Assessment of Adaptive Margins Using a Single Planning Computed Tomography Scan for Bladder Radiotherapy.

INTRODUCTION: A "plan of the day" (PoD) adaptive radiotherapy approach is presented for bladder cancer. The potential reduction in volumes of normal tissue and bowel bag receiving high-dose radiation is evaluated. MATERIALS AND METHODS: Planning computed tomography (pCT) and daily cone beam CT (CBCT) data sets were analyzed for eight previously treated bladder cancer patients. For each patient, a whole bladder clinical target volume (CTV) was delineated on a pCT. Then, the clinical target volume was expanded using five sets of anisotropic or isotropic margins to create five planning target volumes (PTVs). A library of five corresponding treatment PoDs was then created using volumetric modulated arc therapy. Offline PoD selection was performed by three independent radiation therapists (RTs) using daily CBCTs. Dosimetric results were compared between PoD treatments and two conventional treatments using isotropic 1.5- and 2.0-cm margins. RESULTS: The smallest PTV using 1.0-cm isotropic margins was selected most frequently (70%). Three RTs demonstrated good agreement for daily PTV selections, choosing identical PoDs for 51% of all CBCTs. In addition, acceptable dosimetric coverage of the whole bladder was achieved for all PoD selections, similar to standard treatments. The average volume of bowel bag receiving 45 and 50 Gy and normal tissue receiving 95% prescription dose was significantly (P < .01) lower for PoD treatments compared with both conventional treatments. CONCLUSIONS: A PoD strategy using one pCT with isotropic and anisotropic margins can be used to treat bladder cancer and improve sparing of the bowel bag. Minimal dosimetric differences observed between three RTs suggests that PoD strategies are feasible for clinical implementation.

A comparison of volumetric modulated arc therapy and conventional intensity-modulated radiotherapy for frontal and temporal high-grade gliomas.

PURPOSE: Volumetric modulated arc therapy (VMAT), the predecessor to Varian's RapidArc, is a novel extension of intensity-modulated radiotherapy (IMRT) wherein the dose is delivered in a single gantry rotation while the multileaf collimator leaves are in motion. Leaf positions and the weights of field samples along the arc are directly optimized, and a variable dose rate is used. This planning study compared seven-field coplanar IMRT (cIMRT) with VMAT for high-grade gliomas that had planning target volumes (PTVs) overlapping organs at risk (OARs). METHODS AND MATERIALS: 10 previously treated patients were replanned to 60 Gy in 30 fractions with cIMRT and VMAT using the following planning objectives: 98% of PTV covered by 95% isodose without violating OAR and hotspot dose constraints. Mean OAR doses were maximally decreased without reducing PTV coverage or violating hotspot constraints. We compared dose-volume histogram data, monitor units, and treatment times. RESULTS: There was equivalent PTV coverage, homogeneity, and conformality. VMAT significantly reduced maximum and mean retinal, lens, and contralateral optic nerve doses compared with IMRT (p < 0.05). Brainstem, chiasm, and ipsilateral optic nerve doses were similar. For 2-Gy fractions, mean monitor units were as follows: cIMRT = 789 +/- 112 and VMAT = 363 +/- 45 (relative reduction 54%, p = 0.002), and mean treatment times (min) were as follows: cIMRT = 5.1 +/- 0.4 and VMAT = 1.8 +/- 0.1 (relative reduction 65%, p = 0.002). CONCLUSIONS: Compared with cIMRT, VMAT achieved equal or better PTV coverage and OAR sparing while using fewer monitor units and less time to treat high-grade gliomas.

Volumetric modulated arc therapy for delivery of prostate radiotherapy: comparison with intensity-modulated radiotherapy and three-dimensional conformal radiotherapy.

PURPOSE: Volumetric modulated arc therapy (VMAT) is a novel form of intensity-modulated radiotherapy (IMRT) optimization that allows the radiation dose to be delivered in a single gantry rotation of up to 360 degrees , using either a constant dose rate (cdr-VMAT) or variable dose rate (vdr-VMAT) during rotation. The goal of this study was to compare VMAT prostate RT plans with three-dimensional conformal RT (3D-CRT) and IMRT plans. PATIENTS AND METHODS: The 3D-CRT, five-field IMRT, cdr-VMAT, and vdr-VMAT RT plans were created for 10 computed tomography data sets from patients undergoing RT for prostate cancer. The parameters evaluated included the doses to organs at risk, equivalent uniform doses, dose homogeneity and conformality, and monitor units required for delivery of a 2-Gy fraction. RESULTS: The IMRT and both VMAT techniques resulted in lower doses to normal critical structures than 3D-CRT plans for nearly all dosimetric endpoints analyzed. The lowest doses to organs at risk and most favorable equivalent uniform doses were achieved with vdr-VMAT, which was significantly better than IMRT for the rectal and femoral head dosimetric endpoints (p < 0.05) and significantly better than cdr-VMAT for most bladder and rectal endpoints (p < 0.05). The vdr-VMAT and cdr-VMAT plans required fewer monitor units than did the IMRT plans (relative reduction of 42% and 38%, respectively; p = 0.005) but more than for the 3D-CRT plans (p = 0.005). CONCLUSION: The IMRT and VMAT techniques achieved highly conformal treatment plans. The vdr-VMAT technique resulted in more favorable dose distributions than the IMRT or cdr-VMAT techniques, and reduced the monitor units required compared with IMRT.