Why is Volumetric Modulated Arc Therapy Better Than Three Dimensional Conformal Radiotherapy in Prostate Cancer? Dosimetric Analysis from a Tertiary Care Hospital in Saudi Arabia

Background: Prostate cancer is one of the most common cancers among older 
men. It is ranked as the third most common cancer among Saudi men. (As per 
the last Saudi cancer registry,2016) Current protocols for prostate cancer 
external beam radiation therapy (EBRT) commonly use two main techniques for 
treatment planning, including three-dimensional conformal radiation therapy 
(3D-CRT) and intensity-modulated radiation therapy (IMRT) including 
volumetric modulated arc therapy (VMAT).
Objectives: The goal of this study is to compare target volumes and organ at 
risk (OAR) for VMAT versus 3D-CRT plans. 
Materials and methods: Forty patients with localized prostate cancer, 
diagnosed and treated at King Fahad Medical City (KFMC), Riyadh, Saudi 
Arabia were selected retrospectively for this planning study. Patients were 
treated with radical definitive external beam radiation therapy (EBRT) using the 
VMAT technique with a prescribed dose of 78Gy in 39 daily fractions over 
about 8 weeks. Elective pelvic nodal irradiation was not performed. All patients 
were re-planned with six fields of 3D-CRT for study purposes. Treatments were 
delivered using the Trilogy VARIAN Linear Accelerator. Treatment plans were 
done by Eclipse Varian treatment planning system (TPS) version 10, dose 
calculations were performed using Analytical Anisotropic Algorithm (AAA) for 
both VMAT and 3D-CRT techniques. Plans were evaluated using the 
conformity index (CI) and homogeneity index (HI) for target volumes. Mean, 
maximum, and OAR dose volumes were compared between both techniques 
based on QUANTEC normal tissue tolerance doses. Data was analyzed using 
SPSS-23. 
Results: Planning Target Volume (PTV) received a significantly higher 
maximum dose in VMAT than 3D-CRT plans (p=0.000). The HI for PTV was 
better in 3D-CRT compared to VMAT plans (p = 0.010). However, CI was 
significantly better in VMAT vs. 3D-CRT plans (p = 0.002). As expected, 3D-
CRT plans required a smaller number of monitor units (MU) than VMAT plans 
to deliver the same prescribed dose (p = 0.000). VMAT technique resulted in the 
delivery of lower OAR mean doses to the rectum, penile bulb, bone marrow, 
and femoral heads compared to the 3D-CRT technique (p < 0.05); however, 
there was no significant difference between the two techniques for small bowel 
(p=0.234) and bladder (p=0.509). On the other hand, the mean dose was lower 
in 3D than the VMAT plan for testis (p = 0.000). VMAT delivered significantly 
higher maximum doses than 3D-CRT to the bladder and rectum while 3D-CRT 
delivered higher maximum doses to the femoral heads and small bowel. VMAT 
plans resulted in the delivery of significantly lower OAR dose volumes for all 
dosimetric endpoints, except for small bowel (V45) and bone marrow (V5), for 
which there was no significant difference. 
Conclusion: VMAT generated more favorable treatment plans compared to 3D-
CRT, however, 3-D CRT can also achieve QUANTEC goals with required PTV 
coverage. VMAT requires more MU than 3 D-CRT, raising the issue of possible 
second malignancies that need to be clarified by further clinical trials.