Prescription Point Research Articles

Treatment-plan optimization for soft-tissue sarcoma brachytherapy using a genetic algorithm

Purpose: To describe a treatment-plan optimization system for temporary implant of soft-tissue sarcomas using a genetic algorithm, and evaluate its potential advantages over manual planning. Methods and Materials: A planning system that optimizes the distribution of radioactive seeds needed for adequate coverage of the target in the treatment of soft-tissue sarcomas has been designed and implemented. The treatment-planning procedures include simulation, film digitization, target-volume definition, optimized planning, and plan evaluation. The input to the optimization program consists of seed coordinates reconstructed from isocentric films, prescription points, and a list of available seed activities. The optimization is performed using a genetic algorithm. Results: Case studies are presented, which compare plans generated by computer optimization or by trial and error (manually). As expected, computer-optimized plans are often (but not always) superior to manual plans. This is particularly evident for situations where (unavoidably) catheters are far apart or irregularly spaced, in which case the advantages of optimized planning in terms of tumor coverage can be quite dramatic. When the target volume is well contained, the optimized plan minimizes the dose to normal tissue. Computer-based optimization has the additional advantage of being much faster than manual planning; this is valuable because it often reduces the total time the patient will spend in the hospital before implantation. Conclusion: Optimized planning with a genetic algorithm and seeds of different activities significantly improves planning efficiency and generally results in improved plan quality. The utility of this optimization system is not limited to sarcoma implants.

Biological equivalent dose assessment of the consequences of hypofractionated radiotherapy

Purpose: To investigate the changes in biological effective dose (BED) that occur in high-dose regions within a target volume when radiotherapy is hypofractionated. Methods and Materials: By comparing a standard prescription of 2 Gy per fraction that is matched to give the same BED as a hypofractionated schedule at a standard intersectional prescription point, the BED increments for late-tissue effects at a higher dose region within the planning target volume (PTV) are compared. The alternative approach of BED matching between a conventional and hypofractionated schedule at the high-dose region is also considered. The results are presented as a sequence of calculations that can be understood by practicing radiation oncologists and in graphical form. Results: The BED increment at the high-dose region is marginally increased by hypofractionation, although the latter effect is relatively small: up to 5% additional BED due to hypofractionation for a 20% increase in physical dose when the prescribed fraction size is 6–7 Gy. BED matching for late effects between a conventional and hypofractionated schedule at the high-dose region produces lower BED values throughout the remaining PTV, but at the expense of a reduced tumor control BED. Conclusion: Clinical trials that use BED isoeffect matching for late reacting tissue effects to design a hypofractioned test schedule should include comprehensive calculations of the likely BED in high-dose regions.

Radiation treatment for cervical esophagus: patterns of practice study in Canada, 1996

Purpose: To assess the patterns of practice among Canadian radiation oncologists who treat esophageal cancers, using a trans-Canada survey, completed at the end of 1996. Methods and Materials: One of 3 case presentations of different stages of cervical esophageal cancer was randomly assigned and sent to participating radiation oncologists by mail. Respondents were asked to fill in questionnaires regarding treatment techniques and to outline target volumes for the boost phase of radiotherapy. Radiation oncologists from 26 of 27 (96%) of all Canadian centers participated. Results: High-energy X-rays (≥ 10 MV) were employed by 68% of the respondents in part of the treatment course. The majority (83%) of the radiation oncologists used at least two phases of treatment. Very few, 10 of 59 (17%), responses started with multifield treatment. The most frequently used prescription dose was 60 Gy/30 fractions/6 weeks, given with concurrent chemotherapy. Dose prescriptions were to the isocenter in 39 of 48 (81%) or to a particular isodose line in 9 of 48 (19%) of respondents. Conclusion: There was a variety of radiation treatment techniques in this trans-Canada survey. The majority of the patients had combined cisplatin-based chemoradiation. The isocenter was not used consistently as a dose prescription point.

Dose calculation and verification of intensity modulation generated by dynamic multileaf collimators

While the development of inverse planning tools for optimizing dose distributions has come to a level of maturity, intensity modulation has not yet been widely implemented in clinical use because of problems related to its practical delivery and a lack of verification tools and quality assurance (QA) procedures. One of the prerequisites is a dose calculation algorithm that achieves good accuracy. The purpose of this work was twofold. A primary-scatter separation dose model has been extended to account for intensity modulation generated by a dynamic multileaf collimator (MLC). Then the calculation procedures have been tested by comparison with carefully carried out experiments. Intensity modulation is being accounted for by means of a 2D (two-dimensional) matrix of correction factors that modifies the spatial fluence distribution, incident to the patient. The dose calculation for the corresponding open field is then affected by those correction factors. They are used in order to weight separately the primary and the scatter component of the dose at a given point. In order to verify that the calculated dose distributions are in good agreement with measurements on our machine, we have designed a set of test intensity distributions and performed measurements with 6 and 20 MV photons on a Varian Clinac 2300C/D linear accelerator equipped with a 40 leaf pair dynamic MLC. Comparison between calculated and measured dose distributions for a number of representative cases shows, in general, good agreement (within 3% of the normalization in low dose gradient regions and within 3 mm distance-to-dose in high dose gradient regions). For absolute dose calculations (monitor unit calculations), comparison between calculation and measurement reveals good agreement (within 2%) for all tested cases (with the condition that the prescription point is not located on a high dose gradient region).

115 The american brachytherapy society recommendations for high dose rate brachytherapy for carcinoma of the endometrium

Purpose: To develop recommendations for use of high-dose-rate (HDR) brachytherapy in patients with endometrial cancer. Methods: A panel of members of the American Brachytherapy Society (ABS) performed a literature review, supplemented their clinical experience, and formulated recommendations for endometrial HDR brachytherapy. Results: The ABS endorses the National Comprehensive Cancer Network (NCCN) guidelines for indications for radiation therapy for patients with endometrial cancer and the guidelines on HDR quality assurance of the American Association on Physicists in Medicine (AAPM). The ABS made specific recommendations for HDR applicator selection, insertion techniques, target volume definition, dose fractionation, and specifications for postoperative adjuvant vaginal cuff therapy, for vaginal recurrences, and for medically inoperable primary endometrial cancer patients. The ABS recommends that applicator selection should be based on patient and target volume geometry. The dose prescription point should be clearly specified. The treatment plan should be optimized to conform to the target volume whenever possible while recognizing the limitations of computer optimization. Suggested doses were tabulated for treatment with HDR alone, and in combination with external beam radiation therapy (EBRT), when applicable. For intravaginal brachytherapy, the largest diameter applicator should be selected to ensure close mucosal apposition. Doses should be reported both at the vaginal surface and at 0.5-cm depth irrespective of the dose prescription point. For vaginal recurrences, intracavitary brachytherapy should be restricted to patients with nonbulky ( 0.5-cm thick) recurrences should be treated with interstitial techniques. For medically inoperable patients, an appropriate applicator that will allow adequate irradiation of the entire uterus should be selected. Conclusion: Recommendations are made for HDR brachytherapy for endometrial cancer. Practitioners and cooperative groups are encouraged to use these recommendations to formulate their treatment and dose reporting policies. This will lead to meaningful comparisons of reports from different institutions and lead to advances and appropriate use of HDR. © 2000 Elsevier Science Inc.

A variable speed translating couch technique for total body irradiation.

We have developed a variable speed translating patient couch system for the delivery of total body irradiation (TBI). For a whole body Rando-type phantom, dose variation at mid-plane relative to the prescription point (navel) can be as high as 15% (neck or legs) with a constant velocity. By taking into account variations in body thickness, the intensity modulation radiation therapy, resulting from variable velocities, effectively delivers a uniform dose distribution at mid plane. The couch control user interface, technical aspects and dose planning optimization procedure for determining velocity distribution are described.

Limitations of the straight-line assumption for endobronchial HDR brachytherapy treatments.

It is useful to expedite high dose rate brachytherapy endobronchial treatments by using standardized dwell weights and worksheets, avoiding computerized planning where possible. Such methods assume the treatment catheter is straight. This study uses inverse-square considerations to quantify the curvature that invalidates the straight-line approximation. The ratio of the distance between end dwell positions and the active length can be used as a measure of the curvature. The variation in mean dose and maximum dose at the prescription points is presented as a function of active length and curvature, allowing the planner to determine quickly when a pre-treatment computer plan is necessary.

Critical appraisal of a conformal head and neck cancer irradiation avoiding electron beams and field matching

Purpose: In head and neck cancer patients, spinal chains are usually irradiated by a combination of photon and electron beams, requiring high precision in field matching. This study compares a conventional treatment approach where two lateral photon beams are combined to direct electron fields, to a conformal radiotherapy based on five photon fields, covering the whole neck. Methods and Materials: A comparative analysis of dose distributions and dose-volume histograms was carried out in patients with locally advanced head and neck tumors, for which planning target volumes (PTV) were outlined from the base of the skull down to the supraclavicular region. The prescribed dose to PTV (excluding booster irradiation) was 54 Gy, with spinal dose constraint not exceeding 75% of the total dose, whatever the technique. Results: For the new five-field technique, minimum and maximum point doses showed mean deviations, on five patients entered in the study, of 84% and 113% from the ICRU prescription point. In the conventional treatment, the corresponding figures were 73% and 112%, respectively. A positioning error analysis (isocenter displacement of 2 mm, in all directions) did not elicit any systematic difference in five-field treatment plans while hot spots were found with electron fields. Conclusions: The five-field technique appears routinely feasible and compares favorably with the conventional mixed photon- and electron-therapy approach, especially in regard to its better compliance with dose homogeneity requirements and a reduced risk in dose inhomogeneity related to field matching and patient positioning.

Esophageal brachytherapy: a phantom menace?

Concerns have been raised within the past few years regarding the safety and efficacy of esophageal brachytherapy as a boost to concurrent chemoradiation (1). In this issue of the journal, Okawa and colleagues describe the results of a multi-institutional randomized trial of external beam radiation, with or without an intraluminal brachytherapy boost for squamous cancer of the esophagus. This article is an important publication given the relatively few published prospective studies of esophageal brachytherapy. Does this article clarify the risk/benefit of esophageal brachytherapy? Yes and no. This study by Okawa et al. had an unusual design. Patients with squamous cell cancer of the thoracic esophagus were first treated with external beam radiation alone to a dose of 60 Gy in 6–7 weeks. Those patients with a complete or partial response were then randomized to further external beam radiation (10 Gy/5 fractions/1 week) or an intraluminal boost of 10 Gy in 2 fractions 1 week apart. Brachytherapy was either high-dose rate (HDR) or low-dose rate (LDR), utilizing the same total dose, dose/fractionation scheme, and dose prescription point of 5 mm below esophageal submucosa. Patients with progressive or stable disease were only randomized on the study “if intraluminal brachytherapy was indicated.” (Isn’t that the real problem here—we just don’t know when it is indicated!) Approximately one-third of patients were deemed ineligible for randomization after 60 Gy. Chemotherapy was unusual by North American standards, consisting of “maintenance” etoposide following radiation therapy (2). The 5-year actuarial survival rate was not significantly different according to treatment group; 38% and 27% for the patients who did and did not receive the boost by brachytherapy, respectively (p 5 0.385). However, for the subset of patients with tumors 5 cm or less in length, the 5-year actuarial survival was significantly better in those receiving the boost by brachytherapy as opposed to further external beam (64% vs. 31.5%, p 5 0.025). Although not statistically significant, a trend toward improved survival with the brachytherapy boost was suggested in the patients with T1 and T2 tumors. Several questions come to mind while reading the article by Okawa et al. Why randomize patients after 60 Gy external beam rather than prior to external beam radiation? How did investigators decide which patients to exclude from randomization? Why choose the same dose/fractionation scheme for HDR and LDR? How many of the patients received HDR versus LDR brachytherapy? Were the patient sample sizes appropriate for the chosen endpoints of the study? Despite the unusual methodology and statistical design of this study, several interesting points are made. One of the most important points made by Okawa et al. concerns the low incidence of acute or chronic complications observed following brachytherapy. The low incidence of toxicity in this study, as well as several other published reports, suggests that brachytherapy is safe under certain conditions such as: total brachytherapy dose no more than 10 Gy specified at a depth 5 mm below the esophageal mucosa, brachytherapy catheter diameter of 10 mm, no concurrent chemotherapy and brachytherapy (3, 4). Another interesting observation made by Okawa et al. was the significant improvement in cause-specific survival rate following brachytherapy boost in those patients with tumors 5 cm or less in size. The 5-year cause-specific survival rate was 64% for brachytherapy boost, and 31.5% for external beam only (p 5 0.025). There was a tendency to improved cause-specific survival rate for patients with T1-2 disease as opposed to T3-4 tumors. As Okawa et al. point out, small superficial tumors are more effectively encompassed in a relatively uniform brachytherapy dose. However, if uniformity was the major prerequisite for successful treatment, external beam radiation should be superior to brachytherapy. It could be the “nonuniformity” of brachytherapy with hot spots falling within the target that makes brachytherapy more effective than external beam radiation in the treatment of superficial tumors. Concerns regarding the toxicity of esophageal brachytherapy were fueled by the 12% incidence of treatmentrelated esophageal fistulas reported by the Radiation Therapy Oncology Group (RTOG) Phase II study of concurrent chemoradiation followed by brachytherapy boost (1). This unexpectedly high incidence of serious complications, without a corresponding high rate of local tumor control or 1-year survival, stopped further investigation of esophageal

The `equivalent wedge' implementation of the Varian Enhanced Dynamic Wedge (EDW) into a treatment planning system

The purpose of this work was to establish procedures for the implementation of the Varian Enhanced Dynamic Wedge into a treatment planning system (TPS), based as much as possible on simple theoretical considerations and already available data. A method is presented for the calculation (rather than measurement) of off-axis relative wedge transmission curves that are required by the TPS for relative dose calculations. We also present a method for absolute dose (monitor unit) calculations, based on the calculation of an effective wedge factor on the prescription point. A simple formula has been derived for the calculation of the effective wedge factor for the most general case, i.e. an arbitrary effective wedge angle, field size and prescription point. Relative dose calculations have been verified by measurements performed on a Varian Clinac 2300C/D linear accelerator, for 6 MV and 20 MV photon energies. Monitor unit calculations have also been verified experimentally for several cases such as symmetric and asymmetric fields with prescription on the collimator axis or on the geometrical centre of the asymmetric field. The presented technique provides results within 2% for both relative and absolute dose calculations for clinically relevant cases.

Radiation Dose Homogeneity in an EORTC Multicenter Trial on Breast Irradiation

The influence of the radiation dose on local control and cosmetic outcome in breast-conserving treatment for breast cancer is investigated in EORTC trial 22881/10882. In this study 50 Gy is administered to the whole breast, and the effect of an additional dose to the tumor-bearing area (boost dose) is evaluated. The purpose of this analysis is to document the dose homogeneity of the radiation dose as reported in the first 1915 treatment forms, received at the EORTC Data Center. The dose to the prescription Point (A) was within 95 and 110% of 50 Gy in all but 13 (99.3%) patients (median dose 50 Gy) and the minimum and maximum doses in the central plane of the breast were within 95% and 110% of the dose to point A in 82% of patients. The dose to the tumor excision area (point B) was within the homogeneity criteria in 97% of patients, and the boost doses were consistent with randomization in 93% of cases. These data, based on one-third of the randomized patients in trial 22881/10882, demonstrate a high level of homogeneity in radiation doses, despite a number of ambiguities in the protocol, which were efficiently clarified a few months after the beginning of the trial. This high level of consistency is a remarkable achievement given the number of centers, the number of patients, and the number of countries involved in this trial.

Predictive factors for late toxicity after endobronchial brachytherapy: a multivariate analysis

Purpose: To determine the predictive factors associated with hemoptysis and radiation bronchitis after endobronchial brachytherapy by univariate and multivariate analyses Methods and Materials: One hundred forty-nine patients underwent endobronchial brachytherapy and were divided into three therapeutic groups: group 1: patients treated with palliative intent ( n = 47); group 2: patients treated with curative intent (small endobronchial tumors without mediastinal or general dissemination: n = 73); group 3: patients also receiving external irradiation ( n = 29). One hundred twelve patients had previously received external irradiation. Brachytherapy was delivered with a dose per fraction ranging from 4 to 7 Gy and a prescription point between 0.5 and 1.5 cm, usually 1 cm from the source center. Two to six fractions were delivered according to the therapeutic group and clinical situation. The influence of the following variables on the incidence of hemoptysis or radiation bronchitis was studied: age, sex, Karnofsky score, therapeutic group, histologic type, endoscopic tumor length, dose per fraction, total brachytherapy dose, total external beam irradiation dose, total dose (brachytherapy dose plus external irradiation dose), volumes of the 100% and 200% isodoses, and volumes of the 7 and 14 Gy isodoses. Results: We observed 11 hemoptyses (7.4%), 10 were lethal. All but one occurred in patients with progressive disease. Two clinical factors were significantly associated with hemoptysis by univariate analysis: palliative group ( p = 0.009) and endobronchial tumor length ( p = 0.004). No technical factors seem to be implicated in the occurrence of hemoptysis. Only endobronchial tumor length remained in the multivariate model ( p = 0.02). Radiation bronchitis was observed in 13 cases (8.7%). By univariate analysis, a good Karnofsky score ( p = 0.02), curative treatment ( p = 0.02), and tumor location on trachea and main stem bronchus ( p = 0.002) were significantly associated with this complication. Two technical factors were also incriminated: the total dose ( p = 0.04) and the 100% isodose volume ( p = 0.02). By multivariate analysis, only the tumor location retained statistical significance ( p = 0.009). Conclusion: Hemoptysis is most likely due to disease progression, with the bleeding being facilitated by brachytherapy. Some rare cases could be a direct complication of brachytherapy itself, particularly when tumors are located in the upper lobes. In contrast, radiation bronchitis occurred more frequently in patients with controlled disease, and was significantly influenced by tumor location and technical factors (dose and volumes treated). Technical improvements should increase the therapeutic ratio.

Optimized Dose Distribution of a High Dose Rate Vaginal Cylinder

Purpose: To present a comparison of optimized dose distributions for a set of high-dose-rate (HDR) vaginal cylinders calculated by a commercial treatment-planning system with benchmark calculations using Monte-Carlo–calculated dosimetry data. Methods and Materials: Optimized dose distributions using both an isotropic and an anisotropic dose calculation model were obtained for a set of HDR vaginal cylinders. Mathematical optimization techniques available in the computer treatment-planning system were used to calculate dwell times and positions. These dose distributions were compared with benchmark calculations with TG43 formalism and using Monte-Carlo–calculated data. The same dwell times and positions were used for a quantitative comparison of dose calculated with three dose models. Results: The isotropic dose calculation model can result in discrepancies as high as 50%. The anisotropic dose calculation model compared better with benchmark calculations. The differences were more significant at the apex of the vaginal cylinder, which is typically used as the prescription point. Conclusion: Dose calculation models available in a computer treatment-planning system must be evaluated carefully to ensure their correct application. It should also be noted that when optimized dose distribution at a distance from the cylinder surface is calculated using an accurate dose calculation model, the vaginal mucosa dose becomes significantly higher, and therefore should be carefully monitored.

Spatial Reproducibility of the Ring and Tandem High-Dose Rate Cervix Applicator

Purpose: High-dose rate (HDR) applicators for uterine cervix brachytherapy are used with increasing frequency. Because multiple HDR fractions are required for treatment, the applicator position reproducibility is of most importance. To study this effect, the clinical data from patients with uterine cervix cancer were examined retrospectively to evaluate the interfraction geometric variation of the HDR applicator and its potential treatment impact. Methods and Materials: Eighteen patients with invasive cervical cancer who were treated with definitive radiotherapy at William Beaumont Hospital were included in the study. Patients were treated with 45–50.4 Gy megavoltage external beam to the pelvis, and 35 Gy to the prescription point A from 7 fractions of HDR brachytherapy. The 3-dimensional (3D) interfraction geometrical variation of the ring and tandem (R & T) applicator was measured using predefined reference points in the 7 sets of orthogonal simulation films obtained prior to each HDR application. Spatial reproducibility of the R & T insertion and time-trend of the R & T position variation related to patient’s anatomy during the treatment course were analyzed with respect to different groups of patients who had either early or advanced disease. Results: The translational variation of the applicator position for all patients was 6.5, 5.9, and 7.7 mm (one standard deviation), respectively, in the patient’s superior-to-inferior (SI), right-to-left lateral (RL), and anterior-to-posterior (AP) direction. The rotational variation was 3.4, 4.6, and 6.0 degrees (one standard deviation) in the patient’s coronal, transverse, and sagital planes. When the patients were grouped based on early disease or advanced disease, the latter demonstrated substantially larger variation (factor of 2) in the applicator position than the former. Furthermore, the time-trend of position variation was observable for both groups of patients. The variations occurred primarily during the first 3 fractions. Conclusions: Based on the good spatial reproducibility observed in our study, the current clinical procedure for the HDR R & T applicator placement is reliable. Positional reproducibility of the R & T applicator is highly dependent upon the size of tumor volume, which, in turn, deviates the applicator during the early course of HDR brachytherapy. Attention to the construction of the midline block is of paramount importance.

Use of thermoluminescent dosimetry to verify dose compensation in total body irradiation

For Total Body irradiation (TBI) the midline dose along the long axis of the body should vary by no more than ± 10% compared to the dose received at the prescription point. Compensating filters are often made to compensate for contour irregularities. This paper describes an experimental method using Thermoluminescent Dosimetry (TLD) to verify the appropriateness of dose compensation provided by compensating filters. An AP PA treatment geometry was used with an 18 MV photon beam. TLDs were placed in the midplane and on the surface of an anthropomorphic phantom in five different areas. Three different lead compensating filters were designed to provide proper compensation, overcompensation, and undercompensation. The TLD readings were compared with calculated doses and the measurements correlated favorably. The measurements were sensitive enough to detect dose variations due to improper compensation. These results demonstrate that TLD is a reliable method of verifying dose compensation provided by compensating filters in TBI.

Patterns of dose variability in radiation prescription of breast cancer

Purpose: Comparison of radiation outcome of various treatment protocols is difficult due to the variability of dose prescription. A retrospective analysis of the pattern and intercomparison of dose prescriptions is presented for the treatment of breast cancer. Materials and methods: To represent the clinical practice for breast irradiation with tangential fields, commonly used prescription points were chosen that lie on the perpendicular bisector of the chest wall separation ( s) that represents the breast apex height ( h). These points are located at 1.5 cm from the posterior beam edge, at the chest wall-lung interface (2–3 cm), at distances of h 3 and h 2 , and at the isocenter. One hundred consecutive patients treated with intact breast following excisional biopsy were used in this study. For analysis, treatment planning was carried out without lung correction with a 6 MV beam for all patients, even though some of the patients were treated with high energy beams for dose uniformity. Dose distributions were optimized with wedges and beam weights to provide a symmetrical dose distribution on the central axis plane. The statistical analyses of the different parameters, s, h, maximum dose, and doses at various prescription points were carried out. Results: The maximum dose (hot spot) in breast varied from +5% to +27% above the prescribed dose among the patient population. The hot spot was directly related to s, and appeared to be independent of h and the ratio h s . Among 55%, 40%, and 5% of the patients, the magnitude of the hot spot was 5–10%, 10–15%, and > 15%, respectively. Except for the magnitude of the hot spot, the doses at various prescription points were independent of the breast size. For a prescription point at h 3 or at the lung-chest wall interface, the dose variation within ± 1% is observed for 90% of the patient population. On the other hand, the average dose variation is about ± 3% among other protocols with dose prescription point varying up to the h 2 point. With the prescription point at the isocenter, an average and maximum variation of 4–5% and 11% were observed, respectively. The maximum dose inhomogeneity for some patients was significantly higher, i.e. up to +27% even without the lung correction. Conclusions: A wide variation in prescription dose is observed among the different treatment protocols commonly used in breast treatment. For a total dose of 46–50 Gy delivered at 2 Gy/fraction to the breast, the prescribed dose may vary between 50 and 55 Gy and the hot spot dose per fraction may range between 2.3 and 2.5 Gy depending on the protocol and breast size. Thus dose normalization at hot spot and the isocenter should be discouraged unless the total dose to the breast is modified. A uniform definition of dose prescription for breast treatment is greatly required for intercomparison of clinical data.

Dose estimation to critical organs from vertex field treatment of brain tumors

Radiation management of intracranial tumors may require a noncoplanar vertex field that often irradiates the entire length of the body. In view of radiation related risks to the normal tissues dose estimation to the extracranial organs such as the thyroid gland, spinal cord, heart, and genitalia is performed for a vertex field. A vertex field used clinically was reproduced on an anthropomorphic Rando phantom to measure radiation dose to various organs in the primary beam. Three photon beams (4, 6, and 10 MV), and two high energy electron beams (16 and 20 MeV) were used. Dosimetry was performed with an ion chamber sandwiched between phantom slices at the appropriate positions. All doses were normalized to the target dose at a depth of 5 cm. The effect of the head position was studied by rotating the gantry angle up to +/-20 degrees to mimic the extension and flexion of the head. Theoretical calculation was performed using an exponential best fit to the depth dose table to estimate the dose to various points and compare with the measured dose. The measured normalized dose to the cervical cord, thyroid, heart, and female and male gonads are 60, 36, 16, 2.5, and 1.6%, respectively, for a 6 MV photon beam. The dose from 4 MV and 10 MV are slightly lower and higher, respectively. Doses from electron beams are about a factor of 4-10 lower than those of the photon beams. The measured gonadal dose from the primary beam is <5% of the target dose for all energies used in the study. The actual value, however, is dependent on the body structure, length, and the posture of the patient. A +5 degree head flexion had little effect on the dose to the various parts of the body. The head rotations greater than +/-10 degrees produced relatively lower doses by a factor of 10(-2) to the organs at distances greater than 40 cm from the prescription point. The radiation doses to the different critical organs estimated from the fitted curves are lower than the measured doses up to 35%. When a vertex field is used for the treatment of the brain tumors, the entire axial length of the body is irradiated which adds to the integral dose. Unlike the scattered and leakage radiation, the primary dose to extracranial critical organs is greater for higher energies. For a 10 MV beam the ovary and testis at a distance of 80 cm and 90 cm may receive a dose of 4.2 and 3%, respectively, of the target dose. The gonadal dose could be quite significant if the entire treatment is delivered using a vertex field. For pediatric and smaller patients, dose to the critical organs at known distances could be estimated from the empirical equation obtained from the measured data. While the risk-benefit ratio is often evaluated and acceptable for treating malignant tumors, the long-term complications need thorough assessment in younger and curable patients. In view of radiation carcinogenesis and genetic burden, dose reduction to critical organs should be considered using a 3D planning system to arrange beams in other nonaxial planes and by considering electron beams for the vertex field.

Treatment planning for carcinoma of the cervix: A patterns of care study report

Purpose : The Patterns of Care Study (PCS) of patients treated in 1988–1989 included “patterns of treatment planning” for radiotherapy of carcinoma of the uterine cervix. A Consensus Committee of radiation physicists and oncologists established current guidelines and developed questionnaires to assess the treatment planning process (i.e., the general structure, methodology, and tools) of institutions involved in the Patterns of Care Study. This paper reports the findings of the assessment. Methods and Materials : The PCS surveyed 73 radiotherapy facilities, of which 21 ae academic institutions (AC), 26 hospital-based facilities (HB), and 26 free-standing centers (FS). In total, 242 cases were assessed with 39% from academic centers, 33% from hospital-based centers, and 28% from free-standing centers. The survey collected treatment planning information such as the use of computed tomography (CT), simulation procedures, contouring of patient outlines, tumor or target delineation, identification of critical structures, method of dose prescription (point or isodose), etc. Data also obtained concerning implant boost, e.g. radioisotope used, use of midline block for external beam treatment, availability of remote afterloader, practice of interstitial implants, combination with hyperthermia, etc. Results : There is a high degree of compliance relative to the basic treatment planning standards. For example, 171 cases (out of 173) from AC and HB institutions included simulation and 169 used port film; for cases from FS centers, 61 out of 69 involved simulation and 66 out of 69 included port film. Most institutions used linacs (231 out of 242); in five cases, Co-60 units and in six cases betatron was used. In terms of treatment planning, 53% used skin contours, but only 14% had target volume delineation, with AC and HB being slightly more conscientious in these efforts. Critical organs did not appear to be explicitly considered in external beam treatment planning, with only 3% outlining the bladder, 5% the rectum, and less than 1% the small bowel. Only 11% of the centers used CT in treatment planning, and none reported the used of magnetic resonance imaging (MRI). For patients receiving implants, about 40% had midline blocking during external beam treatment, of which one out of three were shielded by standard blocks and two out of three with customized ones. About 11% of the patients receiving implants were treated with remote afterloading devices, 5% received interstitial implants, and none were treated in combination with hyperthermia. Conclusion : The treatment planning aspects of radiotherapy of carcinoma of the cervix have been established by this Patterns of Care Study Survey. There is a high level of uniformity in the approach. Some variations exist among centers in the different strata.

Influence of inguinal node anatomy on radiation therapy techniques

A 1992 GOG (Gynecologic Oncology Group) study suggested that groin irradiation does not control microscopic inguinal node metastases as well as does surgical dissection. That trial has come under some scrutiny, as possible deficiencies in the radiation regimen used might have influenced results. The study received criticism for assigning a prescription point [Rx @ 3 cm below anterior skin surface] to patients which may not have given adequate coverage to the inguinal nodes. The inguinal node depths of 31 patients with cervical, vaginal, or vulvar malignancies were measured from their planning CT scans and then compared to depth of the prescription point designated by the radiation therapists in the aforementioned study. Twenty-four of eighty-one viable superficial inguinal node depth measurements were greater than 3 cm, and all of eighty-four deep inguinal node measurements were outside the three centimeter range. The results of this project would indicate that the depths of patients' inguinal nodes vary enough to justify alterations in some present external beam radiation therapy techniques. Indiscriminate prescription points will no longer suffice if groin radiation is to be effective, and treatment planning must utilize imaging studies to devise radiation regimens that provide optimal dose to the superficial and deep inguinal nodes.

Double-film technique for the determination of the homogeneity of line sources.

A new technique is presented for determining the homogeneity of the linear activity. The method uses two x-ray films that are firmly pressed against opposing sides of a needle containing a radioactive line source. The homogeneity of the linear activity of 68 iridium-192 wires has been measured. About 80% of the measured linear activities showed deviations of no more than 5% from the mean; about 20% of the measured linear activities showed deviations of 5%-10% from the mean. Individual wires may contain deviations of 5% or more extended over several centimeters, which causes the dose in the prescription point to deviate 5% from the desired value. Four wires (8%) were rejected for this reason.