Radiation dosimetry by a new solid-state effect.

Ionizing radiation can induce strong electrical polarization phenomena in dielectic solids. These radiation-induced thermally activated depolarization (RITAD) effects are quite different from radioelectret effects. For nominally pure calcium fluoride samples, the RITAD signals show a signal-to-noise power advantage of 40 decibels over that of thermoluminescence signals measured under the same experimental conditions. Ease of measurement, very high radiation sensitivity, and simple sample fabrication requirements give the RITAD phenomena a great potential for use as a new solid-state dosimetry technique.

Clinic based transfer of the N(D,W)(60Co) calibration coefficient using a linear accelerator.

Ionization chambers used for reference dosimetry require a local secondary standard ionization chamber with a 60Co absorbed dose to water calibration coefficient N(D,W)(60Co) traceable to a national primary standards dosimetry laboratory or an accredited secondary dosimetry calibration laboratory. Clinic based (in-house) transfer of this coefficient to tertiary reference ionization chambers has traditionally been accomplished with chamber cross calibration in water using a 60Co beam; however, access to 60Co teletherapy machines has become increasingly limited for clinic based physicists. In this work, the accuracy of alternative methods of transferring the N(D,W)(60Co) calibration coefficient using 6 and 18 MV photon beams from a linear accelerator in lieu of 60Co has been investigated for five different setups and four commonly used chamber types.

Verification of cell irradiation dose deposition using a radiochromic film.

We describe a technique for the MTT assay that irradiates all cells at once by a combination of couch movement and a step-and-shoot irradiation technique on a linear accelerator with 6 MV and 18 MV photon beams. In two experimental setups, we obtained maximum to minimum dose ranges of 10 for the constant MU/bin (monitor units per bin) setup and 20 for the variable MU/bin technique. The irradiation technique described is dose rate independent and it can be used on any teletherapy irradiation machine. We also employed radiochromic film dosimetry to verify dose delivered in each of the wells within the dish. It is shown that for the lowest doses, relative dose variation within wells reaches a value of 6%. We also demonstrated that the radiochromic film positioned below the 96-well plate does not underestimate dose deposited within each compartment by more than 2% due to the vertical dose gradient.

High dose-rate brachytherapy source position quality assurance using radiochromic film.

Traditionally, radiographic film has been used to verify high-dose-rate brachytherapy source position accuracy by co-registering autoradiographic and diagnostic images of the associated applicator. Filmless PACS-based clinics that do not have access to radiographic film and wet developers may have trouble performing this quality assurance test in a simple and practical manner. We describe an alternative method for quality assurance using radiochromic-type film. In addition to being easy and practical to use, radiochromic film has some advantages in comparison with traditional radiographic film when used for HDR brachytherapy quality assurance.

Accurate skin dose measurements using radiochromic film in clinical applications.

Megavoltage x-ray beams exhibit the well-known phenomena of dose buildup within the first few millimeters of the incident phantom surface, or the skin. Results of the surface dose measurements, however, depend vastly on the measurement technique employed. Our goal in this study was to determine a correction procedure in order to obtain an accurate skin dose estimate at the clinically relevant depth based on radiochromic film measurements. To illustrate this correction, we have used as a reference point a depth of 70 micron. We used the new GAFCHROMIC dosimetry films (HS, XR-T, and EBT) that have effective points of measurement at depths slightly larger than 70 micron. In addition to films, we also used an Attix parallel-plate chamber and a home-built extrapolation chamber to cover tissue-equivalent depths in the range from 4 micron to 1 mm of water-equivalent depth. Our measurements suggest that within the first millimeter of the skin region, the PDD for a 6 MV photon beam and field size of 10 x 10 cm2 increases from 14% to 43%. For the three GAFCHROMIC dosimetry film models, the 6 MV beam entrance skin dose measurement corrections due to their effective point of measurement are as follows: 15% for the EBT, 15% for the HS, and 16% for the XR-T model GAFCHROMIC films. The correction factors for the exit skin dose due to the build-down region are negligible. There is a small field size dependence for the entrance skin dose correction factor when using the EBT GAFCHROMIC film model. Finally, a procedure that uses EBT model GAFCHROMIC film for an accurate measurement of the skin dose in a parallel-opposed pair 6 MV photon beam arrangement is described.

An endocavitary rectal irradiation technique.

An alternative to standard short target-skin distance (TSD) rectal irradiation techniques is presented. The proctoscopic rectal cone is linked outside of the patient to a superficial X ray tube operated at 50 kVp and 25 mA. The cone is rigidly held in place by a hydraulic clamp and linked to the X ray tube with an electromagnetic lock. The X ray beam is filtered by 0.3 mm of aluminum, giving a half-value-layer of 0.27 mm of aluminum and an exposure rate of 720 R/min. (0.186 C/kg-min.) at the treatment TSD of 22 cm. The physical aspects of the technique are described in detail.

Dose distributions around selectron applicators.

Measured and calculated dose distributions around selectron applicators, loaded with 60Co high dose rate pellets, are presented. The effect of the stopping screw, spacers, pellets themselves and the applicator wall on the dose distribution is discussed. The measured dose distribution is in almost perfect agreement with the calculated distribution in planes perpendicular to the applicator axis and containing a source. On the applicator axis directly below the applicator the measured dose amounts to about 75% of the calculated value, when only the stopping screw attenuates the beam from a pellet. When the beam is attenuated by spacers in addition to the stopping screw, the discrepancy between the calculated and measured dose may exceed 50%. Clinically relevant source geometries are also discussed. It is shown that for most regions around the applicator the method of a simple addition of dose contributions from individual point sources is an acceptable approximation for the calculation of dose distributions around the selectron applicators.

Total body irradiation with a sweeping beam.

A technique for total body irradiation, in which the patient lies in the prone or supine position in the beam of a conventional column mounted 4 MV linear accelerator, is described. A sufficiently large radiation field is obtained by rotating the beam in a vertical plane about the source (i.e., sweeping beam) at a source-to-skin distance of 190 cm on the vertical axis. The variation of the midplane dose is less than +/- 5% in parallel-opposed beams, when attenuators are placed over the region containing the lungs and bolus is employed around the head and legs. The percentage depth dose for the sweeping beam is identical to that of a stationary beam for the same collimator setting and source-to-skin distance. A method for monitoring the dose to the patient by means of a thimble ionization chamber located on the vertical beam axis is outlined. The average dose rates used are between 5 and 10 cGy/min. The design and placement of lung attenuators is simple. The treatment technique with the sweeping beam requires minimal modification of a treatment unit and can be applied on any unit which has a head swivel option.

Radiosurgery of cerebral arteriovenous malformations with the dynamic stereotactic irradiation.

From December 1986 through December 1988, 33 patients with inoperable arteriovenous malformation (AVM) were treated in our center with the dynamic stereotactic radiosurgery, which uses a standard 10 MV isocentric linear accelerator. There were 18 females and 15 males with a median age of 26 years (range: 9-69) and a median follow-up time of 16 months (range: 7-32). The arteriovenous malformation volumes treated ranged from 0.2 to 42 cm3. The prescribed doses at the isocenter varied from 50 to 55 Gy and were given as a single fraction in the majority of the patients (31/33). Late complications consisting of intracranial bleeding and/or hemiparesis were observed in three patients. To date, 21 patients underwent repeat angiographic studies at 1 year post-treatment. A complete obliteration of the lesion was achieved in 38% of these patients. For the patients whose arteriovenous malformation nidus was covered by a minimum dose of 25 Gy, the total obliteration rate was 61.5% (8/13), whereas none of the patients who had received less than 25 Gy at the edge of the nidus obtained a total obliteration. Our preliminary analysis at 1 year post-radiosurgery reveals results comparable to those previously reported for other radiosurgical techniques for the same follow-up period.

Dynamic stereotactic radiosurgery.

Two radiosurgical procedures using a stereotactic frame and a linear accelerator X ray beam with a circular field diameter between 0.5 and 3 cm are presented. One technique is based on a single plane rotation (single plane radiosurgery) whereas the other uses simultaneous and continuous motions of both the gantry (approximately 360 degrees) and couch (approximately 180 degrees) during the radiosurgical procedure (dynamic radiosurgery). The dose, typically a few thousand cGy, is prescribed to the 90% isodose line which just covers the target volume. The dose fall-off outside the spherical target volume is considerably sharper for the dynamic rotation than for the single plane rotation, and is comparable to the dose fall-off obtained with the two presently known dedicated radiosurgical techniques: one based on focused cobalt beams and the other on proton beams. The dose fall-off in the dynamic radiosurgery discussed here is also comparable to that of previously described linear accelerator based multiple converging are techniques, making the dynamic radiosurgery an attractive alternative to presently known radiosurgical procedures. The radiation beam parameters are discussed and the stereotactic frame described. The dose distributions for both radiosurgical techniques are calculated in a single plane and then corrected for the attenuation effects in the stereotactic frame (approximately 2%) and for the effects of the dynamic rotation (approximately 2%). The skin doses are 0.7% and 2%, and the lens doses, if the beam passes through the eyes, are 2.5% and 3.5% for the dynamic rotation and single plane rotation, respectively. The scatter and leakage dose for the radiosurgical procedures is typically 0.2% to the patient's thyroid, 0.06% to the breast, and 0.02% to the gonads.

Interstitial pneumonitis following total body irradiation for bone marrow transplantation using two different dose rates.

A total of 22 patients with leukemia (10 ALL, 11 AML, 1 CML) have undergone allogeneic bone marrow transplantation (BMT) by the Quebec Co-operative Group for Marrow Transplantation from 1980 to 1982. All patients received 900 cGy total body irradiation (TBI), in a single fraction, on the day preceding BMT. The first 11 patients were treated on a cobalt unit at a constant dose rate of 4.7 to 6.3 cGy/min. Six of these patients developed interstitial pneumonitis (IP). The clinical course of three patients, two with idiopathic and one with drug-induced pneumonitis, was mild and recovery was complete in all. The other three patients developed severe infectious IP and two died. The next 11 patients were treated with a sweeping beam technique on a 4 MV linear accelerator delivering a total tumor dose of 900 cGy at an average dose rate of 6.0 to 6.5 cGy/min but an instantaneous dose rate of 21.0 to 23.5 cGy/min. Eight patients developed severe IP. Five of these were idiopathic and four died. Three were infectious and all died. The fatality of interstitial pneumonitis appeared to be greater in the group treated with the sweeping beam technique.

A technique using parallel-opposed high energy electron beams for reirradiation of tumors near the spinal cord.

PURPOSE: Previous radiation treatment of patients with recurrent Hodgkin's lymphoma hampers attempts to give these patients further radiation treatment for their recurrence, because of the potential for serious radiation damage to critical normal structures within the treatment volume. The purpose of this paper is to present a technique we developed for treatment of recurrent Hodgkin's lymphoma in mediastinum and hilum. The technique is based on parallel-opposed electron beams with a spinal cord shield in the posterior electron beam. METHODS AND MATERIALS: The patient was treated with parallel-opposed 20 MeV electron beams and the spinal cord shield was shaped in such a way that the total dose from both the anterior and posterior electron beams did not exceed 33% of the prescribed tumor dose. Wax bolus was used to obtain the desired separation for the electron beams. RESULTS: In an electron beam, the dose under the spinal cord shield depends not only on the depth of the spinal cord but also on the width of the shield. For a given shield width, as the cord depth increases the relative dose under the shield first increases, reaches a maximum, and then decreases to approach the open field depth dose data at large depths. The depth at which the relative dose maximum occurs increases slowly with the shielded width. At a given depth, the relative dose decreases with an increasing shield width. CONCLUSION: Parallel-opposed electron beams with a spinal cord shield in the posterior electron beam provide a viable option in treatment of tumors enveloping the spinal cord. A high tumor dose may be prescribed with a concurrent low dose to the spinal cord obtained with an appropriately shaped spinal cord shield.

A technique for fractionated stereotactic radiotherapy in the treatment of intracranial tumors.

PURPOSE: The excellent treatment results obtained with traditional radiosurgery have stimulated attempts to broaden the range of intracranial disorders treated with radiosurgical techniques. For major users of radiosurgery this resulted in a gradual shift from treating vascular diseases in a single session to treating small, well delineated primary tumors on a fractionated basis. In this paper we present the technique currently used in Montreal for the fractionated stereotactic radiotherapy of selected intracranial lesions. METHODS AND MATERIALS: The regimen of six fractions given every other day has been in use for "fractionated stereotactic radiotherapy" in our center for the past 5 years. Our current irradiation technique, however, evolved from our initial method of using the stereotactic frame for target localization and first treatment, and a "halo-ring" with tattoo skin marks for the subsequent treatments. Recently, we developed a more precise irradiation technique, based on an in-house-built stereotactic frame which is left attached to the patient's skull for the duration of the fractionated regimen. Patients are treated with the stereotactic dynamic rotation technique on a 10 MV linear accelerator (linac). RESULTS: In preparation for the first treatment, the stereotactic frame is attached to the patient's skull and the coordinates of the target center are determined. The dose distribution is then calculated, the target coordinates are marked onto a Lucite target localization box, and the patient is placed into the treatment position on the linac with the help of laser positioning devices. The Lucite target localization box is then removed, the target information is tattooed on the patient's skin, and the patient is given the first treatment. The tattoo marks in conjunction with the target information on the Lucite target localization box are used for patient set-up on the linac for the subsequent 5 treatments. The location of the target center is marked with radio-opaque markers on the target localization box and verified with a computerized tomography scanner prior to the second treatment. The same verification is done prior to other treatments when the target center indicated by the target localization box disagrees with that indicated by the tattoo marks. The new position of the target center is then determined and used for treatment positioning. CONCLUSION: The in-house-built frame is inexpensive and easily left attached to the patient's skull for the 12 day duration of the fractionated regimen. Positioning with the Lucite target localization box verified with tattoo marks ensures a high level of precision for individual fractionated treatments.

The use of CT densitometry to predict lung toxicity in bone marrow transplant patients.

Total body irradiation (TBI) is considered an integral part of the preparation of patients with hematological malignancies for marrow transplantation. One of the major causes of death following bone marrow transplantation is interstitial pneumonia. Its pathogenesis is complex but radiation may play a major role in its development. Computed tomography (CT) has been used in animal and human studies as a sensitive non-invasive method for detecting changes in the lung following radiotherapy. In the present study CT scans are studied before and up to 1 year after TBI. Average lung densities measured before TBI showed large variations among the individual patients. On follow-up scans, lung density decreases were measured for patients who did not develop lung complications. Significant lung density increases were measured in patients who subsequently had lung complications. These lung density increases were observed prior to the onset of respiratory complications and could be correlated with the clinical course of the patients, suggesting the possibility for the usage of CT lung densitometry to predict lung complications before the onset of clinical symptoms.

Radiosurgery with high energy photon beams: a comparison among techniques.

The presently known radiosurgical techniques with high energy photon beams are based either on the commercially available Gamma unit utilizing 201 stationary cobalt beams or on isocentric linear accelerators. The techniques using linear accelerators are divided into the single plane rotation, the multiple non-coplanar arcs, and the dynamic rotation. A brief description of these techniques is given, and their physical characteristics, such as precision of dose delivery, dose fall-off outside the target volume, and isodose distributions are discussed. It is shown that the multiple non-coplanar arcs technique and the dynamic rotation give dose distributions similar to those of the Gamma unit, which makes these two linear accelerator based techniques attractive alternatives to radiosurgery with the Gamma unit.

Physical aspects of the angle-beta concept in electron arc therapy.

A technique for the determination of treatment parameters that are required to achieve a desired depth dose distribution in electron arc therapy is discussed and a method for calculating isodose distributions is presented. Both the treatment technique and the dose calculation method rely on the angle beta concept, which uniquely describes the dependence of the radial percentage depth doses in electron arc therapy on the nominal field width, isocenter depth, and virtual source-axis distance. The angle beta concept is discussed in detail and the electron pseudo-arc therapy technique used at McGill is described. Also presented is the method used to achieve dose homogeneity in target volumes treated with the pseudo-arc technique.

Surface applicators for high dose rate brachytherapy in AIDS-related Kaposi's sarcoma.

PURPOSE: The development of commercially available surface applicators using high dose rate remote afterloading devices has enabled radiotherapy centers to treat selected superficial lesions using a remote afterloading brachytherapy unit. The dosimetric parameters of these applicators, the clinical implementation of this technique, and a review of the initial patient treatment regimes are presented. METHODS AND MATERIALS: A set of six fixed-diameter (1, 2, and 3 cm), tungsten/steel surface applicators is available for use with a single stepping-source (Ir-192, 370 GBq) high dose rate afterloader. The source can be positioned either in a parallel or perpendicular orientation to the treatment plane at the center of a conical aperture that sits at an SSD of approximately 15 mm and is used with a 1-mm thick removable plastic cap. The surface dose rates, percent depth dose, and off-axis ratios were measured. A custom-built, ceiling-mounted immobilization device secures the applicator on the surface of the patient's lesion during treatment. RESULTS: Between November 1994, and September 1996, 16 AIDS-related Kaposi's sarcoma patients having a total of 120 lesions have been treated with palliative intent. Treatment sites were distributed between the head and neck, extremity, and torso. Doses ranged from 8 to 20 Gy, with a median dose of 10 Gy delivered in a single fraction. Treatments were well tolerated with minimal skin reaction, except for patients with lesions treated to 20 Gy who developed moderate/severe desquamation. CONCLUSION: Radiotherapy centers equipped with a high dose rate remote afterloading unit may treat small selected surface lesions with commercially available surface applicators. These surface applicators must be used with a protective cap to eliminate electron contamination. The optimal surface dose appears to be either 10 or 15 Gy depending upon the height of the lesion.

Dosimetry of hip irradiation for the prevention of heterotopic bone formation after arthroplasty.

PURPOSE: The dosimetry of hip irradiation for the prevention of heterotopic bone formation following arthroplasty is complicated by the use of custom shielding in the treatment portal, and the fact that irradiation is usually required during a 48 hour period following surgery. Both the machine output and depth dose factors of the resulting fields are modified by the presence of the shielding blocks. A simplified dosimetric approach, based on correction factors for both the output and depth dose as a function of field geometry is presented for various megavoltage energy beams. MATERIALS AND METHODS: Measurements of relative dose factors (RDF) and percentage depth dose (PDD) were carried out for different combinations of field size, block size and separation between adjacent blocks. Both RDF and PDD measurements were made in a water phantom. Ratios of RDF and PDD were obtained by dividing individual measurements or curves by the corresponding values for the open field (i.e., without blocks). The average values of these ratios constitute the correction factors to be applied for a given MU or treatment time calculation. RESULTS: Extensive RDF and PDD measurements reveal that for the field and block dimensions of interest the correction factors for RDF can be parameterized as a function of separation between two adjacent blocks and beam energy alone and the depth correction factors are additionally only a function of depth. The correction factors for depth dose are equally valid for fixed source-skin distance techniques (that use PDD) and fixed source-axis distance techniques (that use TMR). CONCLUSION: A simple model for the calculation of output in hip irradiation is presented for the situation where the use of computer-based algorithms may not be practical. The model accurately predicts the RDF of the treatment portal to within 2% and the PDD to within 2% for the range of field sizes, block sizes, block gaps and beam energies of interest ignoring other variables.

Treatment results of high dose rate brachytherapy in patients with carcinoma of the cervix.

PURPOSE: The combination of external beam irradiation and low-dose-rate brachytherapy is known to be an effective form of treatment in carcinoma of the cervix and any change from this well-established therapeutic combination must be able to equal or improve the treatment results. Since 1984 we have been using high dose rate brachytherapy in conjunction with external beam irradiation for patients with carcinoma of the cervix. This paper reports our long term treatment results in terms of local disease control, survival, and complications. METHODS AND MATERIALS: Between January 1984 and December 1989, 187 previously untreated patients with carcinoma of the cervix underwent combined external beam irradiation and high dose rate brachytherapy. The International Federation of Gynecology and Obstetrics stage distribution of patients was as follows: I B = 15, II A = 35, II B = 68, III A = 9, III B = 54, IV A = 6. External beam irradiation to the whole pelvis was delivered by megavoltage irradiation with once-a-day fractionation, to a median dose of 4600 cGy. High dose rate brachytherapy was delivered by a high-dose-rate remote controlled afterloading unit, containing 20 spherical Cobalt 60 sources with a nominal activity of 19 GBq (0.5 Ci) at the time of installation, giving a typical dose rate to point A of 160 cGy/min, decreasing to about 80 cGy/min at the end of the 5-year study. One to 3 high dose rate brachytherapy treatments delivering 800 to 1000 cGy to point A were given weekly concurrently with the last 2 to 3 weeks of radiation therapy, or following its completion. Maximum rectal and bladder doses were routinely measured for each treatment. RESULTS: Overall 5-year actuarial survivals were as follows: I B = 72%, II A = 65%, II B = 66%, III A = 66%, III B = 45%. Five-year actuarial pelvic control rates were as follows: I B = 66%, II A = 83%, II B = 78%, III A = 88%, III B = 40%. At a median follow-up time of 54 months for patients at risk, 23 patients developed 25 complications attributable to radiotherapy (13 rectal, 3 bladder, 8 small bowel, 1 fistula) at a median time of 18 months following completion of treatment. Thirteen complications (7.6%) were grades 3 or 4. Patients with Stage II disease had a higher incidence of complications than patients with Stages I and III disease (p < 0.05). Rectal complications were significantly higher in patients who received a total rectal dose > 5400 cGy (p = 0.045). CONCLUSION: High-dose-rate brachytherapy treatment results are comparable to those obtained with low dose rate brachytherapy techniques. The use of three high dose rate brachytherapy insertions is a practical, economical, and safe treatment for patients with carcinoma of the cervix.

High dose rate afterloading intracavitary therapy in carcinoma of the cervix.

From January 1984 through December 1986, 87 patients with previously untreated carcinoma of the cervix received external beam pelvic irradiation and high dose rate intracavitary therapy (HDRT). There were 18 Stage IIA patients, 39 Stage IIB, and 30 Stage IIIB. The median age was 60 years and the median follow-up time was 42 months for patients at risk. Radiotherapy consisted of external megavoltage irradiation to the whole pelvis (median dose 4600 cGy) combined with one (6 patients), two (51 patients), or three (30 patients) HDRT insertions. A high dose rate remote afterloading unit with 60Co sources was used to deliver the HDRT. The prescribed dose to point A was between 800 and 1000 cGy per treatment. The dose rate at point A initially was approximately 150 cGy/min and dropped to approximately 100 cGy/min during the duration of the study. Treatments with multiple fractions were given at weekly intervals. The overall actuarial survival at 5 years was 88% for Stage IIA, 64% for Stage IIB and 32% for Stage IIIB patients. Pelvic recurrence remained the major cause of failure. Grade III and IV late complications included proctitis and bowel obstruction in six patients each. We conclude that HDRT results are similar to those obtained with conventional low dose rate intracavitary systems. HDRT is cost effective and minimizes exposure to personnel. Several questions, such as the total number of insertions required, dose per HDRT insertion, and optimal HDRT insertion schedule remain unanswered and further experience is needed to better clarify these issues.