Measurement of dose in the buildup region using fixed‐separation plane‐parallel ionization chambers

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: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Cashmore

2016

“…The Markus chamber is designed for the measurement of surface and buildup dose. Markus‐type chambers are known for their overresponse due to the large separation and their small guard ring, 44 , 45 and different methods have been developed to correct for this overresponse 46 , 47 , 48 , 49 . In this study, all measurements using the Markus ionization chamber were corrected using the Velkley correction as modified by Rawlinson to correct the overresponse of the Markus chamber.…”

Section: Methodsmentioning

confidence: 99%

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.

“…Measurements were performed with a parallel plate (PP) ionization chamber (PS‐033, Capintec, Ramsey, NJ) possessing an entry window thickness of

0 {. 5 mg / cm}^{2}

, a plate separation of 2 mm, and a collecting diameter of 16.2 mm. For each measurement point, the relative ionization was acquired by dividing the charge collected at depth, via a modified Keithley electrometer (Modified K602, CNMC Co., Nashville, TN), by the charge at the depth of

d_{max}

and then corrected to PDD by correcting for bias effects and using the Gerbi method to account for chamber characteristics 13 . This was accomplished in the following method: Bias Correction : All ionization readings were corrected by first accounting for bias effects where,…”

Section: Methodsmentioning

confidence: 99%

Surface and buildup dose characteristics for 6, 10, and 18 MV photons from an Elekta Precise linear accelerator

Klein

Esthappan

2003

Understanding head scatter characteristics of photon beams is vital to properly commission treatment planning (TP) algorithms. Simultaneously, having definitive surface and buildup region dosimetry is important to optimize bolus. The Elekta Precise linacs have unique beam flattening filter configurations for each photon beam (6, 10, and 18 MV) in terms of material and location. We performed a comprehensive set of surface and buildup dose measurements with a thin window parallel‐plate (PP) chamber to examine effects of field size (FS), source‐to‐skin distance (SSD), and attenuating media. Relative ionization data were converted to fractional depth dose (FDD) after correcting for bias effects and using the Gerbi method to account for chamber characteristics. Data were compared with a similar vintage Varian linac. At short SSDs the surface and buildup dose characteristics were similar to published data for Varian and Elekta accelerators. The FDD at surface (FDD0) for 6, 10, and 18 MV photons was 0.171, 0.159, and 0.199, respectively, for a 15×15 cm2, 100 cm SSD field. A blocking tray increased FDD0 to 0.200, 0.200, and 0.256, while the universal wedge decreased FDD0 to 0.107, 0.124, and 0.176. FDD0 increased linearly with FS (~1.16%/cm). FDD0 decreased exponentially for 10 and 18 MV with increasing SSD. However, the 6 MV FDD0 actually increased slightly with increasing SSD. This is likely due to the unique distal flattening filter for 6 MV The measured buildup curves have been used to optimize TP calculations and guide bolus decisions. Overall the FDD0 and buildup doses were very similar to published data. Of interest were the relatively low 10 MV surface doses, and the 6 MV FDD0's dependence on SSD. © 2003 American College of Medical Physics. PACS number(s): 87.53.–j, 87.66.–a