Potential for Isotoxic Re-irradiation Stereotactic Ablative Body Radiotherapy in Locally Recurrent Rectal Cancer

The non-surgical management of locally recurrent rectal cancer (LRRC) is an area of unmet need, with no defined standard treatment and extremely poor outcomes. Patients typically receive radiotherapy during initial multimodality treatment and historically re-irradiation has been limited to conservative doses with subsequent short-term symptom control. Recently, stereotactic ablative body radiotherapy (SABR) has shown promise in re-irradiation of LRRC in England, but is limited to a relatively modest dose prescription of 30Gy in five fractions. We propose that SABR can be achieved in LRRC to higher doses using an isotoxic dose prescription with fixed 15% per annum tissue recovery for acceptable organ at risk (OAR) constraints.

Patients with LRRC at a local centre treated with SABR re-irradiation were audited. Patients were identified, the dose and time since previous radiotherapy determined, re-irradiation OAR constraints calculated and retrospective re-planning carried out.

In patients currently receiving SABR (17 patients, 21 targets), dose escalation above 30 Gy in five fractions was achievable, with a biological effective dose of 80 Gy (alpha/beta = 10) deliverable to 80% or more of the planning target volume in eight of the 21 targets.

Isotoxic SABR re-irradiation should be considered a potential treatment option for LRRC to maximise patient outcomes while limiting excess toxicity. Although probably conservative, clinical outcome data are needed to determine the suitability of OAR constraints using 15% per annum tissue recovery and the impact on local control rates, patient quality of life and overall survival of isotoxic SABR.