ESTRO 2021 Abstract Book

S619

ESTRO 2021

irradiating entire metastases with possibly different doses in every fraction. The optimal s STF scheme is determined by means of a new purpose-specific planning objective (shown in Figure 1) to be added to the BED-based treatment plan optimization, which optimizes the partial dose contributions δ mt ’s of every fraction t to each metastasis m , while ensuring that very similar spatial dose distributions are delivered within each metastasis by all fractions. The potential benefits of spatiotemporal fractionation schemes are evaluated for a clinical case consisting of twenty-nine brain metastases of varying sizes (with volumes ranging from 0.1 to 9.9 cc and mean volume of 0.9 cc). Six-fraction non-coplanar intensity-modulated radiotherapy plans are generated with both STF and s STF schemes and compared to a uniformly fractionated plan. The plan optimization is designed to minimize the mean cumulative biologically effective dose (BED 2Gy ) to the healthy brain while achieving the same clinically prescribed BED 10Gy coverage of 45 Gy in the metastases for all plans.

Results For the same tumor BED 10Gy

in all plans, the mean BED 2Gy to the healthy brain is 12.16 Gy for the uniformly fractionated plan, while it is reduced to 11.41 Gy (-6.2%) and 11.21 Gy (-7.8%) for the s STF and STF plans, respectively. Exemplary dose distributions achieved with STF and s STF schemes are shown in Figure 2. Compared to the STF plan, no spatial fractionation within the metastases is performed with the s STF plan, which tends to irradiate small metastases with high single fraction doses and larger metastases with more uniform doses in every fraction.

Conclusion Spatiotemporally fractionated plans outperform the uniformly fractionated plan in terms of mean BED 2Gy to the healthy brain. While s STF cannot achieve the full BED 2Gy reduction of STF, it improves on uniform fractionation and represents a practical approach to spatiotemporal fractionation with lower hurdles for clinical implementation. This work was supported by grant Spatiotemporal fractionation in radiotherapy (310030_189285/1) of the Swiss National Science Foundation.

PD-0787 Is dosed averaged LET the best RBE descriptor for proton therapy? F. Kalholm 1,2 , N. Bassler 3,4,5,6 , I. Toma-Dasu 1,2

1 Stockholm University, Medical Physics, Stockholm, Sweden; 2 Karolinska Institutet, Department of Oncology and Pathology, Medical Radiation Physics, Stockholm, Sweden; 3 Stockholm Universiry, Medical Physics, Stockholm, Sweden; 4 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus, Denmark; 5 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 6 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark Purpose or Objective For proton therapy, a relative biological effectiveness (RBE) of 1.1 has broadly been applied clinically. However, as unexpected toxicities have been observed by the end of the proton tracks, variable RBE models have been proposed. Typically, the dose averaged linear energy transfer (LETd) has been used as an input