Introduction: T4 nasopharyngeal carcinoma (NPC) with close proximity to critical organs at risk (OARs) is usually underdosed during radiotherapy in order to respect radiation constraints. N3 disease has high risk of distant metastasis. Induction chemotherapy (IC) provides advantages of sparing of OARs during subsequent chemoradiotherapy (CCRT) and early eradication of micrometastasis. However, factors predicting successes of IC in this patient group are not well-studied. Methods: 104 T4 or N3 NPC patients were retrospectively reviewed during 2007-2018. They were planned for IC followed by CCRT using intensitymodulated radiotherapy. Results: In the whole group, five-year failure-free survival (FFS), locoregional failure-free survival (LRFS), distant failure-free survival (DFFS) and overall survival (OS) were 40.9%, 45.7%, 46.9% and 53.6% respectively. Isolated marginal failure rate was 5% (4/80) among patients with primary tumours located close to critical OARs. Pre-IC gross tumour volume primary (GTVp) total volume > 110 cm 3 correlated with worse five-year LRFS (OR 6.37, P = 0.008), DFFS (OR 8.89, P = 0.003) and OS (OR 50.12, P < 0.001). In the T4 subgroup, IC improved D100% GTVp from 61.39 Gy to 64.71 Gy (P < 0.001) and V100% GTVp from 98.78% to 99.28% (P < 0.001). Conclusion: Our study demonstrated improved dosimetric parameters and low isolated marginal failure rate. It supported the use of IC and CCRT for tumours located close to critical OARs. Further research is warranted to compare predictive roles of pre-and post-IC tumour volumes. For high-risk patients being defined by pre-IC volume or other prognostic models, treatment escalation should be considered.

Objective: The aim of this study was to investigate whether omitting elective inguinal irradiation during neoadjuvant or adjuvant (chemo)radiotherapy is feasible for patients with locally advanced low rectal cancer (LALRC) with anal canal invasion (ACI). Study design: A total of 90 LALRC patients with ACI who underwent neoadjuvant or adjuvant (chemo)radiotherapy between 2011 and 2021 were recruited. Inguinal lymph node (ILN) was clinically negative on presentation. Failure pattern, ILN recurrence rate, survival data and prognostic factors were analysed. Results: The 3-year ILN failure rate was 4.94%. 1 patient developed isolated ILN failure, which was successfully salvaged by surgery. The 3-year locoregional disease free survival (LRFS), distant metastatic recurrence free survival (DMRFS), and overall survival (OS) were 81.1%, 77.0%, and 86.8% respectively. In multivariate cox regression analysis, positive pathological lymph node after neoadjuvant treatment predicted worse LRFS (odd ratio [OR] 10.57, P= 0.00001), DMRFS (OR 9.17, P= 0.0002), and OS (OR 12.92, P= 0.0005). Positive tumour resection margin correlated with worse LRFS (OR 23.53, P= 0.001), DMRFS (OR 12.62, P= 0.002) and OS (OR 47.24, P= 0.002). Concurrent chemotherapy to RT was associated with better LRFS (OR 28.32, P= 0.002). ≥ Grade 3 acute and chronic toxicities occurred in 44.4% and 11.1% respectively in patients with elective inguinal radiation compared with 19.8% and 11.1% respectively in patients who did not receive it. Conclusion: Omission of elective inguinal irradiation was associated with a low inguinal failure rate for LALRC with ACI. It can spare patients from unnecessary acute radiotherapy toxicities.