Faculty Bibliography

Purpose: Patients receiving myeloablative total body irradiation (TBI) doses >= 12Gy are at risk of developing interstitial pneumonitis. At our institution, TBI transitioned from extended distance opposed Laterals to image-guided VMAT, in an effort to improve coverage while sparing lungs and kidneys. This work presents a dosimetric comparison between 3D Laterals and VMAT.
Method(s): Nine patients were treated with VMAT as part of an ongoing phase II single-arm clinical trial. VMAT patients were CT-simulated supine, with thermoplastic masks for head/neck, chest/abdomen/pelvis and feet. VMAT planning (12Gy (n=6) or 13.2Gy (n=3) in 8-BID fractions) utilizes 6MV multi-isocentric arcs and AP/PA beams to treat the upper and lower body, respectively. Ten 3D Lateral patients were CT-simulated supine with arms positioned/immobilized for lung shielding, with rice compensation between legs/feet. Laterals plan (12Gy in 8-BID fractions) uses 15MV, beam spoiler, head compensation, and subfields to maintain coverage and mean-lungs dose <10.5Gy. Target (Body-5mm, extending 3mm into lungs and kidneys for VMAT; Body-2cm for Laterals) coverage was evaluated at V100%, and D98% (percentage of Rx). Absolute dose to lungs and kidneys were reportedResults: Median Target V100% and D98% for VMAT was 93.2% (Range: 95.6% to 92.1%) and 90.2% (94.3% to 88.3%), whereas for Laterals V100% and D98% was 57.3% (66.5% to 46.3%) and 80.6% (75.5% to 84%). The median Lung mean dose was 7.6Gy (7.3Gy to 7.9Gy) for VMAT. The median mean dose to kidney was 10.4Gy (10.1Gy to 10.7Gy) for VMAT, and 12.5Gy (11.9Gy to 13.5Gy) for Laterals.
Conclusion(s): We have established a VMAT-TBI program for patients requiring myeloablative irradiation. Improvement in target coverage is demonstrated by V100% and D98%, while reducing the mean dose to the lungs significantly from 10.5Gy to 8Gy

Purpose: In-vivo dosimetry for conventional total body irradiation (TBI) utilize point detectors placed along the patient surface to confirm the delivered dose matches prescription dose. However, in the volumetrically modulated arc therapy (VMAT) approach to TBI, the electronic portal imager device (EPID) can be utilized to acquire a 2-dimensional transmission fluence plane. This work explores the relationship between patient setup accuracy with transit in-vivo dosimetry.
Method(s): A total of 192 fields were investigated. Each VMAT plan consisted of four isocenters: head, chest, abdomen, and pelvis. Prior to treatment, the patient was imaged at the head, pelvis, and chest. Optimal couch shifts were determined for each isocenter under image guidance. The optimal IGRT shifts were determined using an inhouse application that minimized dose deviation using criteria established through plan uncertainty analysis performed in Eclipse. Translational couch residuals were recorded and defined as the difference in the global shift calculated and the optimal couch position with shifts. Transit dosimetry was measured per arc, and analyzed using SNC PerFRACTION with a gamma criteria of 10%/5mm, 5%/5mm, and 5%/7mm.
Result(s): Based on plan uncertainty analysis, clinical threshold for couch residuals were set to 7 mm (5 mm for chest isocenter) as there would be minimal impact on target coverage and organ sparing at those levels. Transit dosimetry showed that the average pass rate across all fields was 99.6%, 97.0%, and 99.2% for 10%/5mm, 5%/5mm, and 5%/7mm gamma criteria, respectively. Pearson correlation tests showed that there was weak correlation between gamma criteria and couch residuals. At stringent 3%/5mm gamma criteria, moderate correlation was found between lateral couch residuals for the head and chest and the head and chest arc analysis.
Conclusion(s): Transit dosimetry showed high pass rates using our couch residual tolerances, which confirmed the plan uncertainty analysis performed during treatment planning

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 5 mm from the patient surface and extended 3 mm into the lungs and kidneys. Two plans were generated for each case: one to a total dose of 1200 cGy in 8 fractions and a second one to a total dose of 1320 cGy in 8 fractions. Plans were calculated using the AAA algorithm and 6 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 1200 cGy total dose the following dosimetric results were achieved: median PTV V100% = 94.5%; median PTV D98% = 89.9%; median lungs Dmean = 763 cGy; median left kidney Dmean = 1058 cGy; and median right kidney Dmean = 1051 cGy. For the plans prescribed to 1320 cGy total dose the following dosimetric results were achieved: median PTV V100% = 95.0%; median PTV D98% = 88.7%; median lungs Dmean = 798 cGy; median left kidney Dmean = 1059 cGy; and median right kidney Dmean = 1064 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 ±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.

Purpose: For the Cyberknife (Accuray Inc. Sunnyvale CA) stereotactic radiosurgery and stereotactic body radiotherapy treatment unit, a series of collimating cones are used with diameters ranging from 5 mm to 60 mm. The measurement of output factors (OFs) in these small radiation fields is challenging due to limitations in detector size. Currently multiple detectors are used for the measurement of output factors, but if not water equivalent, Monte Carlo corrections are required. The Exradin W2 (Standard Imaging Inc., Middleton, WI) plastic scintillation device (PSD) has an active volume of 0.0008 cm³ and is water equivalent. This work reports OFs and scanning data measured by the W2 PSD and a diode in Cyberknife.
Method(s): A Cyberknife M6 and a W2 PSD in a MP3 water phantom (PTW, Freiberg, Germany) were used to take TPR, profile, and OF measurements. The W2 comes with a specialized electrometer, MAX SD, used to measure the Cerenkov light ratio using the manufacturer's recommendation. Profiles, tissue phantom ratios (TPR), and OF values were taken using the clinically relevant (5, 7.5 and 10) mm cones, with OFs normalized to the 60 mm cone and scanning measurements normalized to maximum dose. All results were compared with PTW 60018 diode measurements taken during commissioning.
Result(s): The W2 scintillator required the slowest scan speed (0.1 mm/s) in order to eliminate noise, and TPR data was smoothed using a moving average filter. Comparison between diode and W2 profiles and TPR data showed good agreement. Monte Carlo adjusted diode OFs and W2 measured OFs were within 2.6%.
Conclusion(s): This work demonstrated the feasibility of using a water equivalent, small volume commercial W2 PSD in Cyberknife. Scanning data was near equivalent, but showed significant noise and required smoothing. W2 measured OFs agreed with MC-corrected diode measurements taken during commissioning

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