PTV margin determination in conformal SRT of intracranial lesions

Quantifying 6D tumor motion and calculating PTV margins during liver stereotactic radiotherapy with fiducial tracking

Objective

Our study aims to estimate intra-fraction six-dimensional (6D) tumor motion with rotational correction and the related correlations between motions of different degrees of freedom (DoF), as well as quantify sufficient anisotropic clinical target volume (CTV) to planning target volume (PTV) margins during stereotactic body radiotherapy (SBRT) of liver cancer with fiducial tracking technique.

Methods

A cohort of 12 patients who were implanted with 3 or 4 golden markers were included in this study, and 495 orthogonal kilovoltage (kV) pairs of images acquired during the first fraction were used to extract the spacial position of each golden marker. Translational and rotational motions of tumor were calculated based on the marker coordinates by using an iterative closest point (ICP) algorithm. Moreover, the Pearson product-moment correlation coefficients (r) were applied to quantify the correlations between motions with different degrees of freedom (DoFs). The population mean displacement (MP¯), systematic error (Σ) and random error (σ) were obtained to calculate PTV margins based on published recipes.

Results

The mean translational variability of tumors were 0.56, 1.24 and 3.38 mm in the left-right (LR, X), anterior-posterior (AP, Y), and superior-inferior (SI, Z) directions, respectively. The average rotational angles θX , θY and θZ around the three coordinate axes were 0.88, 1.24 and 1.12, respectively. (|r|>0.4) was obtainted between Y -Z , Y - θZ , Z -θZ and θX - θY . The PTV margins calculated based on 13 published recipes in X, Y, and Z directions were 1.08, 2.26 and 5.42 mm, and the 95% confidence interval (CI) of them were (0.88,1.28), (1.99,2.53) and (4.78,6.05), respectively.

Conclusions

The maximum translational motion was in SI direction, and the largest correlation coefficient of Y-Z was obtained. We recommend margins of 2, 3 and 7 mm in LR, AP and SI directions, respectively.

Total workflow uncertainty of frameless radiosurgery with the Gamma Knife Icon cone-beam computed tomography

Objective

Frameless treatment with the Gamma Knife Icon is still relatively new as a treatment option. As a result, additional confidence/knowledge about the uncertainty that exists within each portion of the treatment workflow could be gained especially regarding steps that have not been previously studied in the literature.

Methods

The Icon base delivery device (Perfexion) uncertainty is quantified and validated. The novel portions of the Icon such as mask immobilization, cone‐beam computed tomography image guidance, and the intrafraction motion management methods are studied specifically and to a greater extent to determine a total workflow uncertainty of frameless treatment with the Icon.

Results

The uncertainty of each treatment workflow step has been identified with the total workflow uncertainty being identified in this work as 1.3 mm with a standard deviation of 0.51 mm.

Conclusion

The total uncertainty of frameless treatment with the Icon has been evaluated and this data may indicate the need for setup margin in this setting with data that could be used by other institutions to calculate needed setup margin per their preferred recipe after validation of this data in their context.

Analytical setup margin for spinal stereotactic body radiotherapy based on measured errors

Background

No consensus currently exists about the correct margin size to use for spinal SBRT. Margins have been proposed to account for various errors individually, but not with all errors combined to result in a single margin value. The purpose of this work was to determine a setup margin for five-fraction spinal SBRT based on known errors during radiotherapy to achieve at least 90% coverage of the clinical target volume with the prescription dose for at least 90% of patients and not exceed a 30 Gy point dose or 23 Gy to 10% of the spinal cord subvolume.

Methods

The random and systematic error components of intrafraction motion, residual setup error, and end-to-end system accuracy were measured. The patient’s surface displacement was measured to quantify intrafraction motion, the residual setup error was quantified by re-registering accepted daily cone beam computed tomography setup images, and the displacement between measured and planned dose profiles in a phantom quantified the end-to-end system accuracy. These errors and parameters were used to identify the minimum acceptable margin size. The margin recommendation was validated by assessing dose delivery across 140 simulated patient plans suffering from various random shifts representative of the measured errors.

Results

The errors were quantified in three dimensions and the analytical margin generated was 2.4 mm. With this margin applied in the superior/inferior direction only, at least 90% of the CTV was covered with the prescription dose for 96% of the 140 patients simulated with minimal negative effect on the spinal cord dose levels.

Conclusions

The findings of this work support that a 2.4 mm margin applied in the superior/inferior direction can achieve at least 90% coverage of the CTV for at least 90% of dual-arc volumetric modulated arc therapy spinal SBRT patients in the presence of errors when immobilized with vacuum bags.

Using 4DCBCT simulation and guidance to evaluate inter-fractional tumor variance during SABR for lung tumor within the lower lobe

Inter-fractional tumor variance would lead to insufficient dosage or overdose in tumor region during lung cancer radiotherapy. However, previous works have not considered influence of inter-fractional tumor amplitude variance at treatment position due to lack of effective evaluation method during radiotherapy, especially for lung tumor within the lower lobe. Our objective was to investigate inter-fractional tumor baseline shift and amplitude variance due to respiratory motion with 4DCBCT simulation and guidance during stereotactic ablative body radiotherapy (SABR) for lung tumor. Subject included 19 patients with lung tumor within the lower lobe. 4DCBCT-simulated images at treatment position were acquired sequentially to determine internal tumor volume (ITV) and reference tumor motion at simulation process. Compared with reference tumor motion, 95 4DCBCT-guided images were acquired during each treatment to evaluate inter-fractional tumor baseline shift and amplitude variance, which were − 0.0 ± 1.3 mm and − 0.2 ± 1.4 mm in left–right(LR) direction, 0.9 ± 2.3 mm and 0.4 ± 2.9 mm in superior-inferior (SI) direction, 0.1 ± 1.5 mm and − 0.4 ± 2.0 mm in anterior–posterior (AP) direction. ITV margin were 3.5 mm, 7.5 mm and 5.3 mm in LR, SI and AP directions with van Herk’s (Int J Radiat Oncol Biol Phys 52(5):1407–1422, 2002) formula. 4DCBCT simulation and guidance is a reliable method to evaluate inter-fractional tumor variance during SABR for lung tumor within the lower lobe. ITV margin of 3.5 mm, 7.5 mm and 5.3 mm in LR, SI and AP directions would ensure greater tumor coverage during SABR for lung tumor within the lower lobe.

The impact of androgen deprivation therapy on setup errors during external beam radiation therapy for prostate cancer

PURPOSE

To determine whether setup errors during external beam radiation therapy (RT) for prostate cancer are influenced by the combination of androgen deprivation treatment (ADT) and RT.

MATERIALS AND METHODS

Data from 175 patients treated for prostate cancer were retrospectively analyzed. Treatment was as follows: concurrent ADT plus RT, 33 patients (19%); neoadjuvant and concurrent ADT plus RT, 91 patients (52%); RT only, 51 patients (29%). Required couch shifts without rotations were recorded for each megavoltage (MV) cone beam computed tomography (CBCT) scan, and corresponding alignment shifts were recorded as left-right (x), superior-inferior (y), and anterior-posterior (z). The nonparametric Mann-Whitney test was used to compare shifts by group. Pearson's correlation coefficient was used to measure the correlation of couch shifts between groups. Mean prostate shifts and standard deviations (SD) were calculated and pooled to obtain mean or group systematic error (M), SD of systematic error (Σ), and SD of random error (σ).

RESULTS

No significant differences were observed in prostate shifts in any direction between the groups. Shifts on CBCT were all less than setup margins. A significant positive correlation was observed between prostate volume and the z‑direction prostate shift (r = 0.19, p = 0.04), regardless of ADT group, but not between volume and x‑ or y‑direction shifts (r = 0.04, p = 0.7; r = 0.03, p = 0.7). Random and systematic errors for all patient cohorts and ADT groups were similar.

CONCLUSION

Hormone therapy given concurrently with RT was not found to significantly impact setup errors. Prostate volume was significantly correlated with shifts in the anterior-posterior direction only.

4DCT and CBCT based PTV margin in Stereotactic Body Radiotherapy(SBRT) of non-small cell lung tumor adhered to chest wall or diaphragm

Background

Large tumor motion often leads to larger treatment volumes, especially the lung tumor located in lower lobe and adhered to chest wall or diaphragm. The purpose of this work is to investigate the impacts of planning target volume (PTV) margin on Stereotactic Body Radiotherapy (SBRT) in non-small cell lung cancer (NSCLC).

Methods

Subjects include 20 patients with the lung tumor located in lower lobe and adhered to chest wall or diaphragm who underwent SBRT. Four-dimensional computed tomography (4DCT) were acquired at simulation to evaluate the tumor intra-fractional centroid and boundary changes, and Cone-beam Computer Tomography (CBCT) were acquired during each treatment to evaluate the tumor inter-fractional set-up displacement. The margin to compensate for tumor variations uncertainties was calculated with various margin calculated recipes published in the exiting literatures.

Results

The means (±standard deviation) of tumor centroid changes were 0.16 (±0.13) cm, 0.22 (±0.15) cm, and 1.37 (±0.81) cm in RL, AP, and SI directions, respectively. The means (±standard deviation) of tumor edge changes were 0.21 (±0.18) cm, 0.50 (±0.23) cm, and 0.19 (±0.44) cm in RL, AP, and SI directions, respectively. The means (±standard deviation) of tumor set-up displacement were 0.03 (±0.24) cm, 0.02 (±0.26) cm, and 0.02 (±0.43) cm in RL, AP, and SI directions, respectively. The PTV margin to compensate for lung cancer tumor variations uncertainties were 0.88, 0.98 and 2.68 cm in RL, AP and SI directions, which were maximal among all margin recipes.

Conclusions

4DCT and CBCT imaging are appropriate to account for the tumor intra-fractional centroid, boundary variations and inter-fractional set-up displacement. The PTV margin to compensate for lung cancer tumor variations uncertainties can be obtained. Our results show that a conventional 1.0 cm margin in the SI plane dose not suffice to compensate the geometrical variety of the tumor located in lower lobe and adhered to chest wall and diaphragm.

Dosimetric impact of intrafractional patient motion in pediatric brain tumor patients

The purpose of this study was to determine the dosimetric consequences of intrafractional patient motion on the clinical target volume (CTV), spinal cord, and optic nerves for non-sedated pediatric brain tumor patients. The patients were immobilized for treatment using a customized thermoplastic full-face mask and bite-block attached to an array of reflectors. The array was optically tracked by infra-red cameras at a frequency of 10 Hz. Patients were localized based on skin/mask marks and weekly films were taken to ensure proper setup. Before each noncoplanar field was delivered, the deviation from baseline of the array was recorded. The systematic error (SE) and random error (RE) were calculated. Direct simulation of the intrafractional motion was used to quantify the dosimetric changes to the targets and critical structures. Nine patients utilizing the optical tracking system were evaluated. The patient cohort had a mean of 31 +/- 1.5 treatment fractions; motion data were acquired for a mean of 26 +/- 6.2 fractions. The mean age was 15.6 +/- 4.1 years. The SE and RE were 0.4 and 1.1 mm in the posterior-anterior, 0.5 and 1.0 mm in left-right, and 0.6 and 1.3 mm in superior-inferior directions, respectively. The dosimetric effects of the motion on the CTV were negligible; however, the dose to the critical structures was increased. Patient motion during treatment does affect the dose to critical structures, therefore, planning risk volumes are needed to properly assess the dose to normal tissues. Because the motion did not affect the dose to the CTV, the 3-mm PTV margin used is sufficient to account for intrafractional motion, given the patient is properly localized at the start of treatment.

Simulation of intrafraction motion and overall geometrical accuracy of a frameless intracranial radiosurgery process

We conducted a comprehensive evaluation of the clinical accuracy of an image-guided frameless intracranial radiosurgery system. All links in the process chain were tested. Using healthy volunteers, we evaluated a novel method to prospectively quantify the range of target motion for optimal determination of the planning target volume (PTV) margin. The overall system isocentric accuracy was tested using a rigid anthropomorphic phantom containing a hidden target. Intrafraction motion was simulated in 5 healthy volunteers. Reinforced head-and-shoulders thermoplastic masks were used for immobilization. The subjects were placed in a treatment position for 15 minutes (the maximum expected time between repeated isocenter localizations) and the six-degrees-of-freedom target displacements were recorded with high frequency by tracking infrared markers. The markers were placed on a customized piece of thermoplastic secured to the head independently of the immobilization mask. Additional data were collected with the subjects holding their breath, talking, and deliberately moving. As compared with fiducial matching, the automatic registration algorithm did not introduce clinically significant errors (<0.3 mm difference). The hidden target test confirmed overall system isocentric accuracy of < or =1 mm (total three-dimensional displacement). The subjects exhibited various patterns and ranges of head motion during the mock treatment. The total displacement vector encompassing 95% of the positional points varied from 0.4 mm to 2.9 mm. Pre-planning motion simulation with optical tracking was tested on volunteers and appears promising for determination of patient-specific PTV margins. Further patient study is necessary and is planned. In the meantime, system accuracy is sufficient for confident clinical use with 3 mm PTV margins.

Dosimetric characteristics of dual-layer multileaf collimation for small-field and intensity-modulated radiation therapy applications

The purpose of the present work was to measure the performance characteristics in the penumbra region and on the leaf‐end of an innovative dual‐layer micro multileaf collimator (DmMLC). The DmMLC consists of two orthogonal (upper and lower) layers of leaves; a standard MLC consists of one layer. The DmMLC provides unique performance characteristics in smoothing dose undulation, reducing leaf‐end transmission, and reducing MLC field dependence of the leaf stepping angle. Two standard MLCs (80‐leaf and 120‐leaf versions: Varian Medical Systems, Palo Alto, CA), a DmMLC (AccuKnife: Initia Medical Technology, Canton, MA), and a Cerrobend (Cerro Metal Products, Bellefonte, PA) block were used in performance studies involving a triangular field, a cross leaf‐end field, and a circular field. Measurements were made with 6‐MV X‐rays and extended dose range film at a depth of 5 cm in Solid Water (Gammex rmi, Middleton, WI) at a source–axis distance of 100 cm. The field penumbra width measured between the 20% and 80% isodose lines through the MLC‐80, MLC‐120, DmMLC, and Cerrobend block were 9.0, 5.0, 3.0, and 2.0 mm respectively. The dose undulation amplitude of the 50% isodose line was measured as 5.5, 2.0, and 0.5 mm for the MLC‐80, MLC‐120, and DmMLC respectively. The planar dose difference between the MLC‐80, MLC‐120, and DmMLC against Cerrobend block was measured as ranging at ±52.5%,±35.0%, and ±20.0% respectively. The leaf‐end transmission was measured at 22.4% in maximum and 15.4% in average when closing a single layer of the DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of the leaf stepping angle with the DmMLC ranged from 45 degrees to 90 degrees. The standard MLC leaf stepping angle ranged from 0 degrees to 90 degrees. In conclusion, the dose undulation, leaf‐end transmission, and MLC field dependence of the leaf stepping angle with the DmMLC were remarkably reduced as compared with those of the standard MLCs. And as compared with Cerrobend block, the DmMLC provided very comparable performance in field‐edge smoothing and in the shaping of complex fields.

PACS numbers: 87.56.Jk, 87.56.Nk, 87.56.Nj, 87.57.Nt

Commissioning and quality assurance of a commercial stereotactic treatment-planning system for extracranial IMRT

A 3D treatment‐planning system (TPS) for stereotactic intensity‐modulated radiotherapy (IMRT) using a micro‐multileaf collimator has been made available by Radionics. In this work, we report our comprehensive quality assurance (QA) procedure for commissioning this TPS. First, the accuracy of stereotaxy established with a body frame was checked to ensure accurate determination of a target position within the planning system. Second, the CT‐to‐electron density conversion curve used in the TPS was fitted to our site‐specific measurement data to ensure the accuracy of dose calculation and measurement verification in a QA phantom. Using the QA phantom, the radiological path lengths were verified against known geometrical depths to ensure the accuracy of the ray‐tracing algorithm. We also checked inter‐ and intraleaf leakage/transmission for adequate jaw settings. Measurements for dose verification were performed in various head/neck and prostate IMRT treatment plans using the patient‐specific optimized fluence maps. Both ion chamber and film were used for point dose and isodose distribution verifications. To ensure that adjacent organs at risk receive dose within the expectation, we used the Monte Carlo method to calculate dose distributions and dose‐volume histograms (DVHs) for these organs at risk. The dosimetric accuracy satisfied the published acceptability criteria. The Monte Carlo calculations confirmed the measured dose distributions for target volumes. For organs located on the beam boundary or outside the beam, some differences in the DVHs were noticed. However, the plans calculated by both methods met our clinical criteria. We conclude that the accuracy of the XKnife™ RT2 treatment‐planning system is adequate for the clinical implementation of stereotactic IMRT.

PACS numbers: 87.53.Xd, 87.53.Ly, 87.53.Wz

[Evaluation of positioning reproducibility using a re-locatable head frame for stereotactic radiotherapy]

In fractionated stereotactic radiotherapy (SRT), the accuracy of patient relocation is very important. The Gill-Thomas-Cosman (GTC) re-locatable stereotactic frame is used for patient immobilization. A depth helmet and measuring probe are used to check the stability of the patient's head position relative to the GTC frame. However, displacement error caused by rotation of the patient's head is not considered in the depth-helmet measurement. Consequently, displacement of the isocenter position cannot be confirmed by the measurement obtained from the depth helmet. In this study, we evaluated the precision of reproducibility by comparing measurement values of the depth helmet with the displacement of anatomical position on a CT image. We analyzed 21 setups of 8 patients immobilized for SRT using the GTC frame, between June 2001 and June 2003. The reproducibility of the GTC frame was checked at each treatment by comparing it with the treatment planning position. The average discrepancy of the GTC frame set-up measured by the depth helmet was 0.6 mm, with a standard deviation of 0.3 mm. The result measured by CT was 0.7 mm, with a standard deviation of 0.4 mm. When the error of each measurement point was within 1.0 mm, the accuracy of relocation of the patient could be considered clinically acceptable. Displacement error not considered in the measurement of the depth helmet could be evaluated by using CT.

An integrated treatment delivery system for CSRS and CSRT and clinical applications

An integrated treatment delivery system for conformal stereotactic radiosurgery (CSRS) and radiotherapy (CSRT) has been developed through a collaboration involving Siemens Medical Systems, Inc., Tyco/Radionics, Inc., and The University of Texas M. D. Anderson Cancer Center. The system consists of a 6-MV linear accelerator (LINAC) equipped with a Tyco/Radionics miniature multileaf collimator (mMLC). For the conventional SRS treatment, the circular collimator housing can be attached to the opening window of the mMLC. The treatment delivery system is integrated with a radiotherapy treatment planning system and a record-and-verify system. The purpose of this study is to report the characteristics, performance, benefits, and the clinical applications of this delivery system. The technical specifications of the LINAC and mMLC were tested, and all the specifications were met. The 80% to 20% penumbral width for each mMLC leaf is approximately 3 mm and is nearly independent of the off-axis positions of a leaf. The maximum interleaf leakage is 1.4% (1.1% on average) and the maximum intra-leaf leakage is 1.0% (0.9% on average). The leaf position precision is better than 0.5 mm for all the leaves. The integration of the SRS/SRT treatment planning system, mMLC, and LINAC has been evaluated successfully for transferring the patient treatment data file through radiotherapy treatment planning system to the patient information and treatment record-and-verify server and the mMLC controller. Subsequently, the auto-sequential treatment delivery for SRS, CSRS/CSRT, and the step-and-shoot intensity-modulated radiotherapy has also been tested successfully. The accuracy of dose delivery was evaluated for a 2-cm spherical target in a Radiological Physics Center SRS head phantom with GAFChromic films and TLD. Five non-coplanar arcs, using a 2-cm diameter circular collimator, were used for this simulation treatment. The accuracy to aim the center of the spherical target was within 0.5 mm and the deviation of dose delivery to the isocenter of the target was within 2% of the calculated dose. For the irregularly shaped tumor, a tissue-equivalent head phantom was used to evaluate the accuracy of dose delivery for using either geometric conformal treatment or IMRT. The accuracy of dose delivery to the isocenter was within 2% and 3% of the calculated dose, respectively. From October 26, 1999 to September 30, 2002, we treated over 400 SRS patients and 70 SRT patients. Four representative cases are presented to illustrate the capabilities of this dedicated unit in performing conventional SRS, CSRS, and CSRT. For all the cases, the geometric conformal-plan dose distributions showed a high degree of conformity to the target shape. The degree of conformity can be evaluated using the target-volume-ratio (TVR). Our preferred TVR values for highly conformed dose distributions range from 1.6 to 2.0. The patient setup reproducibility for the Gill-Thomas-Cosman (GTC) noninvasive head frame ranges from 0.5 to 1 mm, and the head and neck noninvasive frame is within 2 mm. The integrated treatment delivery system offers excellent conformation for complicated planning target volumes with the stereotactic setup approach, ensuring that dose delivery can be achieved within the specified accuracy. In addition, the treatment time is comparable with that of single isocenter multiple-arc treatments.