ESTRO 36 Abstract Book

S510 ESTRO 36 _______________________________________________________________________________________________

beams (0.4 ± 3.2%). Comparing C4 to C2, log file accuracy is analyzed for MC. LF resulted in lower σ values for 20/22 arcs (-5.4 ± 3.4%) and improved pass rates for 14/18 arcs (1.1 ± 1.4%). Comparing C5 to C2, LF and AC QA techniques are compared. The LF technique yielded decreased σ values for 22/22 arcs (-51 ± 7%) and improved pass rates for 18/18 fields (9.9 ± 3.8%). The LF technique also eliminated systematic AC errors; mean dose errors decreased from 3.2% to 0.1%. For 1/2/3/4° LF-CS control point spacing, 1%/1mm pass rates were 80.0 ± 5.0%, 78.0 ± 4.2%, 74.0 ± 5.1%, and 68.8 ± 5.3%. Plan-CS pass rates were 80.2 ± 4.0%. Figure 2 plots difference in pass rates [(LF-CS vs. AC) minus (Plan-CS vs. AC)] as a function of control point spacing for each arc. Calculation times for CS and MC were 12s per control point and 3 minutes per VMAT arc respectively.

technique (orthogonal vs. CBCT and high vs. low quality) on the doses to normal tissue was evaluated using Eclipse, where the imaging doses were used as based plans in the treatment planning process. For breast plans, doses to the heart and lung were evaluated. For head/neck plans, doses to all the normal tissues were compared.

Figure 1. Anthropomorphic phantom (CIRS) with dose measurement points identified. Results Average imaging dose was measured as 1.3, 2.5, 3.7, and 7.6cGy for daily low dose MV pairs, high quality MV pairs, low dose CBCT and high quality CBCT, respectively. Over a 30 fraction treatment with daily IGRT, this equates to 38 - 227cGy. The average agreement between measured and calculated tissue doses due to imaging was 0.4±0.4cGy. The largest difference was 1.3cGy, found in the lung for high quality CBCT imaging (~39cGy over a 30 fraction treatment). With imaging dose incorporated into the treatment planning process, it was possible to create clinically acceptable treatment plans for a range of treatment sites, including breast, head and neck and prostate. The imaging technique did, however, increase the heart and lung dose for breast plans. For an example left breast treatment, the mean heart dose in our original, clinically delivered plan was 60cGy. With daily MV imaging included, this increased to 140, 150, 190 and 260cGy for daily low dose MV pairs, high quality MV pairs, low dose CBCT and high quality CBCT, respectively. The corresponding values for mean lung dose were 360cGy (original clinical) and 470, 490, 510 and 570cGy.

Conclusion MC doses proved more accurate than CS when compared to AC measurement. LF-MC plans yielded superior accuracy and shorter calculation times than LF-CS plans. By cutting out the phantom and comparing LF dose to that of the original plan, systematic error was eliminated and random error greatly reduced. PO-0921 Dose considerations of IGRT using MV projection and MV CBCT on a prototype linear accelerator P. Balter 1 , T. Netherton 1 , Y. Li 1 , P. Nitsch 1 , S. Gao 1 , M. Muruganandham 1 , S. Shaitelman- 1 , S. Frank 1 , S. Hahn 1 , A. Klopp 1 , L. Court 1 1 UT MD Anderson Cancer Center Radiation Physics, Radiation Physics, Houston- TX, USA Purpose or Objective The use of the mega-voltage treatment beam for image- guided patient setup has some potential advantages over kV imaging, especially reduced equipment and QA requirements. One of the challenges that MV imaging introduces is the increase in daily imaging dose. Here we investigate (1) whether the MV imaging dose can be correctly calculated and incorporated into the treatment plan, and (2) the impact of MV imaging dose on the dose to normal tissues such as the lung and heart. Material and Methods MV imaging dose to the lung, heart and other soft tissue was measured using an ion chamber in anthropomorphic thorax phantom (CIRS), and compared with dose calculated in the TPS (Eclipse) for orthogonal MV-MV imaging fields and MV CBCT images using a prototype linear accelerator, each with a low-dose and high-quality mode (total 4 modes). The impact of the imaging

Table 1: Tissue doses(cGy/fraction) at different points in the anthropomorphic phantom. M: Measured. C:

Calculated Conclusion