ESTRO 37 Abstract book

ESTRO 37

S467

PO-0883 Feasibility of a transit in vi vo dosimetry method to monitor cumulative dose in lung SBRT treatments A. Latorre-Musoll 1 , C. Cases Copestake 1 , N. Jornet Sala 1 , T. Eudaldo Puell 1 , M. Lizondo Gisbert 1 , P. Delgado-Tapia 1 , P. Carrasco de Fez 1 , A. Ruiz-Martínez 1 , I. Valverde Pascual 1 , M. Ribas Morales 1 1 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain Purpose or Objective For ITV-based lung SBRT plans, inter/intra-fraction breathing variations may compromise target coverage. In this scenario, we assessed the capability of an EPID-based transit in vivo dosimetry method to monitor the cumulative dose delivered to a target in a moving phantom. Material and Methods We used the Quasar Respiratory Motion Phantom (Modus Medical Devices) equipped with a cylindrical moving wood insert (as lung substitute) and an inner 30mm- diameter plastic sphere (as tumour substitute). We planned two 6MV 3DCRT plans (Varian Eclipse AAA v13.5, 1mm grid resolution, 7.5Gy/fx, V 100%,PTV =95%). In the first plan (PLAN 0mm ) we considered the static case (sphere=GTV=ITV). In the second plan (PLAN 8mm ) we considered an ITV encompassing a virtual excursion of ±8mm of the sphere. In both cases, we added an isotropic margin of 3mm to generate the PTV. We acquired the per-beam transit integrated images of these two plans on a Varian Clinac 2100C/D by using an aS1000 EPID at SDD=150cm. In this setting, we applied several breathing patterns (BP) and breathing amplitudes (BA) to the phantom. We used sinusoidal, “trained” and “free” BP (the two latter were acquired with the Varian RPM system from conveniently selected patients). We changed BA in 2mm steps, from 0 to 20(14mm) for the sinusoidal(trained and free) BP, respectively. For each plan, BP and BA combination, we assessed the correlation between D BA and V BA indexes, defined as: • D BA = D mean,BA / D mean,ref , where D mean was the per- field averaged EPID pixel values with significant transit doses (>70% of D max ); and reference BA was 0mm (static) for PLAN 0mm and 8mm for PLAN 8mm . The lower the target dose due to excessive breathing excursion, the higher the D BA value ( D BA ≥1) due to reduced beam attenuation. • V BA = V 98%,ref / V 98%,BA, where V 98% was based on the

Fig.2a shows moderate correlation between < D BA > and V BA indexes in this case, with the same inflection point at BA≈10mm. For the remaining BP of PLAN 0mm , < D BA > was insensitive to detect variations in V 98% up to –9.1% ( V BA =1.1) (fig.2b).

cumulative dose, determined by recalculating the dose distribution on a set of CT scans of the phantom (one CT scan for each BA step of 2mm). We shifted dose distributions to a reference frame that moved rigidly with the sphere and we weighted and summed them according to the probability density function of the underlying BP.

We did not found significant variations of < D BA

> (nor in

Results For each plan and BP, all fields had similar D BA

V BA

) for PLAN 8mm ,

except for BA≥18mm for sinusoidal BP.

behaviour.

Conclusion In-phantom cumulative dose coverage variations of lung SBRT targets could be monitored by transit mean dose variations only for major BP and BA deviations (>10mm). We are developing new EPID-based image feature driven metrics to increase the sensitivity of the method for moderate BP or BA deviations.

Fig.1 shows D BA average over all radiation fields (< D BA >). Significant variations only arose for BA>10mm for the sinusoidal BP for PLAN 0mm .