Biomedical subjects

Prem Pareek

Publications and source records attributed to Prem Pareek.

5 recordsLinked to original sources

PubMed2005

Image-based intracavitary brachytherapy in the treatment of inoperable uterine cancer: individual dose specification at specific anatomical sites.

PURPOSE: With advances in imaging studies, dose specification for uterine cancer can be defined at specific anatomical sites such as the myometrium or the serosal surface rather than at arbitrary points or milligram-hours. This report presents our experience with image-based brachytherapy for inoperable uterine cancer. METHODS AND MATERIALS: Eight patients with organ-confined uterine cancer (2 Stage I GI, 3 Stage I G2, 3 Stage I G3) underwent definitive radiation therapy because of poor medical condition. All the patients underwent a CT or MRI scan of the pelvis before intracavitary application. Based on the size of the uterine cavity, a single-channel intrauterine applicator was selected for a small uterus, and a multiple-channel intrauterine applicator was used for a large uterus. A CT (n=5) or MRI (n=3) scan of the pelvis was performed with the applicator in place in addition to orthogonal pelvic films. Individualized dose specification was 75Gy to the midmyometrium and limited to 50Gy to the serosal surface of the uterus based on imaging information. RESULTS: Four patients with Stage I G1-2 disease had intracavitary brachytherapy alone. Four patients with Stage I G2-3 disease were treated with a combination of external pelvic radiation and intracavitary brachytherapy. Six patients had low-dose-rate brachytherapy, and 2 patients had high-dose-rate brachytherapy. Five patients had single-channel intrauterine brachytherapy, and 3 patients had multiple-channel brachytherapy. Based on the measurements of the uterine wall thickness by the imaging studies, the dose specification was prescribed to 1.5 cm lateral to the central axis of the uterus in 4 patients, 2.0 cm in 3 patients, and 2.5 cm in 1 patient. The medium followup time after radiation treatment was 38 months. Six patients are alive without evidence of disease, and 2 patients died of other causes. All patients had local control without major side effects. CONCLUSIONS: Image-based brachytherapy based on individualized dose specification at specific anatomical sites can be done easily and provides excellent local control for inoperable uterine cancer.

Aged↗

PubMed2004

Benchmark of PENELOPE code for low-energy photon transport: dose comparisons with MCNP4 and EGS4.

The expanding clinical use of low-energy photon emitting 125I and 103Pd seeds in recent years has led to renewed interest in their dosimetric properties. Numerous papers pointed out that higher accuracy could be obtained in Monte Carlo simulations by utilizing newer libraries for the low-energy photon cross-sections, such as XCOM and EPDL97. The recently developed PENELOPE 2001 Monte Carlo code is user friendly and incorporates photon cross-section data from the EPDL97. The code has been verified for clinical dosimetry of high-energy electron and photon beams, but has not yet been tested at low energies. In the present work, we have benchmarked the PENELOPE code for 10-150 keV photons. We computed radial dose distributions from 0 to 10 cm in water at photon energies of 10-150 keV using both PENELOPE and MCNP4C with either DLC-146 or DLC-200 cross-section libraries, assuming a point source located at the centre of a 30 cm diameter and 20 cm length cylinder. Throughout the energy range of simulated photons (except for 10 keV), PENELOPE agreed within statistical uncertainties (at worst +/- 5%) with MCNP/DLC-146 in the entire region of 1-10 cm and with published EGS4 data up to 5 cm. The dose at 1 cm (or dose rate constant) of PENELOPE agreed with MCNP/DLC-146 and EGS4 data within approximately +/- 2% in the range of 20-150 keV, while MCNP/DLC-200 produced values up to 9% lower in the range of 20-100 keV than PENELOPE or the other codes. However, the differences among the four datasets became negligible above 100 keV.

Benchmarking↗

PubMed2003

Radiography-based treatment planning compared with computed tomography (CT)-based treatment planning for intracavitary brachytherapy in cancer of the cervix: analysis of dose-volume histograms.

PURPOSE: To analyze the dose-volume histograms (DVHs) of the tumor volume and organs at risk by CT-based treatment planning compared with conventional radiography-based treatment planning for intracavitary brachytherapy in cancer of the cervix. METHODS AND MATERIALS: Fifteen consecutive patients with cancer of the cervix (1 IB1, 3 IB2, 7 IIB, 4 IIIB) were treated with plastic CT-compatible HDR intracavitary applicators and underwent postimplant pelvic CT scans with applicators in place. CT-images were transferred to the PLATO treatment planning system. The gross tumor volume (GTV) and organs at risk were digitized. Dwell positions in the uterine tandem and colpostats were identified and registered for each patient. All patients were treated with 6 Gy per fraction to Point A using radiography-based planning. For the CT-based planning, DVHs were performed for the GTV, bladder, rectum, sigmoid colon, and small bowel in the pelvis. The dose delivered to 3% volume of the organs at risk (D3%) was compared with the respective ICRU reference doses. RESULTS: For stage IB(I), IB2, IIB, and IIIB disease the mean GTV was 20.5 cc, 56.6 cc (54.2-57.2), 63.7 cc (55.4-118.9), and 77.6 cc (49.4-102.9), respectively. The 6 Gy pear-shaped volume (PSV) encompassed an average GTV of 98.5%, 89.5%, 79.5%, and 59.5% for stages IBI, IB2, IIB, and IIIB, respectively. The mean dose for the ICRU bladder point and D3% was 3.72 Gy (1.51-5.53) and 4.74 Gy (1.70-10.10), respectively. The mean dose for the ICRU rectal point and D3% was 3.97 Gy (2.09-5.37) and 3.52 Gy (2.05-4.08), respectively. The D3% for the sigmoid colon was highest (3.88 Gy), followed by the rectum (3.52 Gy), and the small bowel (3.36 Gy). CONCLUSION: Radiography-based conventional treatment planning overestimates tumor dose, especially those with more advanced tumors. To correlate DVHs for tumor control, improved tumor imaging is necessary.

Brachytherapy↗

PubMed2003

Custom step wedge blocking using dynamic multileaf collimation for parametrial pelvic boost irradiation following brachytherapy for carcinoma of the cervix.

Carcinoma of the cervix is typically treated with a combination of intracavitary brachytherapy and external beam radiation. The external beam dose is delivered with whole pelvis fields followed by split fields that protect midline organs at risk (bladder and rectum) while treating the parametria. Three approaches have been developed to shield midline structures: a simple rectangular block, a block customized to a single brachytherapy isodose line, and a step wedge filter constructed to conform to multiple brachytherapy isodose lines. A customized step wedge filter has the potential to produce a more homogeneous dose distribution but has not achieved widespread use due to labor intensive construction. We have developed a simple, novel method to produce a custom midline step wedge using dynamic multileaf collimation (dMLC). A comparison of film measurements in a phantom with the dose calculated by a commercial treatment planning system demonstrated agreement within 3% or 3 mm. The technique requires delivery times comparable to conventional techniques.

Algorithms↗

PubMed2002

Postoperative intravaginal brachytherapy for endometrial cancer; dosimetric analysis of vaginal colpostats and cylinder applicators.

PURPOSE: To investigate the dosimetric differences between colpostats and cylinder applicators for intravaginal brachytherapy. METHODS AND MATERIALS: Dose distributions near vaginal colpostats and dome cylinders were computed with a commercial high-dose-rate treatment-planning system and verified by spot measurements by using LiF thermoluminescent dosimeters. Taking source anisotropy into account, dwell times were optimized by the computer by using the polynomial optimization on dose points method to give uniform doses along the lateral surfaces of the applicators. In addition, the effects of vaginal packing and the separation distance between colpostats were studied by computing the dose to the vaginal mucosa, assuming 0.5 and 1.0 cm of vaginal packing and colpostat separation, respectively. RESULTS: The computed and measured doses agreed within +/- 7%. Surface doses were similar for both types of applicators when the effect of shielding in the colpostats was neglected. However, percent depth doses in the anterior/posterior and lateral directions were higher for the cylinder, whereas the dose fall-off along the longitudinal patient axis was less pronounced for the colpostats. When vaginal packing at the anterior and posterior surface of the colpostats was increased from 0 to 5, 10, and 15 mm, the corresponding vaginal dose decreased from 97% of the prescription dose to 60%, 39%, and 26%, respectively. Separating the colpostats from 0 to 5 and 10 mm reduced the surface dose near the bladder/rectum to 80% and 67%, respectively, whereas the respective apex dose decreased from 105% of prescription to 91% and 77%. CONCLUSIONS: Colpostats and cylinder applicators for intracavitary brachytherapy have their advantages and disadvantages in depth dose distribution and clinical use. If treatment is confined to the vaginal apex, either applicator can be used. However, the colpostat separation should be kept to a minimum, and vaginal packing should be applied with great care to avoid generating cold spots along the upper vaginal surface and vaginal cuff.

Brachytherapy↗