The effect of spill change on reliable absolute dosimetry in a synchrotron proton spot scanning system.

Absolute dosimetry measurement is an integral part of Treatment Planning System (TPS) commissioning and it involves measuring the integrated absorbed dose to water for all energies in a pencil beam scanning delivery system. During the commissioning of Singapore's first proton therapy center, a uniform scanned field with an Advanced Markus chamber method was employed for this measurement, and a large dose fluctuation of at least 5% was observed for 10% of the energy layers during repeated measurements. This study aims to understand the root cause of this fluctuation by relating the actual delivered spot information in the log file with the charge measurement by the ion chambers. A dedicated pencil beam dose algorithm was developed, taking into account the log file parameters, to calculate the dose for a single energy layer in a homogeneous water phantom. Three energies, 70.2, 182.7, and 228.7 MeV were used in this study, with the 182.7 MeV energy exhibiting large dose fluctuation. The dose fluctuation was investigated as a function of detector's sizes (pinpoint 3D, Advanced Markus, PTW 34070, and PTW 34089) and water depth (2 , 6, and 20 cm). Twelve ion chambers measurements were performed for each chamber and depth. The comparison of the theoretically predicted integrated dose and the charge measurement served as a validation of the algorithm. About 5.9% and 9.6% dose fluctuation were observed in Advanced Markus and pinpoint 3D measurements at 2 cm depth for 182.7 MeV, while fluctuation of 1.6% and 1.1% were observed in Advanced Markus with 228.7 and 70.2 MeV at similar depth. Fluctuation of less than 0.1% was observed for PTW34070 and PTW 34089 for all energies. The fluctuation was found to diminish with larger spot size at 20 cm depth to 1.3% for 182.7 MeV. The theoretical and measured charge comparison showed a high linear correlation of R 2 > 0.80 ${R^2} > 0.80$ for all datasets, indicating the fluctuation originated from the delivered spot characteristics. The cause of fluctuation was identified to be due to the spill change occurring close to the detector, and since the spot positional deviation profiles were different between two spills, this resulted in local hot spots between columns of spots. The actual position of spill change varies randomly during measurement, which led to a random occurrence of hot spot within the detector's sensitive volume and a fluctuating dose measurement. This is the first report of a dose fluctuation greater than 5% in absolute dosimetry measurement with a uniform scanned field and the cause of the fluctuation has been conclusively determined. It is important to choose the MU and scanning pattern carefully to avoid spill change happening when the spot delivery is near the detector.

Tan HQ ，Lew KS ，Koh CWY ，Wibawa A ，Master Z ，Beltran CJ ，Park SY ，Furutani KM ... -  《-》

A procedure to determine the planar integral spot dose values of proton pencil beam spots.

Anand A ，Sahoo N ，Zhu XR ，Sawakuchi GO ，Poenisch F ，Amos RA ，Ciangaru G ，Titt U ，Suzuki K ，Mohan R ，Gillin MT ... -  《MEDICAL PHYSICS》

Commissioning and beam characterization of the first gantry-mounted accelerator pencil beam scanning proton system.

To present and discuss beam characteristics and commissioning process of the first gantry-mounted accelerator single room pencil beam scanning (PBS) proton system. The Mevion HYPERSCAN employs a design configuration with a synchrocyclotron mounted on the gantry to eliminate the traditional beamline and a nozzle that contains the dosimetry monitoring chambers, the energy modulator (Energy Selector (ES)), and an Adaptive Aperture (AA). To characterize the beam, we measured the integrated depth dose (IDDs) for 12 energies, from highest energy of 227 MeV down to 28 MeV with a range difference ~ 2 cm between the adjacent energies, using a large radius Bragg peak chamber; single-spot profiles in air at five locations along the beam central axis using radiochromic EBT3 film and cross compared with a scintillation detector; and determined the output using a densely packed spot map. To access the performance of AA, we measured interleaf leakage and the penumbra reduction effect. Monte Carlo simulation using TOPAS was performed to study spot size variation along the beam path, beam divergence, and energy spectrum. This proton system is calibrated to deliver 1 Gy dose at 5 cm depth in water using the highest beam energy by delivering 1 MU/spot to a 10 × 10 cm2 map with a 2.5 mm spot spacing. The spot size in air varies from 4 mm to 26 mm from 227 MeV to 28 MeV at the isocenter plane with the nozzle retracted 23.6 cm from isocenter. The beam divergence of 28 MeV beam is ~ 52.7 mrad, which is nearly 22 times that of 227 MeV proton beam. The binary design of the ES has resulted in shifts of the effective SSD toward the isocenter as the energy is modulated lower. The peaks of IDD curves have a constant 80%-80% width of 8.4 mm at all energies. The interleaf leakage of the AA is less than 1.5% at the highest energy; and the AA can reduce the penumbra by 2 mm to 13 mm for the 227 and 28 MeV energies at isocenter plane in air. The unique design of the HYPERSCAN proton system has yielded beam characteristics significantly different from that of other proton systems in terms of the Bragg peak shapes, spot sizes, and the penumbra sharpening effect of the AA. The combination of the ES and AA has made PBS implementation possible without using beam transport line and range shifter devices. Different considerations may be required in treatment planning optimization to account for different design and beam characteristics.

Kang M ，Pang D 《-》

Ultrahigh dose rate pencil beam scanning proton dosimetry using ion chambers and a calorimeter in support of first in-human FLASH clinical trial.

To provide ultrahigh dose rate (UHDR) pencil beam scanning (PBS) proton dosimetry comparison of clinically used plane-parallel ion chambers, PTW (Physikalisch-Technische Werkstaetten) Advanced Markus and IBA (Ion Beam Application) PPC05, with a proton graphite calorimeter in a support of first in-human proton FLASH clinical trial. Absolute dose measurement intercomparison of the plane-parallel plate ion chambers and the proton graphite calorimeter was performed at 5-cm water-equivalent depth using rectangular 250-MeV single-layer treatment plans designed for the first in-human FLASH clinical trial. The dose rate for each field was designed to remain above 60 Gy/s. The ion recombination effects of the plane-parallel plate ion chambers at various bias voltages were also investigated in the range of dose rates between 5 and 60 Gy/s. Two independent model-based extrapolation methods were used to calculate the ion recombination correction factors ks to compare with the two-voltage technique from most widely used clinical protocols. The mean measured dose to water with the proton graphite calorimeter across all the predefined fields is 7.702 ± 0.037 Gy. The average ratio over the predefined fields of the PTW Advanced Markus chamber dose to the calorimeter reference dose is 1.002 ± 0.007, whereas the IBA PPC05 chamber shows ∼3% higher reading of 1.033 ± 0.007. The relative differences in the ks values determined from between the linear and quadratic extrapolation methods and the two-voltage technique for the PTW Advanced Markus chamber are not statistically significant, and the trends of dose rate dependence are similar. The IBA PPC05 shows a flat response in terms of ion recombination effects based on the ks values calculated using the two-voltage technique. Differences in ks values for the PPC05 between the two-voltage technique and other model-based extrapolation methods are not statistically significant at FLASH dose rates. Some of the ks values for the PPC05 that were extrapolated from the three-voltage linear method and the semiempirical model were reported less than unity possibly due to the charge multiplication effect, which was negligible compared to the volume recombination effect in FLASH dose rates. The absolute dose measurements of both PTW Advanced Markus and IBA PPC05 chambers are in a good agreement with the National Physical Laboratory graphite calorimeter reference dose considering overall uncertainties. Both ion chambers also demonstrate good reproducibility as well as stability as reference dosimeters in UHDR PBS proton radiotherapy. The dose rate dependency of the ion recombination effects of both ion chambers in cyclotron generated PBS proton beams is acceptable and therefore, both chambers are suitable to use in clinical practice for the range of dose rates between 5 and 60 Gy/s.

Lee E ，Lourenço AM ，Speth J ，Lee N ，Subiel A ，Romano F ，Thomas R ，Amos RA ，Zhang Y ，Xiao Z ，Mascia A ... -  《-》

Commissioning a 250 MeV research beamline for proton FLASH radiotherapy preclinical experiments.

The potential reduction of normal tissue toxicities during FLASH radiotherapy (FLASH-RT) has inspired many efforts to investigate its underlying mechanism and to translate it into the clinic. Such investigations require experimental platforms of FLASH-RT capabilities. To commission and characterize a 250 MeV proton research beamline with a saturated nozzle monitor ionization chamber for proton FLASH-RT small animal experiments. A 2D strip ionization chamber array (SICA) with high spatiotemporal resolution was used to measure spot dwell times under various beam currents and to quantify dose rates for various field sizes. An Advanced Markus chamber and a Faraday cup were irradiated with spot-scanned uniform fields and nozzle currents from 50 to 215 nA to investigate dose scaling relations. The SICA detector was set up upstream to establish a correlation between SICA signal and delivered dose at isocenter to serve as an in vivo dosimeter and monitor the delivered dose rate. Two off-the-shelf brass blocks were used as apertures to shape the dose laterally. Dose profiles in 2D were measured with an amorphous silicon detector array at a low current of 2 nA and validated with Gafchromic films EBT-XD at high currents of up to 215 nA. Spot dwell times become asymptotically constant as a function of the requested beam current at the nozzle of greater than 30 nA due to the saturation of monitor ionization chamber (MIC). With a saturated nozzle MIC, the delivered dose is always greater than the planned dose, but the desired dose can be achieved by scaling the MU of the field. The delivered doses exhibit excellent linearity with R 2 > 0.99 ${R^2} > 0.99$ with respect to MU, beam current, and the product of MU and beam current. If the total number of spots is less than 100 at a nozzle current of 215 nA, a field-averaged dose rate greater than 40 Gy/s can be achieved. The SICA-based in vivo dosimetry system achieved excellent estimates of the delivered dose with an average (maximum) deviation of 0.02 Gy (0.05 Gy) over a range of delivered doses from 3 to 44 Gy. Using brass aperture blocks reduced the 80%-20% penumbra by 64% from 7.55 to 2.75 mm. The 2D dose profiles measured by the Phoenix detector at 2 nA and the EBT-XD film at 215 nA showed great agreement, with a gamma passing rate of 95.99% using 1 mm/2% criterion. A 250 MeV proton research beamline was successfully commissioned and characterized. Challenges due to the saturated monitor ionization chamber were mitigated by scaling MU and using an in vivo dosimetry system. A simple aperture system was designed and validated to provide sharp dose fall-off for small animal experiments. This experience can serve as a foundation for other centers interested in implementing FLASH radiotherapy preclinical research, especially those equipped with a similar saturated MIC.

Yang Y ，Kang M ，Chen CC ，Hu L ，Yu F ，Tsai P ，Huang S ，Liu J ，Turner R ，Shen B ，Hasan S ，Chhabra AM ，Choi JI ，Bell B ，Pennock M ，Tome WA ，Guha C ，Simone CB 2nd ，Lin H ... -  《-》