中文摘要
放射治療已多年來使用於直腸癌的整合方式之一。手術前放射治療可降低局部復發率，增加切除機會，腸道副作用較少。在手術前先行縮小腫瘤，或可使離肛門口較近的低位腫瘤增加保留肛門手術的機會。合併化學治療與放射治療更促進局部控制功效。在手術前放射治療與手術後放射治療的比較上，手術前放射治療有較高的局部控制率及肛門保留率。大多數直腸癌放射治療的副作用是可處理且可回復的。直腸癌的放射治療技術已有長遠的進步，由二度空間放射治療，進步到三度空間順形治療，到現今的定向強度調控放射治療，及弧形體積調控放射治療。技術的進步使放射線劑量由大範圍骨盆腔照射更集中到直腸腫瘤與淋巴區，讓腹瀉等治療副作用明顯減少。這樣的技術進步也造成新近開發的標靶藥物加入手術前放射治療，以提升病理完全消退率，期望更改善直腸癌整合治療的控制成績。

Abstract
Radiotherapy has been integrated into the multi-modality treatment for rectal cancer. Preoperative radiotherapy is associated with the advantages of decreasing local recurrence rate, improving the resectability, and reducing side effects of bowels. The chance of anal preservation increases by preoperative radiotherapy to shrink the low located rectal tumor. Combined chemotherapy and radiotherapy further increases the local control rate. The comparison between preoperative and postoperative radiotherapy reveals the superior local control and anal preservation rates with preoperative radiotherapy. Most side effects from radiotherapy for rectal cancer are manageable and reversible. There have been tremendous improvements in radiotherapy techniques for rectal cancer. It has been moved from two-dimensional radiotherapy, three-dimensional conformal radiotherapy, step-and-shoot intensity modulated radiation therapy (IMRT), to volumetric modulated arc therapy (VMAT). Such an improvement shifts the radiation dose distribution from large pelvis to focus on the rectal tumor and lymphatics, and significantly reduces the side effects like diarrhea. The technical advancement allows the combination of recently developed target therapy with radiotherapy to increase the pathological complete response and hopefully improve the disease control of rectal cancer.