Improving treatment geometries in total skin electron therapy: Experimental investigation of linac angles and floor scatter dose contributions using Cherenkov imaging.

The purpose of this study was to identify the optimal treatment geometry for total skin electron therapy (TSET) using a new optimization metric from Cherenkov image analysis, and to investigate the sensitivity of the Cherenkov imaging method to floor scatter effects in this unique treatment setup. Cherenkov imaging using an intensified charge coupled device (ICCD) was employed to measure the relative surface dose distribution as a 2D image in the total skin electron treatment plane. A 1.2 m × 2.2 m × 1 cm white polyethylene sheet was placed vertically at a source to surface distance (SSD) of 300 cm, and irradiated with 6 MeV high dose rate TSET beams. The linear accelerator coordinate system used stipulates 0° is the bottom of the gantry arc, and progresses counterclockwise so that gantry angle 270° produces a horizontal beam orthogonal to the treatment plane. First, all unique pairs of treatment beams were analyzed to determine the performance of the currently recommended symmetric treatment angles (±20° from the horizontal), compared to treatment geometries unconstrained to upholding gantry angle symmetry. This was performed on two medical linear accelerators (linacs). Second, the extent of the floor scatter contributions to measured surface dose at the extended SSD required for TSET were imaged using three gantry angles of incidence: 270° (horizontal), 253° (-17°), and 240° (-30°). Images of the surface dose profile at each angle were compared to the standard concrete floor when steel plates, polyvinyl chloride (PVC), and solid water were placed on the ground at the base of the treatment plane. Postprocessing of these images allowed for comparison of floor material-based scatter profiles with previously published simulation results. Analysis of the symmetric treatment geometry (270 ± 20°) and the identified optimal treatment geometry (270 + 23° and 270 - 17°) showed a 16% increase in the 90% isodose area for the latter field pair on the first linac. The optimal asymmetric pair for the second linac (270 + 25° and 270 - 17°) provided a 52% increase in the 90% isodose area when compared to the symmetric geometry. Difference images between Cherenkov images captured with test materials (steel, PVC, and solid water) and the control (concrete floor) demonstrated relative changes in the two-dimensional (2D) dose profile over a 1 × 1.9 m region of interest (ROI) that were consistent with published simulation data. Qualitative observation of the residual images demonstrates localized increases and decreases with respect to the change in floor material and gantry angle. The most significant changes occurred when the beam was most directly impinging the floor (gantry angle 240°, horizontal -30°), where the PVC floor material decreased scatter dose by 1-3% in 7.2% of the total ROI area, and the steel plate increased scatter dose by 1-3% in 7.0% of the total ROI area. An updated Cherenkov imaging method identified asymmetric, machine-dependent TSET field angle pairs that provided much larger 90% isodose areas than the commonly adopted symmetric geometry suggested by Task Group 30 Report 23. A novel demonstration of scatter dose Cherenkov imaging in the TSET field was established.

Andreozzi JM ，Brůža P ，Tendler II ，Mooney KE ，Jarvis LA ，Cammin J ，Li H ，Pogue BW ，Gladstone DJ ... -  《-》

Cherenkov imaging method for rapid optimization of clinical treatment geometry in total skin electron beam therapy.

Andreozzi JM ，Zhang R ，Gladstone DJ ，Williams BB ，Glaser AK ，Pogue BW ，Jarvis LA ... -  《-》

A scattering-foil free electron beam to increase dose rate for total skin electron therapy (TSET).

Electron beams are used at extended distances ranging between 300 to 700 cm to uniformly cover the entirety of the patient's skin for total skin electron therapy (TSET). Even with electron beams utilizing the high dose rate total skin electron (HDTSe) mode from the Varian 23iX or TrueBeam accelerators, the dose rate is only 2500 cGy/min at source-to-surface distance (SSD) = 100 cm. At extended distances, the decrease in dose rate leads to long beam delivery times that can limit or even prevent the use of the treatment for patients who, in their weakened condition, may be unable to stand on their own for extended periods of time. Previously, to increase dose rate, a customized 6 MeV electron beam was created by removing the x-ray target, flattening filter, beam monitor chamber, and so forth. from the beam path (Chen, et at IJROBP 59, 2004) for TSET. Using this scattering-foil free (SFF) electron beam requires the treatment distance be extended to 700 cm to achieve dose uniformity from the single beam. This room size requirement has limited the widespread use of the 6 MeV-SFF beam. This study explores an application of a dual-field technique with a 6 MeV-SFF beam to provide broad and uniform electron fields to reduce the treatment distances in order to overcome treatment room size limitations. The EGSnrc system was used to generate incident beams. Gantry angles between 6 MeV-SFF dual-fields were optimized to achieve the similar patient skin dose distribution resulting from a standard 6 MeV-HDTSe dual-field configuration. The patient skin dose comparisons were performed based on the patient treatment setup geometries using dose-volume-histograms. Similar dose coverage can be achieved between 6 MeV-SFF and 6 MeV-HDTSe beams by reducing gantry angles between dual-field geometries by 8° and 7° at treatment distances of 400 and 500 cm, respectively. To achieve 95% mean dose to the first 5 mm of skin depth in the torso area, the mean dose to depths of 5-10 mm and 10-15 mm below the skin surface was 74% (74%) and 49% (50%) of the prescribed dose when using 6 MeV-SFF (6 MeV-HDTSe) beam, respectively. The 6 MeV-SFF electron beam is feasible to provide similar TSET skin dose coverage at SSD ≥ 400 cm using a dual-field technique. The dose rate of the 6 MeV-SFF beam is about 4 times that of current available 6 MeV-HDTSe beams at treatment distances of 400-500 cm, which significantly shortens the treatment beam-on time and makes TSET available to patients in weakened conditions.

Ding GX ，Chen ZJ ，Homann K 《-》

An investigation of clinical treatment field delivery verification using cherenkov imaging: IMRT positioning shifts and field matching.

Cherenkov light emission has been shown to correlate with ionizing radiation dose delivery in solid tissue. An important clinical application of Cherenkov light is the real-time verification of radiation treatment delivery in vivo. To test the feasibility of treatment field verification, Cherenkov light images were acquired concurrent with radiation beam delivery to standard and anthropomorphic phantoms. Specifically, we tested two clinical treatment scenarios: (a) Observation of field overlaps or gaps in matched 3D fields and (b) Patient positioning shifts during intensity modulated radiation therapy (IMRT) field delivery. Further development of this technique would allow real-time detection of treatment delivery errors on the order of millimeters so that patient safety and treatment quality can be improved. Cherenkov light emission was captured using a PI-MAX4 intensified charge coupled device (ICCD) system (Princeton Instruments). All radiation delivery was performed using a Varian Trilogy linear accelerator (linac) operated at 6 MV or 18 MV for photon and 6 MeV or 16 MeV for electron studies. Field matching studies were conducted with photon and electron beams at gantry angles of 0°, 15°, and 45°. For each modality and gantry angle, a total of three data sets were acquired. Overlap and gap distances of 0, 2, 5, and 10 mm were tested and delivered to solid phantom material of 30 × 30 × 5 cm3 . Phantom materials used were white plastic water and brown solid water. Tests were additionally performed on an anthropomorphic phantom with an irregular surface. Positioning shift studies were performed using IMRT fields delivered to a thoracic anthropomorphic phantom. For thoracic phantom measurements, the camera was placed laterally to observe the entire right side of the phantom. Fields were delivered with known translational patient positioning shifts in four directions. Changes in the Cherenkov fluence were evaluated through the generation of difference maps from unshifted Cherenkov images. All images were evaluated using ImageJ, Python, and MATLAB software packages. For matched fields, Cherenkov images were able to quantitate matched field separations with discrepancies between 2 and 4 mm, depending on gantry angle and beam energy or modality. For all photon and electron beams delivered at a gantry angle of 0°, image analysis indicated average discrepancies of less than 2 mm for all field gaps and overlaps, with 83% of matched fields exhibiting discrepancies less than 1 mm. Beams delivered obliquely to the phantom surface exhibited average discrepancies as high as 4 mm for electron beams delivered at large oblique angles. Finally, for IMRT field delivery, vertical and lateral patient positioning shifts of 2 mm were detected in some cases, indicating the potential detectability threshold of using this technique alone. Our study indicates that Cherenkov imaging can be used to support and bolster current treatment delivery verification techniques, improving our ability to recognize and rectify millimeter-scale delivery and positioning errors.

Black PJ ，Velten C ，Wang YF ，Na YH ，Wuu CS ... -  《-》

Total skin electron therapy (TSET): a reimplementation using radiochromic films and IAEA TRS-398 code of practice.

Schiapparelli P ，Zefiro D ，Massone F ，Taccini G ... -  《MEDICAL PHYSICS》