K Stuhr

Visual sensations during megavoltage radiotherapy to the orbit attributable to Cherenkov radiation

During megavoltage photon and electron beam radiotherapytreatment involving the eye, patients commonly report visual sensations; “nerve stimulation” is the conventional explanation. We propose that the phenomenon can be attributed to Cherenkov radiation inside the eye. The threshold electron energy for Cherenkov radiation in water is 260 keV . The human retina is able to perceive approximately 5–14 visible photons in 0.001 s . A single 500 keV electron traversing 1 mm of water will induce nearly 15 Cherenkov visible range photons. We propose that a portal image involving the eye will produce sufficient Cherenkov radiation to be detected by the retina.

Rotational setup errors in pediatric stereotactic radiation therapy

PURPOSE Stereotactic radiation therapy (SRT) is an increasingly commonly used technique in children. The use of image guidance increases the ability to accurately position patients. With our robotic couch, rotational errors that can be corrected are limited to approximately 3 degrees. Given this limitation, we reviewed the rotational setup errors in our pediatric brain tumor population. METHODS AND MATERIALS We reviewed the rotational corrections for all pediatric (age ≤21 years old) patients treated at our facility from 2009 to 2011. We compared children <5 years old treated to children between 5 and 21 years old (≥5 years old). Also, we analyzed the effect of steroid use and trends in rotational errors over the treatment period in each age group. RESULTS The mean pitch, roll, and yaw rotational setup errors for younger children are -0.70 ± 2.60 degrees, -0.06 ± 1.89 degrees, and 0.69 ± 2.42 degrees, respectively; for children ≥5 years old, they are 0.46 ± 2.09 degrees, -0.06 ± 1.89 degrees, and 0.69 ± 2.42 degrees, respectively. The mean pitch corrections are larger for children <5 years old (P < .001) and the variance of the pitch, roll, and yaw corrections are all larger for children <5 years old (P < .001). The frequency of rotational errors above 3 degrees for pitch, roll, and yaw is 21.7%, 10.6%, and 20.9% for children <5 years old, and 15.6%, 2.1%, and 13.8% for children ≥5 years old. In both age groups, pitch and roll corrections were larger for children treated with steroids. CONCLUSIONS Rotational errors in our pediatric population occur more frequently than previously reported, and are more common in younger children and in children treated with steroids. These rotational set up errors may not be fully correctable due to mechanical and safety limitations. We have altered our planning and treatment process to better account for rotational errors in children receiving SRT.

WE‐G‐BRA‐07: Biological Modeling and Global Normal Lung Tissue Radiological Response in Patients Treated with Stereotactic Body Radiation Therapy for Lung Tumors

Purpose: To model the probability of normal lungtissue density change and obtain dose‐response curves restricted to observed change areas for patients with lungtumors who received stereotactic body radiation therapy(SBRT). Methods: 129 follow‐up CT scans for 36 patients who received SBRT between 2003 and 2009 with a maximum of 5 fractions and a median total dose of 54 Gy (range, 30–60) were analyzed. RT‐induced lung density changes were evaluated after fusion of planning CT scans with post‐RT follow‐up scans corresponding to interval periods of approximately 3, 6, 12, 18, and 24 months after treatment. Each follow‐up scan was manually reviewed and areas of visible density change were contoured. Scans were divided into regions receiving specific 10Gy dose‐bins. Voxel density changes belonging to contoured areas were averaged over each dose‐bin to obtain dose‐response curves (DRC). The occurrence of visible density change depending on the maximum dose to the tumor was also chosen as an end point to model the normal tissue complication probability (NTCP). Results: With an analysis restricted to area of visible change, the density change magnitude appeared to not depend much on the dose bin. The average density increase was smaller at 3 month (214HU), peaked at 6 months (322HU) and slightly decays thereafter without totally vanishing (300HU). This observed trend was similar using rigid or deformable registration. The maximum target dose for a 50% complication probability for changes >200HU was 80Gy according to the NTCP model. Conclusion: Although local correlations between dose and density increase have been reported for conventional and SBRTlungtreatments, our analysis revealed a relatively uniform and stronger density response, while temporal density changes followed a previously reported density change trend (Palma 2010, IJROBP). These discrepancies were likely due to the restriction of the analysis to areas with actual density increase.

MO-D-204B-02: Temporal Dose-Response of Normal Lung Tissue in Patients Treated with Stereotactic Body Radiation Therapy for Lung Tumors

Purpose: To study the temporal changes of normal lungtissuedose‐response for patients with lungtumors who received stereotactic body radiation therapy(SBRT).Methods: Between 2003 and 2009, 63 patients received a hypofractionated treatment with a maximum of 5 fractions and a median total dose of 54 Gy (range, 30–60). RT‐induced lung density changes were evaluated after fusion of planning CT scans with a maximum of 5 post‐RT follow‐up scans corresponding to interval periods of approximately 3, 6, 12, 18, and 24 months after treatment. Patient specific dose‐response curves (DRC) were obtained by averaging CT number changes for regions receiving the same dose at 10 Gy intervals. In a secondary analysis, SBRTdose schedules were converted into 2Gy‐equivalent using a biological equivalent dose (BED) model. Results:At 6 months, transient increases of up to 100% were observed compared to the pre‐treatment lung density. After 12 months, the density changes stabilized to less than 50% of pre‐treatment levels. Different evolutions of the lung density were observed for BED levels of 0–45 Gy and 45–110 Gy. For the late response, no significant increase of lung density was visible for doses below 45 Gy. Above this threshold, the dose‐density change was linearly increasing but not as rapidly as after 6 months. Conclusion: This analysis is the first report, to our knowledge, of SBRT‐induced lung density changes showing that the lung density temporal response varied significantly with the received dose. More than 90% of the lung volume experienced an acute response that reversed to nearly no late response, indicating that the lung morphology was mostly unaffected after 12 months. Since the acute response also partially reversed for the remaining 10%, SBRT was expected to only marginally affect long term lung function.

SU‐GG‐T‐239: Initial Experience of Patient Specific Rotational Quality Assurance for VMAT Using a Cylindrical Diodes Array Detector System

Purpose: Our recent success in commissioning volumetric modulated arc therapy (VMAT) on an Elekta Synergy accelerator calls for a convenient and suitable system for patient specific quality assurance. Traditional 2D diode array devices exhibit significant angular dependence under rotational beams. The newly available cylindrical diode array device ‐ ArcCHECK (Sun Nuclear Corp., FL) was designed specifically for rotational dosimetry by arranging diodes on a cylindrical plane. The purpose of this study is to evaluate the performance of the ArcCHECK device for VMAT. Results are compared with TomoTherapy QA. Method and Material: Both ArcCHECK and MapPHAN were commissioned on a Monaco treatment planning system (Version 2.0.3, CMS Inc., MO). Initial testing of the system involved four representative VMAT cases at various tumor sites: lung,liver, spine and prostate. The planned dose map on the diode plane was extracted from the QA plans for comparison with ArcCHECK measurements. Absolute differences between measured and planned doses were analyzed using 3% dose deviation and 3mm distance‐to‐agreement (DTA). For comparison, the selected plans were also calculated and delivered to a 2D diode array device (at 5cm effective depth) using MapCHECK (Sun Nuclear Corp., FL) integrated with a square solid water phantom (MapPHAN). Results from VMAT were compared with results from 14 QA plans from TomoTherapy. Results: Both diode array devices provide reasonable QA results for all cases. Doses measured with ArcCHECK agreed better with plans, with average passing rates and standard deviations of 98.3 ± 0.5% and 97.0 ± 1.8% for VMAT and Tomo respectively at all tumor sites. QA plans utilizing the 2D MapPHAN had average passing rates of 92.2 ± 2.4% (VMAT) and 90.0 ± 4.7% (Tomo). Conclusion: Compared to the 2D MapPHAN, ArcCHECK provided improved agreement between measured and calculated doses for rotational QA, and is recommended for rotational IMRT QA.

SU-GG-T-390: Dose to Proximal Bronchial Tree in Lung SBRT Treatments: Comparison of Pencil Beam and Monte Carlo Dose Distributions

Purpose: Increased dose to proximal bronchial tree (PBT) has been associated with higher incidence of toxicities in prior lungSBRT trials. In this study, the effect of pencil beam (PB) dose calculations on the PBT dose for targets within the PBT zone was evaluated using Monte Carlo (MC).Methods:LungSBRT plans with PTVs within the PBT zone were generated for 10 patients using a commercial treatment planningsoftware equipped with PB and MC algorithms. PBT within 2 cm of the PTV region was contoured on CT datasets for all patients. Treatment plans were generated using two different dose calculation methods: i) Pencil beam algorithm without inhomogeneity correction (PB‐IHC), and ii) PB with inhomogeneity correction (PB+IHC). Both plans for each patient had identical beam geometry, but different MLC shapes were used to achieve similar conformai dose coverage of PTV. All plans were normalized to have 98% of PTV receive 60Gy in 3 fx. Dose distribution for each plan was recalculated with MC by keeping the monitor units the same in respective plans. Dosimetric comparisons were performed between PB‐IHC, PB+IHC, and MC based dose distributions, where MC dose distributions were assumed to be the gold standard. Results: The MC mean values for PTVD95, PTVDmax, PBTDmean, and PBTDmax outside the PTV region were 67.7Gy, 83.1Gy, 38.5Gy, and 64.5Gy respectively. These values were higher than PB‐IHC by 9%±4.5%, 17%±9%, 19%±6%, and 16%±5.8%, and were lower than PB+IHC by 13%±4.7%, 9%±1.5%, 5%±1.8%, and 8%±2.4%, respectively. Conclusion: Compared to MC, PB+IHC overestimates the dose to both PTV and PBT, and PB‐IHC underestimates the dose to both PTV and PBT. Unrecognized dose hotspots in this region might influence the risk of toxicity to proximal airways. This issue should be considered when analyzing clinical toxicity data calculated using PB‐based algorithms without heterogeneity corrections.

SU‐FF‐J‐73: Targeting Accuracy Using Exac‐Trac® and Synergy® CBCT Image‐Guided Radiotherapy Systems for Lung Stereotactic Body Radiotherapy

Purpose: To analyze the targeting accuracy using Exac‐Trac® (BrainLAB) and Synergy® CBCT (Elekta) systems for lungSBRT, and discuss the difference between these two IGRT systems. Methods: The target localization for lung patients treated with SBRT on Novalis® with Exac‐Trac® system and on Synergy® with CBCT system was analyzed. Prior to each treatment, patients setup correction using Exac‐Trac® is performed by fusing (bone structure matching) two oblique x‐ray images with related DRRs computed from the planning CT, the correction using the CBCT system is by fusing (grey volume registration) CBCT with the planning CT. The position error is adjusted by shifting the table in lateral, longitudinal and vertical directions. If the shifted distance is >5mm in one direction, we usually repeat the image capturing and fusing procedures to double check the position error—the “second” shifting distance. In this study, we used the “second” shifting distance to analyze the targeting accuracy for both systems. Results: Sixteen and twenty‐nine setup correction cases were studied for Exac‐Trac® and Synergy® CBCT systems, respectively. The first shifted distances were in the ranges of 8.6–22.7mm and 3.5–18.3mm, with the average of 15.2mm and 8.2mm. The “second” shifting distances were in the ranges of 0.5–4.9mm and 0.6–3.4mm, with the averages of 2.8mm and 1.9mm, respectively. For both systems there was no correlation between the first shifted and the “second” shifting distances. The average position error after table shifting in Exac‐Trac® was larger than that in Synergy® CBCT system. We think the 3D volume registration is more practical or accurate than the 2D imaging matching for soft tissue localization. Conclusion: Using volume registration, Synergy® CBCT system is more accurate compared to Exac‐Trac® system in IGRT for lungSBRT. Detailed comparison and discussion of targeting accuracy for different tumor locations between the two systems will be reported.

SU‐FF‐T‐350: A Comparative Dosimetric Study of Three‐Dimensional Conformal, Dynamic Conformal Arc, and Intensity Modulated Radiation Therapy for Brain Tumor Treatment

Purpose: To investigate dosimetric differences among three‐dimensional conformal (3D‐CRT), dynamic conformal arc therapy (DCAT) and intensity modulated radiotherapy(IMRT) for braintumortreatment for a broad range of braintumor volumes and shapes in an effort to determine whether a preferred method can be identified based upon pre‐treatment characteristics. Method and Materials: Fifteen patients treated with Novalis were selected. We performed 3D‐CRT, DCAT and IMRT plans for all the cases. The beam numbers in 3D‐CRT or IMRT plans were the same as the arc numbers in the DCAT plans, and the gantry angle of each beam in 3D‐CRT or IMRT plans was the middle angle of each arc in the DCAT plans. The PTV margin was re‐chosen as 1mm, and the specific prescription dose was re‐set to 90% for all the plans. The target coverage at prescription dose (TV90%), conformity index (CI) and heterogeneity index (HI) were used to compare the different plans. V50% and V80% of the organs at risk (OAR) were also calculated. Results: For small braintumors (PTV⩽2cc), three dosimetric parameters had approximate values for both 3D‐CRT and DCAT plans ( TV 90% ¯ ∼93%, CI ¯ ∼1.7, HI ¯ ∼1.4) . The CI for IMRT plans was high ( CI ¯ =3) . For medium braintumors (2cc<PTV⩽100cc), the three plans were competitive with each other. IMRT plans had higher CI and better TV90% and HI. For large braintumors (PTV⩾100cc), IMRT plan had nearly perfect TV90% and HI and the approximate CI values as those in both 3D‐CRT and DCAT plans. Conclusions: DCAT is suitable for most cases in the treatment of braintumors. For a small target, 3D‐CRT is still useful, and IMRT is not recommended. For larger braintumors,IMRT is superior to 3D‐CRT, and very competitive in sparing critical structures near the target, especially for the treatment of a big braintumor.

SU‐FF‐T‐413: The Impact of Heterogeneity Correction On Tumor Dosimetry for Lung Cancer Stereotactic Body Radiation Therapy

Purpose: Stereotactic body radiation therapy(SBRT) for non‐small cell lungcancer has been shown to limit toxicity. Heterogeneity correction on lungcancerradiotherapy has not been recommended by RTOG. In this study, dosimetric difference between the SBRT plans with/without heterogeneity correction is analyzed. Method and Materials: Nine lungcancer patients treated with SBRT techniques using a 6 MV Novalis system were selected. Using the path length algorithm in BrainLAB treatment planning system, all the treatment plans were applied the heterogeneity correction. With same beam parameters, we performed the plans without the heterogeneity correction, and compared the dosimetric difference to the treatment plans. The heterogeneity correction factors (Kc) at iso‐center, target coverage, heterogeneity index (HI), and conformity index (CI) were used in the comparison. Results: The average of Kc at isocenter for 14 planning target volumes (PTV) was 1.002±0.02, only three Kc values (1.07, 0.985, 0.981) were relatively off to the average. Except one case, the other 13 target coverage values of the plans with the heterogeneity correction were better than those without the correction. The maximum difference of the target coverage was ∼7%. All the HI values of the plans with the heterogeneity correction were better than those without the correction. The maximum difference of the HI reached 300%. The difference of the CI between the compared plans was within ∼10%. The CIs of the plans without correction were better than those corrected. For one case — the tumor located in the lung base, the impact of heterogeneity correction was significant. Conclusion: The impact of heterogeneity correction for tumordosimetry on SBRT for lungcancer is case depended. For most primary lungtumors the difference between the plans with/without heterogeneity correction is clinically insignificant. For the tumors near the interface of different mass density, the heterogeneity correction is necessary.