Integral dose comparison of VMAT and IMRT for rectal cancer treatment: A dosimetric analysis

Intensity-modulated radiation therapy (IMRT) combined with chemotherapy followed by surgery is the standard treatment for locally advanced rectal cancer (LARC). IMRT, however, uses a larger number of monitor units (MU) to achieve a highly conformal radiation distribution while causing spillage of radiation to the surrounding normal soft tissues of the body, this phenomenon has led to concerns of increased risk of secondary radiation-induced malignancies for patients with long life expectancies. Volumetric-modulated arc therapy (VMAT) is a newer efficient technique wherein radiotherapy is delivered using a cone beam that rotates around the patient in an arc to generate IMRT-quality dose distributions, VMAT with its reduced MUs leads to a lower radiation dose to the surrounding normal tissue. The Integral dose (ID) describes the energy deposition of radiation within the body and is defined as the product of mass of tissue Irradiated and absorbed dose, normal tissue complication risk and secondary malignancies is directly related to ID. The aim of this study was to analyze whether VMAT can reduce the normal tissue integral dose (NTID) when compared with IMRT. The 32 LARC patients were all first treated with IMRT from 2015 to 2020, then a separate VMAT treatment plan was generated for each patient for comparison. All patients were treated with a cumulative dose of 50 Gy in 25 fractions. The NTID for body normal tissue volume encompassed by 5Gy (221.9 vs 223.2), 10Gy (206.9 vs 208.1), 20 Gy (152.6 vs 165.4), 30 Gy (103.4 vs 108.6), and 40 Gy dose (72.3 vs 74.8), were significantly lower for the VMAT when compared IMRT, while those for urinary bladder, large intestine and small intestine did not show any difference. The normal tissue complication probability (NTCP) and tumor control probability (TCP) did not show any significant difference between both techniques. VMAT for rectal cancer offers an advantage in terms of a shorter treatment time, a highly conformal radiation dose distribution, and a lower NTID when compared with IMRT.