Dosimetric and radiobiological evaluation of four radiation techniques in preoperative rectal cancer radiotherapy

Purpose: To compare tumour dose distribution, conformality, homogeneity, normal tissue avoidance, tumour control probability (TCP) and normal tissue complication probability (NTCP) using 3D conformal radiation therapy (3DCRT), 3- and 4-field intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) in patients with locally advanced rectal cancer. Materials and methods: Twenty-four patients staged T1-3 N+ M-0 with locally advanced rectal cancer underwent neoadjuvant chemoradiation therapy. Four different radiotherapy plans were prepared for each patient: 3DCRT, 3- and 4-field IMRT and VMAT are evaluated for target distribution using CI and homogeneity index (HI), normal tissue avoidance using D-max, V-45, V-40, V-50 and TCP and NTCP using the Lyman-Kutcher-Burman model. Results: VMAT has better HI (HI = 1.32) and 3DCRT exhibited better conformality (CI = 1.05) than the other radiotherapy techniques. With regard to normal tissue avoidance, all radiotherapy plans met the constraints. D-max in the 3DCRT plans was statistically significant (p = 0.04) for bladder and no significant differences in V-40 and V-50. In the bowel bag, no significant differences in D-max for any radiotherapy plan and V-40 was lower in 3DCRT than VMAT (p = 0.024). In the case of femoral heads, 3DCRT has a statistically significant lower dose on D-max than 4-field IMRT (p = 0.00 << 0.05). VMAT has the biggest TCP (80.76%) than the other three radiotherapy plans. With regard to normal tissue complications, probabilities were shown to be very low, of the order of 10(-14) and 10(-41) for bowel bag and femoral heads respectively. Conclusions: It can be concluded that 3DCRT plan improves conformity and decreases radiation sparing in the organ at risks, but the VMAT plan exhibits better homogeneity and greater TCP.