Stereotactic body radiotherapy for medically inoperable patients with stage I non-small cell lung cancer - a first report of toxicity related to COPD/CVD in a non-randomized prospective phase II study

BACKGROUND AND AIMS

In a retrospective study using stereotactic body radiotherapy (SBRT) in medically inoperable patients with stage I NSCLC we previously reported a local control rate of 88% utilizing a median dose of 15Gyx3. This report records the toxicity encountered in a prospective phase II trial, and its relation to coexisting chronic obstructive pulmonary disease (COPD) and cardio vascular disease (CVD).

MATERIAL AND METHODS

Sixty patients were entered in the study between August 2003 and September 2005. Fifty-seven patients (T1 65%, T2 35%) with a median age of 75 years (59-87 years) were evaluable. The baseline mean FEV1% was 64% and median Karnofsky index was 80. A total dose of 45Gy was delivered in three fractions at the 67% isodose of the PTV. Clinical, pulmonary and radiological evaluations were made at 6 weeks, 3, 6, 9, 12, 18, and 36 months post-SBRT. Toxicity was graded according to CTC v2.0 and performance status was graded according to the Karnofsky scale.

RESULTS

At a median follow-up of 23 months, 2 patients had relapsed locally. No grade 4 or 5 toxicity was reported. Grade 3 toxicity was seen in 12 patients (21%). There was no significant decline of FEV1% during follow-up. Low grade pneumonitis developed to the same extent in the CVD 3/17 (18%) and COPD 7/40 (18%) groups. The incidence of fibrosis was 9/17 (53%) and pleural effusions was 8/17 (47%) in the CVD group compared with 13/40 (33%) and 5/40 (13%) in the COPD group.

CONCLUSION

SBRT for stage I NSCLC patients who are medically inoperable because of COPD and CVD results in a favourable local control rate with a low incidence of grade 3 and no grade 4 or 5 toxicity.

Low-dose hypersensitive gammaH2AX response and infrequent apoptosis in epidermis from radiotherapy patients

BACKGROUND AND PURPOSE

A low-dose hypersensitivity to radiation can be observed in vitro for many human cell types in terms of increased cell kill per dose unit for doses below 0.5Gy. Quantification of the double-strand break marker gammaH2AX in samples taken in clinical radiotherapy practice has the potential to provide important information about how induction and repair of severe DNA damage and apoptosis are linked to low-dose hypersensitivity.

MATERIAL AND METHODS

The effects of exposure to low doses (0.05-1.1Gy) were investigated in skin biopsies taken from prostate cancer patients undergoing the first week of radiotherapy. gammaH2AX foci and apoptotic cells were visualised by immunohistochemistry and quantified by image analysis.

RESULTS

The gammaH2AX foci pattern in biopsies taken 30min after a single fraction revealed a low-dose hypersensitivity below 0.3Gy (p=0.0009). The result was consistent for repeated fractions (p=0.00001). No decrease in foci numbers could be detected when comparing biopsies taken 30min and 2h after single fractions of 0.4 and 1.2Gy. The result was consistent for repeated fractions. Only 43 of 168,000 cells in total were identified as apoptotic, yet a dose dependency could be detected after 1week of radiotherapy (p=0.003).

CONCLUSIONS

We describe a method based on gammaH2AX to study DNA damage response and apoptosis in a clinical setting. A gammaH2AX hypersensitive response to low doses can be observed in epidermal skin, already 30min following delivered fraction. A very low frequency of apoptosis in normal epithelium suggests that this effect is not an important part of the in vivo response to low doses.

National HIV treatment guidelines in Tanzania and Ethiopia: are they legitimate rationing tools?

OBJECTIVE

To provide an ethical analysis of whether the Ethiopian and Tanzanian national HIV/AIDS treatment guidelines can be considered legitimate and fair rationing tools.

METHOD

Qualitative study and ethical analysis involving guideline documents and interviews with nine key members involved in the development of the guidelines. The analysis followed an editing organising style. The theoretical framework was a guideline-specific framework based on theories of just resource allocation in healthcare and conditions that ensure fair processes in guideline development. According to this framework, legitimate rationing requires reasons for patient selection to be explicit, public and relevant, and decisions must be open to question and revision.

RESULTS

The only explicit rationing criteria that both guidelines recommended were clinical antiretroviral treatment indications. Explicit non-clinical rationing criteria were expressed in a separate Ethiopian implementation guideline. Neither of the guideline development processes fully satisfies minimal requirements of procedural fairness. There is a lack of transparency. The reasons for decisions are rarely given and are not publicly available. This reduces the opportunity for public questioning, debate and revisions. The guidelines were based on expert opinion and consensus. Recommendations from the WHO were copied without much discussion, disagreement or adjustment.

CONCLUSIONS

The two national HIV treatment guidelines discussed are de facto mechanisms for rationing but were developed using methods that do not fully satisfy the requirements of fair processes.

Factors important for efficacy of stereotactic body radiotherapy of medically inoperable stage I lung cancer. A retrospective analysis of patients treated in the Nordic countries

We reviewed results of SBRT treatment of 138 patients with medically inoperable stage I NSCLC treated during 1996-2003 at five different centres in Sweden and Denmark. Mean age was 74 years (range 56-90) with 69 men and 72 women. SBRT was delivered using a 3D conformal multifield technique and a stereotactic body frame. Doses delivered were 30-48 Gy (65% isodose at the periphery of planning target volume, PTV) in 2-4 fractions. Equivalent dose in 2 Gy fractions (EQD2) was in the range of 50-100 Gy. Mean gross tumour volume (GTV) was 39 cm3 (2-436), and planning target volume was 101 cm3 (11-719). Overall response rate (CR, PR) was 61% (84/138). SD was noted in 36% (50/138). During a median follow-up period of 33 months (1-107), 16 (12%) local failures occurred, ten of which also included distant metastases. Local failure was associated with tumour size, target definition and central or pleura proximity. Distant metastases occurred in 25% (35/138) of the patients. Ninety-one (65%) patients died during follow-up of which 55 patients (60%) died of other causes than lung cancer. Three- and 5-year overall survival was 52 and 26% respectively. Lung cancer specific 3- and 5-year overall survival was 66 and 40% respectively. Fifty nine percent (83/138) of the patients had no side effects. Fourteen patients experienced grade 3-4 toxicity according to radiation therapy oncology group (RTOG). EQD2 (> v.s.<55.6 Gy) showed a statistically significant benefit survival for the higher doses. SBRT for stage I NSCLC results in favourable local control not inferior to fractionated RT and with acceptable toxicity.

Does electron and proton therapy reduce the risk of radiation induced cancer after spinal irradiation for childhood medulloblastoma? A comparative treatment planning study

The aim of this treatment planning comparison study was to explore different spinal irradiation techniques with respect to the risk of late side-effects, particularly radiation-induced cancer. The radiotherapy techniques compared were conventional photon therapy, intensity modulated x-ray therapy (IMXT), conventional electron therapy, intensity/energy modulated electron therapy (IMET) and proton therapy (IMPT).CT images for radiotherapy use from five children, median age 8 and diagnosed with medulloblastoma, were selected for this study. Target volumes and organs at risk were defined in 3-D. Treatment plans using conventional photon therapy, IMXT, conventional electron therapy, IMET and IMPT were set up. The probability of normal tissue complication (NTCP) and the risk of cancer induction were calculated using models with parameters-sets taken from published data for the general population; dose data were taken from dose volume histograms (DVH). Similar dose distributions in the targets were achieved with all techniques but the absorbed doses in the organs-at-risk varied significantly between the different techniques. The NTCP models based on available data predicted very low probabilities for side-effects in all cases. However, the effective mean doses outside the target volumes, and thus the predicted risk of cancer induction, varied significantly between the techniques. The highest lifetime risk of secondary cancers was estimated for IMXT (30%). The lowest risk was found with IMPT (4%). The risks associated with conventional photon therapy, electron therapy and IMET were 20%, 21% and 15%, respectively. This model study shows that spinal irradiation of young children with photon and electron techniques results in a substantial risk of radiation-induced secondary cancers. Multiple beam IMXT seems to be associated with a particularly high risk of secondary cancer induction. To minimise this risk, IMPT should be the treatment of choice. If proton therapy is not available, advanced electron therapy may provide a better alternative.

Stereotactic hypofractionated radiotherapy for stage I non-small cell lung cancer—Mature results for medically inoperable patients

Medically inoperable patients with stage I NSCLC are mainly offered conventionally fractionated radiotherapy with a limited chance of local control and some toxicity. A technique for stereotactic precision therapy for extracranial tumors using a linear accelerator and a body frame for patient immobilization was applied in an attempt to improve the local control and decrease toxicity for consecutive patients with inoperable stage I NSCLC at Sahlgrenska University hospital since 1998. A hypofractionated schedule with three fractions of 15Gy to a total of 45 Gy during 1 week was used which represents a biological equivalent dose (BED) of 112.5 Gy. Planning target volume (PTV) was a 5mm margin around the tumor in the transversal plane and 10mm in the cranial-caudal direction and the dose was prescribed in the periphery of the PTV. Forty-five patients were treated between September 98 and March 03, 25 men and 20 women, median age 74 years (58-84) and median Karnofsky 80 (100-60). TNM: 18 T1N0, 27 T2N0.

HISTOLOGY

18 squamous cell carcinoma, 15 adenocarcinoma, 3 NSCLC and histology was missing in nine patients. The majority, 51%, did not experience any toxicity at all, four had esophagitis grade I, nine had skin reactions, four had transient chest pain and four had infections. Late toxicity was two rib fractures and three patients with atelectasias. After a median follow-up of 43 months had nine patients developed local recurrence or never achieved local control, two had regional recurrence and nine distant metastases. The 1-, 2-, 3- and 5-year overall survival was 80, 71, 55 and 30%, respectively, with a median survival of 39 months. No prognostic factor for survival could be identified among histology, tumor stage and size, gender and age. We think this hypofractionated stereotactic radiotherapy shows encouraging survival and a relatively low toxicity in this elderly population with substantial comorbidity. A multicenter randomized trial comparing this treatment with conventional fractionated radiotherapy is under way.

DNA double strand break quantification in skin biopsies

BACKGROUND AND PURPOSE

Following induction of double strand breaks the histone H2AX is rapidly phosphorylated at regions flanking the breaks resulting in nuclear gamma H2AX foci. The purpose of this study was to use this endogenous signalling system to quantify the in vivo response to radiation in normal tissue.

PATIENTS AND METHODS

Skin biopsies were taken from prostate cancer patients undergoing radiotherapy with a curative intent. Biopsies were taken at locations corresponding to 5 different doses in the range below 1.1 Gy per fraction. Biopsies were taken from patients 30 min following the first fraction and then once again following the fraction given after the first weekend break in the treatment course. The DNA double strand breaks were visualised as gamma H2AX foci using immunohistochemistry. Images were acquired using a CCD-camera and a fluorescence microscope and the gamma H2AX foci were quantified using digital image analysis including the basic procedures of top-hat transformation, threshold setting and labelling.

RESULTS

Repeated assessments of the biopsies showed a high reproducibility in quantifying the number of foci per DNA area of the nucleated cells in epidermis. The reproducibility was equally good for the two biopsy occasions. A linear dose response was observed for the epidermis in the dose region 0-1 Gy.

CONCLUSIONS

We have established a method to measure the relative amount of DNA double strand breaks by detecting gamma H2AX foci in patients exposed to radiotherapy. The method provides a tool to study induction and repair of DNA double strand breaks and has the potential to predict individual radiosensitivity.

Radiation therapy dose delivery

In an investigation by the Swedish Cancer Society, the present status critical issues and future aspects and potentials in each of nine major areas of radiation therapy research were described by an expert group. The report presented here deals with radiation therapy dose delivery. dose distributions, beam shaping and intensity modulation.

The potential of proton and light ion beams in radiotherapy

In an investigation by the Swedish Cancer Society, the present status, critical issues and future aspects and potentials were described by an expert group for each of nine major areas of radiation therapy research. The present report deals with the potential of proton and light ion beams in radiotherapy.

The Quality Assurance programme of the Radiotherapy Group of the European Organization for Research and Treatment of Cancer (EORTC): a critical appraisal of 20 years of continuous efforts

In 1982, the European Organization for Research and Treatment of Cancer (EORTC) Radiotherapy Group established the Quality Assurance (QA) programme. During the past 20 years, QA procedures have become a major part of the activities of the group. The methodology and steps of the QA programme over the past 20 years are briefly described. Problems and conclusions arising from the results of the long-lasting QA programme in the EORTC radiotherapy group are discussed and emphasised. The EORTC radiotherapy group continues to lead QA in the European radiotherapy community. Future challenges and perspectives are proposed.

Systematic set-up errors for IMRT in the head and neck region: effect on dose distribution

BACKGROUND AND PURPOSE

There is a general concern about intensity modulated radiation therapy (IMRT) treatments being more sensitive to patient positioning than conventional treatments. The aim of this study was to evaluate the International Commission on radiation units and measurements (ICRU) method for taking systematic set-up errors into account for IMRT treatments and to compare the effects on the dose distribution with the effects of conventional treatments.

MATERIAL AND METHODS

A planning margin to account for set-up errors was added to the clinical target volumes and to the spinal cords, for three head and neck patients, according to the ICRU. No margin was added to organs at risk with mainly parallel structure if they were situated adjacent to the target volume, for example, the parotid glands. The effects of set-up errors in six IMRT plans and three conventional plans were simulated in the planning system and analysed with physical dose parameters.

RESULTS AND CONCLUSIONS

In general, the ICRU method of taking set-up errors into account works satisfactorily for IMRT treatments as well as for conventional treatments with no difference between the treatment techniques. The sensitivity to set-up errors regarding the target volume is dependent on the quality of the treatment plan, i.e. the part of the target covered with a dose >95 and <105% and the effect in the critical organs is dependent on the sharpness of the dose gradients outside the critical organ. However, the method makes it difficult to include organs at risk with mainly parallel structure if they are situated adjacent to the target volume.

Intensity modulated radiotherapy treatment planning for dynamic multileaf collimator delivery: influence of different parameters on dose distributions

BACKGROUND AND PURPOSE

This is an investigation of a dose-based conjugated gradient optimization method (implemented in the CadPlan/Helios system) applied for head and neck tumours. Optimized field fluence distributions are created and transformed into dynamic multileaf collimator (MLC) movements. The aim was to gain knowledge about the influence of different parameters on the dose distribution and how to use the optimization algorithm in an optimum way.

MATERIAL AND METHODS

Parameters such as the number of beams, collimator angle and constraints and weight factors have been investigated. Dose escalation to the target volume, the target volume in the build-up region and the way of prescribing the target dose were also investigated. The dose distributions were mainly analysed with physical parameters.

RESULTS AND CONCLUSIONS

The optimization algorithm is well suited to create clinical Intensity modulated radiation therapy (IMRT) treatment plans for head and neck tumours even when the target volume is situated in the build-up region. The number of beams is a critical parameter and has a great influence on the dose distribution. The choice of collimator angles is not a critical parameter. The constraints and weight factors have a great influence on the dose distribution and varying these could easily control priorities regarding dose to the target volume or to the surrounding critical organs. Because of dose variations inside the target volume, prescribing to, normalizing to and reporting the mean dose in the target volume for IMRT treatment plans is preferable to the absorbed dose at a point, for example the isocentre point.

Development and diseases of the pancreas

The pancreas is a vital gland of exocrine and endocrine function. It is the target of two main affections: diabetes and pancreatic cancer. We describe the tissue interactions, signaling pathways and intracellular targets that are involved in the emergence of the pancreas primordium and its proliferation, morphogenesis and differentiation. It appears that several genes of developmental relevance have an adult function and are involved in pancreas affections. Embryological experimentation in animals contributed to provide candidate genes for human disease and holds promise for future treatments.

Normal tissue response to low doses of radiotherapy assessed by molecular markers--a study of skin in patients treated for prostate cancer

The aim of this study was to evaluate normal tissue response by molecular markers to multifraction low doses of ionizing radiation, with the focus on changes in repopulation, estimated using Ki-67 as the proliferation marker, and on expressions of the p53 and p21 proteins, identified as key proteins in the DNA damage checkpoint. Repeated skin biopsies were taken from patients treated for prostate cancer with radiotherapy. The expressions of Ki-67, p53 and p21 of the keratinocytes in the basal cell layer of the epidermis were quantified immunohistochemically. The dose to the basal layer was 1.1 Gy per fraction, given five times per week for seven weeks. The indices of the three markers were determined over the whole period. A significant suppression of the Ki-67 index was observed during the first weeks, followed by a significant gradual increase in the Ki-67 index over the last weeks. The p53 and p21 protein levels were almost zero in the unirradiated skin. Upon irradiation, both the p53 and p21 index increased in a pattern very congruent to the Ki-67 index. In conclusion, daily fractions of about 1 Gy to the skin resulted in, for the keratinocytes in the basal layer, a cell growth arrest for a couple of weeks and a subsequent acceleration in repopulation during the following weeks of irradiation. The present findings also provided novel insights into the role of the p53/p21 pathway in the response of a normal epithelium to ionizing radiation as it is applied in radiotherapy.

Swedish Cancer Society radiation therapy research investigation

In an investigation by the Swedish Cancer Society, the present status, critical issues and future aspects and prospects were described by an expert group for each of nine major areas of radiation research. A summary of the investigation is presented in this report. A more extensive summary (in Swedish) can be found at www.Cancerfonden.se. It is concluded that radiation therapy plays an increasingly important role in curative and palliative tumour treatment and presents a considerable challenge to research. Several suggestions are made that could improve the possibilities for high-quality radiation therapy research in Sweden.

Hyperfractionated-accelerated or conventionally fractionated radiotherapy for early glottic cancer

PURPOSE

To evaluate the effect of shortening overall treatment time by hyperfractionated-accelerated radiotherapy for T2N(0)M(0) glottic carcinomas. Results for local control and survival were calculated and compared to those for T1N(0)M(0) tumors treated with a once-a-day fractionated schedule.

METHODS AND MATERIALS

Between 1990 and 1998, 92 patients with T1N(0)M(0) and 45 patients with T2N(0)M(0) glottic cancers were treated with radical radiotherapy. The T1N(0)M(0) tumors were treated with a once-a-day fractionated schedule lasting 6.5 weeks to a total dose of 62.4 Gy. The T2N(0)M(0) tumors received a split-course hyperfractionated-accelerated treatment over a total of 4.5 weeks to a total dose of 64.6 Gy.

RESULTS

The 5-year local control was 85% for T1N(0)M(0) and 88% for T2N(0)M(0), whereas the 5-year locoregional control was 85% for both groups. The 5-year overall survival was 70% and 53% for T1N(0)M(0) and T2N(0)M(0), respectively. No significant statistical difference was found between the two groups for the parameters analyzed. The number of serious late complications was few and comparable for the two groups.

CONCLUSIONS

Hyperfractionated-accelerated radiotherapy proved beneficial for T2N(0)M(0) glottic cancer, giving local control rates comparable to those for T1N(0)M(0) tumors.

[Destiny-zapping--medical education, students' world view and the ethical challenge]

Three medical students describe their search for professional and personal identity midway through medical school. The article focuses on their concrete experience of human suffering and vulnerability, which is set against elements from the relational ethics of Danish philosopher KE Løgstrup. Løgstrup's ontology is based on a relational understanding of being human, and implicitly opposes the strongly objectivating and individualised view of human existence promoted through the experiences of everyday medical education.

Clinical impact of dosimetry quality assurance programmes assessed by radiobiological modelling of data from the thermoluminescent dosimetry study of the European Organization for Research and Treatment of Cancer

The European Organization for Research and Treatment of Cancer (EORTC) Radiotherapy Group initiated its mailed thermoluminescence dosimetry (TLD) programme in 1986. The aim of the present study was to evaluate the clinical relevance of variations in beam output detected in the period 1993 to 1996. A total of 140 beam outputs were checked (26 for cobalt-60 units and 114 for linear accelerators) in 35 centres. Clinical dose-response data for tumour control and normal tissue morbidity were used to assess the variation in clinical outcome resulting from variability in beam output. For 75 checked beams with nominal accelerating potentials (n. a.p.) of 6 MV or less the mean ratio, +/- standard deviation (S.D.) of measured to stated output was 1.004+/-0.020. For 65 beams with n. a.p. of 8 MV or more, the ratio was 1.009+/-0.021. Even with this relatively high level of precision, broad distributions of estimated tumour control or normal tissue morbidity were found. In the 10% of the beams with the most pronounced underdosage, the loss in tumour control probability was estimated at 7-8 percentage points. Likewise, in the 10% of the beams with the most pronounced overdosage, the increase in mild/moderate morbidity was 19-22 percentage points. For severe morbidity the same beams raised the estimated incidence of severe complications from 5% to 9-10%. An estimation of the loss of uncomplicated cure probability was about 1% for both high and low energy beams. Sequential mailings considerably improved the uniformity of clinical outcome. We conclude that small deviations in beam output may lead to clinically important variations in outcome. Substantial reductions in the variation between measured and stated output can be achieved by sequential mailings. Mailed TLD checks should be an integral part of a continuously ongoing quality assurance activity in radiotherapy.

Soft tissue sarcoma after treatment for breast cancer--a Swedish population-based study

The aim was to quantify the risk of post-treatment sarcoma in breast cancer patients. All 122,991 women with a breast cancer from 1958 to 1992 in the Swedish Cancer Register were followed up for soft tissue sarcomas and 116 were found, giving a standardised incidence ratio of 1.9 (95% CI 1.5-2.2). The absolute risk was 1.3 per 10(4) person-years. The sarcomas were located in the breast region or on the ipsilateral arm in 63% (67/106). There were 40 angiosarcomas and 76 sarcomas of other types. In a case-control study, angiosarcoma correlated significantly with lymphoedema of the arm, odds ratio (OR) 9.5 (95% CI 3.2-28.0), but no correlation with radiotherapy was observed. For other types of sarcoma there was a correlation with the integral dose. The dose-response relationship indicated that the risk increased linearly with the integral dose to 150-200 J and stabilised at higher energies. The OR was 2.4 (95% CI 1.4-4.2) for an energy of 50 J, approximately corresponding to the radiation of the breast after breast-conserving surgery. Thus, only oedema of the arm correlated with angiosarcoma, but for other types of sarcoma the integral dose of radiotherapy was a predictor of the risk.

Absorbed dose determination at short distance from 60Co and 192Ir brachytherapy sources

This paper presents two methods for absorbed dose determination with ionization chambers at short distance from 60Co and 192Ir brachytherapy sources. The methods are modifications of the Bragg-Gray and large cavity principles given in the IAEA code of practice for high- and medium-energy photon beams. A non-uniformity correction factor to account for the non-uniform electron fluence in the air cavity is introduced into the methods. The absorbed dose rates were determined from ionization chamber measurements at distances between 1.5 and 5.0 cm from the brachytherapy sources. The agreement between the two methods is excellent in 60Co brachytherapy dosimetry. For 192Ir dosimetry, the difference is less than 2.5% at all distances. In absorbed dose rate calculations with the 60Co brachytherapy source, the ratios between calculated and experimentally determined absorbed dose rates are 0.987 and 0.994 depending on the method used for absorption and scatter correction. In 192Ir dosimetry, the large cavity principle gives almost identical values to those which can be obtained with the AAPM recommendations. Using the chambers according to the Bragg-Gray principle in 192Ir dosimetry, the agreement with AAPM calculated absorbed dose rates is within 2.5% at all distances. The uncertainty, expressed as one standard deviation, in the experimentally determined absorbed dose is estimated to be between 3 and 4%.

Correction factors for Farmer-type chambers for absorbed dose determination in 60Co and 192Ir brachytherapy dosimetry

This paper presents experimentally determined correction factors for Farmer-type chambers for absorbed dose determination in 60Co and 192Ir brachytherapy dosimetry. The correction factors were determined from measurements made in a PMMA phantom and calculation of ratios of measured charges. The ratios were corrected for the different volumes of the ionization chambers, determined in external high-energy electron beams. The correction factors for the central electrode effect and the wall material dependency in 60Co brachytherapy dosimetry agree with those used in external 60Co beam dosimetry. In 192Ir dosimetry, the central aluminium electrode increases the response of an NE2571 chamber compared with that of a chamber with a central graphite electrode. The increase is 1.1 and 2.1% at 1.5 and 5.0 g cm(-2) distance, respectively. Similar values are obtained with an NE2577 chamber. The wall correction factor in 192Ir dosimetry for a chamber with an A-150 wall has been determined to be 1.018, independent of the measurement distance. For a graphite walled chamber, the correction factor is 0.996 and 1.001 at 1.5 and 5.0 g cm(-2) distance, respectively. The values of the wall correction factors are evaluated by a theory presented. If the chamber is used according to the 'large cavity' principle, the correction factor to account for the replacement of the phantom material by the ionization chamber was determined to be 0.982 for an NE2571 chamber when used with a Delrin cap, and 0.978 for an NE2581 when used with a polystyrene cap. The correction factors for the 'large cavity' principle are valid at both 60Co and 192Ir qualities.

Calculation of absorbed dose distributions from dynamic wedges

In radiotherapy with photon beams, the use of dynamic wedges, which are obtained by the movement of one of the jaws, offers an increasing flexibility relative to the traditional use of metal wedges. But it is a disadvantage for the measurement of absorbed dose distributions, because the absorbed dose at each measurement point can only be obtained after a complete movement of the jaw. Consequently, for radiotherapy planning, an algorithm should be available that does not require measurements for any specific dynamically wedged beam, but is based on only a modest number of measurements. In this paper, an algorithm for the calculation of the dose distribution from dynamic wedges is described. This algorithm uses the convolution of pencil beam kernels with a non-uniform field function. These pencil beam kernels are derived from empirical data resulting from measurements of the open beam only.

Dose accuracy check of the 3D electron beam algorithm in a treatment planning system

The accuracy of the recently implemented three-dimensional electron beam dose calculating algorithm in CADPLAN version 2.62 manufactured by Varian Dosetek was investigated. The algorithm uses a generalized Gaussian pencil beam model and the dose distributions are calculated as the sum of three weighted Gaussians. To use the calculating program in an optimum way, one needs to know the dose calculation accuracy of the algorithm as well as its limitations. This investigation includes comparisons of measured relative dose distributions with calculated dose distributions and also comparisons of measured and calculated monitor units. The geometries tested were quadratic fields, irregularly shaped fields, oblique fields, irregularly shaped phantom surfaces and internal heterogeneities and were most often irradiated with 8 and 20 MeV electrons. The results indicate that the algorithm is well suited for clinical three-dimensional dose planning. Some deviations occurred but they were most often within the limits of international criteria of acceptability.

Practical implementation of enhanced dynamic wedge in the CadPlan treatment planning system

The Varian CadPlan algorithm for computation of relative dose distributions and monitor unit calculations for Enhanced Dynamic Wedge (EDW) fields is based on a combination of open field beam data and Segmented Treatment data Tables. Calculation of dose by the pencil beam convolution model uses scatter kernels and boundary kernels to create the distribution. The principles of the pencil beam convolution model is presented. Comparison of measured and calculated monitor units and relative dose distributions showed good agreement and the deviations are within international accepted tolerans. Test results indicate that the EDW model works satisfactorily for all energies and wedge angles.