The effect of different lung densities on the accuracy of various radiotherapy dose calculation methods: implications for tumour coverage.

PURPOSE: To evaluate against Monte-Carlo the performance of various dose calculations algorithms regarding lung tumour coverage in stereotactic body radiotherapy (SBRT) conditions. MATERIALS AND METHODS: Dose distributions in virtual lung phantoms have been calculated using four commercial Treatment Planning System (TPS) algorithms and one Monte Carlo (MC) system (EGSnrc). We compared the performance of the algorithms in calculating the target dose for different degrees of lung inflation. The phantoms had a cubic 'body' and 'lung' and a central 2-cm diameter spherical 'tumour' (the body and tumour have unit density). The lung tissue was assigned five densities (rho(lung)): 0.01, 0.1, 0.2, 0.4 and 1g/cm(3). Four-field treatment plans were calculated with 6- and 18 MV narrow beams for each value of rho(lung). We considered the Pencil Beam Convolution (PBC(Ecl)) and the Analytical Anisotropic Algorithm (AAA(Ecl)) from Varian Eclipse and the Pencil Beam Convolution (PBC(OMP)) and the Collapsed Cone Convolution (CCC(OMP)) algorithms from Oncentra MasterPlan. RESULTS: When changing rho(lung) from 0.4 to 0.1g/cm(3), the MC median target dose decreased from 89.2% to 74.9% for 6 MV and from 83.3% to 61.6% for 18 MV (of dose maximum in the homogenous case at both energies), while for both PB algorithms the median target dose was virtually independent of lung density. CONCLUSIONS: Both PB algorithms overestimated the target dose, the overestimation increasing as rho(lung) decreased. Concerning target dose, the AAA(Ecl) and CCC(OMP) algorithms appear to be adequate alternatives to MC.

Intra- and interfraction breathing variations during curative radiotherapy for lung cancer.

BACKGROUND AND PURPOSE: This study aimed at quantifying the breathing variations among lung cancer patients over full courses of fractionated radiotherapy. The intention was to relate these variations to the margins assigned to lung tumours, to account for respiratory motion, in fractionated radiotherapy. MATERIALS AND METHODS: Eleven lung cancer patients were included in the study. The patients' chest wall motions were monitored as a surrogate measure for breathing motion during each fraction of radiotherapy by use of an external optical marker. The exhale level variations were evaluated with respect to exhale points and fraction-baseline, defined for intra- and interfraction variations respectively. The breathing amplitude was evaluated as breathing cycle amplitudes and fraction-max-amplitudes defined for intra- and interfraction breathing, respectively. RESULTS: The breathing variations over a full treatment course, including both intra- and interfraction variations, were 15.2mm (median over the patient population), range 5.5-26.7mm, with the variations in exhale level as the major contributing factor. The median interfraction span in exhale level was 14.8mm, whereas the median fraction-max-amplitude was 6.1mm (median of patient individual SD 1.4). The median intrafraction span in exhale level was 1.6mm, and the median breathing cycle amplitude was 4.0mm (median of patient individual SD 1.4). CONCLUSIONS: The variations in externally measured exhale levels are larger than variations in breathing amplitude. The interfraction variations in exhale level are in general are up to 10 times larger than intrafraction variations. Margins to account for respiratory motion cannot safely be based on one planning session, especially not if relying on measuring external marker motion. Margins for lung tumours should include interfraction variations in breathing.

Cardiac and pulmonary complication probabilities for breast cancer patients after routine end-inspiration gated radiotherapy.

PURPOSE: Substantial reductions of radiation doses to heart and lung can be achieved using breathing adaptation of adjuvant radiotherapy following conservative surgery for breast cancer. The purpose of this study was to estimate the radiobiological implications after routine use of an end-inspiration gated treatment, and to compare the results with predictions based on pre-clinical CT-studies. PATIENTS AND METHODS: Nineteen consecutive patients with axillary lymph node-positive left-sided breast cancer were referred for adjuvant radiotherapy after breast conserving surgery. Treatment was performed with gating in the end-inspiration phase of audio-coached enhanced free breathing. The target intended to encompass the remaining breast, ipsilateral internal mammary and periclavicular nodes, and the prescription dose was 48Gy in 24 fractions. A three-field mono-isocentric conformal technique using deep tangentials and a supraclavicular field was employed. NTCPs were calculated using the relative seriality model for the heart, and the model proposed by Burman et al. for the lung. The observed values were compared to those predicted from two previous CT-studies for a deep inspiration breath-hold technique and an uncoached end-inspiration gating technique. RESULTS: The ipsilateral lung V(50) (relative volume receiving more than 50% of the prescription dose) had a median value of 23.7% (range 10.8-35.1%) over the patient population. The corresponding median lung pneumonitis probability was 1.1% (range 0-14%). The median heart V(50) was 0.8% (range 0-19.1%) with a corresponding median cardiac mortality NTCP of 0.1% (range 0-5.7%). These results compare well with the predictions of our previous CT-studies. There is a significant reduction in dose to the left anterior descending coronary artery for the enhanced end-inspiration gating technique compared to the uncoached end-inspiration technique employed in the CT-studies. CONCLUSIONS: In a routine clinical practice involving adjuvant breast radiotherapy gated in an enhanced end-inspiration phase, remarkably low doses to organs at risk are observed. The corresponding cardiac and pulmonary complication risks are of the order of 1% and smaller.

Reduction of cardiac and pulmonary complication probabilities after breathing adapted radiotherapy for breast cancer.

PURPOSE: Substantial reductions of cardio-pulmonary radiation doses can be achieved using voluntary deep inspiration breath-hold (DIBH) or free breathing inspiration gating (IG) in radiotherapy after conserving surgery for breast cancer. The purpose of this study is to evaluate the radiobiological implications of such dosimetric benefits. METHODS AND MATERIALS: Patients from previously reported studies were pooled for a total of 33 patients. All patients underwent DIBH and free breathing (FB) scans, and 17 patients underwent an additional IG scan. Tangential conformal treatment plans covering the remaining breast, internal mammary, and periclavicular nodes were optimized for each scan, prescription dose 48 Gy. Normal tissue complication probabilities were calculated using the relative seriality model for the heart, and the model proposed by Burman et al. for the lung. RESULTS: Previous computed tomography studies showed that both voluntary DIBH and IG provided reduction of the lung V50 (relative volume receiving more than 50% of prescription dose) on the order of 30-40%, and a 80-90% reduction of the heart V50 for left-sided cancers. Corresponding pneumonitis probability of 28.1% (range, 0.7-95.6%) for FB could be reduced to 2.6% (range, 0.1-40.1%) for IG, and 4.3% (range, 0.1-59%) for DIBH. The cardiac mortality probability could be reduced from 4.8% (range, 0.1-23.4%) in FB to 0.5% (range, 0.1-2.6%) for IG and 0.1% (range, 0-3.0%) for DIBH. CONCLUSIONS: Remarkable potential is shown for simple voluntary DIBH and free breathing IG to reduce the risk of both cardiac mortality and pneumonitis for the common technique of adjuvant tangential breast irradiation.

Respiratory motion prediction by using the adaptive neuro fuzzy inference system (ANFIS).

The quality of radiation therapy delivered for treating cancer patients is related to set-up errors and organ motion. Due to the margins needed to ensure adequate target coverage, many breast cancer patients have been shown to develop late side effects such as pneumonitis and cardiac damage. Breathing-adapted radiation therapy offers the potential for precise radiation dose delivery to a moving target and thereby reduces the side effects substantially. However, the basic requirement for breathing-adapted radiation therapy is to track and predict the target as precisely as possible. Recent studies have addressed the problem of organ motion prediction by using different methods including artificial neural network and model based approaches. In this study, we propose to use a hybrid intelligent system called ANFIS (the adaptive neuro fuzzy inference system) for predicting respiratory motion in breast cancer patients. In ANFIS, we combine both the learning capabilities of a neural network and reasoning capabilities of fuzzy logic in order to give enhanced prediction capabilities, as compared to using a single methodology alone. After training ANFIS and checking for prediction accuracy on 11 breast cancer patients, it was found that the RMSE (root-mean-square error) can be reduced to sub-millimetre accuracy over a period of 20 s provided the patient is assisted with coaching. The average RMSE for the un-coached patients was 35% of the respiratory amplitude and for the coached patients 6% of the respiratory amplitude.