Radiation Dose–Volume Effects in the Lung

The three dimensional dose/volume/outcome data for lung are reviewed in detail. The rate of symptomatic pneumonitis is related to many dosimetric parameters, and there are no evident threshold “tolerance dose/volume” levels. There are strong volume and fractionation effects

Inclusion of geometric uncertainties in treatment plan evaluation

PURPOSE: To correctly evaluate realistic treatment plans in terms of absorbed dose to the clinical target volume (CTV), equivalent uniform dose (EUD), and tumor control probability (TCP) in the presence of execution (random) and preparation (systematic) geometric errors. MATERIALS AND METHODS: The dose matrix is blurred with all execution errors to estimate the total dose distribution of all fractions. To include preparation errors, the CTV is randomly displaced (and optionally rotated) many times with respect to its planned position while computing the dose, EUD, and TCP for the CTV using the blurred dose matrix. Probability distributions of these parameters are computed by combining the results with the probability of each particular preparation error. We verified the method by comparing it with an analytic solution. Next, idealized and realistic prostate plans were tested with varying margins and varying execution and preparation error levels. RESULTS: Probability levels for the minimum dose, computed with the new method, are within 1% of the analytic solution. The impact of rotations depends strongly on the CTV shape. A margin of 10 mm between the CTV and planning target volume is adequate for three-field prostate treatments given the accuracy level in our department; i.e., the TCP in a population of patients, TCP(pop), is reduced by less than 1% due to geometric errors. When reducing the margin to 6 mm, the dose must be increased from 80 to 87 Gy to maintain the same TCP(pop). Only in regions with a high-dose gradient does such a margin reduction lead to a decrease in normal tissue dose for the same TCP(pop). Based on a rough correspondence of 84% minimum dose with 98% EUD, a margin recipe was defined. To give 90% of patients at least 98% EUD, the planning target volume margin must be approximately 2.5 Sigma + 0.7 sigma - 3 mm, where Sigma and sigma are the combined standard deviations of the preparation and execution errors. This recipe corresponds accurately with 1% TCP(pop) loss for prostate plans with clinically reasonable values of Sigma and sigma. CONCLUSION: The new method computes in a few minutes the influence of geometric errors on the statistics of target dose and TCP(pop) in clinical treatment plans. Too small margins lead to a significant loss of TCP(pop) that is difficult to compensate for by dose escalatio

ROLE OF INTENSITY-MODULATED RADIOTHERAPY IN REDUCING TOXICITY IN DOSE ESCALATION FOR LOCALIZED PROSTATE CANCER

Purpose: To compare the acute and late gastrointestinal (GI) and genitourinary (GU) toxicity in prostate cancer patients treated to a total dose of 78 Gy with either a three-conformal radiotherapy technique with a sequential boost (SEQ) or a simultaneous integrated boost using intensity-modulated radiotherapy (SIB-IMRT). Patients and Methods: A total of 78 prostate cancer patients participating in the randomized Dutch trial comparing 68 Gy and 78 Gy were the subject of this analysis. They were all treated at the same institution to a total dose of 78 Gy. The median follow-up was 76 and 56 months for the SEQ and SIB-TMRT groups, respectively. The primary endpoints were acute and late GI and GU toxicity. Results: A significantly lower incidence of acute Grade 2 or greater GI toxicity occurred in patients treated with SIB-IMRT compared with SEQ (20% vs. 61 %,p = 0.001). For acute GU toxicity and late GI and GU toxicity, the incidence was lower after SIB-IMRT, but these differences were not statistically significant. No statistically significant difference were found in the 5-year freedom from biochemical failure rate (Phoenix definition) between the two groups (70% for the SIB-IMRT group vs. 61 % for the SEQ group,p = 0.3). The same was true for the 5-year freedom from clinical failure rate (90% vs. 72%,p = 0.07). Conclusion: The results of our study have shown that SIB-IMRT reduced the toxicity without compromising the outcome in patients with localized prostate cancer treated to 78 Gy radiation. (c) 2009 Elsevier Inc