Real-time dose reconstruction for wedged photon beams: a generalized procedure

Abstract

A practical and accurate generalized procedure to reconstruct the isocenter dose D(iso) for 3D conformal radiotherapy (3DCRT) has been developed for X-ray open beams supplied by linacs of different manufacturers and equipped with aSi electronic portal imaging devices (aSi EPIDs). This paper reports an extension of the method, to be applied at the wedged X-ray beams characterized by the wedge attenuation factor W(AF). Using water-equivalent solid phantoms (SPs) of different thicknesses, w, and photon square fields of sizes, L, the generalized midplane doses D(0)(W(AF), w/2,L) and generalized transit signals s(t)(0)(W(AF),w,L) by 38 beams of six different linacs were determined. The generalized data were fitted by surface equations and used together with the information of the ‘record & verify’ network of the centers. In this manner, for every beam, the D(iso) reconstruction was obtained in about 25 seconds after the treatment. To test the in vivo dosimetric procedure, six pelvic treatments that used conformed wedged beams were carried out with three linacs of different manufacturers. For every beam, the comparison between the reconstructed D(iso) and the D(iso,TPS) computed by the TPS, resulted in an acceptable tolerance level of ±5%, estimated for this kind of treatment. Generally the in vivo dosimetry methods that use EPIDs require: (i) a special effort for the dosimetric commissioning with SPs of different thicknesses, and (ii) extra time for the analysis of the EPID signals. The proposed procedure simplifies the commissioning step and supplies for Varian, Elekta, and Siemens linacs equipped with the aSi EPIDs a quasi-real time in vivo dosimetry for open and wedged 3DCRT fields.

Figure 1

Central sections of the different wedges supplied by Varian for 15°, 30°, 45°, and 60°, by Siemens for 15°, 30°, 45°, and 60°, and by Elekta for 60°.

Figure 2

Dose at the SAD and ${\mathrm{D}}_{\text{SAD},\mathrm{W}}$ (a) determined for a wedged beam with the ion chamber at the reference depth ${\mathrm{d}}_{\text{ref}}=10\hspace{0.17em}\text{cm}$ in water phantom and field size $10\times 10\hspace{0.17em}{\text{cm}}^2$; as in Fig. 2(a), the dose ${\mathrm{D}}_{\text{SAD},0}$ was measured in water phantom for the open beam (b); measurements (c) of the dose $\mathrm{D}(\mathrm{w}/2\mathrm{L})$ at the SP midplane and the transit signal ${\mathrm{s}}_{\mathrm{t}}(\mathrm{w},\mathrm{L})$ by the EPID using a square field size L.

Figure 3

Linear fits by the experimental ${\mathrm{s}}_{\mathrm{t}}^0{({\mathrm{W}}_{\text{AF},}\mathrm{w},10)}_{\text{MV}}$ values (obtained with a field $\mathrm{L}=10\times 10{\text{cm}}^2$) as function of ${\mathrm{W}}_{\text{AF}}$, for the 6 MV (dotted line), 10 MV (thin line), and 15 MV (heavy line) beams. The fits reported here were obtained by SP thicknesses $\mathrm{w}=10,22$, and 42 cm and using $10\times 10\hspace{0.17em}{\text{cm}}^2$ fields.

Figure 4

Three surfaces (a) that fitted the ${\mathrm{s}}_{\mathrm{t}}^0{({\mathrm{W}}_{\text{AF},}\mathrm{w},10)}_{\text{MV}}$ data for the square field width $\mathrm{L}=4,10$, and 20 cm from the bottom to the top; three surfaces (b) that fitted the ${\mathrm{s}}_{\mathrm{t}}^0({\mathrm{W}}_{\text{AF},}\mathrm{w},\mathrm{L})$ data for square field sides 4, 10, and 20 cm from the bottom to the top.

Figure 5

Averaged factors $\mathrm{f}{(22,\mathrm{L},\mathrm{d})}_{6\text{MV}}$ for different ${\mathrm{W}}_{\text{AF}}$ indexes obtained by the SP thickness of $\mathrm{w}=22\hspace{0.17em}\text{cm}$ and using beams of 6 MV supplied by Varian (Δ), Elekta (□), and Siemens (⋄) linacs. The data are reported for $\mathrm{L}=4\hspace{0.17em}\text{cm}$ (open symbol), $\mathrm{L}=10\hspace{0.17em}\text{cm}$ (green symbol), and $\mathrm{L}=20\hspace{0.17em}\text{cm}$ (black symbol).

Figure 6

Histogram of 131 tests carried out for 12 pelvic treatments using wedged beams of 6 MV (black), 10 MV (grey), and 15 MV (white) supplied by three linacs (Varian, Elekta and Siemens).