T LaCouture

Dose tolerance limits and dose volume histogram evaluation for stereotactic body radiotherapy

Almost 20 years ago, Emami et al. presented a comprehensive set of dose tolerance limits for normal tissue organs to therapeutic radiation, which has proven essential to the field of radiation oncology. The paradigm of stereotactic body radiotherapy (SBRT) has dramatically different dosing schemes but, to date, there has still been no comprehensive set of SBRT normal organ dose tolerance limits. As an initial step toward that goal, we performed an extensive review of the literature to compare dose limits utilized and reported in existing publications. The impact on dose tolerance limits of some key aspects of the methods and materials of the various authors is discussed. We have organized a table of 500 dose tolerance limits of normal structures for SBRT. We still observed several dose limits that are unknown or not validated. Data for SBRT dose tolerance limits are still preliminary and further clinical trials and validation are required. This manuscript presents an extensive collection of normal organ dose tolerance limits to facilitate both clinical application and further research. PACS numbers: 87.53.Ly, 87.55.dk

Dosimetric investigation of accelerated partial breast irradiation (APBI) using CyberKnife

PURPOSE To investigate the dosimetric feasibility of accelerated partial breast irradiation (APBI) using CyberKnife. METHODS Fourteen previously treated patients with early-stage breast cancer were selected for a retrospective study. Six of these patients had been treated to 38.5 Gy in 10 fractions in a phase III accelerated partial breast trial and the rest of the patients were treated to 50.4 Gy in 28 fractions. In this planning study, the guidelines in the protocol for the phase III partial breast trial were followed for organ delineation and CyberKnife planning. The achievable dosimetric parameters from all CyberKnife plans were compared to Intensity-modulated radiation therapy (IMRT) and 3D-CRT methods. The reproducibility of the dose delivery with and without respiratory motion was assessed through delivering a patient plan to a breast phantom. Different dose calculation algorithms were also compared between ray tracing and Monte Carlo. RESULTS For all the patients in the study, the dosimetric parameters met the guidelines from the NSABP B39∕RTOG 0413 protocol strictly. The mean PTV volume covered by 100% of the prescription dose was 95.7 ± 0.7% (94.7%-97.1%). The mean maximal dose was 104 ± 2% of the prescription dose. The mean V(50%) and mean V(100%) to the ipsilateral normal breast were 23.1 ± 11.6% and 9.0 ± 5.8%, respectively. The conformity index of all plans was 1.14 ± 0.04. The maximum dose to the contralateral breast varied from 1.3 cGy to 111 cGy. The mean V(5%) and mean V(30%) to the contralateral and ipsilateral lungs were 1.0 ± 1.6% and 1.3 ± 1.2%, respectively. In our study, the mean V(5%) to the heart was 0.2 ± 0.5% for right-sided tumors and 9.4 ± 10.1% for left-sided tumors. Compared with IMRT and 3D-CRT planning, the PTV coverage from CyberKnife planning was the highest, and the ratio of V(20%) to V(100%) of the breast from CyberKnife planning was the smallest. The heart and lung doses were similar in all the techniques except that the V(5%) for the lung and heart in CyberKnife planning was slightly higher. CONCLUSIONS The dosimetric feasibility of APBI using CyberKnife was investigated in this retrospective study. All the dosimetric parameters strictly met the guidelines from the NSABP B39∕RTOG 0413 protocol. With advanced real-time tracking capability, CyberKnife should provide better target coverage and spare nearby critical organs for APBI treatment.

Low toxicity for lung tumors near the mediastinum treated with stereotactic body radiation therapy.

PURPOSE To report the local control, survival, and low toxicity observed at the Cooper University Hospital CyberKnife Center post stereotactic body radiation therapy (SBRT) in the treatment of lung tumors near the mediastinum. METHODS AND MATERIALS Twenty-four medically inoperable lung cancer patients with tumors near the mediastinum were treated using the Accuray CyberKnife system (Accuray, Sunnyvale, CA) with Monte Carlo dose calculations and heterogeneity corrections from July 2008 to May 2010. The prescription dose ranged from 28.5 Gy to 60 Gy in 3-5 fractions. For conventional fractionation schemes, Emami et al(1) organized the dose tolerance limits into a unified format for clinical utility and partitioned them into 2 risk levels (5% and 50%) with preset volumes for most critical structures throughout the body. In contrast, statistical SBRT dose tolerance limits for mediastinal structures have not been established yet. We have sufficient experience at least to begin organizing a unified format with low-risk and high-risk partitions and preset volumes for 1-5 fractions exposing mediastinal structures. With the help of the (dose-volume histogram) DVH Evaluator, a software tool developed by our senior author, each treatment plan was assessed for safety and feasibility prior to treatment. The DVH Evaluator was also used to analyze the follow-up data and to create graphs of risk, called DVH Risk Maps, superimposing clinical data onto the unified SBRT dose tolerance limits. RESULTS It was not feasible to prescribe the doses of peripheral lung lesions for all tumors near the mediastinum because of known toxicity. The crude local tumor control rate achieved in our series was 92%. Median survival was 26.8 months for the primary lung cases and 9.6 months for the metastatic cases. No patients experienced grade 3 or higher toxicities. CONCLUSIONS We affirm that SBRT is feasible in the treatment of centrally located lung cancers when the dose tolerance limits of critical structures are diligently respected. The low adverse event rates that we have experienced, combined with a good local tumor control rate, are encouraging.

Small Bowel Dose Tolerance for Stereotactic Body Radiation Therapy

Inconsistencies permeate the literature regarding small bowel dose tolerance limits for stereotactic body radiation therapy (SBRT) treatments. In this review, we organized these diverse published limits with MD Anderson at Cooper data into a unified framework, constructing the dose-volume histogram (DVH) Risk Map, demonstrating low-risk and high-risk SBRT dose tolerance limits for small bowel. Statistical models of clinical data from 2 institutions were used to assess the safety spectrum of doses used in the exposure of the gastrointestinal tract in SBRT; 30% of the analyzed cases had vascular endothelial growth factor inhibitors (VEGFI) or other biological agents within 2 years before or after SBRT. For every dose tolerance limit in the DVH Risk Map, the probit dose-response model was used to estimate the risk level from our clinical data. Using the current literature, 21Gy to 5cc of small bowel in 3 fractions has low toxicity and is reasonably safe, with 6.5% estimated risk of grade 3 or higher complications, per Common Terminology Criteria for Adverse Events version 4.0. In the same fractionation for the same volume, if lower risk is required, 16.2Gy has an estimated risk of only 2.5%. Other volumes and fractionations are also reviewed; for all analyzed high-risk small bowel limits, the risk is 8.2% or less, and the low-risk limits have 4% or lower estimated risk. The results support current clinical practice, with some possibility for dose escalation.

Introduction and Clinical Overview of the DVH Risk Map

Radiation oncologists need reliable estimates of risk for various fractionation schemes for all critical anatomical structures throughout the body, in a clinically convenient format. Reliable estimation theory can become fairly complex, however, and estimates of risk continue to evolve as the literature matures. To navigate through this efficiently, a dose-volume histogram (DVH) Risk Map was created, which provides a comparison of radiation tolerance limits as a function of dose, fractionation, volume, and risk level. The graphical portion of the DVH Risk Map helps clinicians to easily visualize the trends, whereas the tabular portion provides quantitative precision for clinical implementation. The DVH Risk Map for rib tolerance from stereotactic ablative body radiotherapy (SABR) and stereotactic body radiation therapy (SBRT) is used as an example in this overview; the 5% and 50% risk levels for 1-5 fractions for 5 different volumes are given. Other articles throughout this issue of Seminars in Radiation Oncology present analysis of new clinical datasets including the DVH Risk Maps for other anatomical structures throughout the body.

SU-E-J-214: Assessment of Rotational Lung Tumor Motion and Its Influence On Treatment Margins for Stereotactic Body Radiosurgery (SBRT)

Purpose: To quantify rotational motion of lung tumors under radiotherapy based on fiducial imaging. Tumor rotational motion in lung has not been well understood due to difficulties of imaging and target delineation. In this study the rotational motion of fiducial clusters were measured for assessing the treatment margins necessary for adequate dose coverage to CTV. Methods: 25 patients who underwent CyberKnife based SBRT were recruited. Three to five fiducials were implanted in or near the tumor. The reference fiducial locations were determined using a breath‐hold CT. Orthogonal X‐ray image pairs were acquired for modeling and tumor tracking during treatment. These images were used to reconstruct the fiducial locations in 3D. A rigid‐body registration was derived between the measured and reference fiducial locations. The mean distance between the corresponding fiducial pairs was used to evaluate the registration. 2796 pairs of images in 106 fractions of treatment were analyzed. The rotational motion of fiducial clusters was used to simulate the tumor rotational motion for assessing the adequacy of the PTV margins. A margin of 3mm was used to expand the upper lobe (UL) target and 5 mm for lower lobe (LL) target. Additional tracking errors were included in the analysis for tumor coverage under alignment of the rotated CTV with the PTV. Results: The tumor rotational angles in LL and UL were 0.25°±5.7° vs 0.40°±2.1° in roll, — 0.21°±7.3° vs 0.05°±1.8° in pitch, and 0.23°±5.3° vs. 0.1°±2.1° in yaw, respectively. In 94.4% (LL) and 97.1% (UL) of the total imaged tumor locations, the CTV was 100% covered by the corresponding PTV. The mean missing coverage of the CTV for the rest locations were of 4.4% and 1%, respectively. Conclusion: The reported angles were highly correlated to the distance to the diaphragm. Appropriate margins need to be applied for different lobes to ensure CTV coverage.

SU‐E‐J‐147: Internal Brain Motion Between CT and MR Scanning

Purpose: To evaluate the internal brain motion between two imaging studies of CT and MRI. A study with 30 healthy volunteers with MRI scans in 4 different positions showed significant brain to skull motion up to 1 cm. Such motion among patients for radiotherapy to the brain is evaluated in this study. Methods: Twenty‐five patients underwent MRI and CT scans in the same day for radiotherapy planning were recruited. A whole brain fusion was first performed. Three to five pairs of control points were selected on both CT and MRI for starting an automated intensity based registration. The fusion was reviewed and fine‐tuned for the best skull‐to‐skull matching. To study potential internal brain motion, a subsequent fine‐tuning of the fusion was performed by matching the visible features, such as gyri, sulci and fissures, near the tumor site. The second fusion was reviewed and fine‐tuned by two physicists until the best visible feature matching could be agreed upon. The resulting rotation and translation between the whole brain and feature‐based fusions indicated potential internal brain motion between the two scans. Results: Between two fusions the mean internal shifts in x (LR), y (AP) and z (SI) were 0.34±0.95 mm, 0.21±1.18 mm and −0.34±0.8 mm, respectively. The mean overall shift was 1.4±1.1 mm, and the largest shift was 3.5 mm. The mean rotation angles were 0.22±1.32° (pitch), 0.14±0.4° (yaw) and 0.08±0.53° (roll), respectively. The pitch motion was predominant (head up and down) due to difference of couch tops of CT and MRI scanners. Conclusion: Our study showed small but measurable internal brain motion among radiotherapy patients with typical clinical setting for CT and MRI imaging. Therefore the CT‐MRI fusion should be carefully checked for internal structure matching. An additional treatment margin may be needed if an internal motion is observed.

SU‐E‐T‐883: Clinical Application of Monte Carlo for SBRT: Mediastinal Lung

Purpose: Treatment planning systems have provided Monte Carlo dose calculations for several years but many physicians are still hesitant to use them clinically due to lack of data. Therefore we recalculated 200 Ray Tracing treatment plans using Monte Carlo with heterogeneity corrections and compared to SBRT dose tolerance limits. Methods: From among these 200 CyberKnife cases, 25 mediastinal lung cases are presented in this study. An extensive literature review obtained 105 published SBRT dose tolerance limits for the mediastinal critical structures aorta, bronchi, esophagus, heart, and trachea. These limits were partitioned into high‐risk and low‐risk categories. The DVH Evaluator software tool was used to generate DVH Risk Maps for these critical structures, which superimpose a) published dose tolerance limits b) unified high‐risk and low‐risk trends and c) published adverse event doses, onto Monte Carlo patient data to assess risk of adverse events. Results: Recalculated treatment plan data is within the expected range of published SBRT dose tolerance limits, providing optimism for clinical use. None of the patients experienced any Grade 3 or higher adverse events. The low‐risk dose tolerance limits were exceeded 22 times in these cases with no severe adverse event, thus helping to validate their safety. Conclusions: The range of doses calculated by Monte Carlo for our historical patient data is compatible with published SBRT dose tolerance limits. SBRT dose tolerance limits should be fine‐tuned by Monte Carlo dose calculations in long‐term statistical followup studies. Disclosure: The first author has developed the DVH Evaluator software.

MO‐D‐BRB‐07: Phantom Validation and Clinical Application of Monte Carlo for Small Field SBRT

Purpose: Small field dosimetry is challenging in homogeneous medium and extremely difficult in an inhomogeneous medium. Monte Carlo dose calculation algorithms are considered as the most accurate for treatment planning. We present our validation of the Monte Carlo algorithm in the Accuray Multiplan system using measurements in a cork phantom. We also recalculated Ray Tracing treatment plans with the Monte Carlo algorithm and compared to SBRT dose tolerance limits. Methods: In our validation measurements with a cork phantom, an Exradin A16 ion chamber was used for collimators from 60mm to 20mm on a CyberKnife, and a PTW 60012 stereotactic diode for collimators from 60mm to 5mm. A literature review of more than 500 published SBRT dose tolerance limits was partitioned into high‐risk and low‐risk categories. Two hundred CyberKnife treatment plans were recalculated using Monte Carlo and compared to the dose limits. The DVH Evaluator software tool was used to generate DVH Risk Maps for 25 critical structures throughout the body, which superimpose a) published dose tolerance limits b) unified high‐risk and low‐risk trends and c) published adverse event doses, onto Monte Carlo patient doses to assess risk of adverse events. Results: The Monte Carlo calculations matched the Exradin A16 measurements to within 2.5% for field sizes down to 20mm, and matched the PTW 60012 measurements to within 2.5% for all field sizes down to 5mm. Recalculated treatment plan data is within the expected range of published SBRT dose tolerance limits, providing optimism for clinical use. Conclusions: The Accuray MultiPlan Monte Carlo algorithm is accurate even for small fields in heterogeneous media. The range of doses calculated by Monte Carlo for our patient data is compatible with published SBRT dose tolerance limits. SBRT dose tolerance limits should be fine‐tuned by Monte Carlo dose calculations in long‐term statistical followup studies. Disclosure: The first author has developed the DVH Evaluator software.

SU‐E‐T‐892: Calculation Uncertainty in CyberKnife Dosimetric Parameters of Brainstem

Purpose: It is important to understand the effect of dose calculation on the dosimetric parameters of a critical structure, which are often used to evaluate the risk of toxicity. We present the variation of some dosimetric parameters for brainstem as computed by two different dose algorithms. Methods: Both Ray Tracing and Monte Carlodose algorithms are commissioned for the CyberKnife planning system at Cooper. Study was performed for twenty seven intracranial patients treated with CyberKnife. Dose calculation was carried out in Multiplan workstation by Monte Carlo algorithm with heterogeneity corrections as well as Ray Tracing algorithm without heterogeneity corrections. Brainstem was delineated by the attending radiation oncologist during planning process. The same prescription and optimization as the actual treatment were employed in dose calculation for each patient. Results: Principle dosimetric parameters (D50, D10, D1cc, D0.1cc and maximum dose) of brainstem are derived for each patient. The difference in percentage between Ray Tracing and Monte Carlo calculation is plotted. Significant variation has been observed for each dosimetric parameter between two different dose algorithms. Also plotted is the variation of those dosimetric parameters against the volume of brainstem. Conclusions: The value of dosimetric parameters can be considerately different depending on dose algorithm used in calculation. The evaluation of dose tolerance for a critical structure using those dosimetric parameters needs to be cautious about dose computing methods.