ESTRO 2021 Abstract Book

S1052

ESTRO 2021

Conclusion Our findings have demonstrated that pre-treatment anemia was associated with worse clinical outcomes, especially in female patients. Correcting blood Hb levels might improve tumor response and survival in LARC patients who are candidates for neoadjuvant chemo-radiotherapy. A prospective evaluation is warranted to confirm these results.

PO-1275 Potential for Isotoxic Re-Irradiation SABR in Locally Recurrent Rectal Cancer M. Robinson 1 , S. O’Cathail 2 , A. Duffton 3 , K. Aitken 4 , R. Muirhead 5

1 University of Oxford, MRC Oxford Institute for Radiation Oncology, Oxford, United Kingdom; 2 University of Glasgow, Institute of Cancer Sciences, Glasgow, United Kingdom; 3 Beatson West of Scotland Cancer Centre, Oncology, Glasgow, United Kingdom; 4 Royal Marsden NHS Trust, Oncology, London, United Kingdom; 5 Oxford University Hospitals NHS Trust, Oncology, Oxford, United Kingdom Purpose or Objective Locally recurrent rectal cancer (LRRC) is a life altering diagnosis. It results in poor quality of life with symptoms including pain, bleeding, fistula’s, abscesses, hydronephrosis and faecal discharge. Median overall survival is significantly lower than metastatic rectal cancer at around 10 months. Radical pelvic exenteration is the only potential route to long term control, with complete resection (R 0 ) achieved in ~55% of cases. Most patients with LRRC had radiotherapy as part of their initial multimodality treatment. Re-irradiation offers good symptom palliation though effects last <1yr. SABR is increasingly used for the treatment of LRRC with the aim of extending the duration of local control. In England, SABR re-irradiation is available for previously irradiation pelvic recurrences, a small proportion of which are LRRC. Standard of care (SoC) re- irradiation SABR is 30Gy in 5#. Given the modest biological effective dose from this prescription, and that the consequences of not achieving local control in LRRC are so significant, increasing dose could significantly improve patient outcomes. We hypothesise that it is feasible to meaningfully increase dose above current SoC using an individualised isotoxic dose prescription. Materials and Methods We identified all LRRC patients treated with SoC dose re-irradiation SABR at local institution from 2015 to 2020. A predetermined meaningful increase in dose of isotoxic SABR was set at achieving an ablative threshold of 80Gy BED (alpha/beta 10Gy) to >80% of PTV. Dose constraints for each patient were recalculated retrospectively based on national consensus OAR constraints and a fixed 15% annual tissue recovery post previous radiotherapy. Clinical SABR plans were reviewed to determine limiting small volume (0.5cc) OAR dose and suitable individualised isotoxic dose prescription determined based on recalculated OAR constraints. Isotoxic dose prescription was capped at 50Gy in 5#, equivalent to 100Gy BED. Results Eighteen local recurrences were identified. Median time since previous radiotherapy was 3.8yrs (1.3-7yrs). Two patients were excluded from analysis; one due to <6 months since previous radiotherapy and the other due to limiting OAR not being considered a re-irradiation structure by the treating clinician as a consequence of anatomical changes post-surgical resection. Dose escalation was achievable above SoC in 78% of cases, see table one. An ablative threshold BED to 80% of PTV was achievable in 28%. In 22% isotoxic dose was decreased but in only 2 cases was BED decreased >2Gy.