Jason Rownd

Evaluation of Kodak EDR2 film for dose verification of intensity modulated radiation therapy delivered by a static multileaf collimator.

A new type of radiographic film, Kodak EDR2 film, was evaluated for dose verification of intensity modulated radiation therapy (IMRT) delivered by a static multileaf collimator (SMLC). A sensitometric curve of EDR2 film irradiated by a 6 MV x-ray beam was compared with that of Kodak X-OMAT V (XV) film. The effects of field size, depth and dose rate on the sensitometric curve were also studied. It is found that EDR2 film is much less sensitive than XV film. In high-energy x-ray beams, the double hit process is the dominant mechanism that renders the grains on EDR2 films developable. As a result, in the dose range that is commonly used for film dosimetry for IMRT and conventional external beam therapy, the sensitometric curves of EDR2 films cannot be approximated as a linear function, OD = c * D. Within experimental uncertainty, the film sensitivity does not depend on the dose rate (50 vs 300 MU/min) or dose per pulse (from 1.0 x 10(-4) to 4.21 x 10(-4) Gy/pulse). Field sizes and depths (up to field size of 10 x 10 cm2 and depth = 10 cm) have little effect on the sensitometric curves. Percent depth doses (PDDs) for both 6 and 23 MV x rays were measured with both EDR2 and XV films and compared with ion chamber data. Film data are within 2.5% of the ion chamber results. Dose profiles measured with EDR2 film are consistent with those measured with an ion chamber. Examples of measured IMRT isodose distributions versus calculated isodoses are presented. We have used EDR2 films for verification of all IMRT patients treated by SMLC in our clinic. In most cases, with EDR2 film, actual clinical daily fraction doses can be used for verification of composite isodose distributions of SMLC-based IMRT.

American Brachytherapy Society consensus guidelines for adjuvant vaginal cuff brachytherapy after hysterectomy

PURPOSE To develop recommendations for the use of adjuvant vaginal cuff brachytherapy after hysterectomy and update previous American Brachytherapy Society (ABS) guidelines. METHODS AND MATERIALS A panel of members of the ABS performed a literature review, supplemented their clinical experience, and formulated recommendations for adjuvant vaginal cuff brachytherapy. RESULTS The ABS endorses the National Comprehensive Cancer Network guidelines for indications for radiation therapy for patients with endometrial cancer and cervical cancer and the guidelines on quality assurance of the American Association on Physicists in Medicine. The ABS made specific recommendations for applicator selection, insertion techniques, target volume definition, dose fractionation, and specifications for postoperative adjuvant vaginal cuff therapy. The ABS recommends that applicator selection should be based on patient anatomy, target volume geometry, and physician judgment. The dose prescription point should be clearly specified. Suggested doses were tabulated for treatment with brachytherapy alone, and in combination with external beam radiation therapy, when applicable. A properly fitted brachytherapy applicator should be selected that conforms to the vaginal apex and achieves mucosal contact with optimal tumor and normal tissue dosimetry. Dose prescription points may be individually selected but doses should be reported at the vaginal surface and at 0.5-cm depth. CONCLUSIONS Recommendations are made for adjuvant vaginal cuff brachytherapy. Practitioners and cooperative groups are encouraged to use these recommendations to formulate their treatment and dose reporting policies. These recommendations will permit meaningful comparisons of reports from different institutions and lead to better and more appropriate use of vaginal brachytherapy.

American Brachytherapy Society consensus guidelines for interstitial brachytherapy for vaginal cancer

PURPOSE To present recommendations for the use of interstitial brachytherapy in patients with vaginal cancer or recurrent endometrial cancer in the vagina. METHODS A panel of members of the American Brachytherapy Society reviewed the literature, supplemented that with their clinical experience, and formulated recommendations for interstitial brachytherapy for primary or recurrent cancers in the vagina. RESULTS Patients with bulky disease (approximately >0.5cm thick) should be considered for treatment with interstitial brachytherapy. The American Brachytherapy Society reports specific recommendations for techniques, target volume definition, and dose-fractionation schemes. Three-dimensional treatment planning is recommended with CT scan and/or MRI. The treatment plan should be optimized to conform to the clinical target volume and should reduce the dose to critical organs, including the rectum, bladder, urethra, and sigmoid colon. Suggested doses in combination with external beam radiation therapy and summated equivalent doses in 2Gy fractions are tabulated. CONCLUSION Recommendations are made for interstitial brachytherapy for vaginal cancer and recurrent disease in the vagina. Practitioners and cooperative groups are encouraged to use these recommendations to formulate treatment and dose-reporting policies. Such a process will result in meaningful outcome comparisons, promote technical advances, and lead to appropriate utilization of these techniques.

Characteristics of sensitometric curves of radiographic films.

A new type of radiographic film, EDR (extended dose range) film, has been recently become available for film dosimetry. It is particularly attractive for composite isodose verification of intensity modulated radiation therapy because of its low sensitivity relative to the more common Kodak XV film. For XV film, the relationship between optical density and dose, commonly known as the sensitometric curve, depends linearly on the dose at low densities. Unlike XV film, the sensitometric curve of EDR film irradiated by megavoltage x rays is not linearly dependent on the dose at low densities. In this work, to understand the mechanisms governing the shape of the sensitometric curves, EDR film was studied with kilovoltage x rays, 60Co gamma rays, megavoltage x rays, and electron beams. As a comparison, XV film was also studied with the same beams mentioned above. The model originally developed by Silberstein [J. Opt. Soc. Am. 35, 93-107, 1945)] is used to fit experimental data. It is found that the single hit model can be used to predict the sensitometric curve for XV films irradiated by all beams used in this work and for EDR films exposed to kilovoltage x rays. For EDR film irradiated by 60Co gamma rays, megavoltage x rays, and electron beams, the double hit model is used to fit the sensitometric curves. For doses less than 100 cGy, a systematic difference between measured densities and that predicted by the double hit model is observed. Possible causes of the observed differences are discussed. The results of this work provide a theoretical explanation of the sensitometric behavior of EDR film.

Comparision of four different dose specification methods for high-dose-rate intracavitary radiation for treatment of cervical cancer

PURPOSE To compare the dose delivered to target tissues and dose-limiting structures as defined by specific dose points with high-dose-rate intracavitary brachytherapy using tandem and ring or tandem and ovoids applicators, and to provide a reasonable approach to dose optimization. METHODS AND MATERIALS Dosimetry was obtained using four different dose specifications: (1) 100% of the dose prescribed in a tapered fashion along the tandem and 140% at the ovoid/ring surface, (2) 100% of the dose prescribed along the tandem and 100% at the ovoid/ring surface, (3) 100% of the dose prescribed to point A without any additional applicator specification points, and (4) nonoptimized plan using relative dwell weighting to simulate classic Fletcher low-dose-rate (LDR) loading with the dose specified at point A. Point doses were recorded at A, B, and T (cervical tumor point), ICRU rectum, and ovoid/ring surface. RESULTS For the tandem and ovoids applicators, significant differences were found among the four different dose specification methods for point T and vaginal mucosal doses. When the dose was optimized to point A alone, the ovoid dwell weights were reduced, resulting in higher point T doses and underdosing of the vaginal mucosa. Fixed weighting based on Fletcher LDR loading specifications resulted in higher vaginal mucosa doses. For the tandem and ring applicators, significant differences were observed for vaginal mucosal doses and the ICRU rectal dose. Optimization to point A alone resulted in widely varying dosimetric distributions and vaginal mucosa doses up to 632% of the prescription dose. With nonoptimized fixed weighting, the vaginal wall dose and ICRU rectal dose were increased. CONCLUSION Prescribing to dose optimization points in a tapered fashion along the tandem and at the ovoid/ring surface results in a pear-shaped dose distribution resembling classic LDR systems. The other dose specification methods may result in underdosing of important target tissues or overdosing of adjacent dose-limiting structures.

Comparison of dosimetric characteristics of Siemens virtual and physical wedges

Dosimetric properties of Virtual Wedge (VW) and physical wedge (PW) in 6 and 23 MV photon beams from a Siemens Primus linear accelerator, including wedge factors, depth doses, dose profiles, peripheral doses and surface doses, are compared. While there is a great difference in absolute values of wedge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have a stronger depth dependence than VWF due to beam hardening in PW fields. VW dose profiles in the wedge direction, in general, match very well with PW, except in the toe area of large wedge angles with large field sizes. Dose profiles in the nonwedge direction show a significant reduction in PW fields due to off-axis beam softening and oblique filtration. PW fields have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open fields while PW fields have lower surface doses. Surface doses for both VW and PW increase with field size and slightly with wedge angle. For VW fields with wedge angles 45 degrees and less, the initial gap up to 3 cm is dosimetrically acceptable when compared to dose profiles of PW. VW fields in general use less monitor units than PW fields.

CT-guided high-dose-rate dose prescription for cervical carcinoma: the importance of uterine wall thickness •

PURPOSE To evaluate the use of CT imaging to guide dose prescription for high-dose-rate brachytherapy in cervical cancer, by defining the uterine wall thickness and the proximity of the rectosigmoid, bladder, and small bowel. METHODS AND MATERIALS From 1994-2000, 40 patients with cervical cancer underwent treatment with external beam radiation therapy plus high-dose-rate brachytherapy. A pelvic CT scan was performed following applicator placement, and was analyzed to determine anterior and posterior uterine wall thickness, and the distance to the bladder and rectosigmoid. The dose prescription was initially at Point A, but in some patients, was altered based on the thickness of the uterine wall. RESULTS Measurements in the region of the lower and upper uterine wall were analyzed separately. Nearly half (46%) of the patients had an average anterior or posterior wall thickness of <20 mm. A correlation was observed between thinner anterior lower uterine walls and increased late bladder and ureteral toxicity, and between thinner anterior upper uterine walls and increased late small-bowel toxicity. CONCLUSIONS A significant number of patients have uterine walls <2 cm in thickness. The use of CT imaging can identify patients in whom the conventional dose specification point may need to be altered to maintain the therapeutic ratio.

Dependence of virtual wedge factor on dose calibration and monitor units.

One of the important features of the Siemens Virtual Wedge (VW) is that the VW factor (VWF) is approximately equal to unity for all beams with a total deviation for a given wedge no greater than 0.05, as specified by Siemens. In this note we report the observed dependence of VWF on dose calibration (cGy/MU), monitor units (MU), and beam tuning for a Primus, a linear accelerator with two dose-rate ranges available for VW operation. The VWF is defined as the ratio of doses measured on the beam central axis for the wedge field to the open field; the open field dose is always measured with the nominal high dose-rate beam. When VW operates in the high dose-rate range, the VWF is independent of calibration (cGy/MU). When VW works in the low dose-rate range, the VWF varies linearly with the calibration of the low dose-rate mode. For a linear accelerator that has only one dose-rate range for VW, there is no observable dependence of VWF on the calibration. We also studied the monitor unit dependence of VWF. A discontinuity in VWF was observed at the switching point between the high and low dose-rate ranges. Working with Siemens, we have investigated causes of this discontinuity. As a result of this investigation, the discontinuity in VWF as a function monitor unit is practically removed.

Image-Based Approaches to Interstitial Brachytherapy

For patients with tumors where the dose distribution of standard gynecologic brachytherapy applicators does not adequately cover the area at risk, interstitial brachytherapy represents an excellent alternative. After completion of external beam radiation, interstitial brachytherapy escalates the radiation dose to the regions of remaining gross residual disease for patients with all gynecologic malignancies. Interstitial catheters, placed either through a perineal template or free-hand, provide high-dose regions of radiation directly in the target. The use of 3D imaging before, during and after insertion, with ultrasound, CT, MRI, or PET scanning, results in an increase in accuracy that may change the total number of inserted catheters, may results in a higher degree of dose conformality and avoidance of normal tissues and may improve patient outcomes.

Clinical implementation of AAPM TG61 protocol for kilovoltage x-ray beam dosimetry

Historically, there have been a variety of dosimetry protocols used for kilovoltage x-ray therapy beams with a set of conversion factors and correction factors taken from different references. Corresponding to the continued installation and use of kilovoltage machines, the American Association of Physicists in Medicine (AAPM) presented a unified protocol developed by Task Group 61 (TG61). TG61 determines the absorbed dose to water with an ionization chamber calibrated in air in terms of air kerma (Nk). TG61 presents both an in-air method and an in-phantom method. In this work we only examine the TG61 in-air method. Our traditional dosimetry procedure, which is based upon NCRP Report 69 and on material found in standard medical physics texts, has been compared to the TG61. A variety of kilovoltage beam energies were examined with a set of various field sizes and source to surface distances. TG61 published updated data for the mass absorption coefficient ratios, backscatter factors, and the average energy per ion pair factor. The following conclusions have been reached: (1) Our traditional procedures and the TG61 protocol for in-air measurements are equivalent. (2) The conversion and correction factors used in TG61 are different by up to 4.5% compared to the old factors that we have used. (3) The application of the TG61 factors can result in up to 5% differences in the determination of the absorbed dose.