Role of deformable image registration for delivered dose accumulation of adaptive external beam radiation therapy and brachytherapy in cervical cancer.

PURPOSE: Deformable image registration (DIR) can be used to accumulate the absorbed dose distribution of daily image-guided adaptive external beam radiation treatment (EBRT) and brachytherapy (BT). Since dose-volume parameter addition assumes a uniform delivered EBRT dose around the planned BT boost, the added value of DIR over direct addition was investigated for dose accumulation in bladder and rectum. MATERIAL AND METHODS: For 10 patients (EBRT 46/46.2 GyEQD2, EBRT + BT: D90 85-90 GyEQD2, in equivalent dose in 2 Gy fractions), the actually delivered dose from adaptive volumetric-modulated arc therapy (VMAT)/intensity-modulated radiotherapy (IMRT) EBRT was calculated using the daily anatomy from the cone-beam computed tomography (CBCT) scans acquired prior to irradiation. The CBCT of the first EBRT fraction and the BT planning MRI were registered using DIR. The cumulative dose to the 2 cm3 with the highest dose (D2cm3) from EBRT and BT to the bladder and rectum was calculated and compared to direct addition assuming a uniform EBRT dose (UD). RESULTS: Differences (DIR-UD) in the total EBRT + BT dose ranged between -0.2-3.9 GyEQD2 (bladder) and -1.0-3.7 GyEQD2 (rectum). The total EBRT + BT dose calculated with DIR was at most 104% of the dose calculated with the UD method. CONCLUSIONS: Differences between UD and DIR were small (< 3.9 GyEQD2). The dose delivered with adaptive VMAT/IMRT EBRT to bladder and rectum near the planned BT boost can be considered uniform for the evaluation of bladder/rectum D2cm3.

Dose warping uncertainties for the accumulated rectal wall dose in cervical cancer brachytherapy.

PURPOSE: Structure-based deformable image registration (DIR) can be used to calculate accumulated dose volume histogram parameters for cervical cancer brachytherapy (BT). The purpose of this study is to investigate dose warping uncertainties for the accumulated dose to the 2 cm3 receiving the highest dose [Formula: see text] in the rectal wall, using a physically realistic model (PRM) describing rectal wall deformation. METHODS AND MATERIALS: For 10 patients, treated with MRI-guided pulsed dose rate BT (two times 24 × 0.75 Gy, given in two applications BT1 and BT2), the planning images were registered with structure-based DIR. The resulting transformation vectors were used to accumulate the total rectum dose from BT. To investigate the dose warping uncertainty, a PRM describing rectal deformation was used. For point pairs on rectumBT1 and rectumBT2 that were at the same location according to the PRM, the dose for BT1 and BT2 was added (DPRM) and compared to the DIR-accumulated dose (DDIR) in the BT2 point. The remaining distance after DIR between corresponding point pairs, defined as the residual distance, was calculated. RESULTS: For points within the [Formula: see text] volume, more than 75% was part of the [Formula: see text] volume according to both PRM and DIR. The absolute dose difference was <7.3 GyEQD2, and the median (95th percentile) of the residual distance was 8.7 (22) mm. CONCLUSIONS: DIR corresponded with the PRM for on average 75% of the [Formula: see text] volume. Local absolute dose differences and residual distances were large. Care should therefore be taken with DIR for dose-warping purposes in BT.

Image Distortions on a Plastic Interstitial Computed Tomography/Magnetic Resonance Brachytherapy Applicator at 3 Tesla Magnetic Resonance Imaging and Their Dosimetric Impact.

PURPOSE: To quantify magnetic resonance imaging (MRI) distortions on a plastic intracavitary/interstitial applicator with plastic needles at a field strength of 3 T and to determine the dosimetric impact, using patient data. METHODS AND MATERIALS: For 11 cervical cancer patients, our clinical MRI protocol was extended with 3 scans. From the first scan, a multi-echo acquisition, a map of the magnetic field (B0) was calculated and used to quantify the field inhomogeneity. The expected displacements of the applicator were quantified for the clinical sequence using the measured field inhomogeneity and the clinical sequence's bandwidth. The second and third scan were our routine clinical sequence (duration: <5 minutes each), acquired consecutively using opposing readout directions. The displacement of the applicator between these scans is approximately twice the displacement due to B0 inhomogeneity. The impact of the displacement on the dose was determined by reconstructing the applicator on both scans. The applicator was then shifted and rotated the same distance as the observed displacement to create a worst-case scenario (ie, twice the actual displacement due to B0 inhomogeneity). Next, the dose to 98%/90% (D98/D90) of the clinical target volume at high risk, as well as the dose to the most irradiated 2 cm3 for bladder and rectum, were calculated for the original plan as well as the shifted plan. RESULTS: For a volume of interest containing the intrauterine device and the ovoids the 95th percentile of the absolute displacement ranged between 0.2 and 0.75 mm, over all patients. For all patients, the difference in D98/D90 in the opposing readout scans with the original plan was at most 4.7%/4.3%. For the dose to the most irradiated 2 cm3 of bladder/rectum, the difference was at most 6.0%/6.3%. CONCLUSIONS: The dosimetric impact of distortions on this plastic applicator with plastic needles is limited. Applicator reconstruction for brachytherapy planning purposes is feasible at 3 T MRI.

Structure-based deformable image registration: Added value for dose accumulation of external beam radiotherapy and brachytherapy in cervical cancer.

BACKGROUND AND PURPOSE: Structure-based deformable image registration (DIR) can be used to calculate accumulated brachytherapy (BT) and external-beam radiation therapy (EBRT) dose-volume histogram (DVH) parameters in cervical cancer. Since direct parameter addition does not take dose non-uniformity into account, the added value of DIR over addition methods was investigated for bladder and rectum. MATERIALS AND METHODS: For twelve patients (EBRT: 46Gy, EBRT+BT: D90 85-90GyEQD2 in equivalent dose in 2Gy fractions) the EBRT planning CT and BT planning MRI were registered using DIR. Affected lymph nodes, located far from the BT boost region, received an EBRT boost (9.2Gy) not contributing to the BT boost dose. Cumulative bladder/rectum D2cm3/D1cm3 were calculated and compared to direct addition methods, assuming uniform EBRT doses (UD), or overlapping high dose volumes (OHD). RESULTS: Between the three methods, the maximum differences in the cumulative DVH parameters were 3.2GyEQD2 (bladder) and 3.3GyEQD2 (rectum). The difference between DIR and UD was <1.8GyEQD2 for both organs. CONCLUSIONS: The UD method provides a better estimate of D2cm3/D1cm3 than the OHD method. There is no added value of DIR since differences with direct addition methods are clinically insignificant. EBRT dose distributions can be considered uniform in bladder and rectum for the evaluated dose parameters.