Abstract:

A National Project for the in-vivo dosimetry in radiotherapy: first results

A.Piermattei1

, L. Azario1, S. Cilla

2 , A.Fidanzio

1, F.Greco

1, M.T.Russo

3 , S. Zucca

4

1Istituto di Fisica e Unità Operativa di Fisica Sanitaria, Università Cattolica del S. Cuore, Roma

2Unità Operativa di Fisica Sanitaria, Fondazione per la Ricerca e Cura ‘Giovanni Paolo II’, Campobasso

3Unità Operative di Radioterapia e Fisica Sanitaria, Ospedale Belcolle, Viterbo

4Unità Operativa di Fisica Sanitaria, Presidio Oncologico Businco, Cagliari

The work reports the first results of a National Project financed by the National Institute of Nuclear Physics for the development of in-vivo dosimetric procedures in radiotherapy. The first objective has been the development of a generalized procedure for the in-vivo dose reconstruction at the isocenter point, Diso , for 3D conformal radiotherapy treatments, with open and wedged x-ray beams supplied by linacs of different manufacturers and equipped with aSi EPID. This way the commissioning procedure is greatly simplified and applicable at Elekta, Siemens and Varian linacs and the results of the in-vivo tests are reported in quasi-real time.

The method is based on two calibration procedures for x-ray beams and aSi EPIDs. In particular, generalized mid-plane doses in solid water phantom D

0, and transit signal s

0t values were obtained by 19

open and 38 wedged beams of 8 different linacs. Generalized ratios F°= st°/ D° were fitted by surface equations and together with other parameters were used by the algorithm to convert the integral transit signals St, in reconstructed doses Diso, to compare with the Diso,TPS (computed by the TPS) using a tolerance level of 5% for pelvic, breast, head tumors and lung cancer treatments. Moreover the dedicated software supplied for each beam, the γ-analysis for a 2D comparison between portal images, using alert criteria γmean >0.5 and Pγ >1 >10%.

The generalized in-vivo dosimetry procedure has been adopted by 8 centers that used different linacs . The results of about 5000 tests showed that about 85% of the tests were well-within tolerance levels. The dose discrepancies were due to patient's set-up misalignment, presence of attenuating media in the field, patient's morphological changes, no correct wedge TPS implementation, no correct CT number calibration, dose output variations, and CT simulator laser misalignments. The results were supplied in quasi-real time because the dedicated software used the information of the 'record and verify' network of the centers. Consequently, the extra-time needed to obtain the results after the dose delivery was about 20 seconds per beam.

At present, the second objective of the National Project is the application of the same method at IMRT beams. The same algorithm developed for open beams can be utilized for the IMRT step-shoot and slide-window beams, adopting corrections that take into account (i) the effect of the radiation transmitted through the MLC on the integral signal St and (ii) the effect of the fluence inhomogeneity on the generalized ratios F

0. However, the preliminary in-vivo results obtained for step-shoot and slide-window IMRT with different

linacs confirm that it is possible (i) to reduce strongly the commissioning time and (ii) to obtain in a quasi-real time the results of tests. The success of this step could encourage future developments of the project for modulated arc-radiotherapy in-vivo dosimetry.

Contributo orale

Stand Best Medical International

Area poster

In-vivo dose reconstruction for tangential treatments of breast cancer : a generalized procedure

F. Greco1, L. Azario1, A. Fidanzio1, S. Cilla2, M. Russo3, S. Zucca4, L. C. Orlandini5, M. Betti5, A. Piermattei1

1Istituto di Fisica e Unità Operativa di Fisica Sanitaria, Università Cattolica del S. Cuore, Roma 2Unità Operativa di Fisica Sanitaria, Fondazione per la Ricerca e Cura ‘Giovanni Paolo II’, Campobasso 3Unità Operativa di Radioterapia, Ospedale Belcolle, Viterbo 4Unità Operativa di Fisica Sanitaria, Presidio Oncologico Businco, Cagliari 5Unità Operativa di Fisica Medica, Centro Oncologico Fiorentino, Firenze

Introduction The work is part of a National Project financed by the National Institute of Nuclear Physics for the development of in-vivo dosimetric procedures in radiotherapy. In particular the work reports a practical and accurate generalized procedure for linacs Varian, Elekta and Siemens, equipped with aSi Electronic Portal Imaging Devices (aSi EPIDs) for in-vivo dose reconstruction at breast mid-thickness in tangential x-ray treatments carried out by physical and virtual wedge. The generalized procedure is easily commissioned, due to the strong reduction of the time required by implementation measurements. Moreover a dedicated software, that can collect informations from the ‘Record & Verify’ (R&V) network of the centres, supplies the in-vivo results in quasi-real time. This last objective is particularly recommended for this kind of quality control in radiotherapy. The authors are now developing a new generation software in cooperation with Best Medical Italy, to be commercialized. Materials and methods Transit dosimetry method The method is based on correlation ratios between the EPID transit signals and the doses at mid-thickness measured along the chords, w, of cylindrical water phantoms (CWP) [1]. An ion-chamber (0.125 cm3 in volume, PTW Freiburg) was positioned at the Source to Axis Distance (SAD), along the beam central axis to determine the dose values, Dm, at the mid-thickness of the chord (fig.1). For the transit signal, st, measurements the EPID was positioned at Source-EPID-Distance (SED) equal to 159 cm. In order to generalized the method, two calibration factors have been adopted. The first one, k0, takes into account the x-ray beam output and is defined by

(1) where DSAD,w is the dose in water phantom at the depth of 10 cm, SSD=90 cm, for wedged beam 10×10 cm2. This way the doses, Dm, were normalized by the respective factor k0, obtaining a set of generalized doses, D0, that were independent of the MU calibration adopted by the center [3]. The second factor, ks, takes into account the EPID sensitivity and is defined by (2) where sr,t, is a reference signal obtained irradiating a standard solid water phantom 22 cm thick for a 10×10 cm2 wedged beam. The transit signals, st, by different aSi EPIDs, were multiplied by the ks, obtaining a set of generalized transit signals, s0, that were independent both from MU calibration and aSi EPID sensitivity [3]. The correlation ratios, s0/D0, were fitted by linear function as follows (3) where the parameters A(w) and B(w) were polynomial functions of w and WAF is the wedge attenuation factor defined in previous paper[2].The reconstructed dose, Dm,R, at the breast mid-thickness for a tangential beam is obtained by (4) where St is the EPID integral signal corrected by the factor ks and the factor k0; d is the distance between mid- thickness and the isocenter point; F0 is the correlation function defined by equation 3; and the function f(d) takes into account the scatter contribution on the EPID. Software A dedicated software was developed for dose reconstruction using the informations stored in R&V system. The software can be configured by users introducing the factors WAF, k0, ks and was developed in two modules. The first one was developed for a ‘pre-treatment step’ that uses the Digital Imaging and COmmunications in Medicine (DICOM) files supplied by the CT scanner and the Treatment Planning System (TPS). The patient CT scan is used to determine the water-equivalent thicknesses, w; the DICOM files from TPS, for every tangential beam, supplied the dose at mid-thickness computed by TPS, Dm,TPS. The second module was developed for the ‘post-treatment step’. The DICOM files from EPID, obtained for each beam, after the patient daily treatment, supplies the integral signal St to be used in equation 4 for the dose reconstruction Dm,R. For every beam, the software supplies the comparison between the Dm,R and Dm,TPS in terms of R values. The tolerance level of the R values was estimated in 5%. Moreover, the software supplies transit signal profiles crossing the beam central axis to help to investigate the causes of the possible dose variations.

0

SAD,w

1cGy/ UMk =

D

s

r,t

1cCU / UMk =

s

0

AF AFF (W , w) A(w) W B(w)

sm,R t 2

0 0

AF

k f (d)D S

k SAD dF (W , w)

SAD

Results 900 tests were carried out in three centers that used different linacs. In particular 300 tests were performed by Varian linacs with physical wedges, 300 tests by Elekta linacs with physical wedges and 300 tests by Siemens linacs with virtual wedges. The R values resulted well-within the tolerance level in 93% of tests. The remaining 7% of the tests, outside the tolerance level, were due to incorrect set-up of patient (5%), output factor variation (1%), wing board attenuation (1%). Figure 3 shows the results of the IVD for a single patient as displayed on the computer screen. The figure 3a shows the CT scan and the beam geometrical edges for beam at 53°. Figure 3b shows two transit signals profiles obtained at the beginning of the treatment and after three fractions, respectively. The second profile shows a loss of patient set-up (shift of the beam inner edge towards the chest wall), responsible of an increase of the radiological thickness, w, and R equal to 0.90. In this case the MPI was not carried out before the irradiation. Figure 3c reports the R values resulted in five checks both the tangential beams.

Conclusion

The IVD method here reported avoids the measurements in CWP and it can be easily included in the Quality Assurance program of the center. Moreover the use of the dedicated software supplied the dosimetric results in quasi real-time as urged by recent recommendations. At the moment, the objective of the National Project is the application of the IVD method at IMRT and arc-radiotherapy treatments.

Figure 3 a) The patient CT scan crossed by the beam central axis (dashed line) at 53°and the beam geometrical boundaries (continuous lines); b) Signal EPID profiles obtained for the beam at 53°, at the beginning of the treatment (heavy line) and at second therapy fraction (thin-line). c) Diagram of five R ratios obtained for the two tangential beams at 53° (◊) and 133° ( ).

Figure 2. Immediately after the therapy session a radiology technician can visualize on a monitor (on the right) the dose obtained by the IVD method.

a)

(1) A. Fidanzio, F. Greco, A. Mameli, L. Azario, M. Balducci, M. A. Gambacorta, V. Frascino, S. Cilla, D.Sabatino, A. Piermattei, Breast in vivo dosimetry by EPID, Journal of Applied Clinical Medical Physics 11 (2010) 249-262. (2) A.Piermattei, F. Greco, L.Azario, A. Porcelli, S. Cilla, D. Sabatino, A. Russo, G. D’Onofrio, M. Russo, A. Fidanzio, Real time dose reconstruction for wedged photon beams: a generalized procedure, JACMP 12(4) (2011) 124-138. (3) A.Piermattei, F.Greco, L.Azario, A.Porcelli, S.Cilla, S.Zucca, A.Russo, E.D.Castro, M.Russo, R.Caivano, V.Fusco, A.Morganti, A.Fidanzio, A National Project for in-vivo dosimetry procedures in radiotherapy: first results, NIM B 274 (2012) 42-50.

Figure 1. Experimental set-up

53°

a)

The software is able to supply the result after the therapy session assuring a quasi-realtime in-vivo dosimetry (IVD) (fig.2).