AUTHOR=Sharfo Abdul Wahab M. , Rossi Linda , Dirkx Maarten L. P. , Breedveld Sebastiaan , Aluwini Shafak , Heijmen Ben J. M. 

TITLE=Complementing Prostate SBRT VMAT With a Two-Beam Non-Coplanar IMRT Class Solution to Enhance Rectum and Bladder Sparing With Minimum Increase in Treatment Time

JOURNAL=Frontiers in Oncology

VOLUME=11

YEAR=2021

URL=https://www.frontiersin.org/journals/oncology/articles/10.3389/fonc.2021.620978

DOI=10.3389/fonc.2021.620978

ISSN=2234-943X

ABSTRACT=<sec><title>Purpose</title><p>Enhance rectum and bladder sparing in prostate SBRT with minimum increase in treatment time by complementing dual-arc coplanar VMAT with a two-beam non-coplanar IMRT class solution (CS).</p></sec><sec><title>Methods</title><p>For twenty patients, an optimizer for automated multi-criterial planning with integrated beam angle optimization (BAO) was used to generate dual-arc VMAT plans, supplemented with five non-coplanar IMRT beams with individually optimized orientations (VMAT+5). In all plan generations, reduction of high rectum dose had the highest priority after obtaining adequate PTV coverage. A CS with two most preferred directions in VMAT+5 and largest rectum dose reductions compared to dual-arc VMAT was then selected to define VMAT+CS. VMAT+CS was compared with automatically generated <italic>i)</italic> dual-arc coplanar VMAT plans (VMAT), <italic>ii)</italic> VMAT+5 plans, and <italic>iii)</italic> IMRT plans with 30 patient-specific non-coplanar beam orientations (30-NCP). Plans were generated for a 4 x 9.5 Gy fractionation scheme. Differences in PTV doses, healthy tissue sparing, and computation and treatment delivery times were quantified.</p></sec><sec><title>Results</title><p>For equal PTV coverage, VMAT+CS, consisting of dual-arc VMAT supplemented with two fixed, non-coplanar IMRT beams with fixed Gantry/Couch angles of 65°/30° and 295°/-30°, significantly reduced OAR doses and the dose bath, compared to dual-arc VMAT. Mean relative differences in rectum D<sub>mean</sub>, D<sub>1cc</sub>, V<sub>40GyEq</sub> and V<sub>60GyEq</sub> were 19.4 ± 10.6%, 4.2 ± 2.7%, 34.9 ± 20.3%, and 39.7 ± 23.2%, respectively (all <italic>p</italic>&lt;0.001). There was no difference in bladder D<sub>1cc</sub>, while bladder D<sub>mean</sub> reduced by 17.9 ± 11.0% (<italic>p</italic>&lt;0.001). Also, the clinically evaluated urethra D<sub>5%</sub>, D<sub>10%</sub>, and D<sub>50%</sub> showed small, but statistically significant improvements. All patient V<sub>X</sub> with X = 2, 5, 10, 20, and 30 Gy were reduced with VMAT+CS, with a maximum relative reduction for V<sub>10Gy</sub> of 19.0 ± 7.3% (<italic>p</italic>&lt;0.001). Total delivery times with VMAT+CS only increased by 1.9 ± 0.7 min compared to VMAT (9.1 ± 0.7 min). The dosimetric quality of VMAT+CS plans was equivalent to VMAT+5, while optimization times were reduced by a factor of 25 due to avoidance of individualized BAO. Compared to VMAT+CS, the 30-NCP plans were only favorable in terms of dose bath, at the cost of much enhanced optimization and delivery times.</p></sec><sec><title>Conclusions</title><p>The proposed two-beam non-coplanar class solution to complement coplanar dual-arc VMAT resulted in substantial plan quality improvements for OARs (especially rectum) and reduced irradiated patient volumes with minor increases in treatment delivery times.</p></sec>