Photon beam skin dose analyses for different clinical setups

As the use of linear accelerators operating in flattening filter‐free (FFF) modes becomes more widespread, it is important to have an understanding of the surface doses delivered to patients with these beams. Flattening filter removal alters the beam quality and relative contributions of low‐energy X‐rays and contamination electrons in the beam. Having dosimetric data to describe the surface dose and buildup regions under a range of conditions for FFF beams is important if clinical decisions are to be made. An Elekta Synergy linac with standard MLCi head has been commissioned to run at 6 MV and 10 MV running with the flattening filter in or out. In this linac the 6 MV FFF beam has been energy‐matched to the clinical beam on the central axis (normalD10). The 10 MV beam energy has not been adjusted. The flattening filter in both cases is replaced by a thin (2 mm) stainless steel plate. A thin window parallel plate chamber has been used to measure a comprehensive set of surface dose data in these beams for variations in field size and SSD, and for the presence of attenuators (wedge, shadow tray, and treatment couch). Surface doses are generally higher in FFF beams for small field sizes and lower for large field sizes with a crossover at 10×10 cm2 at 6 MV and 25×25 cm2 at 10 MV. This trend is also seen in the presence of the wedge, shadow tray, and treatment couch. Only small differences (<0.5%) are seen between the beams on varying SSD. At both 6 and 10 MV the filter‐free beams show far less variation with field size than conventional beams. By removing the flattening filter, a source of contamination electrons is exchanged for a source of low‐energy photons (as these are no longer attenuated). In practice these two components almost balance out. No significant effects on surface dose are expected by the introduction of FFF delivery.PACS number(s): 87.53.Bn, 87.55.ne, 87.56.bd

Section: Introductionmentioning

confidence: 99%

Surface dose variations in 6 and 10 MV flattened and flattening filter‐free (FFF) photon beams

Cashmore

2016

“…Head‐scattered photons have a lower average energy compared to the primary photon field, contributing significantly at shallower depths relative to primary beam photons. Therapy field accessories such as trays and wedges ( 12 – 14 ) and oblique incidence of the beam (15) also affect the surface dose. Lee et al (5) reported surface buildup dose measurements for IMRT that included effects of the patient mask material and oblique incidence.…”

Section: Introductionmentioning

confidence: 99%

Surface buildup dose dependence on photon field delivery technique for IMRT

Yokoyama

Roberson

Litzenberg

et al. 2004

The more complex delivery techniques required for implementation of intensity‐modulated radiotherapy (IMRT) based on inverse planning optimization have changed the relationship between dose at depth and dose at buildup regions near the surface. Surface buildup dose is dependent on electron contamination primarily from the unblocked view of the flattening filter and secondarily from air and collimation systems. To evaluate the impact of beam segmentation on buildup dose, measurements were performed with 10×10 cm2 fields, which were delivered with 3 static 3.5×10 cm2 or 3×10 cm2 strips, 5 static 2×10 cm2 strips, 10 static 1×10 cm2 strips, and 1.1×10 cm2 dynamic delivery, compared with a 10×10 cm2 open field. Measurements were performed in water and Solid Water using parallel plate chambers, a stereotactic diode, and thermoluminescent dosimeters (TLDs) for a 6 MV X‐ray beam. Depth doses at 2 mm depth (relative to dose at 10 cm depth) were lower by 6%, 7%, 11%, and 10% for the above field delivery techniques, respectively, compared to the open field. These differences are most influenced by differences in multileaf collimator (MLC) transmission contributing to the useful beam. An example IMRT field was also studied to assess variations due to delivery technique (static vs. dynamic) and intensity level. Buildup dose is weakly dependent on the multileaf delivery technique for efficient IMRT fields.PACS numbers: 87.53.‐j, 87.53.Dq

“…The surface dose in particular can be one of the limiting factors in treatment plans with high doses to a target and is the focus of this work (38) . Similar studies in the past have been completed for beam modifiers such as wedges, MLC, and block trays 39 , 40 , 41 , 42 . Beam perturbation can be quantified by measuring changes in the percentage depth dose (PDD) profiles particularly in the buildup region, including surface dose, with the transmission MP placed in the beam.…”

Section: Introductionmentioning

confidence: 86%

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

Alrowaili

Lerch

Petasecca

et al. 2016

The main objective of this study is to demonstrate the performance characteristics of the Magic Plate (MP) system when operated upstream of the patient in transmission mode (MPTM). The MPTM is an essential component of a real‐time QA system designed for operation during radiotherapy treatment. Of particular interest is a quantitative study into the influence of the MP on the radiation beam quality at several field sizes and linear accelerator potential differences. The impact is measured through beam perturbation effects such as changes in the skin dose and/or percentage depth dose (PDD) (both in and out of field). The MP was placed in the block tray of a Varian linac head operated at 6, 10 and 18 MV beam energy. To optimize the MPTM operational setup, two conditions were investigated and each setup was compared to the case where no MP is positioned in place (i.e., open field): (i) MPTM alone and (ii) MPTM with a thin passive contamination electron filter. The in‐field and out‐of‐field surface doses of a solid water phantom were investigated for both setups using a Markus plane parallel (Model N23343) and Attix parallel‐plate, MRI model 449 ionization chambers. In addition, the effect on the 2D dose distribution measured by the Delta4 QA system was also investigated. The transmission factor for both of these MPTM setups in the central axis was also investigated using a Farmer ionization chamber (Model 2571A) and an Attix ionization chamber. Measurements were performed for different irradiation field sizes of 5×5 cm2 and 10×10 cm2. The change in the surface dose relative to dmax was measured to be less than 0.5% for the 6 MV, 10 MV, and 18 MV energy beams. Transmission factors measured for both set ups (i & ii above) with 6 MV, 10 MV, and 18 MV at a depth of dmax and a depth of 10 cm were all within 1.6% of open field. The impact of both the bare MPTM and the MPTM with 1 mm buildup on 3D dose distribution in comparison to the open field investigated using the Delta4 system and both the MPTM versions passed standard clinical gamma analysis criteria. Two MPTM operational setups were studied and presented in this article. The results indicate that both versions may be suitable for the new real‐time megavoltage photon treatment delivery QA system under development. However, the bare MPTM appears to be slightly better suited of the two MP versions, as it minimally perturbs the radiation field and does not lead to any significant increase in skin dose to the patient.PACS number(s): 87.50.up, 87.53.Bn, 87.55.N, 87.55.Qr, 87.56.Fc.