Abstract

Roberta MUNI¹, Paolo Colleoni², Marco Fortunato², Mohamed Ilyes Benfetima³, Claudia Bianchi⁴, Micaela Motta⁵, Stefano Andreoli², Fabiola Cretti², Francesco Romeo Filippone¹, Laura Maffioletti¹, Fabio Piccoli¹, Elisabetta Vitali¹, Suela Vukcaj⁶, Giulia Rinaldi⁶, Luigi Franco Cazzaniga¹

¹Ospedale Papa Giovanni XXIII, Radiation Oncology, Bergamo, Italy; ²Ospedale Papa Giovanni XXIII, Medical Physics, Bergamo, Italy; ³International Centre for Theoretical Physics, Medical Physics, Trieste, Italy; ⁴Ospedale Papa Giovanni XXIII, Medical Physics , Bergamo, Italy; ⁵Ospedale Papa Giovanni XXIII, Radiation Oncology, ergamo, Italy; ⁶Ospedale Papa Giovanni XXIII, Radiation Oncology , Bergamo, Italy

HyperArc (HA) is a non-coplanar volumetric modulated arc therapy (VMAT) treatment approach with single isocenter for intracranial stereotactic radiotherapy. HA provide the advantage of delivering high dose to target volume while minimizing dose to normal tissue for single or multiple targets compared to the conventional VMAT technique.

Stereotactic radiotherapy for brain metastases resection cavities represents a challenge because of large volume and irregular shape with related high toxicity.

HA plans were calculated for 10 consecutive patients who received hypofractionated stereotactic radiotherapy on resection cavity with VMAT Varian RapidArc (RA).

HA plans were generated with 6MV FFF for a TrueBeam LINAC equipped with 2.5 mm central leaves. RA plans previously delivered on a TRILOGY LINAC equipped with 5 mm leaves MLC with 6MV. All plans were calculated with Acuros XB algorithm on Eclipse TPS.

Homogeneity index (HI), RTOG and Paddick conformity index (CI), gradient index (GI) and V2Gy-25Gy for normal brain tissue were compared as indicator of PTV coverage and brain tissue spare. Monitor units (MU) per fraction were evaluated as an indicator of irradiation efficiency.

Dose was prescribed to the minimum of the target (100% of the prescription dose to 100% of  PTV) for both HA and RA plans. For HA plans Dmax was limited to 130% of the prescription dose.

Total dose was 27-35 Gy in 3-5 fractions.  Mean PTV was 31.37 cc (range 7.20-81.31).

HA and RA provided comparable RTOG CI (mean±SD 1.14±0.03 vs.1.16 ±0.04, p=0.28) and Paddick CI (0.88±0.02 vs 0.87±0.30, p=0.29).

HI (1.22±0.06 vs 1.15±0.05, p<0.01) and GI (2.31±0.22 vs 2.59±0.34, p<0.01) values were more favorable for HA than RA.

V2Gy of normal brain tissue values were similar for HA and RA plans (539.8±249.4 vs 568.1±257.9 cc, p=0.11)

Low-to-moderate dose spreads (V4Gy-V18Gy) were significantly reduced (p<0.01) in the HA plans over that of RA.  Mean V18Gy was 24.6±14.2 cc vs 26.4±17.5 cc (p<0.01) and V25Gy was 11.7±7.9 cc vs 11.8±8.7 cc (p=0.35) for HA and RA respectively.

Mean MU per fraction were 2910±1129 vs 3740±1061 for HA and RA plans respectively.

HA plans provide steeper dose fall-off and comparable conformity with respect to the delivered RA plans.

HA plans reduce MU delivering by one third resulting in more efficient irradiation technique.