Yongbok Kim

Registration of MR prostate images with biomechanical modeling and nonlinear parameter estimation

Magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) have been shown to be very useful for identifying prostate cancers. For high sensitivity, the MRI/MRSI examination is often acquired with an endorectal probe that may cause a substantial deformation of the prostate and surrounding soft tissues. Such a probe is removed prior to radiation therapy treatment. To register diagnostic probe-in magnetic resonance (MR) images to therapeutic probe-out MR images for treatment planning, a new deformable image registration method is developed based on biomechanical modeling of soft tissues and estimation of uncertain tissue parameters using nonlinear optimization. Given two-dimensional (2-D) segmented probe-in and probe-out images, a finite element method (FEM) is used to estimate the deformation of the prostate and surrounding tissues due to displacements and forces resulting from the endorectal probe. Since FEM requires tissue stiffness properties and external force values as input, the method estimates uncertain parameters using nonlinear local optimization. The registration method is evaluated using images from five balloon and five rigid endorectal probe patient cases. It requires on average 37 s of computation time on a 1.6 GHz Pentium-M PC. Comparing the prostate outline in deformed probe-out images to corresponding probe-in images, the method obtains a mean Dice Similarity Coefficient (DSC) of 97.5% for the balloon probe cases and 98.1% for the rigid probe cases. The method improves significantly over previous methods (P < 0.05) with greater improvement for balloon probe cases with larger tissue deformations.

Measurement of craniocaudal catheter displacement between fractions in computed tomography-based high dose rate brachytherapy of prostate cancer.

The objective of the present work was to measure the craniocaudal displacement of catheters occurring between consecutive fractions of transrectal ultrasound (TRUS)‐guided high dose rate (HDR) prostate brachytherapy. Ten consecutive patients were treated with 2 fractions of 9.5‐Gy TRUS‐guided HDR brachytherapy, with dental putty being used for the fixation of catheters. For each patient, a computed tomography (CT) scan with 3‐mm slice thickness was acquired before each of the 2 fractions. Two different references were used to measure the catheter displacement between fractions: the ischial bone as a bony marker (BM) and the center of two gold markers (COGM) implanted in the prostate. Catheter displacement was calculated by multiplying the thickness of the CT slice by the difference in number of CT slices between the reference slice and the slice containing the tip of a catheter. The average magnitude of caudal catheter displacement was 2.7 mm (range: −6.0 mm to 13.5 mm) for the BM method and 5.4 mm (range: −3.75 mm to 18.0 mm) for the COGM method. The measurement data obtained from the BM and COGM methods verified that prostate movement and catheter displacement both occurred independently between fractions. The most anterior and medial two catheters (catheter positions 8 and 12) had the greatest tendency to be displaced in the caudal direction because they were located at the most distant position from the fulcrum, making them susceptible to rotation of the dental putty in the lateral plane because of the movement of the patients’ legs between fractions. In conclusion, the combination of the BM and COGM methods can demonstrate prostate and catheter movement relative to the BM between fractions. Our technique found a pattern of catheter displacement. Based on that finding, further improvement of our results may be possible by modification of our current technique. PACS number: 87.53.Jw

Predicting chest wall pain from lung stereotactic body radiotherapy for different fractionation schemes.

PURPOSE Recent studies with two fractionation schemes predicted that the volume of chest wall receiving >30 Gy (V30) correlated with chest wall pain after stereotactic body radiation therapy (SBRT) to the lung. This study developed a predictive model of chest wall pain incorporating radiobiologic effects, using clinical data from four distinct SBRT fractionation schemes. METHODS AND MATERIALS 102 SBRT patients were treated with four different fractionations: 60 Gy in three fractions, 50 Gy in five fractions, 48 Gy in four fractions, and 50 Gy in 10 fractions. To account for radiobiologic effects, a modified equivalent uniform dose (mEUD) model calculated the dose to the chest wall with volume weighting. For comparison, V30 and maximum point dose were also reported. Using univariable logistic regression, the association of radiation dose and clinical variables with chest wall pain was assessed by uncertainty coefficient (U) and C statistic (C) of receiver operator curve. The significant associations from the univariable model were verified with a multivariable model. RESULTS 106 lesions in 102 patients with a mean age of 72 were included, with a mean of 25.5 (range, 12-55) months of follow-up. Twenty patients reported chest wall pain at a mean time of 8.1 (95% confidence interval, 6.3-9.8) months after treatment. The mEUD models, V30, and maximum point dose were significant predictors of chest wall pain (p < 0.0005). mEUD improved prediction of chest wall pain compared with V30 (C = 0.79 vs. 0.77 and U = 0.16 vs. 0.11). The mEUD with moderate weighting (a = 5) better predicted chest wall pain than did mEUD without weighting (a = 1) (C = 0.79 vs. 0.77 and U = 0.16 vs. 0.14). Body mass index (BMI) was significantly associated with chest wall pain (p = 0.008). On multivariable analysis, mEUD and BMI remained significant predictors of chest wall pain (p = 0.0003 and 0.03, respectively). CONCLUSION mEUD with moderate weighting better predicted chest wall pain than did V30, indicating that a small chest wall volume receiving a high radiation dose is responsible for chest wall pain. Independently of dose to the chest wall, BMI also correlated with chest wall pain.

Expandable and rigid endorectal coils for prostate MRI: Impact on prostate distortion and rigid image registration

Endorectal coils (ERCs) are used for acquiring high spatial resolution magnetic resonance (MR) images of the human prostate. The goal of this study is to determine the impact of an expandable versus a rigid ERC on changes in the location and deformation of the prostate gland and subsequently on registering prostate images acquired with and without an ERC. Sagittal and axial T2 weighted MR images were acquired from 25 patients receiving a combined MR imaging/MR spectroscopic imaging staging exam for prostate cancer. Within the same exam, images were acquired using an external pelvic phased array coil both alone and in combination with either an expandable ERC (MedRad, Pittsburgh, PA) or a rigid ERC (USA Instruments, Aurora, OH). Rotations, translations and deformations caused by the ERC were measured and compared. The ability to register images acquired with and without the ERC using a manual rigid-body registration was assessed using a similarity index (SI). Both ERCs caused the prostate to tilt anteriorly with an average tilt of 18.5° (17.4±9.9 and 19.5±11.3°, mean±standarddeviation, for expandable and rigid ERC, respectively). However, the expandable coil caused a significantly larger distortion of the prostate as compared to the rigid coil; compressing the prostate in the anterior/posterior direction by 4.1±3.0mm vs 1.2±2.2mm (14.5% vs 4.8%) (p<0.0001), and widening the prostate in the right/left direction by 3.8±3.7mm vs 1.5±3.1mm (8.3% vs 3.4%) (p=0.004). Additionally, the ability to manually align prostate images acquired with and without ERC was significantly (p<0.0001) better for the rigid coil (SI=0.941±0.008 vs 0.899±0.033, for the rigid and expandable coils, respectively). In conclusion, the manual rigid-body alignment of prostate MR images acquired with and without the ERC can be improved through the use of a rigid ERC.

Long-term Cosmesis After Lumpectomy and Brachytherapy in the Management of Carcinoma of the Previously Irradiated Breast

Objective:To evaluate the cosmetic outcome of brachytherapy after lumpectomy in the management of carcinoma of the previously irradiated breast. Materials and Methods:Between January 1998 and April 2008, 26 patients with TIS or T1 breast carcinoma were offered interstitial or intracavitary brachytherapy after lumpectomy in a previously irradiated breast as an alternative to salvage mastectomy. Twenty-five of 26 patients had prior lumpectomy followed by standard postoperative external beam radiotherapy for early stage carcinoma of the breast [dose range 5000–6040 cGy]. One patient developed breast cancer after full mantle irradiation [4500 cGy to the mediastinum and axillae] for Hodgkin Lymphoma 27 years earlier. All tumors were excised with final margins of resection free of disease per National Surgical Adjuvant Breast and Bowel Project definition. After lumpectomy, tumor bed implantation was carried out utilizing a low dose rate interstitial technique in 22 patients, whereas 3 patients were treated with high dose rate brachytherapy using the MammoSite brachytherapy catheter and 1 patient was treated with high dose rate brachytherapy using the Contura catheter. The low dose rate treatment consisted of 4500 to 5000 cGy at 35 to 50 cGy per hour to the tumor bed plus a 1.0 cm margin, whereas the high dose rate treatment consisted of 3400 cGy in twice daily fractions of 340 cGy currently used in National Surgical Adjuvant Breast and Bowel Project B-39. Cosmesis was graded according to the B-39 cosmesis scale for de novo treatments: grade I as excellent, grade II as good, Grade III as fair, and Grade IV as poor. Results:Eighteen patients were scored as grade I, 6 as grade II, and 2 as grade III. No patient received a Grade IV score. All 4 balloon brachytherapy patients were scored as Grade I. Twenty-five of 26 patients remained free of local failure with a median follow-up of 38 months (range 6–75 months). The only patient to develop a second local recurrence was graded as grade I before salvage mastectomy. Two patients developed wound dehiscence after immediate postlumpectomy implantation. Two have succumbed to metastatic breast carcinoma at 17 and 24 months after salvage implant therapy. An additional patient has succumbed to chronic obstructive pulmonary disease. All patients, regardless of cosmesis grade were satisfied with their decision for repeat conservation therapy. Factors affecting cosmesis were distance from the implant to the skin, the type of device used, and the amount of residual breast tissue after repeat lumpectomy. Conclusions:The cosmetic effect of brachytherapy after lumpectomy in the management of recurrent carcinoma of the previously irradiated breast is acceptable in highly selected patients. Intracavitary technique may provide superior cosmetic results for patients retreated with brachytherapy for salvage.

A feasibility study of using conventional jaws to deliver IMRT plans in the treatment of prostate cancer.

The aim of this study is to investigate the feasibility of using conventional jaws to deliver inverse planned intensity-modulated radiotherapy (IMRT) plans for patients with prostate cancer. For ten patients, each had one three-dimensional conformal plan (3D plan) and seven inverse IMRT plans using direct aperture optimization. For IMRT plans using conventional jaws (JO plans), the number of apertures per beam angle was set from two to seven while three apertures per beam angle were set for the multi-leaf collimator (MLC) plans. To evaluate each planning method, we compared average dose volume histograms (DVH), the conformal index (COIN), total number of segments and total number of monitor units. Among the JO plans with the number of apertures per beam angle varying from two to seven, no difference was observed in the average DVHs, and the plan conformal index became saturated after four apertures per beam angle. Subsequently, JO plans with four apertures per beam angle (JO-4A) were compared with 3D and MLC plans. Based on the average DVHs, no difference was found among 3D, JO-4A and MLC plans with regard to the planning target volume and rectum, but the DVHs for the bladder and penile bulb were significantly better with inverse IMRT plans than those with 3D plans. When compared with the plan conformity, the average COIN values for 3D, JO-4A and MLC plans were 0.61 +/- 0.07, 0.73 +/- 0.05 and 0.83 +/- 0.05, respectively. In conclusion, inverse IMRT plans using conventional jaws are clinically feasible, achieving better plan quality than 3D-CRT plans.

Dose uncertainty due to computed tomography (CT) slice thickness in CT‐based high dose rate brachytherapy of the prostate cancer

In computed tomography (CT)-based high dose rate (HDR) brachytherapy, the uncertainty in the localization of the longitudinal catheter-tip positions due to the discrete CT slice thickness, results in a delivered dose uncertainty. Catheter coordinates were extracted from five patients treated for prostate cancer, and three simulation scenarios were followed to mimic the longitudinal imprecision of the catheter tips, hence the dwell positions. All catheters were displaced (1) forward, (2) backward, or (3) randomly distributed within the space defined by one CT slice thickness, for thicknesses ranging from 2 to 5 mm. Average and standard deviation values of the relative dose variations are reported for the various catheter displacement scenarios. Also, the dose points were grouped according to their relative position in the prostate, inner, peripheral and outer area of prostate and base, median and apex zones, in order to estimate the spatial sensitivity of the dose errors. For scenarios (1) and (2), the dose uncertainties due to the finite slice thickness increase linearly with the slice spacing, from 3% to 8% for the slice thickness values ranging from 2 to 5 mm, respectively. The more realistic scenario (3) yields average errors ranging from 0.7% to 1.7%. The apex and the base show larger dose errors and variability of dose errors than the median of the prostate. No statistical difference was observed among different transversal sections of the prostate. A CT slice thickness of 3 mm appears to be a good compromise showing an acceptable average dose uncertainty of 1%, without unduly increasing the number of slices.

INVESTIGATION OF INTERFRACTION VARIATIONS OF MAMMOSITE BALLOON APPLICATOR IN HIGH-DOSE-RATE BRACHYTHERAPY OF PARTIAL BREAST IRRADIATION

PURPOSE To measure the interfraction changes of the MammoSite applicator and evaluate their dosimetric effect on target coverage and sparing of organs at risk. METHODS AND MATERIALS A retrospective evaluation of the data from 19 patients who received 10 fractions (34 Gy) of high-dose-rate partial breast irradiation was performed. A computed tomography-based treatment plan was generated for Fraction 1, and a computed tomography scan was acquired just before the delivery of each fraction to ensure a consistent shape of the balloon. The eccentricity, asymmetry, and planning target volume (PTV) for plan evaluation purposes (PTV_EVAL), as well as trapped air gaps, were measured for all patients. Furthermore, 169 computed tomography-based treatment plans were retrospectively generated for Fractions 2-10. Interfraction dosimetric variations were evaluated using the %PTV_EVAL coverage, target dose homogeneity index, target dose conformal index, and maximum doses to the organs at risks. RESULTS The average variation of eccentricity and asymmetry from Fraction 1 values of 3.5% and 1.1 mm was -0.4% +/- 1.6% and -0.1 +/- 0.6 mm. The average trapped air gap volume was dramatically reduced from before treatment (3.7 cm(3)) to Fraction 1 (0.8 cm(3)). The PTV_EVAL volume change was insignificant. The average variation for the %PTV_EVAL, target dose homogeneity, and target dose conformal index from Fraction 1 values of 94.7%, 0.64, and 0.85 was 0.15% +/- 2.4%, -0.35 +/- 2.4%, and -0.34 +/- 4.9%, respectively. The average Fraction 1 maximum skin and ipsilateral lung dose of 3.2 Gy and 2.0 Gy varied by 0.08 +/- 0.47 and -0.16 +/- 0.29 Gy, respectively. CONCLUSION The interfraction variations were patient specific and fraction dependent. Although the average interfraction dose variations for the target and organs at risk were not clinically significant, the maximum variations could be clinically significant.

Dosimetric comparison of partial and whole breast external beam irradiation in the treatment of early stage breast cancer.

A dosimetric comparison was performed on external-beam three-dimensional conformal partial breast irradiation (PBI) and whole breast irradiation (WBI) plans for patients enrolled in the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-39/Radiation Therapy Oncology Group (RTOG) 0413 protocol at our institution. Twenty-four consecutive patients were treated with either PBI (12 patients) or WBI (12 patients). In the PBI arm, the lumpectomy cavity was treated to a total dose of 38.5 Gy at 3.85 Gy per fraction twice daily using a four-field noncoplanar beam setup. A minimum 6 h interval was required between fractions. In the WBI arm, the whole breast including the entirety of the lumpectomy cavity was treated to a total dose of 50.4 Gy at 1.8 Gy per fraction daily using opposed tangential beams. The lumpectomy cavity volume, planning target volume for evaluation (PTV_EVAL), and critical structure volumes were contoured for both the PBI and WBI patients. Dosimetric parameters, dose volume histograms (DVHs), and generalized equivalent uniform dose (gEUD) for target and critical structures were compared. Dosimetric results show the PBI plans, compared to the WBI plans, have smaller hot spots in the PTV_EVAL (maximum dose: 104.2% versus 110.9%) and reduced dose to the ipsilateral breast (V50: 48.6% versus 92.1% and V100: 10.2% versus 50.5%), contralateral breast (V3: 0.16% versus 2.04%), ipsilateral lung (V30: 5.8% versus 12.7%), and thyroid (maximum dose: 0.5% versus 2.0%) with p values < or = 0.01. However, similar dose coverage of the PTV_EVAL (98% for PBI and 99% for WBI, on average) was observed and the dose difference for other critical structures was clinically insignificant in both arms. The gEUD data analysis showed the reduction of dose to the ipsilateral breast and lung, contralateral breast and thyroid. In addition, preliminary dermatologic adverse event assessment data suggested reduced skin toxicity for patients treated with the PBI technique.

The use of the MammoSite balloon applicator in re-irradiation of the breast

PURPOSE To examine the feasibility of using the MammoSite brachytherapy applicator in the retreatment of the previously irradiated breast. METHODS AND MATERIALS Between March 2004 and March 2007, three patients previously treated with external beam radiotherapy were retreated using the MammoSite brachytherapy device. Two patients were treated for an ipsilateral breast tumor recurrence after breast conservation surgery and postoperative irradiation, whereas the third patient developed an in-field breast cancer likely associated with Hodgkins disease mantle irradiation 27 years before. The recurrent histology of two was ductal carcinoma in situ ([DCIS] one originally presenting as infiltrating ductal carcinoma [IDC] and the other as DCIS), whereas the Hodgkins disease patient presented with IDC. All patients received a twice-daily tumor dose of 3400 cGy at 340 cGy/fraction. The mean maximum skin dose was 53.4% (range, 49.5-60.3%) of the prescribed dose. RESULTS With a mean followup of 32 months, no patient developed a local recurrence. Cosmesis in all three cases as graded by the National Surgical Adjuvant Breast and Bowel Project, cosmesis criteria was excellent (Grade I) in all cases. Dosimetric calculations demonstrated that the device allows for appropriate local irradiation while sparing the previously irradiated skin of the involved breast as defined by the protocol standard. CONCLUSIONS Use of the MammoSite device in the treatment of the previously irradiated breast is feasible and may provide adequate local control as well as acceptable cosmesis in carefully selected patients.