ESTRO 37 Abstract book

ESTRO 37

S476

The beam widths used for CBCT scans influence the SSDE factors. Therefore, the use of SSDE factors derived for conventional CT scans will lead to under/overestimation particularly for small patients. It is recommended that SSDE factors be established for CBCT scans taking into account the influence of the beam width. PO-0896 Accuracy of stereotactic body radiotherapy dose calculation algorithms outside treatment field T. Viren 1 , S. Salomaa 2 , J. Seppälä 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 University of Eastern Finland, Department of Environmental and Biological Science, Kuopio, Finland Purpose or Objective The aim of the present study is to evaluate the accuracy of two dose calculation algorithms in out-of-field area and to estimate the contribution of calculation error to the mean dose. Material and Methods Cubic water phantom was created in MultiPlan treatment planning system (TPS) (Accuray, USA) and dose from perpendicular radiation beams was calculated into phantom by Monte Carlo (MC) and Raytracing (RT) algorithms. Plans were created for circular 50 and 15mm IRIS and Fixed collimators (CybreKnife, Accuray, USA) using three SSD distances (785, 650 and 500mm). Subsequently, beam profiles (x and y directions at beam central axis (CA), length of scan 460 mm) at five different depths (15, 50, 100, 200 and 300mm) and depth doses of fields corresponding the calculations were measured in water phantom (Blue Phantom 2 , IBA, Germany). The measurements were conducted using farmer type ionization chamber (sn. 009362, PTW, Germany) and IBA electrometer (sn. 15123, IBA, Germany). In addition, absolute dose measurements were conducted in a depth of 15 mm at beam CA using calibrated electrometer (2570/1 NE Technologies, UK) and farmer ionization chamber (sn. 1610, NE Technologies, UK). Absolute dose measurements were used to convert the measured relative beam profiles to absolute doses. To estimate contribution of head leakage on measured profiles the measurements were repeated with 0-fields (closed IRIS and fixed block collimator). Finally, the measured mean dose to the whole water phantom was estimated and the results were compared to calculated mean dose. To include the effect of head leakage the calculated mean dose the mean dose in out-of-field area (dose <5% of CA dose) was estimated based on 0-field measurements and added to mean doses calculated with TPS. Results As compared to measurements the MC algorithm systematically underestimated the out-of-field doses. The difference between measurement and calculation decreased as the SSD and measurement depth increased (Fig 1). The RT algorithm was highly inaccurate outside the treatment field (Fig 1). Up to 76% difference between the measured and calculated (MC algorithm) mean dose was detected (Tab. 1). When the calculated mean doses were corrected with the approximated head leakage the difference between measured and calculated mean doses was significantly reduced (Tab 1). Table 1. Difference between measured and calculated (MC) mean dose to water phantom with and without correction of estimated head leakage.

Purpose or Objective The concept of size-specific dose estimate (SSDE) was proposed to overcome limitations of CT dose index (CTDI), which is measured in head and body phantoms with diameters of 16 cm and 32 cm, respectively. SSDE accounts for the patient size by applying appropriate factors to CTDI values. SSDE factors for conventional CT scans acquired with fan beams (≤40 mm) were reported by AAPM TG-204 and TG-220. The aim of this study is to derive SSDE factors for wide beams for cone beam CT (CBCT) scans and to investigate influence of the beam For CT scans, SSDE factors were derived by normalizing absorbed dose measured in various diameters and phantoms and linked to water equivalent diameters with respect to volume weighted CTDI (CTDI vol ), which is based on CTDI 100 . The CTDI 100 has limitations for wide beams, but the absorbed dose for different diameters can be normalized with respect to a CTDI value that is corrected for wide beams as recommend by IEC (CTDI IEC ). Monte Carlo simulations were used to simulate a Varina kV system, on-board-imager (OBI), using BEAMnrc code, and to calculate doses with Cavity code. Absorbed doses D(0) were measured over a length of 20 mm at center and periphery of 900 mm long water phantoms of diameters of 8 – 44 cm. D(0) values were measured for head and body scans using 120 kV and four beam widths (80, 160, 240, 320) mm. CTDI IEC for the beam widths used were assessed in the standard CTDI PMMA head and body phantoms and dose measurements free in air using a 100 mm long pencil chamber. Values of D(0) w were assessed in a manner similar to that used for CTDI w and normalized with respect to CTDI IEC of the same beam to derive SSDE Figure 1 (a – b) shows SSDE factors derived for head and body CBCT scans, compared to those derived by AAPM for conventional CT scans. CBCT factors varied by ±30% for the head scan, but the variations were more significant for the body scan reaching 70% at small diameters and decreasing with phantom diameters. The body CBCT factors were about a factor of two more than those for the head, and this is in agreement with results derived by AAPM. For head and body scans, the beam width was found to play a role in determining the factors, and this influence was raised by increasing the beam width. For example, the factors for 16 cm of both scans were higher than those for 8 cm by up to 22 % and increased to 41% width on the factors. Material and Methods factors. Results

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Conclusion