Blocked field effects on collimator scatter factors

In routine dosimetry we assume separability of the collimator (Sc) and phantom (Sp) scatter components that together comprise the total scatter factor (Sc,p). In practice, the addition of blocking also affects the photon fluence attributable to the treatment head and flattening filter in a complicated way. The reduced aperture blocks out some of the head scatter contribution, while the block and tray add back secondary scatter. In the following we present techniques for directly measuring the aperture effect on Sc in air or in a full-scatter phantom. The change in Sc is found to be a scaleable quantity that can be modelled as a simple linear fit to the ratio of projected open-to-blocked equivalent square fields. Measurements have been made for 6, 18 and 24 MV photon beams on one Varian 2500 and two Varian 2100c accelerators. Results indicate a progressive loss of collimator scatter contribution with increased field blocking that is amplified with increasing energy. Block and tray scatter only contribute significantly to Sc for large fields and treatment distances of 80 cm or less. Application of these corrections in monitor unit calculations is presented.

Cost-benefit analysis of 3D conformal radiation therapy

BACKGROUND

The potential of 3D conformal radiation therapy to improve the efficacy of radiotherapy is one of the most important technical advances in recent years. The potential benefit of improved local control from dose escalation must be weighed against the potential cost of increased complications to normal tissue.

METHOD AND RESULTS

We evaluated the cost-benefit of 3D conformal radiation therapy in terms of the benefit to tumor cure weighed against the cost of complications to normal tissue. Assessment of current data shows that problems remain in adequately defining variables that contribute to both the tumor cure probability (benefit) and normal tissue complication probability (cost). For tumor cure probability, identifying the dose escalation needed for cure for the most tumor sites as well as precisely defining tumor volume remain problematic. For normal tissue complication probability, inadequate clinical data on toxicity to normal tissue for different tumor sites continue to make it difficult to use methods that estimate complications, such as dose-volume histograms as proposed by Lyman, to the clinical setting.

CONCLUSION

3D conformal radiation therapy is a promising new technology that may substantially improve the efficacy of radiation therapy. More clinical research, however, is needed to recognize the costs and benefits of this new technology: 1. Tumor control probability: More information is needed on the accurate definition of the target volume as well as on the amount of radiation necessary to cure different tumor sites. 2. Normal tissue complication probability: Methods for determining toxicity to normal tissue are needed. Current calculations based on various models are insufficient.

Plane-parallel ionization chamber response in the buildup region of obliquely incident photon beams

Fixed-separation plane-parallel ionization chambers have been shown to overestimate the dose in the buildup region of normally incident high-energy photon beams. This work shows that these ionization chambers exhibit an even greater over-response in the buildup region of obliquely incident photon beams. This over-response at oblique incidence is greatest at the surface of the phantom and increases with increasing angle of beam incidence. In addition, the magnitude of the over-response depends on field size, beam energy, and chamber construction. This study shows that plane-parallel ionization chambers can over-respond by more than a factor of 2.3 at the phantom surface for obliquely incident high-energy photon fields.

The use of tertiary collimation for spinal irradiation with extended SSD electron fields

PURPOSE

The spine can be treated with an electron beam when its maximum posterior depth is within the therapeutic range of electrons. Electron fields treated at extended source-to-surface distances (SSDs), however, have larger penumbras and narrower therapeutic isodose widths relative to those at the standard SSD of 100 cm. We investigated the use of tertiary collimation close to the patient surface for these fields to sharpen the penumbra, minimizing dose to normal tissue and maximizing target coverage.

METHODS AND MATERIALS

Using film dosimetry in a polystyrene phantom, we measured the dose distribution for electron fields at extended SSD under varying collimation conditions. Beam penumbra and therapeutic width as a function of depth, SSD, applicator insert size, and tertiary collimator opening were determined. We also measured the dose distributions in the junction region for various gaps between x-ray fields and an electron field as used for craniospinal irradiation.

RESULTS

Measurements show that tertiary collimation close to the skin surface reduces penumbra width (lateral distance between the 90 and 20% isodose lines) by 56% and increases therapeutic isodose width (lateral width of the 90% isodose curve) by 25% at a depth of dmax relative to standard collimation. These numbers change to 23 and 13%, respectively, at an average depth of the spine. When lateral brain and posterior spine fields are used to irradiate the entire craniospinal axis, tertiary collimation aids in reducing the volume of the hot spot in the junction region by as much as 10% without compromising target coverage.

CONCLUSIONS

Tertiary collimation for extended SSD electron fields is preferable to standard collimation in order to minimize dose to normal tissue and increase target coverage. This technique can be applied to both spinal and craniospinal irradiation. Support structures for the tertiary blocking are needed because the weight of the lead is usually too great for placement on the skin.

Improved dose distribution with a universal acrylic breast compensator

Wedge filters frequently do not provide optimal distribution in the chest wall, lung, and areolar-nipple complex because they extend through the entire width of tangential radiation fields for breast cancer. In addition, medial wedge filters increase dose to the contralateral breast. In order to overcome these shortcomings, a new method of acrylic compensation was investigated. A universal acrylic breast compensator (ABC) was designed, with patient data, to extend anteriorly from the central axis. To accurately display build-up data, the ABC was entered into the treatment planning system as a wedge filter. Dose homogeneity within the compensated breast, chest wall, and lung volumes was assessed using 3-D planning and dose volume histograms. Dose to the contralateral breast was evaluated using thermoluminescent dosimetry in a polystyrene phantom. Opened, wedged, and compensated conditions were irradiated using the above described techniques. Due to lack of extension of the ABC posteriorly to the central axis (allowing lung inhomogeneity to correct for increased patient thickness), ABCs reduced the high dose regions in the chest wall and lung as compared to wedge filters. ABCs reduced hot spot in the areolar-nipple complex more efficiently as compared to wedge filters. ABCs also reduced dose to the contralateral breast by more than 20% relative to wedge filters. Wedge filters can be inefficient because of lung transmission and contour topography. Excessive dose to the chest wall, lung, and areolar-nipple complex, and contralateral breast can be alleviated with the use of a universal acrylic breast compensator. Its convenience and dosimetric advantages both in treatment planning and daily application offer a useful tool in the management of breast cancer.

Collimator (head) scatter at extended distances in linear accelerator-generated photon beams

PURPOSE

A calculation formalism is proposed to predict variation of head scatter as a function of field size and treatment distance.

METHODS AND MATERIALS

Assuming that the head scatter for the linear accelerator studied was contributed predominantly by the flattening filter, a formalism was devised to predict beam intensity as a function of distance from the target position. The method used the concept of an equivalent collimator field in which a given field at any distance can be equated to a field at the isocenter such that the extent of the flattening filter seen at the two positions is the same.

RESULTS

The equation derived from the concept of equivalent collimator field size predicated change in head scatter with distance to within 0.5% for collimator field sizes ranging from 8 x 8 to 40 x 40 cm and distances up to 300 cm from the target.

CONCLUSIONS

Considering flattening filter to be the main source of head scatter, the observed deviation from inverse square law for extended treatment distances can be accounted for by an equivalent collimator field size, which sees the same extent of the flattening filter at the isocenter as the field at the given distance.

Calculation of dose in asymmetric photon fields

A method is introduced to calculate monitor units to points off axis. Extensive data are presented comparing this method with measured values of dose per monitor unit on the central ray of asymmetric fields produced by a variety of linear accelerators. The technique demonstrates improvement over existing methods that use large-field profile data. The method is found to be both simple and accurate: Agreement within +/- 2% is obtained using parameters readily available within the clinic.

Molecular characterisation of red cell glucose-6-phosphate dehydrogenase deficiency in north-west Pakistan

233 Pushtoons (129 males and 104 females), 51 Punjabi Muslims (29 males and 22 females) and 21 Afghans (15 males and 6 females) were screened for the presence of red cell glucose-6-phosphate dehydrogenase (G6PD) variants by a dye decolouration screening test and starch gel electrophoresis. The overall frequency of G6PD deficiency in males was found to be about 10%. 17 male G6PD-deficient samples were further investigated for the C-->T substitution at nucleotide (nt) 563 (the Mediterranean mutation) and the C-->T substitution at nt 1311 (the 'silent' allele) of the G6PD gene by PCR amplification followed by digestion with appropriate restriction enzymes. 10 of the 13 Pushtoon, 2 Punjabi and 1 Afghan males had the 563 mutations. Only 1 (Punjabi) out of 13 G6PD-deficient males with the 563 mutation had the silent mutation at nt 1311. The frequency of the silent mutation was found to be about 0.20 in the 60 Pushtoon and 19 Punjabi non-deficient males.

Dosimetric effects of abutting extended source to surface distance electron fields with photon fields in the treatment of head and neck cancers

PURPOSE

In the management of head and neck cancer, it is often necessary to junction photon and electron fields. When patients are treated supine, electron cones or applicators may have to be positioned at source to surface distance (SSD) greater than the standard 100 cm because of the patient's shoulders. We have studied the dosimetric effects of abutting 6 MV photon fields with 9 MeV electron fields at extended SSDs to assess changes in the 90% isodose width, dose uniformity in the target, and the extent of hot and cold spots in the junction region.

METHODS AND MATERIALS

Four independent film studies were conducted, in a polystyrene phantom, for evaluation of the dose distribution in the junction region. Measured distributions were also compared with computer generated distributions using a treatment planning computer system.

RESULTS

At the junction line between photon and extended SSD electron beams, hot and cold spots were observed. A 20% hot spot occurred on the photon side because of the electron scatter from the adjoining field. The width of this hot spot increased in dimension but not in magnitude as electron SSDs were increased. The cold spot occurred on the electron side due to the decrease in 90% isodose width at extended electron SSDs. This cold spot was minimal (less than 10%) at shorter electron SSDs, but increased as electron SSDs approached 120 cm. Computer dosimetry underestimated these results because of limitations of the electron beam treatment planning algorithm.

CONCLUSION

These hot and cold spots may be clinically acceptable compared to the loss of treatment accuracy and the added possibility of more extensive hot or cold spots if the patient were to be removed from the original supine position and repositioned on his/her side.

Comparative studies on the composition of two indigenously produced varieties of jaman (eugenia jambolana) fruits

Eugenia Jambolana (Syzigium Cumini) locally known as Jaman, is being used in the indigenous Medicinal system for treatments of various ailments including some related to nutrition. No study appears to have been made previously regarding the composition of these native varieties produced in Northern Pakistan. Two varieties of fruits have been analyzed for physico chemical characteristics and their values compared. The findings indicate that both varieties contain adequate amount of nutrients and minerals; in general, variety I being richer in the former and II in the latter.

Dosimetry of wedged fields with asymmetric collimation

Shaping of wedged fields with asymmetric collimators (independent jaws) produces dosimetric effects which need to be taken into account when calculating isodose distributions and accelerator monitor units. These effects were studied using ion chamber dosimetry in a water phantom. Wedged fields of 30 degrees to 60 degrees wedge angles were shaped by an asymmetric collimator. A system of dose calculation was developed to take into account the changes in dose in the open portion of the field. The formalism utilizes regular symmetric field parameters (e.g., Sc, Sp, and TPR). Calculated point doses and profiles agreed well with the measured data.

Report of the Cancer Therapy Evaluation Program monitoring plan for secondary acute myeloid leukemia following treatment with epipodophyllotoxins

BACKGROUND

Recent reports have documented the occurrence of treatment-related acute myeloid leukemia (AML) following therapy with epipodophyllotoxins. These reports have led to growing concern among oncologists, which could lead to premature abandonment of these agents at a time when the relationship between cumulative dose of epipodophyllotoxin and risk of treatment-related AML has not been determined.

PURPOSE

Because of the increasingly important role of epipodophyllotoxins in the treatment of several types of adult and pediatric tumors, the Cancer Therapy Evaluation Program (CTEP) of the National Cancer Institute (NCI) has developed a monitoring plan to obtain reliable estimates of the risk of treatment-related AML following epipodophyllotoxin treatment.

METHODS

We identified 12 NCI-supported Cooperative Group clinical trials in which patients with solid tumors are being treated with epipodophyllotoxins at different cumulative doses. One trial is using a moderate dose of teniposide (900 mg/m2), and 11 trials are using etoposide at a low dose (< 1500 mg/m2), a moderate dose (1500-3999 mg/m2), or a high dose (> or = 4000 mg/m2). Cases of treatment-related AML and treatment-related myelodysplastic syndrome (MDS) (hereafter referred to as treatment-related AML/MDS) occurring in these trials are reported to CTEP, with initial analysis for each cumulative dose group triggered by the reporting of four cases of treatment-related AML/MDS in that group. For each analysis, total patient follow-up for the group is determined and cumulative 6-year incidence rate is calculated.

RESULTS

Three cases of treatment-related AML and one case of treatment-related MDS (with documented monosomy 7) were reported in a group of 207 patients who received etoposide at a low cumulative dose. The calculated 6-year rate of development of treatment-related AML/MDS was 3.2% (95% upper confidence interval bounded by 7.2%).

CONCLUSIONS

The 6-year cumulative rate of treatment-related AML/MDS (3.2%) is within the range previously reported for alkylator-based regimens that did not include epipodophyllotoxins.

IMPLICATIONS

Previous reports have suggested that higher cumulative doses of alkylators are associated with increased risk of treatment-related AML, and a critical goal of the monitoring plan is to determine whether a similar relationship exists for the epipodophyllotoxins. Estimates will be developed for leukemogenic risk for the moderate- and high-cumulative-dose groups when four cases of treatment-related AML/MDS have been identified within each group.

Double-blind comparison of etodolac SR and diclofenac SR in the treatment of patients with degenerative joint disease of the knee

An on-going multi-centre, double-blind, parallel-group study is being carried out to compare the efficacy and tolerability of sustained-release (SR) formulations of etodolac and diclofenac in patients with degenerative joint disease (osteoarthritis) of the knee. An interim analysis of the findings has been made for 64 patients from two centres which have now completed their part in the study. Thirty-two patients were randomly assigned to receive 600 mg etodolac SR once daily for 4 weeks; the remaining 32 patients received 100 mg diclofenac SR. Primary efficacy assessments rated on a 5-point categorical scale were patient and physician overall assessments of the patient's condition, night pain and pain intensity. Secondary efficacy parameters included weight-bearing pain, stiffness duration, joint tenderness on pressure, degree of swelling and erythema, degree of knee flexion and time to walk 15 metres. The results showed that for both etodolac SR and diclofenac SR treatment groups there was an improvement from baseline in all efficacy parameters at the last visit and no statistically significant difference was observed between treatments. However, although not statistically significant, the improvement rate in the patient's condition at Week 2 was slightly greater in the etodolac SR treatment group, suggesting that improvement may occur more rapidly with etodolac SR than with diclofenac SR. With regard to tolerability, 5 patients in the etodolac SR treatment group and 3 in the diclofenac SR group withdrew from the study because of adverse reactions. Two events (dyspepsia and mouth ulceration) in the etodolac SR group and 4 events (headache, glossitis, depression and insomnia) in the diclofenac SR group were considered to be definitely drug-related. Dyspepsia was reported by 3 patients (1 withdrawal) treated with etodolac SR and by 4 patients (2 withdrawals) treated with diclofenac SR. A statistically significant decrease was observed in haemoglobin and haematocrit values after 4 weeks of treatment in the diclofenac SR group, but this was not considered to be clinically important. In addition, there were no clinically significant changes in blood chemistry and urinalysis for either treatments. In conclusion, the results of the present study indicate that 600 mg etodolac SR once daily for 4 weeks is effective in the treatment of patients with degenerative joint disease of the knee, as is 100 mg diclofenac SR. In addition, both drugs have comparable tolerability profiles.

Beta irradiation of recurrent ptergia: results and complications

Although postoperative beta irradiation for prevention of pterygium recurrence is widely used, its complication rate has not been widely appreciated. Thirty-six patients underwent pterygium excision followed by irradiation with 90Sr to between 1600 and 5300 cGy (median 2400) in two to four fractions over 7 to 27 days. A median of three fields were used in each treatment course (range two to 10). Most patients had undergone previous treatment for their pterygia, including surgery alone (25 patients), or both surgery and irradiation (5 patients). Overall, recurrences developed in 10 eyes (28%). Recurrence occurred in one eye after re-irradiation (20%), compared to 9 of 31 (29%) previously non-irradiated eyes (p greater than 0.1). There was no association with number of excisions and recurrences, although all but 6 patients had undergone more than one excision. Thirteen patients (36%) developed complications including: epithelial defect or corneal thinning (3), symblepharon (5), cataract (4) and corneal ulceration (1). Complications developed in 4 of the 5 (80%) re-irradiated eyes compared to 9 of 31 (29%) previously non-irradiated eyes (p less than 0.05). The power of the statistical analysis was limited by sample size, but no significant association was observed between the development of complications or recurrences and total dose, number of abutting fields, number of previous surgical excisions or patient age, even when re-irradiated patients were excluded. Recently described calibration uncertainties with the 90Sr applicator may explain in part these complications. Alternatively, technical factors such as the number of fields or volume treated may play a role. Excessive complications and recurrences with the use of postoperative beta irradiation in this series emphasize the danger of re-treatment and the need for alternative safe and effective therapies.

Replacement correction (P(repl)) for ion chamber dosimetry

The use of replacement correction when depth dose distribution is measured with a cylindrical ion chamber is discussed. A review of relevant data shows that the displacement of the effective point of measurement is an appropriate method to make replacement correction for the entire depth ionization curve.

Border separation for adjacent orthogonal fields

Field border separations for adjacent orthogonal fields can be calculated geometrically, given the validity of some important assumptions such as beam alignment and field uniformity. Thermoluminescent dosimetry (TLD) measurements were used to investigate dose uniformity across field junctions as a function of field separation and, in particular, to review the CCSG recommendation for the treatment of medulloblastoma with separate head and spine fields.

Tracheal soiling with blood during intranasal surgery--comparison of two endotracheal tubes

Sixty adult patients, ASA Classes I & II, were involved in a study to compare the effectiveness of Mallinckrodt Hi-Lo-Evac tube and Portex blue line tube in preventing soiling of the lower airways during intranasal surgery. The Hi-Lo-Evac tube with and without pack was significantly more effective than the Portex tube with pharyngeal pack (P less than 0.002) and (P less than 0.01 respectively). There was no significant difference when the Hi-Lo-Evac tube was used with or without a pack (P greater than 0.2). The more effective protection of the lower airways by the Hi-Lo-Evac tube is attributed to the facility of subglottic aspiration during surgery. It is suggested that the Hi-Lo-Evac tube could be used with safety during intranasal surgery in order to reduce postoperative morbidity associated with the use of pharyngeal pack.

Are heat stroke patients fluid depleted? Importance of monitoring central venous pressure as a simple guideline for fluid therapy

During pilgrimage season (Hajj) in Saudi Arabia 34 patients with heat stroke (HS) were centrally cannulated to assess their state of hydration and fluid requirement during cooling period. Central venous pressure (C.V.P.) measurements indicated that most victims of heat stroke had normal C.V.P. on arrival at heat stroke centres and may not be fluid depleted. Twenty-two patients (64.7%) had normal or above normal C.V.P. Twelve patients (35.3%) had zero or below zero C.V.P. Six patients (17.6%) had above 10 cmH2O (range 10-26 cmH2O) and could have developed acute congestive heat failure and pulmonary edema if they had been transfused at the standard recommended rate of 3-4 litres of fluid during an average cooling time of 1 h as has been practiced in the heat stroke centres to date. This study also showed that heat stroke patients should not be briskly transfused because the heart may be affected by heat stroke per se and an unmonitored challenge by brisk i.v. therapy during cooling (which on its own increases preload on the heart due to peripheral vasoconstriction) can lead to acute overload problems. An average of 1 litre of normal saline or Ringer's lactate (crystalloids) was sufficient to normalize C.V.P. during the cooling period and to restore an optimal state of hydration without predisposing to congestive cardiac failure and pulmonary edema--the potential to develop disastrous adult respiratory distress syndrome and disseminated intravascular coagulopathy.

Determination of surface dose rate from a 90Sr ophthalmic applicator

Surface dose rates from two 90Sr ophthalmic applicators were measured using thin thermoluminescent dosimeter (TLD) chips. The TLD's were calibrated against the 90Sr source itself. The calibration technique is described. The results were compared to those obtained by three other techniques. The calibration in terms of roentgen equivalent betas per second provided by the manufacturer was found to be approximately 30% lower when compared to our measured dose rates. The surface dose rates obtained with the TLD's calibrated against 6-MV x rays were 18% higher. One of the two sources was sent to both the National Institute of Standards and Technology (NIST) and Amersham International for calibration. NIST and Amersham use extrapolation chambers as a Bragg-Gray cavity to determine the surface dose rate. NIST results gave values higher than ours by approximately 5%. The Amersham results were approximately 35% lower than ours.