Consideration of the radiation dose delivered away from the treatment field to patients in radiotherapy

The purpose of this work was to experimentally investigate the out‐of‐field dose in a water phantom, with several high energy electron beams used in external beam radiotherapy (RT). The study was carried out for 6, 9, 12, and 18 MeV electron beams, on three different linear accelerators, each equipped with a specific applicator. Measurements were performed in a water phantom, at different depths, for different applicator sizes, and off‐axis distances up to 70 cm from beam central axis (CAX). Thermoluminescent powder dosimeters (TLD‐700) were used. For given cases, TLD measurements were compared to EBT3 films and parallel‐plane ionization chamber measurements. Also, out‐of‐field doses at 10 cm depth, with and without applicator, were evaluated. With the Siemens applicators, a peak dose appears at about 12–15 cm out of the field edge, at 1 cm depth, for all field sizes and energies. For the Siemens Primus, with a 10×10cm2 applicator, this peak reaches 2.3%, 1%, 0.9% and 1.3% of the maximum central axis dose (Dmax) for 6, 9, 12 and 18 MeV electron beams, respectively. For the Siemens Oncor, with a 10×10cm2 applicator, this peak dose reaches 0.8%, 1%, 1.4%, and 1.6% of Dmax for 6, 9, 12, and 14 MeV, respectively, and these values increase with applicator size. For the Varian 2300C/D, the doses at 12.5 cm out of the field edge are 0.3%, 0.6%, 0.5%, and 1.1% of Dmax for 6, 9, 12, and 18 MeV, respectively, and increase with applicator size. No peak dose is evidenced for the Varian applicator for these energies. In summary, the out‐of‐field dose from electron beams increases with the beam energy and the applicator size, and decreases with the distance from the beam central axis and the depth in water. It also considerably depends on the applicator types. Our results can be of interest for the dose estimations delivered in healthy tissues outside the treatment field for the RT patient, as well as in studies exploring RT long‐term effects.PACS number(s): 87.53.Bn, 87.56.bd, 87.56.J‐

< 5 cm

) (5) .…”

Section: Introductionmentioning

confidence: 99%

Experimental assessment of out‐of‐field dose components in high energy electron beams used in external beam radiotherapy

Alabdoaburas

Mège

Chavaudra

et al. 2015

“…Although the two contouring methods (AIP and REF) for conventional 3D dose calculation result in significant differences in predicted healthy organ doses (several tens of percent), there was ultimately no strong trend for over‐ or underprediction of doses to particular organs. This negates the possibility of applying generic correction factors (or similar approaches) to doses estimated using AIP‐based 3D methods, and implies that, particularly for sensitive patient groups, 36 , 37 , 38 , 39 the 4D method ought to be employed. In the long term, it would be preferable for the 4D approach to be employed for all patients, such that accurate dose‐outcome correlation can be recorded and accurate tissue complication probabilities be established.…”

Section: Discussionmentioning

confidence: 99%

Evaluation of dosimetric misrepresentations from 3D conventional planning of liver SBRT using 4D deformable dose integration

Yeo

Taylor

Supple

et al. 2014

Self Cite

The purpose of this study is to evaluate dosimetric errors in 3D conventional planning of stereotactic body radiotherapy (SBRT) by using a 4D deformable image registration (DIR)‐based dose‐warping and integration technique. Respiratory‐correlated 4D CT image sets with 10 phases were acquired for four consecutive patients with five liver tumors. Average intensity projection (AIP) images were used to generate 3D conventional plans of SBRT. Quasi‐4D path‐integrated dose accumulation was performed over all 10 phases using dose‐warping techniques based on DIR. This result was compared to the conventional plan in order to evaluate the appropriateness of 3D (static) dose calculations. In addition, we consider whether organ dose metrics derived from contours defined on the average intensity projection (AIP), or on a reference phase, provide the better approximation of the 4D values. The impact of using fewer (<10) phases was also explored. The AIP‐based 3D planning approach overestimated doses to targets by 1.4% to 8.7% (mean 4.2%) and underestimated dose to normal liver by up to 8% (mean −5.5%; range −2.3% to −8.0%), compared to the 4D methodology. The homogeneity of the dose distribution was overestimated when using conventional 3D calculations by up to 24%. OAR doses estimated by 3D planning were, on average, within 10% of the 4D calculations; however, differences of up to 100% were observed. Four‐dimensional dose calculation using 3 phases gave a reasonable approximation of that calculated from the full 10 phases for all patients, which is potentially useful from a workload perspective. 4D evaluation showed that conventional 3D planning on an AIP can significantly overestimate target dose (ITV and GTV+5mm), underestimate normal liver dose, and overestimate dose homogeneity. Implementing nonadaptive quasi‐4D dose calculation can highlight the potential limitation of 3D conventional SBRT planning and the resultant misrepresentations of dose in some regions affected by motion and deformation. Where the 4D approach is unavailable, contouring on the full expiration phase may yield more accurate dose calculations, most relevant in the case of the healthy liver, but the absolute dose differences are in general small for the other healthy organs. The technique has the potential to quantify under‐ and over‐dosage and improve treatment plan evaluation, retrospective plan analysis, and clinical outcome correlation.PACS numbers: 87.55.‐x, 87.55.D‐, 87.55.de, 87.55.dk, 87.55.Qr, 87.57.nj

“…Calculated electron spectra in water are then used to evaluate stopping‐power ratios and thus determine the extent of the influence of changing spectra on absorbed dose calculations. Such investigations are not only undertaken within the primary field, but also out‐of‐field, which is of increasing interest in the context of out‐of‐field dose in stereotactic radiotherapy and associated risks of radiocarcinogenesis, 15 , 16 in particular for pediatric patients (17) . Note that, in the present work, the term ‘primary field’ refers to the nominal treatment beam and 'out‐of‐field' refers to regions beyond the primary field.…”

Section: Introductionmentioning

confidence: 99%

The influence of field size on stopping‐power ratios in‐ and out‐of‐field: quantitative data for the BrainLAB m3 micro‐multileaf collimator

Taylor

Kairn²,

Kron

et al. 2012

The objective of this work is to quantify the systematic errors introduced by the common assumption of invariant secondary electron spectra with changing field sizes, as relevant to stereotactic radiotherapy and other treatment modes incorporating small beam segments delivered with a linac‐based stereotactic unit. The EGSnrc/BEAMnrc Monte Carlo radiation transport code was used to construct a dosimetrically‐matched model of a Varian 600C linear accelerator with mounted BrainLAB micro‐multileaf collimator. Stopping‐power ratios were calculated for field sizes ranging from 6×6 mm2 up to the maximum (98×98 mm2), and differences between these and the reference field were computed. Quantitative stopping power data for the BrainLAB micro‐multileaf collimator has been compiled. Field size dependent differences to reference conditions increase with decreasing field size and increasing depth, but remain a fraction of a percent for all field sizes studied. However, for dosimetry outside the primary field, errors induced by the assumption of invariant electron spectra can be greater than 1%, increasing with field size. It is also shown that simplification of the Spencer‐Attix formulation by ignoring secondary electrons below the cutoff kinetic energy applied to the integration results in underestimation of stopping‐power ratios of about 0.3% (and is independent of field size and depth). This work is the first to quantify stopping powers from a BrainLAB micro‐multileaf collimator. Many earlier studies model simplified beams, ignoring collimator scatter, which is shown to significantly influence the spectrum. Importantly, we have confirmed that the assumption of unchanging electron spectra with varying field sizes is justifiable when performing (typical) in‐field dosimetry of stereotactic fields. Clinicians and physicists undertaking precise out‐of‐field measurements for the purposes of risk estimation, ought to be aware that the more pronounced spectral variation results in stopping powers (and hence doses) that differ more than for in‐field dosimetry.PACS number: 87.10.RT; 87.53.Ly; 87.56.jf; 87.56.jk