EP-1557 RADIOTHERAPY OF SOFT-TISSUE SARCOMA OF LOWER EXTREMITY WITH HIGH BONE SPARING WITH RAPIDARC AND PROTONS

Purpose/Objective: To investigate the advanced radiotherapy treatments of soft-tissue sarcoma with adequate target coverage and bone high sparing with volumetric modulated arc therapy (RapidArc) and proton beams. Materials and Methods: Ten patients presenting soft tissue sarcoma of the leg, and treated with RapidArc in author’s institutions were collected for the study. Dose was prescribed to 66.5 Gy in 25 fractions to mean planning target volume (PTV), and significant maximum dose to the bone was limited to 50 Gy. Plans were prepared using RapidArc with flattened 6 MV (6X) and unflattened 10 MV (10FFF) photon beams from a Varian TrueBeam facility, and using proton beams up to 250 MeV. RapidArc photon plans were computed with AAA and Acuros XB in the two options of dose to medium and dose to water. Proton plans were prepared using intensity modulated spot scanned proton beams. Plan evaluation was assessed by dose volume histogram analysis, comparing different dose distributions and different dose calculation algorithms (and dose to medium or to water option). Results: All plans acceptably met the criteria of target coverage (V95%>90-95%) and bone sparing (D1cm3<50 Gy). Better coverage was shown for proton plans with respect to 6X RapidArc photon plans, at 95% of the prescription dose (V95% about 5% higher for protons, p<0.05). Dose was more homogeneously distributed in the proton cases: Standard Deviation 2.3Gy for protons, 3.0-3.3Gy for photons, p<0.05. A small difference tending to be significant was found between 6X and 10FFF plans, in favor of using flattened beams. Bone significant maximum dose objective of 50Gy was met in all cases, photon RapidArc and protons, with no significant differences. The largest difference between proton and photon plans in Bone tissue is reflected in the medium/low doses. Similar results were found for surrounding normal tissue. For Healthy Tissue, the low dose bath is minimized with protons, where the Dose Integral is about halved with respect to all photon plans. Interesting result is the significant (p<<0.01) lower Dose Integral when FFF photon beams are used in place of more conventional flattened beams. Dose to water calculations resulted in bone doses of about 5% higher than for dose to medium calculations. A deep understanding of the two calculation options would lead the user to properly choose between the two modalities of dose to water, or the dose to medium (that better reflects the physical dose to the specific tissue). Conclusions: A substantial equivalence between RapidArc technique for photon beams and proton plans for the management of soft-tissue sarcoma of the lower extremity was here presented, yielding good target coverage to 66.5 Gy prescription, while keeping the maximum significant dose to the bone below the 50 Gy minimizing the risk of bone fractures.