Comparison of a lung fitting algorithm with CT data for tangential fields in radiotherapy of the breast.

A method of estimating the shape and position of the lung in tangential breast fields is presented for patients who have not been CT scanned. Using the Osiris system, the external contour is obtained optically, and an estimated lung structure superimposed on the transverse outlines based on the measured lung depth in the tangential fields and an analysis of the typical lung shapes obtained from CT images. The accuracy of this fit was determined by comparison with a set of 64 CT images imported into the Osiris system. Dose distributions were calculated by two treatment planning systems: ADAC Pinnacle and GE Target2. The computed dose distributions for 6 MV photons were compared against measured doses in a specialized breast phantom. For the worst case of lung fit compared with CT, the dosimetric error (based upon ADAC Pinnacle calculations) was 2.0% in the shadow of the lung. For the complete patient data set, the relative dose errors to these points were reduced from a mean value of 8.4% and standard deviation (SD)=1.8% (no lung correction) to a mean of 0.2% and SD=1.0% (lung correction using fitted lung). It was also found that for every 1 cm of lung path length the dose to the breast along that path length increased by approximately 1%. The results of these investigations indicated that the lung fit model was satisfactory for routine clinical use, so that good dosimetric results can be obtained using lung correction without the need for CT imaging.

Calibration of CT Hounsfield units for radiotherapy treatment planning of patients with metallic hip prostheses: the use of the extended CT-scale.

Heterogeneity corrections for radiotherapy dose calculations are based on the electron density of the disturbing heterogeneity. However, when CT planning a radiotherapy treatment, where metallic hip implants are present, considerable artefacts are seen in the images. Often, an additional problem arises whereby no information regarding the artificial hip's composition and geometry is available. This study investigates whether the extended CT range can be used to determine the composition (hence electron density) of artificial hips in radiotherapy patients. Two CT-calibration methods were evaluated, one based on material substitution, the other a stoichiometric calibration. We also evaluate whether the physical dimensions of metal prostheses can be accurately imaged for subsequent use in treatment planning computers. Neither calibration method successfully predicted electron densities. However, the limited range of implant-materials used in patients means that the extended CT range can still successfully distinguish between implant densities. The physical dimensions can be determined to +/-2 mm by establishing the required windowing of displays for each material. The cross-sectional area of the prosthesis and the presence of other high-density objects in a CT slice can influence the generated CT number and careful design of calibration phantoms is essential.

Commissioning and quality assurance of the Pinnacle(3) radiotherapy treatment planning system for external beam photons.

The commissioning of a Pinnacle(3) treatment planning system is described. Four Elekta linear accelerators were commissioned for external beam photons. Measured data were used to derive parameter values for the Pinnacle(3) beam model by (1). fitting a Monte Carlo model of the accelerator head to measured data and then extracting the parameters for the Pinnacle(3) beam model, and by (2). using the auto-modelling facility within Pinnacle(3). Both of these methods yielded dose distributions in accord with published recommendations. A separate small-field beam model, customized for an in-house compact blocking system, was also created, which satisfied appropriate acceptance criteria for stereotactically guided conformal brain treatments. Inhomogeneous, oblique, asymmetrical and irregular fields were also assessed, with calculated and measured doses agreeing to within +/-3%. Dose-volume histogram calculation was found to be accurate to within +/-5% dose or volume for a grid size of 4 mm x 4 mm x 4 mm, with better accuracy being achieved for finer grids. Isocentric doses were compared between Pinnacle(3)'s collapsed cone convolution algorithm and the Bentley-Milan algorithm within the Target-2 treatment planning system. Dose differences were generally less than 3% in the dose prescribed, with larger values for breast plans, where the Pinnacle(3) algorithm calculated scatter more accurately. Pelvic and thoracic plans were also verified using an anthropomorphic phantom, with local dose differences between calculated and delivered dose of up to 8%, but mainly less than 3%, and with no systematic difference. Ionization chamber verifications using START and RT-01 trial procedures demonstrated differences between calculated and measured doses of less than 2%. Following satisfactory performance in the commissioning process, Pinnacle(3) has now been introduced into routine clinical use.

The delivery of intensity modulated radiotherapy to the breast using multiple static fields.

BACKGROUND AND PURPOSE: To develop a method of using a multileaf collimator (MLC) to deliver intensity modulated radiotherapy (IMRT) for tangential breast fields, using an MLC to deliver a set of multiple static fields (MSFs). MATERIALS AND METHODS: An electronic portal imaging device (EPID) is used to obtain thickness maps of medial and lateral tangential breast fields. From these IMRT deliveries are designed to minimize the volume of breast above 105% of prescribed dose. The deliveries are universally-wedged beams augmented with a set of low dose shaped irradiations. Dosimetric and planning QA of this method has been compared with the standard, wedged treatment and the corresponding treatment using physical compensators. Several options for delivering the MSF treatment are presented. RESULTS: The MSF technique was found to be superior to the standard technique (P value=0.002) and comparable with the compensated technique. Both IMRT methods reduced the volume of breast above 105% dose from a mean value of 12.0% of the total breast volume to approximately 2.8% of the total breast volume. CONCLUSIONS: This MSF method may be used to reduce the high dose volume in tangential breast irradiation significantly. This may have consequences for long-term side effects, particularly cosmesis.

A quantitative treatment planning study evaluating the potential of dose escalation in conformal radiotherapy of the oesophagus.

BACKGROUND AND PURPOSE: This study aims to evaluate the reduction in radiation dose to normal thoracic structures through the use of conformal radiotherapy techniques in the treatment of oesophageal cancer, and to quantify the resultant potential for dose escalation. MATERIALS AND METHODS: Three different CT-derived treatment plans were created and compared for each of ten patients. A two-phase treatment with conventional straight-edged fields and standard blocks (CV2), a two-phase conformal plan (CF2), and a three-phase conformal plan where the third phase was delivered to the gross tumour only (CF3), were considered for each patient. Escalated dose levels were determined for techniques CF2 and CF3, which by virtue of the conformal field shaping, did not increase the mean lung dose. The resulting increase in tumour control probability (TCP) was estimated. RESULTS: A two-phase conformal technique (CF2) reduced the volume of lung irradiated to 18 Gy from 19.7+/-11.8 (1 SD) to 17.1+/-12.3% (P=0.004), and reduced the normal tissue complication probability (NTCP) from 2.4+/-4.0 to 0.7+/-1.6% (P=0.02) for a standard prescribed dose of 55 Gy. Consequently, technique CF2 permitted a target dose of 59.1+/-3.2 Gy without increasing the mean lung dose. Technique CF3 facilitated a prescribed dose of 60.7+/-4.3 Gy to the target, the additional 5 Gy increasing the TCP from 53. 1+/-5.5 to 68.9+/-4.1%. When the spinal cord tolerance was raised from 45 to 48 Gy, technique CF3 allowed 63.6+/-4.l Gy to be delivered to the target, thereby increasing the TCP to 78.1+/-3.2%. CONCLUSIONS: Conformal radiotherapy techniques offer the potential for a 5-10 Gy escalation in dose delivered to the oesophagus, without increasing the mean lung dose. This is expected to increase local tumour control by 15-25%.

A comparison of conventional and conformal radiotherapy of the oesophagus: work in progress.

A retrospective treatment planning study was carried out in five patients to assess the effectiveness of conformal radiotherapy of the oesophagus. A two-phase conventional treatment plan was created for each patient, with a prescribed dose of 55 Gy. This plan was compared with a single-phase conformal plan consisting of the same field arrangement as the second phase of the conventional treatment, but with conformal blocks shaped to the beam's eye view of the planning target volume. A further comparison was made between the conventional plan and a two-phase plan using the same beam angles and weights as the conventional plan, but with conformal field shapes. The effectiveness of each treatment plan was assessed using dose--volume histograms and normal tissue complication probabilities for the lungs. On average, the single-phase conformal technique increased the mean lung dose from 22.5% (+/- 6.2 SD) of the prescribed dose to 29.5% (+/- 5.2 SD) compared with the conventional technique (p = 0.0001). This indicates that this technique did not offer any benefit in terms of reducing the risk of pneumonitis. However, the two-phase conformal technique reduced the mean lung dose from 22.5% (+/- 6.2 SD) of the prescribed dose to 19.8% (+/- 4.6 SD)(p = 0.03), showing that this technique should reduce the risk of pneumonitis. Further work is underway to study more patients and to investigate tumour control probability and dose escalation.

Whole body doses from linear accelerator-based stereotactic radiotherapy.

PURPOSE: The objective of this work was to measure whole body radiation doses in a humanoid phantom from linear accelerator-based cranial stereotactic radiosurgery/therapy (SRS/T), using different beam arrangements. METHODS AND MATERIALS: A standard noncoplanar five-arc beam arrangement and a four-arc technique without a sagittal arc were used to deliver 20 Gy in a single fraction to a midline spherical target volume in the corpus callosum region of an Alderson-Rando anthropomorphic phantom using (i) a 20-mm and (ii) a 40-mm circular collimator. Whole body dose measurements were made using lithium fluoride thermoluminescent dosimetry. Whole body isodose plots in the sagittal and coronal planes and organ doses were compared for the two arcing beam arrangements. An ionization chamber was used to record the exit dose at intervals along the length of the phantom at midline and 4.5 cm off-axis for (i) a single fixed field and (ii) a solitary 90 degrees sagittal arc using a 40-mm circular collimator. RESULTS: The sagittal arc was the major contributor to neck and trunk doses when the five- and four-arc arrangements were compared, with fourfold greater thyroid dose. The gonad dose was increased by the sagittal arc, but was largely due to leakage radiation. The dose from a fixed field exiting down the long axis of the phantom was tenfold greater than that from a solitary 90 degrees sagittal arc. When the fixed field or arc traversed the lung or exited through the pharynx and major upper airways, the dose measurements below the diaphragm were 30-40% higher than those along the exit path of maximum soft tissue density. CONCLUSION: When SRS/T is used in nonmalignant conditions such as cranial arteriovenous malformations or benign tumors the exit paths of arcing beams or fixed fields should be taken into account when deciding upon the final treatment plan. Such consideration should minimize the risk of radiation-induced malignancy, notably in the thyroid gland of younger patients.

Beam data measurements for dynamic wedges on Varian 600C (6 MV) and 2100C (6 and 10 MV) linear accelerators.

The measurement of beam data for dynamic wedge dosimetry requires the integration of radiation dose at points across the radiation field during the dose delivery. The different measurement techniques required when using a linear diode array to measure beam profiles and when using ionization chambers to measure depth doses and effective wedge factors are described. The segmented treatment tables (STTS), which specify the delivered dose as a function of jaw position, are used by the control algorithm to deliver dynamic wedge fields. The reproducibility achieved using STTS is very consistent (and the stability of the accelerators is very good) so there is no significant variation in effective wedge factor or profile shape. There is a unique set of 132 STTS for each energy and design of treatment machine, which encompasses all the dynamic wedge data. There are significant discontinuities of up to 14% in wedge factors at certain field sizes. This means that wedge factors have to be measured at small increments (0.5 cm) in field size, as it is the width of the dynamic wedge field that determines the STT used. Considerable care must be taken when implementing these data on a current generation treatment planning computer.