Intrafractional motion and dosimetric analysis in prostate stereotactic body radiation therapy with auto beam hold technique.

Objective: To summarize our institutional prostate stereotactic body radiation therapy (SBRT) experience using auto beam hold (ABH) technique for intrafractional prostate motion and assess ABH tolerance of 10-millimeter (mm) diameter.Approach: Thirty-two patients (160 fractions) treated using ABH technique between 01/2018 and 03/2021 were analyzed. During treatment, kV images were acquired every 20-degree gantry rotation to visualize 3-4 gold fiducials within prostate to track target motion. If the fiducial center fell outside the tolerance circle (diameter = 10 mm), beam was automatically turned off for reimaging and repositioning. Number of beam holds and couch translational movement magnitudes were recorded. Dosimetric differences from intrafractional motion were calculated by shifting planned isocenter.Main Results: Couch movement magnitude (mean ± SD) in vertical, longitudinal and lateral directions were -0.7 ± 2.5, 1.4 ± 2.9 and -0.1 ± 0.9 mm, respectively. For most fractions (77.5%), no correction was necessary. Number of fractions requiring one, two, or three corrections were 15.6%, 5.6% and 1.3%, respectively. Of the 49 corrections, couch shifts greater than 3 mm were seen primarily in the vertical (31%) and longitudinal (39%) directions; corresponding couch shifts greater than 5 mm occurred in 2% and 6% of cases. Dosimetrically, 100% coverage decreased less than 2% for clinical target volume (CTV) (-1 ± 2%) and less than 10% for PTV (-10 ± 6%). Dose to bladder, bowel and urethra tended to increase (Bladder: ΔD10%:184 ± 466 cGy, ΔD40%:139 ± 241 cGy, Bowel: ΔD1 cm3:54 ± 129 cGy; ΔD5 cm3:44 ± 116 cGy, Urethra: ΔD0.03 cm3:1 ± 1%). Doses to the rectum tended to decrease (Rectum: ΔD1 cm3:-206 ± 564 cGy, ΔD10%:-97 ± 426 cGy; ΔD20%:-50 ± 251 cGy).Significance: With the transition from conventionally fractionated intensity modulated radiation therapy to SBRT for localized prostate cancer treatment, it is imperative to ensure that dose delivery is spatially accurate for appropriate coverage to target volumes and limiting dose to surrounding organs. Intrafractional motion monitoring can be achieved using triggered imaging to image fiducial markers and ABH to allow for reimaging and repositioning for excessive motion.

Usefulness of EZFluence software for radiotherapy planning of breast cancer treatment.

This study compared the EZFluence planning technique for irradiation of the breast with commonly used Field-in-Field (FiF) technique by analyzing the dose uniformity, the dose to the lung, heart, and other organs at risk, the total Monitor Unit (MU), and the time spent for planning. Two different 3-dimensional conformal dose plans were created for 20 breast cancer patients. Six patients were treated to a dose of 5000 cGy in 25 fractions and 14 were treated to a dose of 4256 cGy in 16 fractions. Average breast volume was 800 cc (range 128 to 1892 cc). For the FiF technique, the planner manually created between 2 to 4 subfields per gantry angle and sequentially blocked the 115% and 110% isodose line until a homogenous dose distribution was achieved. For the EZFluence technique, the planner implemented the EZFluence script that created an optimal fluence pattern, which was then imported into Eclipse where dose was calculated. Both techniques were optimized to make sure 95% of the breast planning target volume (PTV) received at least 95% of the prescribed dose. Compared to FiF technique, the plans produced by using EZFluence technique, showed the MU increased by 36.9% (p = 0.0002), whereas the planning time decreased significantly by 84.6% (p = 0.00001). The mean heart dose and the relative volume of the heart receiving ≥ 30 Gy (V30) were similar for both techniques. The mean lung dose and the relative volume of lung receiving ≥ 20 Gy (V20) were also comparable between 2 techniques. The contralateral breast mean dose and its relative volume receiving ≥ 3 Gy (V3) and ≥10 Gy (V10) were equally spared and avoided. EZFluence planning technique yielded a 4.6% (p = 0.04) reduction in PTV receiving 105% of the prescribed dose (V105) for the large breast with separation > 22 cm and PTV volume > 650 cc. The EZFluence planning technique yielded the overall comparable or improved dosimetry while significantly reducing planning time.

Dose rate response of Digital Megavolt Imager detector for flattening filter-free beams.

In this study we investigated the dose rate response characteristics of the Digital Megavolt Imager (DMI) detector, including panel saturation, linearity, and imager ghosting effects for flattening filter-free (FFF) beams. The DMI detector dose rate response characteristics were measured as a function of dose rate on a Varian TrueBeam machine. Images were acquired at dose rates ranging from 400 to 1400 MU/min for 6XFFF and 400 to 2400 MU/min for 10XFFF. Line profiles and central portal doses derived from the images were analyzed and compared. The linearity was verified by acquiring images with incremental Monitor Unit (MU) ranging from 5 to 500 MU. Ghosting effects were studied at different dose rates. Finally, for validation, test plans with optimal fluence were created and measured with different dose rates. All test plans were analyzed with a Gamma criteria of 3%-3 mm and 10% dose threshold. Our study showed that there was no panel saturation observed from the profile comparison even at the maximum dose rate of 2400 MU/min. The central portal doses showed a slight decrease (1.013-1.008 cGy/MU for 6XFFF, and 1.020-1.009 cGy/MU for 10XFFF) when dose rate increased (400-1400 MU/min for 6XFFF, and 400-2400 MU/min for 10XFFF). The linearity of the DMI detector response was better than 0.5% in the range of 20-500 MU for all energies. The residual image was extremely small and statistically undetectable. The Gamma index measured with the test plans decreased from 100% to 97.8% for 6XFFF when dose rate increased from 400 to 1400 MU/min. For 10XFFF, the Gamma index decreased from 99.9% to 91.5% when dose rate increased from 400 to 2400 MU/min. We concluded that the Portal Dosimetry system for the TrueBeam using DMI detector can be reliably used for IMRT and VMAT QA for FFF energies.

Relationship between dosimetric leaf gap and dose calculation errors for high definition multi-leaf collimators in radiotherapy.

BACKGROUND AND PURPOSE: Dosimetric leaf gap (DLG) is a parameter to model the round-leaf-end effect of multi-leaf collimators (MLC) that is important for treatment planning dose calculations in radiotherapy. In this study we investigated on the relationship between the DLG values and the dose calculation errors for a high-definition MLC. MATERIALS AND METHODS: Three sets of experiments were conducted: (1) physical DLG measurements using sweeping-gap technique, (2) DLG adjustment based on spine radiosurgery plan measurements, and (3) DLG verification using films and ion-chambers (IC). All experiments were conducted on a Varian Edge machine equipped with HD120 MLC for 6X, 6XFFF, and 10XFFF (FFF: flattening filter free). The Analytical Anisotropic Algorithm was used for all dose calculations. RESULTS: The measured physical DLGs were 0.39 mm, 0.27 mm, and 0.42 mm for 6X, 6XFFF, and 10XFFF respectively. The calculated doses were lower by 4.2% (6X), 3.7% (6XFFF), and 6.8% (10XFFF) than the measured, while the adjusted DLG values with minimum errors were 1.1 mm, 0.9 mm, and 1.5 mm. The IC measurement errors were < 1%, and the film gamma pass rates (3%/3 mm) were greater than 97% for the spine plans. CONCLUSIONS: The calculated doses were systematically lower than measured doses with the physical DLG values. It was necessary to increase the DLG values to minimize the dose calculation uncertainty. The optimal DLG values may be specific to individual MLCs and beams and, thus, careful evaluation and verification are warranted.

Use of 3D printers to create a patient-specific 3D bolus for external beam therapy.

The purpose of this paper is to demonstrate that an inexpensive 3D printer can be used to manufacture patient-specific bolus for external beam therapy, and to show we can accurately model this printed bolus in our treatment planning system for accurate treatment delivery. Percent depth-dose measurements and tissue maximum ratios were used to determine the characteristics of the printing materials, acrylonitrile butadiene styrene and polylactic acid, as bolus material with physical density of 1.04 and 1.2 g/cm3, and electron density of 3.38 × 10²³ electrons/cm3 and 3.80 × 10²³ electrons/ cm3, respectively. Dose plane comparisons using Gafchromic EBT2 film and the RANDO phantom were used to verify accurate treatment planning. We accurately modeled a printing material in Eclipse treatment planning system, assigning it a Hounsfield unit of 260. We were also able to verify accurate treatment planning using gamma analysis for dose plane comparisons. With gamma criteria of 5% dose difference and 2 mm DTA, we were able to have 86.5% points passing, and with gamma criteria of 5% dose difference and 3 mm DTA, we were able to have 95% points passing. We were able to create a patient-specific bolus using an inexpensive 3D printer and model it in our treatment planning system for accurate treatment delivery.