Impact of 18F-FDG PET/CT on target volume delineation in recurrent or residual gynaecologic carcinoma.

BACKGROUND: To evaluate the impact of 18F-FDG PET/CT on target volume delineation in gynaecological cancer. METHODS: F-FDG PET/CT based RT treatment planning was performed in 10 patients with locally recurrent (n = 5) or post-surgical residual gynaecological cancer (n = 5). The gross tumor volume (GTV) was defined by 4 experienced radiation oncologists first using contrast enhanced CT (GTVCT) and secondly using the fused 18F-FDG PET/CT datasets (GTVPET/CT). In addition, the GTV was delineated using the signal-to-background (SBR) ratio-based adaptive thresholding technique (GTVSBR). Overlap analysis were conducted to assess geographic mismatches between the GTVs delineated using the different techniques. Inter- and intra-observer variability were also assessed. RESULTS: The mean GTVCT (43.65 cm3) was larger than the mean GTVPET/CT (33.06 cm3), p = 0.02. In 6 patients, GTVPET/CT added substantial tumor extension outside the GTVCT even though 90.4% of the GTVPET/CT was included in the GTVCT and 30.2% of the GTVCT was found outside the GTVPET/CT. The inter- and intra-observer variability was not significantly reduced with the inclusion of 18F-FDG PET imaging (p = 0.23 and p = 0.18, respectively). The GTVSBR was smaller than GTVCT p ≤ 0.005 and GTVPET/CT p ≤ 0.005. CONCLUSIONS: The use of 18F-FDG PET/CT images for target volume delineation of recurrent or post-surgical residual gynaecological cancer alters the GTV in the majority of patients compared to standard CT-definition. The use of SBR-based auto-delineation showed significantly smaller GTVs. The use of PET/CT based target volume delineation may improve the accuracy of RT treatment planning in gynaecologic cancer.

High-dose-rate brachytherapy boost to the dominant intra-prostatic tumor region: hemi-irradiation of prostate cancer.

BACKGROUND: To assess the feasibility, toxicity, and outcome of prostate hemi-irradiation with a high-dose-rate brachytherapy (HDR-BT) boost for patients presumed to harbor dominant intra-prostatic tumors in a single lobe. METHODS: After 3D conformal external radiotherapy (3DCRT) to 64-64.4 Gy, 77 patients with non-metastatic locally aggressive prostate cancer have been treated from 2000 to 2004, with HDR-BT using temporary open MRI-guided (192) Ir implants, to escalate the dose in the boost region. Twenty patients (26%) had one lobe involvement (i.e., one sided endorectal MRI, rectal examination, and biopsies) and were boosted to one side of the gland only. A dose of 12, 14, and 16 Gy in two fractions was delivered to 5, 6, and 9 patients, respectively. RESULTS: After a median follow-up 69 months, no differences in late rectal toxicity were observed between the unilaterally and bilaterally irradiated cohorts. Although, grade 2 late urinary toxicity was worse in the hemi-irradiated group (P = 0.03), severe grade ≥3 late urinary toxicity at 5 years was not different: 10% versus 8.8% in the unilaterally and bilaterally irradiated cohorts, respectively. Grade 4 late urinary toxicity, however, was exclusively observed in patients boosted to both lobes (5/57, 8.8%). Five-year biochemical relapse-free survival was 79.7% versus 70.5% for the unilateral and bilateral boost groups, respectively (P = 0.99). CONCLUSION: Prostate hemi-irradiation with a HDR-BT boost to the dominant tumor region may be considered when rectal examination, MRI, and biopsies suggest one lobe involvement. Nevertheless, strict dosimetric optimization is needed in order to further reduce the risk of late severe toxicity.

Hypofractionated boost with high-dose-rate brachytherapy and open magnetic resonance imaging-guided implants for locally aggressive prostate cancer: a sequential dose-escalation pilot study.

PURPOSE: To evaluate the feasibility, tolerance, and preliminary outcome of an open MRI-guided prostate partial-volume high-dose-rate brachytherapy (HDR-BT) schedule in a group of selected patients with nonmetastatic, locally aggressive prostatic tumors. METHODS AND MATERIALS: After conventional fractionated three-dimensional conformal external radiotherapy to 64-64.4 Gy, 77 patients with nonmetastatic, locally aggressive (e.g., perineural invasion and/or Gleason score 8-10) prostate cancer were treated from June 2000 to August 2004, with HDR-BT using temporary open MRI-guided (192)Ir implants, to escalate the dose in the boost region. Nineteen, 21, and 37 patients were sequentially treated with 2 fractions of 6 Gy, 7 Gy, and 8 Gy each, respectively. Neoadjuvant androgen deprivation was given to 62 patients for 6-24 months. Acute and late toxicity were scored according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer scoring system. RESULTS: All 77 patients completed treatment as planned. Only 2 patients presented with Grade > or =3 acute urinary toxicity. The 3-year probability of Grade > or =2 late urinary and low gastrointestinal toxicity-free survival was 91.4% +/- 3.4% and 94.4% +/- 2.7%, respectively. Rates of 3-year biochemical disease-free survival (bDFS) and disease-specific survival were 87.1% +/- 4.1% and 100%, respectively. CONCLUSIONS: Boosting a partial volume of the prostate with hypofractionated HDR-BT for aggressive prostate cancer was feasible and showed limited long-term toxicity, which compared favorably with other dose-escalation methods in the literature. Preliminary bDFS was encouraging if one considers the negatively selected population of high-risk patients in this study.

Absolute depth-dose-rate measurements for an 192Ir HDR brachytherapy source in water using MOSFET detectors.

Reported MOSFET measurements concern mostly external radiotherapy and in vivo dosimetry. In this paper, we apply the technique for absolute dosimetry in the context of HDR brachytherapy using an 192Ir source. Measured radial dose rate distributions in water for different planes perpendicular to the source axis are presented and special attention is paid to the calibration of the R and K type detectors, and to the determination of appropriate correction factors for the sensitivity variation with the increase of the threshold voltage and the energy dependence. The experimental results are compared with Monte Carlo simulated dose rate distributions. The experimental results show a good agreement with the Monte Carlo simulations: the discrepancy between experimental and Monte Carlo results being within 5% for 82% of the points and within 10% for 95% of the points. Moreover, all points except two are found to lie within the experimental uncertainties, confirming thereby the quality of the results obtained.

(144)Ce as a potential candidate for interstitial and intravascular brachytherapy.

PURPOSE: To investigate the suitability of (144)Ce for both interstitial and intravascular brachytherapy applications. METHODS AND MATERIALS: Monte Carlo calculations of radial dose rate distributions in water were performed for (144)Ce in a spring-shaped source and compared with two commonly used interstitial and intravascular sources, (192)Ir and (32)P. The numeric simulations were checked experimentally with a calibrated ionization chamber in a water phantom. Other source characteristics, such as half-life and specific activity, were also compared. RESULTS: For interstitial brachytherapy, (144)Ce presents dosimetric advantages over (192)Ir in terms of higher dose rate at shorter distances and lower irradiation of organs outside the tumor. The source size and shape reduce the anisotropy and the number of dwell positions necessary. The longer half-life of (144)Ce might also be advantageous over (192)Ir. For intravascular brachytherapy, (144)Ce permits the treatment of larger arteries as compared with (32)P, compensates centering errors more effectively, and has a more suitable half-life. The experimental validation showed good agreement (within 10%) with the Monte Carlo simulated dose rate distributions. CONCLUSIONS: There are certain potential advantages of using (144)Ce as a source for both interstitial and intravascular brachytherapy. The basis for this finding is provided by the Monte Carlo radial dose rate comparisons with (192)Ir and (32)P, as well as by such characteristics as half-life and specific activity.

Fractionated stereotactic radiotherapy boost for gynecologic tumors: an alternative to brachytherapy?

PURPOSE: A brachytherapy (BT) boost to the vaginal vault is considered standard treatment for many endometrial or cervical cancers. We aimed to challenge this treatment standard by using stereotactic radiotherapy (SRT) with a linac-based micromultileaf collimator technique. METHODS AND MATERIALS: Since January 2002, 16 patients with either endometrial (9) or cervical (7) cancer have been treated with a final boost to the areas at higher risk for relapse. In 14 patients, the target volume included the vaginal vault, the upper vagina, the parametria, or (if not operated) the uterus (clinical target volume [CTV]). In 2 patients with local relapse, the CTV was the tumor in the vaginal stump. Margins of 6-10 mm were added to the CTV to define the planning target volume (PTV). Hypofractionated dynamic-arc or intensity-modulated radiotherapy techniques were used. Postoperative treatment was delivered in 12 patients (2 x 7 Gy to the PTV with a 4-7-day interval between fractions). In the 4 nonoperated patients, a dose of 4 Gy/fraction in 5 fractions with 2 to 3 days' interval was delivered. Patients were immobilized in a customized vacuum body cast and optimally repositioned with an infrared-guided system developed for extracranial SRT. To further optimize daily repositioning and target immobilization, an inflated rectal balloon was used during each treatment fraction. In 10 patients, CT resimulation was performed before the last boost fraction to assess for repositioning reproducibility via CT-to-CT registration and to estimate PTV safety margins around the CTV. Finally, a comparative treatment planning study between BT and SRT was performed in 2 patients with an operated endometrial Stage I cancer. RESULTS: No patient developed severe acute urinary or low-intestinal toxicity. No patient developed urinary late effects (>6 months). One patient with a vaginal relapse previously irradiated to the pelvic region presented with Grade 3 rectal bleeding 18 months after retreatment. A second patient known to suffer from irritable bowel syndrome presented with Grade 1 abdominal pain after treatment. The estimated PTV margins around the CTV were 9-10 mm with infrared marker registration. External SRT succeeded in improving dose homogeneity to the PTV and in reducing the maximum dose to the rectum, when compared to BT. CONCLUSION: These results suggest that the use of external SRT to deliver a final boost to the areas at higher risk for relapse in endometrial or cervical cancer is feasible, well tolerated, and may well be considered an acceptable alternative to BT.

Dosimetric characterization of radioactive sources employed in prostate cancer therapy.

PURPOSE: Three types of radiation sources are employed currently in the radiation treatment of prostate cancer, namely, external, implant, and high-dose-rate (HDR) sources using an afterloader method. The present article provides a detailed dosimetric characterization of several commercially available implant sources and an HDR source employing the same stochastic code and dataset. METHODS AND MATERIALS: The radioactive implants considered are (125)I seeds: models 6701, 6702 and 6711, (103)Pd seed: model 200, and a high-dose-rate (192)Ir source: microSelectron-HDR model V7.0x. Detailed modeling of the sources and their associated X-rays and gamma rays has been carried out using the stochastic code MCNP4C. A sensitivity study has been conducted to quantify effects of varying the composition and density of the tissue equivalent material, and a dosimetric comparison is made for different media (tissue equivalent, solid-water, water, and air). Furthermore, a set of measurements using thermoluminescent dosimeters has been done to provide experimental validation of some of the calculational results obtained. RESULTS: Effectively, high-precision dosimetric values (Monte-Carlo statistical 1-sigma error <1%) are provided in tabulated form over a wide range to enable therapy planning as well as to check numerical values calculated by other methods. A subset of calculated dosimetric values has been experimentally validated by using thermoluminescent dosimeters. CONCLUSIONS: A detailed comparison of results obtained for the radial dose distribution function, anisotropy factor, and dose rate constant as defined in the TG-43 protocol has indicated reasonable agreement with the values reported in the literature.