Commissioning of a 3D image-based treatment planning system for high-dose-rate brachytherapy of cervical cancer.

The objective of this work is to present commissioning procedures to clinically implement a three-dimensional (3D), image-based, treatment-planning system (TPS) for high-dose-rate (HDR) brachytherapy (BT) for gynecological (GYN) cancer. The physical dimensions of the GYN applicators and their values in the virtual applicator library were varied by 0.4 mm of their nominal values. Reconstruction uncertainties of the titanium tandem and ovoids (T&O) were less than 0.4 mm on CT phantom studies and on average between 0.8-1.0 mm on MRI when compared with X-rays. In-house software, HDRCalculator, was developed to check HDR plan parameters such as independently verifying active tandem or cylinder probe length and ovoid or cylinder size, source calibration and treatment date, and differences between average Point A dose and prescription dose. Dose-volume histograms were validated using another independent TPS. Comprehensive procedures to commission volume optimization algorithms and process in 3D image-based planning were presented. For the difference between line and volume optimizations, the average absolute differences as a percentage were 1.4% for total reference air KERMA (TRAK) and 1.1% for Point A dose. Volume optimization consistency tests between versions resulted in average absolute differences in 0.2% for TRAK and 0.9 s (0.2%) for total treatment time. The data revealed that the optimizer should run for at least 1 min in order to avoid more than 0.6% dwell time changes. For clinical GYN T&O cases, three different volume optimization techniques (graphical optimization, pure inverse planning, and hybrid inverse optimization) were investigated by comparing them against a conventional Point A technique. End-to-end testing was performed using a T&O phantom to ensure no errors or inconsistencies occurred from imaging through to planning and delivery. The proposed commissioning procedures provide a clinically safe implementation technique for 3D image-based TPS for HDR BT for GYN cancer.

Bladder and rectum dose estimations on digitized radiographs for vaginal brachytherapy after hysterectomy.

The purpose of this study was to evaluate the feasibility of assessing bladder and rectal point doses, using orthogonal radiographs without treatment planning, for vaginal cylinder applicator (VC), high-dose-rate (HDR) vaginal cuff brachytherapy (BT) after hysterectomy. Thirty-three VC HDR BT treatment plans from 31 postop- erative endometrial cancer patients were retrospectively analyzed. Single-channel VC with four differing diameters - 2.0 cm, 2.3 cm, 2.6 cm, and 3.0 cm - were analyzed. Dose-distance modeling was performed to estimate bladder and rectal point doses by measuring distances on each orthogonal radiograph without treat- ment planning. The estimated doses were then compared with doses calculated on treatment planning system (TPS). Their percent (%) dose differences were recorded. Analysis was performed for each VC size, ICRU bladder and rectal points, and the closest rectal point. The estimated doses obtained from dose-distance modeling displayed on average less than 2.5% difference when compared with TPS doses at ICRU bladder and rectal points for each VC size. Dose percent differences between estimated values and TPS values were on average 1.9% and 2.5% for ICRU blad- der and rectal point, respectively, regardless of VC sizes. Dose-distance modeling for closest rectal point presented on average 5.4% dose difference when compared with TPS values of all VC sizes. It was feasible to estimate rectal and bladder point doses by measuring distances on orthogonal radiographs without treatment plan- ning. Percent dose differences were 2.5% less for both ICRU bladder and rectal points, regardless of VC sizes. The use of closest rectal point is not recommended for estimating rectal dose. 

First 100 cases at a low volume prostate brachytherapy institution: learning curve and the importance of continuous quality improvement.

INTRODUCTION: We report the first 100 patients who underwent prostate brachytherapy as monotherapy with 125I at an institution with moderate volume radical prostatectomy but low volume brachytherapy (<2 cases per month). Learning curve and quality improvement was assessed by way of achieving prescription dose targets. MATERIALS AND METHODS: From May 2002 to August 2006, 100 patients underwent prostate 125I brachytherapy monotherapy via preplanned approach. Preoperative planned dose to 100% of prostate gland (D100) was 145 Gy and postoperative confirmed dose was assessed by computed tomography. The cohort was divided into quartiles and recurrence was assessed using Kaplan-Meier analysis. RESULTS: Patient quartiles were of similar age and Gleason grade, while PSA was slightly higher in the first group. Postoperative D90 increased after the first quartile (p = < 0.0001) reaching targeted values. Kaplan-Meier survival analysis revealed that 5 year recurrence-free survivals by Phoenix definition was 96%-100% in all groups while by ASTRO definition there was a decrease in recurrence for later cases. CONCLUSIONS: At our low volume institution during the first 100 brachytherapy cases, a learning curve for radiation dosimetry was evident, which improved after 25 patients. Preplanned dose-volume parameters were adjusted, enabling the achievement of post-implant goals emphasizing the importance of continuous quality improvement. Although recurrence data is limited by sample size and moderate follow up, there was a discrepancy between the Phoenix and ASTRO definition when evaluating recurrence.

Optimal number of beams for stereotactic body radiotherapy of lung and liver lesions.

PURPOSE: The aim of this study was to determine the optimal number of coplanar and noncoplanar external beams in the setting of stereotactic body radiotherapy (SBRT). METHODS AND MATERIALS: Spherical targets were delineated within 2 separate extracranial sites, the lung and liver, with diameters varying from 2 cm to 7 cm to cover the range of volumes used in SBRT. Treatment plans were created for all target volumes using 5 to 15 geometrically optimized coplanar and noncoplanar conformal beams. Dose gradient and normal tissue complication probability (NTCP) were evaluated for each set of beam configurations and for each target size. RESULTS: For all lung and liver target volumes, the dose gradient improved with an increase in beam number from 5 to 15 for both coplanar and noncoplanar beam configurations. NTCP decreased as the beam number increased from 5 to 9 beams for all target sizes for both coplanar and noncoplanar beams. There is no significant improvement in NTCP when more than 9 beams were used for treatment planning regardless of target size. CONCLUSION: Based on dosimetric criteria, the optimal number of external beams is 13 to 15 for SBRT using either coplanar or noncoplanar beam bouquets. Simple biologic models indicate that the optimal number of beams is 9 for SBRT of lung and liver lesions >2 cm, whereas smaller lesions may benefit from plans using up to 13 beams.

Distortions induced by radioactive seeds into interstitial brachytherapy dose distributions.

In a previous article, we presented development and verification of an integral transport equation-based deterministic algorithm for computing three-dimensional brachytherapy dose distributions. Recently, we have included fluorescence radiation physics and parallel computation to the standing algorithms so that we can compute dose distributions for a large set of seeds without resorting to the superposition methods. The introduction of parallel computing capability provided a means to compute the dose distribution for multiple seeds in a simultaneous manner. This provided a way to study strong heterogeneity and shadow effects induced by the presence of multiple seeds in an interstitial brachytherapy implant. This article presents the algorithm for computing fluorescence radiation, algorithm for parallel computing, and display results for an 81-seed implant that has a perfect and imperfect lattice. The dosimetry data for a single model 6711 seeds is presented for verification and heterogeneity factor computations using simultaneous and superposition techniques are presented.

High resolution (3 Tesla) MRI-guided conformal brachytherapy for cervical cancer: consequences of different high-risk CTV sizes.

PURPOSE: To evaluate conventional brachytherapy (BT) plans using dose-volume parameters and high resolution (3 Tesla) MRI datasets, and to quantify dosimetric benefits and limitations when MRI-guided, conformal BT (MRIG-CBT) plans are generated. MATERIAL AND METHODS: Fifty-five clinical high-dose-rate BT plans from 14 cervical cancer patients were retrospectively studied. All conventional plans were created using MRI with titanium tandem-and-ovoid applicator (T&O) for delivery. For each conventional plan, a MRIG-CBT plan was retrospectively generated using hybrid inverse optimization. Three categories of high risk (HR)-CTV were considered based on volume: non-bulky (< 20 cc), low-bulky (> 20 cc and < 40 cc) and bulky (≥ 40 cc). Dose-volume metrics of D90 of HR-CTV and D2cc and D0.1cc of rectum, bladder, and sigmoid colon were analyzed. RESULTS: Tumor coverage (HR-CTV D90) of the conventional plans was considerably affected by the HR-CTV size. Sixteen percent of the plans covered HR-CTV D90 with the prescription dose within 5%. At least one OAR had D2cc values over the GEC-ESTRO recommended limits in 52.7% of the conventional plans. MRIG-CBT plans showed improved target coverage for HR-CTV D90 of 98 and 97% of the prescribed dose for non-bulky and low-bulky tumors, respectively. No MRIG-CBT plans surpassed the D2cc limits of any OAR. Only small improvements (D90 of 80%) were found for large targets (> 40 cc) when using T&O applicator approach. CONCLUSIONS: MRIG-CBT plans displayed considerable improvement for tumor coverage and OAR sparing over conventional treatment. When the HR-CTV volume exceeded 40 cc, its improvements were diminished when using a conventional intracavitary applicator.

Evaluation of artifacts and distortions of titanium applicators on 3.0-Tesla MRI: feasibility of titanium applicators in MRI-guided brachytherapy for gynecological cancer.

PURPOSE: The aim of this study was to characterize the levels of artifacts and distortions of titanium applicators on 3.0-Tesla magnetic resonance imaging (MRI). METHODS AND MATERIALS: Fletcher-Suit-Delclos-style tandem and ovoids (T&O) and tandem and ring applicator (T&R) were examined. The quality assurance (QA) phantoms for each applicator were designed and filled with copper sulphate solution (1.5 g/l). The artifacts were quantified with the registration of corresponding computed tomography (CT) images. A favorable MR sequence was searched in terms of artifacts. Using the sequence, the artifacts were determined. The geometric distortions induced by the applicators were quantified through each registration of CT and MRI without applicators. The artifacts of T&O were also evaluated on in vivo MRI datasets of 5 patients. RESULTS: T1-weighted MRI with 1-mm slice thickness was found as a favorable MR sequence. Applying the sequence, the artifacts at the tandem tip of T&O and T&R were determined as 1.5 ± 0.5 mm in a superior direction in phantom studies. In the ovoids of T&O, we found artifacts less than 1.5 ± 0.5 mm. The artifacts of a T&O tandem in vivo were found as less than 2.6 ± 1.3 mm on T1-weighted MRI, whereas less than 6.9 ± 3.4 mm on T2-weighted MRI. No more than 1.2 ± 0.6 mm (3.0 ± 1.5 mm) of distortions, due to a titanium applicator, were measured on T1-weighted MRI (T2-). CONCLUSION: In 3.0-Tesla MRI, we found the artifact widths at the tip of tandem were less than 1.5 ± 0.5 mm for both T&O and T&R when using T1-weighted MRI in phantom studies. However, exclusive 3.0-Tesla MRI-guided brachytherapy planning with a titanium applicator should be cautiously implemented.