Anthony Neal

Correlation of breast dose heterogeneity with breast size using 3D CT planning and dose-volume histograms

Abstract The aim of this study was to evaluate the effect of breast size on dose heterogeneity. Twenty women underwent a planning CT scan of the thorax. A three-dimensional treatment plan was devised for each patient using a standard technique of isocentric medial and lateral wedged tangential fields. Three-dimensional dose distributions were derived using an equivalent path length (EPL) inhomogeneity correction and cumulative dose-volume histogram (DVH) data calculated for the breast. Analysis of the DVHs for each patient reveals that 0.2–23.8% of the breast received an absorbed dose outside the desired 95–105% of that prescribed at the isocentre. The degree of dose heterogeneity was most strongly correlated with breast volume ( r = 0.70, 95% confidence interval (C.I.) 0.37–0.87). There was also a positive correlation for breast dose heterogeneity versus brassiere (bra) cup size (Spearman rank correlation ϱ = 0.62), breast area ( r = 0.39, 95% C.I. −0.06–0.71) and chest wall separation ( r = 0.31, 95% C.I. −0.15–0.66). We conclude that breast size is an important determinant of dose heterogeneity within the breast.

Accuracy of pelvic radiotherapy: prospective analysis of 90 patients in a randomised trial of blocked versus standard radiotherapy

The aim of this study was to assess the accuracy of pelvic radiotherapy during a trial of blocked radiotherapy at the Royal Marsden Hospital, UK. Prospective evaluation was performed on 90 patients receiving CT planned pelvic radiotherapy using weekly anterior-posterior and lateral portal films. Field placement errors (FPEs) were calculated by comparing field centres of each film with a designated point of interest. Data was evaluated to calculate the overall treatment simulator differences, the number of error free treatments, and mean treatment-simulator position and to evaluate the role of systematic versus random errors. Age, weight, disease site, position of treatment, fractionation, blocked versus conventional techniques were assessed for their effect on treatment accuracy. The mean absolute error between treatment and simulator films was anterior right-left (ARL) 0.25 cm, anterior superior-inferior (ASI) 0.32 cm, lateral anterior-posterior (LAP) 0.42 cm, and lateral superior-inferior (LSI) 0.28 cm. On average the field centre was displaced by 0.66 cm (standard deviation, S.D. = 0.34) from that intended. On each treatment day 29% of anterior films and 45% of lateral films had at least one 0.5 cm error. Overall 59% of treatments had at least one 0.5 cm error and 9% a 1.0 cm error. The field centre was more than 0.5 cm from the position intended in 66% of treatments and over 1 cm for 14% of treatments. Analysis of variance showed that both random and systematic errors occurred in all directions. Though random errors were of similar magnitude in all direction (variance sigma 2 = 0.06-0.09 cm2); systematic errors showed a 4-fold variation being greatest in the LAP direction (sigma 2 = 0.19 cm2) and least the ARL direction (sigma 2 = 0.048 cm2). No factor consistently predicted for worse outcome in all directions. Hypofractionated treatments were less accurate in the LSI direction (P > 0.05). Systematic errors were associated in the ARL direction with hypofractionation (P < 0.01) and, in the LSI direction with weight (P < 0.03) and age (P < 0.05). We conclude that significant random and systematic errors can occur during pelvic radiotherapy especially in the LAP direction. These results suggest that in the absence of a customised immobilisation device, to cover 95% of errors, margins of 0.6 cm for RL and SI directions and 0.9 cm for AP direction should be allowed between the planning and clinical target volumes. However, ideally, each centre should determine their own margin requirements according to local clinical practice.

Estimating the volume of lung irradiated during tangential breast irradiation using the central lung distance.

The aim of this study was to evaluate the central lung distance (CLD) as a surrogate for the volume of lung irradiated during tangential breast radiotherapy. 20 women underwent a planning computed tomography (CT) scan and three-dimensional (3D) treatment planning for both breasts. The field size was perturbed in a systematic manner to give a number of plans with the CLD increasing from 0 to 30 mm. The volume of lung irradiated was determined directly using dose-volume histograms of the lung and correlated with the known CLD. The results indicate that absolute and percentage lung volumes increase with increasing CLD according to a quadratic relationship which is different for both left and right lungs. It is concluded that while there is no substitute for a 3D plan and a lung dose-volume histogram calculation, CLD may be used as a guide to the volume of lung included within the tangential fields used for breast radiotherapy.

The optimisation of wedge filters in radiotherapy of the prostate

A treatment plan optimisation algorithm has been applied to 12 patients with early prostate cancer in order to determine the optimum beam-weights and wedge angles for a standard conformal three-field treatment technique. The optimisation algorithm was based on fast-stimulated-annealing using a cost function designed to achieve a uniform dose in the planning-target-volume (PTV) and to minimise the integral doses to the organs-at-risk. The algorithm has been applied to standard conformal three-field plans created by an experienced human planner, and run in three PLAN MODES: (1) where the wedge angles were fixed by the human planner and only the beam-weights were optimised; (2) where both the wedge angles and beam-weights were optimised; and (3) where both the wedge angles and beam-weights were optimised and a non-uniform dose was prescribed to the PTV. In the latter PLAN MODE, a uniform 100% dose was prescribed to all of the PTV except for that region that overlaps with the rectum where a lower (e.g., 90%) dose was prescribed. The resulting optimised plans have been compared with those of the human planner who found beam-weights by conventional forward planning techniques. Plans were compared on the basis of dose statistics, normal-tissue-complication-probability (NTCP) and tumour-control-probability (TCP). The results of the comparison showed that all three PLAN MODES produced plans with slightly higher TCP for the same rectal NTCP, than the human planner. The best results were observed for PLAN MODE 3, where an average increase in TCP of 0.73% (+/- 0.20, 95% confidence interval) was predicted by the biological models. This increase arises from a beneficial dose gradient which is produced across the tumour. Although the TCP gain is small it comes with no increase in treatment complexity, and could translate into increased cures given the large numbers of patients being referred. A study of the beam-weights and wedge angles chosen by the optimisation algorithm revealed significant inter-patient variability the implications of which are examined. Probably the most significant benefit of the algorithm is the time saved (about a factor of 10) in computing optimised beam-weights and wedge angles for this simple plan.