Targeted Intervention to Improve the Quality of Head and Neck Radiation Therapy Treatment Planning in the Netherlands: Short and Long-Term Impact.

PURPOSE: To benchmark and improve, through means of a targeted intervention, the quality of intensity modulated radiation therapy treatment planning for locally advanced head and neck cancer (HNC) in the Netherlands. The short and long-term impact of this intervention was assessed. METHODS AND MATERIALS: A delineated computed tomography-scan of an oropharynx HNC case was sent to all 15 Dutch radiation therapy centers treating HNC. Aims for planning target volume and organ-at-risk (OAR) dosimetry were established by consensus. Each center generated a treatment plan. In a targeted intervention, OAR sparing of all plans was discussed, and centers with the best OAR sparing shared their planning strategies. Impact of the intervention was assessed by (1) short-term (half a year after intervention) replanning of the original case and (2) long-term (1 and 3 years after intervention) planning of new cases. RESULTS: Benchmarking revealed substantial difference in OAR doses. Initial mean doses were 22 Gy (range, 15-31 Gy), 35 Gy (18-49 Gy), and 37 Gy (20-46 Gy) for the contralateral parotid gland, contralateral submandibular gland, and combined swallowing structures, respectively. Replanning after targeted intervention significantly reduced mean doses and variation, but clinically relevant differences still remained: 18 Gy (14-22 Gy), 28 Gy (17-45 Gy), and 29 Gy (18-39 Gy), respectively. One and 3 years later the variation remained stable. CONCLUSIONS: Despite many years of HNC intensity modulated radiation therapy experience, initial treatment plans showed surprisingly large variations. The simple targeted intervention used in this analysis improved OAR sparing, and its impact was durable; however, fairly large dose differences still continue to exist. Additional work is needed to understand these variations and to minimize them. A national radiation oncology platform can be instrumental for developing and maintaining high-quality planning protocols.

The sensitivity of dose distributions for organ motion and set-up uncertainties in prostate IMRT.

BACKGROUND AND PURPOSE: To determine the effect of organ motion and set-up uncertainties on IMRT dose distributions for prostate. METHODS: For five patients, IMRT techniques were designed to irradiate the CTV (prostate plus seminal vesicles). Technique I delivered 78 Gy to PTV1 (CTV+10 mm margin). Technique II delivered 68 Gy to PTV1, and a 10 Gy boost to PTV2 (CTV+an anisotropic margin of 0 to 5 mm). Technique III delivered 68 Gy to PTV1 and simultaneously 78 Gy to PTV2. Uncertainties were simulated using population statistics of organ motion and set-up accuracy. The average TCP (TCPpop) of the CTV and average NTCP (NTCPpop) of the rectal wall were calculated. RESULTS: The planning TCP was a good predictor for TCPpop for Techniques I and II. Technique III was sensitive for geometrical uncertainties, reducing TCPpop by 0.8 to 2.4% compared to planning. NTCPpop was reduced for Technique III by a factor 2.6 compared to Technique I. For all plans, the planning NTCP was strongly correlated with NTCPpop. CONCLUSIONS: Dose distributions created with Techniques I and II are insensitive for geometrical uncertainties, while Technique III resulted in a reduction of TCPpop. This reduction can be compensated by a small dose escalation, while still resulting in an NTCPpop of the rectal wall that is lower or comparable to Technique I.

Comparison between manual and automatic segment generation in step-and-shoot IMRT of prostate cancer.

PURPOSE: To compare two methods to generate treatment plans for intensity-modulated radiotherapy (IMRT) of prostate cancer, delivered in a step-and-shoot mode. The first method uses fluence optimization (inverse planning) followed by conversion of the fluence weight map into a limited number of segments. In the second method, segments are manually assigned using a class solution (forward planning), followed by computer optimization of the segment weights. METHODS: Treatment plans for IMRT, utilizing a simultaneous integrated boost, were created. Plans comprise a five-field technique to deliver 78 Gy to the prostate plus seminal vesicles. Five patients were evaluated. Optimization objectives of both planning approaches concerned dose coverage of the target volumes and the dose distribution in the rectal wall. The two methods were evaluated by comparing dose distributions, the complexity of the resulting plan and the time expenditure to generate and to deliver the plan. RESULTS: For both planning approaches 99% of the target volumes received 95% of the prescribed dose, which complies with our planning objectives. Inverse planning resulted in more conformal dose distributions than forward planning (conformity index: 1.37 versus 1.51). Inverse planning reduced the dose to the rectal wall compared to a manually designed plan, albeit to a small extent. The theoretical probability of severe rectal proctitis and/or stenosis was reduced on average by 1.9% with inverse planning. Maximal sparing of the rectal wall was achieved with inverse planning for a patient whose target volume was partly wrapped around the rectum. The number of segments generated with inverse planning ranged between 33 and 52, and between 9 and 13 segments for manually created segments. CONCLUSION: Dose coverage of the planning target volumes is adequate for both approaches of planning. Inverse planning results in slightly better dose distributions with respect to the rectal wall compared to manual planning, at the cost of an increase of the number of segments by a factor of 3.

A comparison of forward and inverse treatment planning for intensity-modulated radiotherapy of head and neck cancer.

BACKGROUND AND PURPOSE: To compare intensity-modulated treatment plans of patients with head and neck cancer generated by forward and inverse planning. MATERIALS AND METHODS: Ten intensity-modulated treatment plans, planned and treated with a step&shoot technique using a forward planning approach, were retrospectively re-planned with an inverse planning algorithm. For this purpose, two strategies were applied. First, inverse planning was performed with the same beam directions as forward planning. In addition, nine equidistant, coplanar incidences were used. The main objective of the optimisation process was the sparing of the parotid glands beside an adequate treatment of the planning target volume (PTV). Inverse planning was performed both with pencil beam and Monte Carlo dose computation to investigate the influence of dose computation on the result of the optimisation. RESULTS: In most cases, both inverse planning strategies managed to improve the treatment plans distinctly due to a better target coverage, a better sparing of the parotid glands or both. A reduction of the mean dose by 3-11Gy for at least one of the parotid glands could be achieved for most of the patients. For three patients, inverse planning allowed to spare a parotid gland that had to be sacrificed by forward planning. Inverse planning increased the number of segments compared to forward planning by a factor of about 3; from 9-15 to 27-46. No significant differences for PTV and parotid glands between both inverse planning approaches were found. Also, the use of Monte Carlo instead of pencil beam dose computation did not influence the results significantly. CONCLUSION: The results demonstrate the potential of inverse planning to improve intensity-modulated treatment plans for head and neck cases compared to forward planning while retaining clinical utility in terms of treatment time and quality assurance.

Importance of accurate dose calculations outside segment edges in intensity modulated radiotherapy treatment planning.

BACKGROUND AND PURPOSE: To assess the effect of differences in the calculation of the dose outside segment edges on the overall dose distribution and the optimisation process of intensity modulated radiation therapy (IMRT) treatment plans. PATIENTS AND METHODS: Accuracy of dose calculations of two treatment planning systems (TPS1 and TPS2) was assessed, to ensure that they are both suitable for IMRT treatment planning according to published guidelines. Successively, 10 treatment plans for patients with prostate and head and neck tumours were calculated in both systems. The calculations were compared in selected points as well as in combination with volumetric parameters concerning the planning target volume (PTV) and organs at risk. RESULTS: For both planning systems, the calculations agree within 2.0% or 3 mm with the measurements in the high-dose region for single and multiple segment dose distributions. The accuracy of the dose calculation is within the tolerances proposed by recent recommendations. Below 35% of the prescribed dose, TPS1 overestimates and TPS2 underestimates the measured dose values, TPS2 being closer to the experimental data. The differences between TPS1 and TPS2 in the calculation of the dose outside segments explain the differences (up to 50% of the local value) found in point dose comparisons. For the prostate plans, the discrepancies between the TPS do not translate into differences in PTV coverage, normal tissue complication probability (NTCP) values and results of the plan optimisation process. The dose-volume histograms (DVH) of the rectal wall differ below 60 Gy, thus affecting the plan optimisation if a cost function would operate in this dose region. For the head and neck cases, the two systems give different evaluations of the DVH points for the PTV (up to 22% differences in target coverage) and the parotid mean dose (1.0-3.0 Gy). Also the results of the optimisation are influenced by the choice of the dose calculation algorithm. CONCLUSIONS: In IMRT, the accuracy of the dose calculation outside segment edges is important for the determination of the dose to both organs at risks and target volumes and for a correct outcome of the optimisation process. This aspect should therefore be of major concern in the commissioning of a TPS intended for use in IMRT. Fulfilment of the accuracy criteria valid for conformal radiotherapy is not sufficient. Three-dimensional evaluation of the dose distribution is needed in order to assess the impact of dose calculation accuracy outside the segment edges on the total dose delivered to patients treated with IMRT.

Interinstitutional variations of sensitometric curves of radiographic dosimetric films.

Depth and field size dependence of the sensitometric curves of radiographic films have been studied by various groups. Limited information is, however, available on the magnitude of the variations in sensitometric curves applied in clinical practice in different institutions. In this study we assessed in a systematic way the effect of the various parameters influencing the shape of the sensitometric curve: batch composition, irradiation conditions, film processing, and film scanning. Two types of film, Kodak X-Omat V and CEA TVS, were irradiated, processed, and analyzed in three different institutions. The interinstitutional variation of the sensitometric curves, expressed as the OD variation at 50 cGy, can be up to 32% and is mainly caused by differences in film processing and to a lesser degree to differences in batch composition, film scanning, and irradiation conditions. For the Kodak films, the average OD difference at 50 cGy between the three institutions is 17% as a result of differences in batch composition and 25% due to differences in processing conditions. For the CEA films these data are 6% and 24%, respectively. The long-term variation of the sensitometric curves of KODAK films in one institution was smaller than the differences in batch composition between the three institutions. The sensitometric curves of CEA films showed in one institution a large variation with time; the shape gradually varied from sigmoidal to quasilinear. By using relative OD values rather than absolute OD values, variations in sensitometric curves of KODAK films can be reduced to 2%. Consequently, one sensitometric curve is sufficient to derive relative dose values. If processing conditions are well controlled, it might therefore be advantageous to determine the absolute OD only at one or two dose values, in combination with a "universal" relative sensitometric curve.

Reduction of rectal dose by integration of the boost in the large-field treatment plan for prostate irradiation.

PURPOSE: To reduce the dose in the rectal wall from prostate irradiation at high dose levels. METHODS AND MATERIALS: Treatment plans in which the boost fields were integrated into the large fields (simultaneous integrated boost [SIB]) were compared with plans in which the large fields and boost fields were planned individually and applied in a sequential manner (sequential boost). Two target volumes were delineated: PTV1, the target volume of the large fields that is irradiated to 68 Gy, and PTV2, the target volume of the boost fields that is irradiated to 10 Gy. The sequential boost and the SIB were normalized to the mean dose in PTV2, being 78 Gy. We used a five-field intensity-modulated radiotherapy (IMRT) technique, applied in a step and shoot mode, and included beam weight optimization. A set of 5 patients with varying degree of overlap between PTV1 and the rectal wall was used for analysis. RESULTS: The SIB resulted in a reduction of the dose in the rectal wall. Rectal normal tissue complication probability (NTCP) decreased for the SIB, on average, by a factor of almost 2, compared with the sequential boost. CONCLUSION: The SIB reduced the dose in the rectal wall, compared with the sequential boost technique.