Dosimetric study of RapidArc plans and conventional intensity modulated radiotherapy for prostate cancer involving seminal vesicles and pelvis lymph nodes
Purpose: The main purpose of this study is to (1) identify the continual diversity between conventional fixed field intensity modulation radiotherapy (IMRT) and RapidArc (RA) for high-risk prostate cancer; and (2) determine potential benefits and drawbacks of using for this type of treatment.
Methods: A cohort of 20 prostate cases including prostate, seminal vesicles and pelvic lymph nodes was selected for this study. The primary planning target volume (PTVP) and boost planning target volume (PTVB) were contoured. The total prescription dose was 75.6 Gy (45 Gy to PTVP and an additional 21.6 Gy to PTVB). Two plans were generated for each PTV: multiple 7-fields for IMRT and two arcs for RA.
Results: A Sigma index (IMRT: 2.75 ± 0.581; RA: 2.8 ± 0.738) for PTVP and (IMRT: 2.0 ± 0.484; RA: 2.1 ± 0.464) for PTVB indicated similar dose homogeneity inside the PTV. Conformity index (IMRT: 0.96 ± 0.047; RA: 0.95 ± 0.059) for PTVP and (IMRT: 0.97 ± 0.015; RA: 0.96 ± 0.014) for PTVB was comparable for both the techniques. IMRT offered lower mean dose to organ at risks (OARs) compared to RA plans. Normal tissue integral dose in IMRT plan resulted 0.87% lower than RA plans. All the plans displayed significant increase (2.50 times for PTVP and 1.72 for PTBB) in the average number of necessary monitor units (MUs) with IMRT beam. Treatment delivery time of RA was 2 ‒ 6 minutes shorter than IMRT treatment.
Conclusion: For PTV including pelvic lymph nodes, seminal vesicles and prostate, IMRT offered a greater degree of OARs sparing. For PTV including seminal vesicles and prostate, RA with two arcs provided comparable plan with IMRT. RA also improved the treatment efficiency due to smaller number of MUs required.
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