The investigators propose to conduct an observational study on consecutive patients with low-lying or mid rectal cancers smaller than 5 cm in length and less than 50% of rectal circumferential extent. The aim of this study is to test a hypothesis that escalation of either radiation or chemotherapy dose of the routine preoperative radio(chemo)therapy leads to an increase of clinical complete response rate. The planned sample size of 23 patients was calculated based on the assumption that clinical complete response rate after routine preoperative radio(chemo)therapy is 34% [1] and expected rate after radio(chemo)therapy dose escalation is 75% [2-4]. An endorectal high dose rate iridium brachytherapy boost (2 fractions of 10 Gy) will be added after the routine preoperative treatment consisted of external beam radiotherapy (5 × 5 Gy) combined with sequential 3 cycles of consolidation FOLFOX4. However, for patients with involvement of the anal canal, additional 3 cycles of consolidation FOLFOX4 (6 cycles in total) will be added instead of brachytherapy boost to avoid severe post-radiation toxicity.

Inclusion Criteria:

1. Adenocarcinoma of the rectum classified clinically as T2, T3a-c or T4b but only with superficial infiltration of the vagina or prostate; tumour accessible by digital rectal examination; tumour ≤5 cm in length and ≤50% of rectal circumferential extent; N0-N2, but mesorectal node <7 mm and no lateral nodes enlargement); extramural vascular invasion but not larger than 7 mm in the largest dimension; M0.
2. Operable patient
3. Patient having read the information note and having signed the informed consent
4. Follow-up possible

Exclusion Criteria:

1. Inoperable patient
2. cT1, cT3d, large T4, tumour ≥5cm or ≥50 of rectal circumferential extent
3. Enlargement of lateral nodes or any node ≥8 mm in diameter
4. Patient presenting metastasis at diagnosis
5. Previous pelvic irradiation
6. Simultaneous progressive another cancer
7. Close follow-up uncertain.