Isotoxic simultaneous integrated boost to dominant intra-prostatic lesions using stereotactic ablative radiotherapy and volumetric modulated arc therapy


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Louise Murray1,2, John Lilley1, Christopher Thompson1, Jonathan Sykes1, Kevin Franks1, David Sebag-Montefiore1,2, Vivian Cosgrove1, Ann Henry1
1St James's Institute of Oncology, Leeds, UK, 2University of Leeds, Leeds, UK

Background

An isotoxic approach was used to investigate simultaneous integrated boost delivery to dominant intra-prostatic lesions (DILs) using stereotactic ablative radiotherapy (SABR) and volumetric modulated arc therapy (VMAT).

Method

T2-weighted, diffusion-weighted and dynamic contrast-enhanced MRI sequences were used to define DILs in 10 prostate datasets. CTV_DIL consisted of the combined DIL volumes from each sequence, expanded by 4mm to PTV_DIL. CTV_prostate was the prostate, expanded by 6mm to PTV_prostate.

The PTV_prostate was prescribed 42.7Gy in 7 fractions. Conformity index (CI; 95% isodose volume/PTV volume) was restricted to ≤1.2 and R50 (50% isodose volume/PTV volume) was limited to ≤5.0. The dose prescribed to each PTV_DIL was increased in 5% increments starting at 115% of the PTV_prostate dose until organ-at-risk (OAR) or conformity limits were reached.

Plans were repeated with inclusion of the proximal seminal vesicles (proxSV) within a PTV prescription of 32.4Gy in 7 fractions.

Monaco v3.3 was used with a Monte Carlo algorithm, VMAT and the Agility MLC head.

Results

17 PTV_DILs were defined (1, 2 and 3 DILs in 5, 3 and 2 cases). Median PTV_DIL volume was 3.7cm3 (range 1.2-51.8cm3).

When treating the prostate alone, the median isotoxic dose prescribed to a PTV_DIL was 125% (range 110-140%) of the PTV_prostate dose. The median D50% received by a PTV_DIL was 55.6Gy (range 49.2-62.9Gy). Plans were conformal with short estimated delivery times. DIL location influenced which OAR was dose-limiting.

When also treating the proxSV, the same isotoxic doses could be prescribed, but with significant increases in CI (all ≤1.2) and R50 (all ≤5.0), and small reductions in PTV_DIL median dose.

Conclusion

In conjunction with robust image guidance, this linac-based SABR solution offers the potential to boost DILs while respecting OAR and conformity limits, both in low risk disease, when treating prostate alone, and in higher risk cases, when also treating proxSV.

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