Eric D. Slessinger

Portal dose images I: Quantitative treatment plan verification

The comparison of a predicted portal dose image, calculated during treatment planning, with the measured image obtained during treatment is proposed as an approach to verify the correct implementation of a patient treatment plan. The comparison inherently verifies both the geometric alignment and the dose delivered. Feasibility studies were conducted with 60Co irradiation of a modular plastic phantom, an anthropomorphic phantom and a patient with lung cancer. Calculations were made with the 3-dimensional scatter ray-trace Delta Volume method. Calculated distributions and/or selected points of transmitted dose correction factors were compared with measurements made with TLD, scanning ionization chamber and film. For the two phantom studies, excellent agreement, usually to within 3%, was achieved when positioning of the phantoms were accurate. The patient study showed that selected point comparisons were inadequate in identifying the cause of errors when disagreement occurred. Simple subtraction of the calculated and measured images showed a 4 mm translational misalignment. The results are encouraging and demonstrate that portal dose images can be used to detect large geometric and dosimetric discrepancies between treatment plan calculations and measurements. The results also show that perfect verification is virtually impossible in the clinical situation. More work is required to use the verification information for improving the estimation of dose to the patient.

Interinstitutional experience in verification of external photon dose calculations

Under the auspices of NCI contracts, four institutions have collaborated to assess the accuracy of the pixel-based dose calculation methods they employ for external photon treatment planning. The approach relied on comparing calculations using each groups algorithm with measurements in phantoms of increasing complexity. The first set of measurements consisted of ionization chamber measurements in water phantoms in normally incident square fields, an elongated field, a wedged field, a blocked field, and an obliquely incident beam. The second group of measurements was carried out using thermoluminescent dosimeters in phantoms designed to investigate the effects of surface curvature, high density heterogeneities, and low density heterogeneities. The final study tested the entire treatment planning system, including CT data conversion, in an anthropomorphic phantom. Overall, good agreement between calculation and measurements was found for all algorithms. Regions in which discrepancies were observed are pointed out, areas for algorithm improvement are identified and the clinical import of algorithm accuracy is discussed.

Design of an afterloading vaginal applicator (miralva)

A vaginal applicator was designed and constructed that incorporates two ovoid sources and a central tandem which can be utilized to treat the entire vagina (alone or in combination with the uterine cervix). The average surface dose rate around the 2 cm ovoids is 120 cGy/hr and in the 2.5 cm diameter vaginal cylinder about 100 cGy/hr with usual loading of 20 mg Ra eq 137-Cs sources in the ovoids and 10-15 mg Ra eq 137-Cs sources in the cylinder. The applicator has vaginal apex caps and additional cylinder sleeves that allow for increased dimensions. The tandem in the uterus can be utilized when clinically indicated using standard loadings, depending on the depth of the uterus (20-10-10 or 20-10 mg Ra eq). When the tandem and vaginal cylinder are utilized the strength of the sources in the ovoids should be 15 mg Ra eq. The vaginal cylinder or uterine tandem never carry an active source at the level of the ovoids. Thermoluminescent dosimetry measurements throughout the surface of the applicator showed close agreement with the computer dose calculations (within +/- 2%). The acronym MIRALVA describes the device (Mallinckrodt Institute of Radiology Afterloading Vaginal Applicator).

Independent collimator dosimetry for a dual photon energy linear accelerator

PURPOSE The independent collimator feature in medical linear accelerators can define radiation fields that are asymmetric with respect to the flattening filter and oblique to the incident surface. Prior to clinical implementation, it is necessary to evaluate the dosimetry of this non-standard treatment delivery technique. An investigation of the independent collimator dosimetry for 6 MV and 18 MV x-ray beams has been undertaken. METHODS AND MATERIALS Dose to tissue in free space, percent depth dose and dose distribution were measured and compared to that for symmetric field collimation. RESULTS The dosimetry results were consistent for both photon modes. Dose in free space with asymmetric collimation can be calculated from the corresponding symmetric field dose in free space to within 1.2 +/- 0.7% by applying an appropriate off-axis factor. Asymmetric field percent depth dose differs from symmetric field percent depth dose on average by 1.1 +/- 0.7% for 6 MV and by 0.7 +/- 0.5% for 18 MV for field sizes ranging from 5 x 5 to 20 x 20, centered 3 cm and 10 cm off-axis. The measured isodose curves demonstrate divergence effects and reduced doses (less than 3%) adjacent to the field edge closest to the flattening filter center. This dose asymmetry result is identical to that from secondary collimation. CONCLUSION The methodology for clinical implementation of the independent collimator feature is straightforward. However, accurate representation of the isodose distributions by commercial radiotherapy treatment planning systems requires special dose calculation algorithms.

Mini-colpostats in the treatment of carcinoma of the uterine cervix.

Between 1976 and 1982, 293 patients were treated for carcinoma of the uterine cervix at Washington University by definitive radiotherapy consisting of external beam therapy and two standard Fletcher-Suit applications (tandem plus vaginal colpostats). In ninety-nine patients (34%) mini-colpostats (MC) were used for one or both of their intracavitary insertions while 194 (66%) patients were treated twice with regular Fletcher-Suit colpostats (RC). The frequency of MC use was related to the age and parity of the patients. The distribution by stage of MC and RC groups was not significantly different. Pelvic failure in the MC group was similar to that of the RC group (21% vs 24%). Five-year disease-free survival was also similar between the two groups: 86% vs 80% Stage IB, 57% vs 61% Stage IIA, 47% vs 52% Stage IIB, and 27% vs 45% Stage III for MC and RC groups, respectively. The rate of major complications (grade 3) was 15% in the MC group and 8% in the RC group (p = 0.08). Careful phantom dosimetric studies in both types of colpostats and correlations of dose distributions at various points in the pelvis with frequency of rectal and bladder complications were carried out. The bladder and rectum received a 5-10% higher mean radiation dose (Gy) in the MC group than in the RC group despite lower overall exposure (milligram-hours). Thermoluminescent dosimetry in a polystyrene phantom demonstrates that approximately 10% higher doses are delivered to the bladder, rectum, and point A with an MC system as compared to an RC system, for constant exposure in mgh. Phantom measurements of a newer MC with bladder and rectal shielding demonstrate no influence on the bladder and rectal point dose at a source separation of 3 cm; midline points of the bladder and rectum are not within the full shadow of the shields even if the colpostats are flush with the tandem. Implications for therapy are discussed.

Reduction in radiation exposure to nursing personnel with the use of remote afterloading brachytherapy devices

The radiation exposure to nursing personnel from patients with brachytherapy implants on a large brachytherapy service were reviewed. Exposure to nurses, as determined by TLD monitors, indicates a 7-fold reduction in exposure after the implementation of the use of remote afterloading devices. Quarterly TLD monitor data for six quarters prior to the use of remote afterloading devices demonstrate an average projected annual dose equivalent to the nurses of 152 and 154 mrem (1.5 mSv). After the implementation of the remote afterloading devices, the quarterly TLD monitor data indicate an average dose equivalent per nurse of 23 and 19 mrem (0.2 mSv). This is an 87% reduction in exposure to nurses with the use of these devices (p less than 0.01).

Dosimetry and dose specification for a new gynecological brachytherapy applicator

A new afterloadable gynecological intracavitary applicator has been designed and is now in use for the treatment of a wide range of vaginal, cervical, and endometrial cancers in a single application, with a dose distribution that more closely matches the prescribed treatment than previous methods. The plexiglass applicator consists of a bulbous section that is inserted up to the vaginal apex and loaded with right and left ovoid sources. The distal portion is cylindrical with a central channel for tandem sources. A straight tandem is used when the target volume extends to the vaginal apex, but an intrauterine tandem can also be used. Extensive dosimetric and radiographic evaluations were performed to guide design refinements and to validate the surface dose predictions of the brachytherapy treatment planning system. The final applicator design delivers 110-120 cGy/hr to the vaginal apex surface and 95-100 cGy/hr to the distal vaginal surfaces when loaded with a 144.6 U (20 mgRaEq) cesium tube in each ovoid channel and 72.3, 72.3, and 144.6 U (10, 10, and 20 mgRaEq) cesium tubes in the vaginal cylinder channel. A system for treatment dose specification has been established that includes dose tables for manually calculating surface doses to reference points. A dose distribution comparison with a sequential colpostat-vaginal cylinder treatment demonstrates that dose delivery is more precise and uniform with this new applicator.

Improvements in brachytherapy quality assurance

Quality assurance (QA) for a busy brachytherapy service is a demanding task which requires the involvement of the entire brachytherapy team. This communication describes the QA program for brachytherapy procedures and source identification and inventory which is currently practiced at the Radiation Oncology Center, Mallinckrodt Institute of Radiology.

Clinical evaluation of an interstitial remote afterloading device for multichannel intracavitary irradiation

PURPOSE The purpose of this report is to describe the clinical implementation and evaluation of an interstitial remote afterloading device for multichannel intracavitary brachytherapy. METHODS AND MATERIALS Two 15-channel low dose rate devices were adapted for use with Fletcher-Suit tandem and ovoids and Simon-Heyman capsules. The technical records for 103 intracavitary brachytherapy procedures performed from February 1989 through February 1991 were reviewed. RESULTS Isodose distributions from fixed source trains for Microselectron low dose rate gynecologic applicators closely approximate standard manual afterloading sources and applicators. Device malfunctions occurred in 51% (53 out of 103) of the procedures. Malfunctions by applicator type were 70% (51 out of 73) for tandem, ovoids, and capsules, 12% (2 out of 17) for tandem and ovoids, and none (0 out of 13) for ovoids only. The most common malfunction occurred during source transfer. Total implant time was prolonged 0 to 4% by malfunction and 10% by patient care interruptions, depending on applicator type. CONCLUSION The adaptation of the Microselectron device for multichannel gynecologic intracavitary brachytherapy results in similar dose distributions as standard manual after loading sources and a decreased radiation exposure to nursing personnel. The system has a high rate of malfunctions but a low overall prolongation of implant time due to malfunction.

The relationship of clinical factors and radiation exposure rates from iodine-131 treated thyroid carcinoma patients☆

Iodine-131 (I-131) was administered orally 58 times over a 31-month period to patients with thyroid carcinoma. Federal regulations (10 CFR 35) require that a patient remain hospitalized until total body activity is less than 30 mCi. This mandated a hospitalization of more than 48 hours in 10 of 58 patients (17%), and greater than 72 hours in 1 of 58 patients (2%). During each administration of I-131 exposure rates were periodically measured throughout the hospitalization at the surface of the neck, 1 meter from the neck surface, the surface of the abdomen, 1 meter from the anterior abdominal surface, and 1 meter from the lateral abdominal surface. Semi-log regression curves were generated for exposure rate versus time for various parameters, including tumor histology, extent of surgery, gender, age, TSH, site of measurement, and administered activity. The endpoint evaluated was the regressed time to reduce the exposure rate to 10% of the initial value. None of the clinical characteristics tested revealed any differences in decay times. The measurements taken 1 meter anterior to the stomach and neck surface were found to correlate best with decay time. We conclude that exposure rate assessments for the purpose of hospital discharge should be made at 1 meter anterior to the stomach or neck, and that individual patient measurements are necessary because none of the clinical parameters were predictive of I-131 elimination.