Mm In AP Direction Research Articles

Intrafraction Dosimetric Evaluation of Biology-Guided Radiotherapy to a Target Under Respiratory Motion

To evaluate the reproducibility and variability of biology-guided radiotherapy (BgRT) treatments using a large anthropomorphic phantom modeling the motion amplitude of a lung tumor. RefleXion X1 is equipped with two opposing 90 degrees PET detector arcs to capture the radionuclide emissions and direct the 6MV Linac to treat the lesions in real time. A custom-built phantom filled with a liquid [¹⁸F]Fluorodeoxyglucose (FDG) solution was used. Fillable target and OAR structures were 3D printed and attached to motion stages. The GTV = CTV was matched to the spherical 22 mm diameter target, and the PTV was a 5 mm expansion from the CTV volume. The Biology Tracking Zone (BTZ) was generated after adding 5 mm margin to the motion extent of the CTV. The OAR was a large C-shape annulus (emulating a heart) that was approximately 3 cm from the target. The 3D independent motion trajectory of the target was designed to mimic lung motion: range of +5.8 mm to -4.9 mm in LR, range of +14.4 mm to -11.3 mm in SI, and range of +5.2 mm to -5.1 mm in AP directions. The OAR motion waveform used a 1D sinusoidal pattern with a 5 mm amplitude in SI direction. The target and the OAR were filled with 40 kBq/mL while the background had 5 kBq/mL FDG. A BgRT Modeling (imaging-only) PET acquisition was performed using RefleXion X1 and used to generate a 4-fraction BgRT treatment plan prescribing 10 Gy/fraction to PTV. For each delivery, target, OAR and background were filled with the same FDG concentrations as in the BgRT Modeling PET planning scan. Dosimetry to the target and OAR were both measured using an ion-chamber (Exradin A14SL) and film in the coronal plane through the center of the GTV for all 4 fractions. The mean activity concentration within the (BTZ) was 7.4 ± 0.8 kBq/mL. The calculated signal-to-noise ratio metric (Normalized Target Signal) within the BTZ was 4.0 ± 0.3. Total treatment times were all less than 35 minutes (34.3 ± 0.2). Prescription dose coverage to the CTV for all 4 fractions was 100%. Ion chamber measurements in the CTV were -1.6 ± 1.3% relative to the planned dose over the active area of the ion-chamber. Minimum and maximum doses to the CTV, measured on film, were -7.7 ± 2.2% and 1.3 ± 1.4%, calculated relative to the planned dose distribution, respectively. The OAR maximum point dose measured on film was -8.7 ± 2.9%, calculated relative to the maximum OAR dose predicted on the bounded dose-volume histogram. Based on this initial study, accurate and reproducible dosimetry can be achieved for targets under respiratory motion using biology-guided radiotherapy over the course of a complete course of treatment. Further studies are needed to evaluate the intrafraction dosimetry of BgRT delivery under various motion models and tumor sizes.

Reducing planning target volume margins decreases dose to parotid glands in head and neck cancers - a dosimetric analysis

Introduction: Radiotherapy in head and neck cancers is treated for several weeks and daily setup and reproducibility is a challenge. This daily variability causes setup errors which accounts planning target volume margins. Reduced PTV margins have to be taken to decrease the dose to the parotid glands, without compromising on loco regional control rates. The present study is done to identify setup errors and see the feasibility to decrease the PTV margins by creating dummy radiotherapy plans in order to decrease dose to parotid glands. Material and Methods: 420 portal images were evaluated for setup errors in three dimensions (Antero Posterior, Left to Right and Superior to Inferior) which were performed in ten patients of oropharyngeal squamous cell carcinoma. All patients were treated in supine position using immobilization cast. After target volume delineation a PTV margin of 7mm was given. Dosimetric parameters of PTV and organs at risk were assessed. PTV margins were calculated according to three methods proposed by Stroom, Van Herk and ICRU 62. Dummy radiotherapy plans were generated using new PTV margins and compared with 7mm PTV margins. The data was analyzed using 3-way ANNOVA test for statistical significance. Results: The optimum PTV margins were 4mm in LR and SI direction and 7mm in AP direction. The PTV parameters (V95, D95, Dmax, Dmean, HI and CI) had no significant difference among different radiotherapy plans with different PTV margins. There was a significant decrease in the dose to right parotid (39.12 Gy to 32.88Gy; p-0.04), left parotid (37.90 to 31.21Gy; p-0.03) and parotid combined (38.65 to 31.45 Gy; p-0.01) when 7mm PTV margins were reduced to 4mm PTV margins. The results of dummy radiotherapy plans using asymmetric PTV margins (LR-4mm, SI-4mm and AP-7mm) and symmetrical PTV margins (4mm in all directions) are compared with PTV margins (7mm in all directions), in terms of PTV and OAR dosimetric parameters. Conclusion: The decreased PTV margins of 4mm decreases the dose to the parotid significantly. The implementation of radiotherapy plans needs to be supplemented by daily IGRT.

A preclinical method for evaluating the kinematics of knee prostheses

The primary intent of anatomical knee implants is to replicate the motions of a normal knee joint. In developing such designs, a preclinical evaluation of kinematic behavior is needed. This study introduces an in vitro testing method for recording movements of the knee joint. A novel testing jig was developed and incorporated into a knee simulator setup alongside a motion capture system to directly track the medial and lateral movements of a knee prosthesis. The test system developed in this study required a number of factors to be validated; (i) gait inputs to the knee simulator (result: 0.37–1.575% error), (ii) validity of global coordinate system in the motion capture system, (iii) the position of flexion facet centers (FFCs) detected by the motion capture system (result: a maximum error of 0.08 mm in AP direction and 0.3 mm in SI direction), (iv) local coordinate system in the motion capture system (result: 1.09% error for the measurement of flexion angle), (v) that FFC results were in good agreement with inputs. In conclusion, the system developed in this study for recording FFC is a direct and reliable in vitro test method for analyzing the kinematics of a knee prosthesis.

Intra-fraction motion of the prostate during treatment with helical tomotherapy

Background and purposeTo measure the geometric uncertainty resulting from intra-fraction motion and intra-observer image matching, for patients having image-guided prostate radiotherapy on TomoTherapy. Material and methodsAll patients had already been selected for prostate radiotherapy on TomoTherapy, with daily MV-CT imaging. The study involved performing an additional MV-CT image at the end of treatment, on 5 occasions during the course of 37 treatments. 54 patients were recruited to the study. A new formula was derived to calculate the PTV margin for intra-fraction motion. ResultsThe mean values of the intra-fraction differences were 0.0mm, 0.5mm, 0.5mm and 0.0° for LR, SI, AP and roll, respectively. The corresponding standard deviations were 1.1mm, 0.8mm, 0.8mm and 0.6° for systematic uncertainties (Σ), 1.3mm, 2.0mm, 2.2mm and 0.3° for random uncertainties (σ). This intra-fraction motion requires margins of 2.2mm in LR, 2.1mm in SI and 2.1mm in AP directions. Inclusion of estimates of the effect of rotations and matching errors increases these margins to approximately 4mm in LR and 5mm in SI and AP directions. ConclusionsA new margin recipe has been developed to calculate margins for intra-fraction motion. This recipe is applicable to any measurement technique that is based on the difference between images taken before and after treatment.

Optimal Planning Target Volume Margins for Elective Pelvic Lymphatic Radiotherapy in High-Risk Prostate Cancer Patients

Purpose. High-risk prostate cancer patients often receive radiotherapy (RT) to pelvic lymphatics (PLs). The aim of this study was to determine the safety margin around clinical target volume for PL (PL-CTV) to construct planning target volume for PL (PL-PTV) and for planning elective PL irradiation. Methods and Materials. Six patients who received RT to PL as part of prostate cancer treatment were identified. To determine average daily shifts of PL, the right and left IVs were contoured at 3 predetermined slices on the daily MV scans and their daily shifts were measured at these 3 levels using a measuring tool. Results. A total of 1,932 observations were made. Daily shifts of IV were random in distribution, and the largest observed shift was 13.6 mm in lateral and 15.4 mm in AP directions. The mean lateral and AP shifts of IV were 2.1 mm (±2.2) and 3.5 mm (±2.7), respectively. The data suggest that AP and lateral margins of 8.9 mm and 6.5 mm are necessary. Conclusions. With daily alignment to the prostate, we recommend an additional PL-CTV to PL-PTV conversion margin of 9 mm (AP) and 7 mm (lateral) to account for daily displacement of PL relative to the prostate.

SU-E-J-28: Comparison of IGRT Shift Data Between Prostate Gland and Prostate Bed Obtained from Ct-On-Rails.

The daily shifts of prostate gland have been intensively reported in literatures. However, few papers reported daily shifts of prostate bed due to several practical difficulties (e.g. limited soft tissue contrast in MVCT and CBCT and significant deformation of prostate cavity). We have routinely performed IGRT for both prostate gland and bed with ct-on-rails, and the superior image quality allows us not only to differentiate both bony anatomy and soft tissue contrast of prostate gland and bed. In this study, we investigated if the shift of prostate bed is signifiant difference from that of prostate gland. we reviewed shift data of 50 prostate gland patients who underwent 43 fractions and 22 patients of prostatectomy underwent 37 fractions. In total 2150 CT scans were reviewed for prostate gland and 814 scans for prostate bed. Of the reviewed 814 CT images from 22 prostate bed patients, the standard deviation of shift was found to be 5.9 mm in AP direction (ranges from -22.4mm to 22mm), 3.2mm in SI direction (ranges from -14mm to 14mm), and 4.1mm in lateral direction (ranges from -15mm to 22mm). Of the 2150 CT images of prostate gland from 50 patients, the standard deviation of the shift was found to be 5.4 mm in AP direction (-20mm to 18 mm), 5.0mm in SI direction (-26mm to 20mm), and 4.3mm in lateral direction (range from-15 to 30mm). F tests of systematic /random shift distribution in three orthogonal directions between prostate gland and prostate bed were subsequently performed, it was found that the systematic shift in SI direction for prostate bed is smaller than for prostate gland (p=0.003). Our result suggests no significant difference existing in shift between prostate bed and gland. Therefore strategies for daily prostate gland motion can be directly applied to prostate bed.

SU‐FF‐J‐02: 3D Target Localization Using Cone‐Beam CT for Head and Neck IMRT Patients

Purpose: To evaluate clinical effectiveness of target localization using cone‐beam CT (CBCT) for H&N IMRT treatment. Method and Materials: All H&N patients treated with an IRB approved protocol were imaged on the first day of treatment and weekly thereafter with Varian On‐Board Imager. After the patient was aligned with skin markers, 2D kilovoltage portal images were acquired and compared to the planning DRRs for set‐up corrections (2D‐based correction). Then, CBCT images were acquired and registered with the planning CT by matching both soft tissue and bony landmarks, which generated the 3D‐based corrections. The difference between two correction measurements yielded the treatment isocenter variation using the 3D/3D matching method. Dose distributions with different isocenter deviations were re‐constructed on the planning CT. The treatment DVHs of PTVs and critical structures were compared with those in the original plan. Results: A total of 12 CBCT were analyzed for 3 patients during their first 4‐weeks of treatment. Compared to set‐up variations, anatomical shrinkage/deformation was negligible. The mean isocenter deviations after 2D‐based corrections were 1.1mm in AP direction (max 3.0mm), 2.6mm in lateral direction (max 5.0mm), and 3.8mm in SI direction (max 6.0mm). For the worst isocenter deviation, the reconstructed mean PTV dose was reduced by 3.6% from the planned mean value. For two patients, the mean brainstem dose increased from 33% to 49.5% and from 36.9% to 66.8%, respectively, in worst cases after 2D‐based correction. Other critical structures that received significantly higher doses were cord (max dose changed from 41.8% to 64%), eye (mean dose changed from 37.5% to 83.9%) and parotid (mean dose changed from 58% to 72.5%). Conclusion: For H&N IMRT treatment, CBCT image‐guidance improves localization accuracy compared to 2D‐based technique and should be the method of choice for target localization when tight margins are applied.Partially supported by Varian research grant.