characterization of Interfractional Anatomic Variations in Abdomen

Characterization of Interfractional Anatomic Variations in Abdomen

SU-GG-T-454: Dosimetric Impact of Inter-Fractional Variations in Proton Therapy of Prostate Cancer: Assessment of Dose Accumulation and Plan Robustness Using Daily In-Room CT Images

Purpose: To quantify the dosimetric impact of inter-fractional anatomical variations during the course of fractionated proton therapy of prostate cancer, and assess the robustness of the standard treatment planning techniques. Method and Materials: We used pre-treatment simulation and daily in-room CT from ten prostate carcinoma patients. Treatment plans for proton therapy (39 fractions, 2 Gy/fraction) were created, with 25 fractions targeting PTV50Gy (expanded from prostate and seminal vesicles to account for intra-fraction motion) followed by 14 boost fractions targeting PTV78Gy (expanded from prostate). The plan was then applied to daily CT, with beams aligned to the prostate center. For three patients having sufficient daily imaging volume, contours were drawn and plans were evaluated for all CT sets. For the other seven patients, the plans were evaluated at the beginning, mid and end point of the treatment course. The daily CT was registered to the simulation CT through deformable registration, and fractional doses were summed to yield the accumulated dose. Results: The fractional PTV78Gy coverage fluctuated within ∼6% around the planned value, mainly due to the deformation of prostate. Loss of coverage was observed in the AP and SI direction, when PTV78Gy deformed and approached the field edge. Lateral coverage was stable even in the presence of considerable anatomical variations. The accumulated PTV78Gy coverage was slightly lower than planned, but remained acceptable. Although bladder dose per fraction increased in all three patients due to reduced daily volume, the accumulated dose remained within tolerance. No increase in the rectum dose was observed, most likely due to the smearing of the daily high-dose volumes. Conclusion: The results confirm that standard proton therapy planning techniques are generally robust to inter-fractional anatomical variations. Deviations in the dose to normal organs can be reduced with efforts aimed to maintain consistent levels of rectal and bladder filling.