Dosimetric Comparison of Volumetric Modulated Arc Therapy (VMAT) with and without Flattening Filter for Stereotactic Body Radiation Therapy (SBRT) in Localised Prostate Cancer

Abstract

Stereotactic body radiation therapy (SBRT) without flattening filter (FFF) for localised prostate cancer reduces the treatment fractionation and has the potential to reduce treatment time per fraction. This could reduce the strain in resources as the patient throughput can potentially increase with a reduced treatment time slot allotted to each patient. Hence the key focus of the study is to compare SBRT for localised prostate cancer with or without FFF based on these three points: 1. Treatment time per fraction 2. Planning Target Volume (PTV) coverage 3. Dose to Organs at Risk (OARs) Doses for Rectum, Bladder, Penile Bulb, Femoral Heads, Skin and Urethra Twelve consecutive patients were retrospectively selected. The target volumes and critical structures for each patient were contoured according to Radiation Therapy Oncology Group (RTOG) 0938 protocol (2014). A dose fractionation of 3625cGy in 5 fractions was used. All treatment plans with a single arc were generated using the same treatment planning system and delivered using the same linear accelerator with FFF capability. Treatment time and total number of monitor units for each treatment plan was recorded. Dosimetric data comprising of PTV coverage, dose conformity and OARs doses were extracted from the dose volume histograms of the treatment plans. Descriptive statistics including mean values and standard deviation were collected. Paired samples t-test was used as a statistical test. To be considered statistically significant, a p value of less than or equal to 0.05 was used. For all treatment plans, except for prostate maximum point dose (VMAT vs FFF: 3843.1 cGy vs 3852.8 cGy, p<0.05), dose to 90% of rectum (VMAT vs FFF: 119.4 cGy vs 122.4 cGy, p<0.05), dose to 90% of bladder (VMAT vs FFF: 74.1 cGy vs 79.3 cGy, p<0.05), dose to less than 3 cc of penile bulb (VMAT vs FFF: 356.8 cGy vs 327.7 cGy, p<0.05), dose to less than 10 cc of bilateral femoral heads (VMAT vs FFF: 1,048.7 cGy vs 1,100.4 cGy, p=0.05), minor differences existed in dose distributions using VMAT and FFF. No differences were detected between the two beam qualities with respect to PTV coverage and conformity. The mean treatment time, measured in minutes, decreased significantly with FFF (VMAT vs FFF: 3.0 min vs 1.4 min, p<0.05). This study found that plans with or without flattening filter were of comparable plan quality in terms of PTV coverage and OARs doses. The main advantage of FFF SBRT treatment over non-FFF is the 53% reduction in treatment time. A significant amount of time were saved for both the prostate patients and the radiation oncology department. In conclusion FFF SBRT for prostate cancer should be implemented in the radiation oncology department for the benefit of both patients and the department.