Comparison of kV-CBCT dose measurement techniques for annual Linac QA [Meeting Abstract]
Purpose: Modern linear accelerators (LINACs) are equipped with kV-imaging systems for 2D-images and 3D-cone-beam CT (CBCT). The delivered single kV-CBCT-dose is small compared to the treatment dose but can add up to a cumulative dose of 1-3% for multi-fraction courses according to TG180. Thus, TG142 recommends an annual assessment of the imaging dose. The purpose of this project was to review the current practice, compare approaches suggested in recent literature and standardize annual kV-CBCT dose measurements.
Method(s): Current institutional practice is to measure the CBCT dose with the 10cmlong RaySafe X2 pencil-ion-chamber inserted in the center position of a CIRS lung phantom. We introduced the PMMA-CTDI phantom in a satellite-location for all measurements. Dose was measured at 3/6/9/12 o'clock and center positions of a 32cm-wide CTDI phantom to determine max dose heterogeneity for a half-trajectory imaging protocol and at the 12 o'clock and center position of either the head or abdomen phantom for 11 protocols ranging from 80kVp/100mAs to 140kVp/1688mAs using the Raysafe X2, the Standard Imaging A101 pencil-ion-chamber, and the A12 Farmer-ion-chamber. Measurements were repeated with decreased kV-tube-collimator opening as suggested by Varian.
Result(s): The dose discrepancy due to chamber position was largest between the 6 and 3 o'clock positions (10%) and the average was close to the peripheral measurement at 12 o'clock. All three detectors showed a linear dependency. Reducing CBCT beam collimation increased measurement complexity with only minimally improved agreement between measured and displayed CTDIw values. The standard deviation for dose measurements at different machines reduced from 22%/12% for the lung phantom to 7%/3% (max/mean) for the CTDI phantom.
Conclusion(s): Dose measurements at 12 o'clock and center position of CTDI-phantom have been defined as our new standard using un-modified treatment imaging protocols; conversion factors for center-only or lung-phantom measurements have been provided
Streamlining complex multi-isocentric VMAT based treatment delivery using a newly developed software tool [Meeting Abstract]
Purpose: Multi-isocentric treatment delivery for CSI and TBI poses specific challenges for treatment delivery. We have developed a software tool to streamline all aspects of delivery for therapists and physicists at the machine, as well as to inform attending physicians of setup variability and image residuals at different locations.
Method(s): Our institution delivers VMAT-based CSI and TBI with up to 3 and 7 isocenters, respectively. A software tool was developed to assist with treatment delivery including initial patient setup, patient imaging, automatic calculation of the optimal global shift based on each isocenter's ideal shift, and automatic calculation of each isocenter's couch coordinates. Initial treatment couch coordinates are queried via the Eclipse scripting API. The global shift was calculated prioritizing the head isocenter for CSI treatments and the chest isocenter for TBI treatments by first maximizing residual tolerance at any other location prior to accepting any residual deviation at these locations. Maximum residuals tolerance was determined based on target margins, plan uncertainty and as per physician instructions. Delivery parameters are reported to a document uploaded to ARIA via API.
Result(s): The developed tool was employed for 11 cases. The software tool replaced the need for plan shift comments or instructions for therapists. In particular, its use eliminated the need to provide isocenter shifts to therapists by directly providing final couch parameters for treatment, greatly reducing the risk of delivery errors. The software effectively informed the therapists if any expected tolerance was surpassed, triggering a patient setup evaluation.
Conclusion(s): The described software tool is a core component to our multi-isocenter treatment programs and has streamlined delivery of these complex techniques that would otherwise require complicated instructions, including multiple shifts and on-the-fly calculations of optimal image alignment based on multiple imaging locations. This has substantially reduced the possibility of delivery errors
Automatic treatment planning for VMAT-based total body irradiation using Eclipse scripting
The purpose of this work is to establish an automated approach for a multiple isocenter volumetric arc therapy (VMAT)-based TBI treatment planning approach. Five anonymized full-body CT imaging sets were used. A script was developed to automate and standardize the treatment planning process using the Varian Eclipse v15.6 Scripting API. The script generates two treatment plans: a head-first VMAT-based plan for upper body coverage using four isocenters and a total of eight full arcs; and a feet-first AP/PA plan with three isocenters that covers the lower extremities of the patient. PTV was the entire body cropped 5Â mm from the patient surface and extended 3Â mm into the lungs and kidneys. Two plans were generated for each case: one to a total dose of 1200Â cGy in 8 fractions and a second one to a total dose of 1320Â cGy in 8 fractions. Plans were calculated using the AAA algorithm and 6Â MV photon energy. One plan was created and delivered to an anthropomorphic phantom containing 12 OSLDs for in-vivo dose verification. For the plans prescribed to 1200Â cGy total dose the following dosimetric results were achieved: median PTV V100%Â =Â 94.5%; median PTV D98%Â =Â 89.9%; median lungs DmeanÂ =Â 763Â cGy; median left kidney DmeanÂ =Â 1058Â cGy; and median right kidney DmeanÂ =Â 1051Â cGy. For the plans prescribed to 1320Â cGy total dose the following dosimetric results were achieved: median PTV V100%Â =Â 95.0%; median PTV D98%Â =Â 88.7%; median lungs DmeanÂ =Â 798Â cGy; median left kidney DmeanÂ =Â 1059Â cGy; and median right kidney DmeanÂ =Â 1064Â cGy. Maximum dose objective was met for all cases. The dose deviation between the treatment planning dose and the dose measured by the OSLDs was within Â±4%. In summary, we have demonstrated that scripting can produce high-quality plans based on predefined dose objectives and can decrease planning time by automatic target and optimization contours generation, plan creation, field and isocenter placement, and optimization objectives setup.
VMAT-based total body irradiation treatment plans with eclipse scripting for field configuration: A dosimetric evaluation [Meeting Abstract]
Purpose: Radiation induced interstitial pneumonitis and late renal dysfunction are major concerns for patients undergoing total body irradiation (TBI). The purpose of this work is to evaluate the dosimetry of VMAT-based TBI plans generated using Varian Eclipse scripting.
Method(s): Three full-body CT datasets (two patients, one anthropomorphic CIRS phantom) were used. An in-house Eclipse script was developed to generate optimized field arrangements using the body contour, user origin, and couch longitudinal travel. Plans consisted of a lower-body AP/PA portion and an upper-body VMAT portion (8 full arcs with 4-isocenters). Treatment plans to 1320 cGy (165 cGy x 8fx) were generated with dose directives: [PTV V100% >=90- 95%; Total lung Dmean <900 cGy; Kidneys Dmean <1100 cGy]. All plans used 6MV photons and were calculated using the AAA algorithm. Upperbody VMAT plan dosimetry was evaluated 'in-phantom' placing 12 OSLDs in different key locations (lung, kidneys, bone, and soft tissue). Additionally, dosimetric verification was performed for the three plans using Varian portal dosimetry, PerFraction(SNC) and ArcCheck(SNC) with a global gamma criterion of 2%/2 mm.
Result(s): Planning objectives were met for the three treatment plans with the following averages: PTV V100% = 94.02%, total lung Dmean = 872.9 cGy, and kidneys Dmean = 1075.8 cGy. The dose deviation between Eclipse and the OSLDs (relative to the prescribed dose) averaged 0.98%, with each individual dose deviation within +/-4%. Dose ranged between 52.5 cGy (lung) and 187.5 cGy (bone) for OSLD measurements. The average passing rate for all 24 fields (8 per plan) was 98.0%, 99.76% and 98.6% for portal dosimetry, PerFraction and ArcCheck respectively. The lowest passing rate of any individual field was 95.4%, 99.0% and 91.8% for portal dosimetry, PerFraction and ArcCheck respectively.
Conclusion(s): Eclipse scripting can assist in creating robust multi-isocentric VMATbased TBI treatment plans to block lungs and kidneys without compromising target coverage. Dosimetric accuracy and deliverability was confirmed using in-phantom OSLD dosimetry, Varian portal dosimetry, PerFraction and Arc-Check verification
Code of Ethics for the American Association of Physicists in Medicine (Revised): Report of Task Group 109
The American Association of Physicists in Medicine (AAPM) has established a comprehensive Code of Ethics for its members. The Code is a formal part of AAPM governance, maintained as Professional Policy 24, and includes both principles of ethical practice and the rules by which a complaint will be adjudicated. The structure and content of the Code have been crafted to also serve the much broader purpose of giving practical ethical guidance to AAPM members for making sound decisions in their professional lives. The Code is structured in four major sections: a Preamble, a set of ten guiding Principles, Guidelines that elucidate the application of the Principles in various practice settings, and the formal Complaint process. Guidelines have been included to address evolving social and cultural norms, such as the use of social media and the broadening scope of considerations important in an evolving workplace. The document presented here is the first major revision of the AAPM Code of Ethics since 2008. This revision was approved by the Board of Directors to become effective 1 January 2019.
MRI Based Treatment Planning of Spinal Stereotactic Radiation Therapy [Meeting Abstract]
NYU approach to CT-based planning total body irradiation (TBI) [Meeting Abstract]
Purpose: TBI treatment at our institution has moved from traditional hand calculation to CT-based planning to incorporate dose heterogeneities and organs at risk dose limits. The main objective of this work is to report our institutional experience with CT-based TBI and to show a comparison with the traditional approach. Methods: Ten patients were CT simulated supine with arms immobilized for lung shielding. Legs are separated to achieve a width similar to umbilicus separation; rice bags were placed between the legs for compensation. Four plans (P1, P2, P3 and P4) were created for each patient, all prescribed at midplane-umbilicus. The first three plans use lateral 15X beams, with head compensation. P1 was planned using a hand calculation. P2 includes heterogeneity corrections and inferior subfield to improve coverage. P3 includes heterogeneity corrections, inferior subfield, and adjustment of field weights to maintain coverage while keeping mean lung doses below 10.5 Gy (prescription dose 12 Gy). P4 uses AP-PA 6X beams. Dose to target (mean, max, D98%, D95%, min), mean lung and liver doses are calculated for all plans; reported doses when unitless and normalized to prescription dose. Results: Coverage of the target (Body-2 cm), indicated by D98% was 84.1 +/- 2.8, 84.7 +/- 3.9, 81.0 +/- 1.8, and 92.2 +/- 1.9 whereas the maximum doses were 123 +/- 5, 135 +/- 4, 129 +/- 4, and 124 +/- 5 for P1, P2, P3, and P4 respectively. The mean relative lung and liver doses were lowest for P3 with values of 87.8 +/- 0.5 and 89.8 +/- 3.4. The largest mean lung dose (12.5 Gy) was observed for P4 plan as expected, showing the necessity of using lung shielding. Conclusion: We are able to achieve target coverage of D98% >80%, keeping the mean lung and liver doses <90% of prescription using optimal arm positioning and subfields. This approach is easy to implement without the complexity of introducing lung shielding required with the use of 6X AP-PAbeams
Spatial and Dosimetric Comparison of Tandem/Ring Applicator Against Adjustable Tandem/Ovoid and Tandem/Split-Ring for Intracavitary Brachytherapy Treatment of Cervical Cancer [Meeting Abstract]
Implications of Monte Carlo Dose Calculation for Structures of Very Small Volume Compared with Measurement [Meeting Abstract]
Dosimetric Validation of Mask-Based Stereotactic Gamma Knife Radiosurgery Using Pseudo in Vivo 3D Dosimetry: End-To-End SRS Quality Assurance [Meeting Abstract]