Geometric leaf placement strategies.

Geometric leaf placement strategies for multileaf collimators (MLCs) typically involve the expansion of the beam's-eye-view contour of a target by a uniform MLC margin, followed by movement of the leaves until some point on each leaf end touches the expanded contour. Film-based dose-distribution measurements have been made to determine appropriate MLC margins--characterized through an index d90--for multileaves set using one particular strategy to straight lines lying at various angles to the direction of leaf travel. Simple trigonometric relationships exist between different geometric leaf placement strategies and are used to generalize the results of the film work into d90 values for several different strategies. Measured d90 values vary both with angle and leaf placement strategy. A model has been derived that explains and describes quite well the observed variations of d90 with angle. The d90 angular variations of the strategies studied differ substantially, and geometric and dosimetric reasoning suggests that the best strategy is the one with the least angular variation. Using this criterion, the best straightforwardly implementable strategy studied is a 'touch circle' approach for which semicircles are imagined to be inscribed within leaf ends, the leaves being moved until the semicircles just touch the expanded target outline

Early mucosal reactions during and after head-and-neck radiotherapy: dependence of treatment tolerance on radiation dose and schedule duration.

PURPOSE: To more precisely localize the dose-time boundary between head-and-neck radiotherapy schedules inducing tolerable and intolerable early mucosal reactions. METHODS AND MATERIALS: Total cell-kill biologically effective doses (BED(CK)) have been calculated for 84 schedules, including incomplete repair effects, but making no other corrections for the effect of schedule duration T. [BED(CK),T] scatterplots are graphed, overlying BED(CKboundary)(T) curves on the plots and using discriminant analysis to optimize BED(CKboundary)(T) to best represent the boundary between the tolerable and intolerable schedules. RESULTS: More overlap than expected is seen between the tolerable and intolerable treatments in the 84-schedule [BED(CK),T] scatterplot, but this was largely eliminated by removing gap and tolerated accelerating schedules from the plot. For the remaining 57 predominantly regular schedules, the BED(CKboundary)(T) boundary increases with increasing T (p = 0.0001), curving upwards significantly nonlinearly (p = 0.00007) and continuing to curve beyond 15 days (p = 0.035). The regular schedule BED(CKboundary)(T) boundary does not describe tolerability well for accelerating schedules (p = 0.002), with several tolerated accelerating schedules lying above the boundary where regular schedules would be intolerable. Gap schedule tolerability also is not adequately described by the regular schedule boundary (p = 0.04), although no systematic offset exists between the regular boundary and the overall gap schedule tolerability pattern. CONCLUSIONS: All schedules analyzed (regular, gap, and accelerating) with BED(CK) values below BED(CKboundary)(T)=69.5x(T/32.2)/sin((T/32.2)((radians)))-3.5Gy(10)(forT< or =50 days) are tolerable, and many lying above the boundary are intolerable. The accelerating schedules analyzed were tolerated better overall than are the regular schedules with similar [BED(CK),T] values

Optimizing collimator margins for isotoxically dose-escalated conformal radiation therapy of non-small cell lung cancer.

PURPOSE: Isotoxic dose escalation schedules such as IDEAL-CRT [isotoxic dose escalation and acceleration in lung cancer chemoradiation therapy] (ISRCTN12155469) individualize doses prescribed to lung tumors, generating a fixed modeled risk of radiation pneumonitis. Because the beam penumbra is broadened in lung, the choice of collimator margin is an important element of the optimization of isotoxic conformal radiation therapy for lung cancer. METHODS AND MATERIALS: Twelve patients with stage I-III non-small cell lung cancer (NSCLC) were replanned retrospectively using a range of collimator margins. For each plan, the prescribed dose was calculated according to the IDEAL-CRT isotoxic prescription method, and the absolute dose (D99) delivered to 99% of the planning target volume (PTV) was determined. RESULTS: Reducing the multileaf collimator margin from the widely used 7 mm to a value of 2 mm produced gains of 2.1 to 15.6 Gy in absolute PTV D99, with a mean gain ± 1 standard error of the mean of 6.2 ± 1.1 Gy (2-sided P<.001). CONCLUSIONS: For NSCLC patients treated with conformal radiation therapy and an isotoxic dose prescription, absolute doses in the PTV may be increased by using smaller collimator margins, reductions in relative coverage being offset by increases in prescribed dose

Independent Validation of Rectal Dose-volume Constraints using MRC RT01 (ISRCTN47772397) Trial Data

Introduction: Treatment plan evaluation requires knowledge of the effect of the plan, not only on the intended target, but also the surrounding normal tissues that are unavoidably irradiated. Recent literature has provided estimations of tolerance doses and proposed dose-volume constraints for many of the organs at risk. However, very few of these recommendations have been independently validated. This study details how constraints proposed for the rectum were tested using data from the RT01 randomised prostate radiotherapy trial. Method: An independent validation of the rectal dose-volume constraints used in the CHHiP trial and proposed recently by Fiorino et al. was performed. The constraints were applied retrospectively to the treatment plans collected from the RT01 trial. Odds ratios (OR) were calculated to compare the reported incidence of specific late rectal toxicity end points in the group of patients whose treatment plan met a specified dose-volume constraint compared to the group of patients who failed that constraint. Results: Statistically significant ORs were observed for every constraint tested (except 75 Gy) for at least one clinical end point. For the CHHiP constraints between 60 and 70 Gy, the ORs calculated for rectal bleeding (RMH score defined in protocol) exceeded 2.5 (P!0.02). Similarly the ORs for CHHiP constraints between 30 and 65 Gy exceeded 2.4 (P!0.021) for urgency (UCLA PCI). The Fiorino constraints between 40 and 60 Gy resulted in ORs O2 (P!0.02) for loose stools (UCLA PCI) Conclusion: Implementing rectal dose-volume constraints from 30 Gy up to the prescription dose will result in a decrease in the incidence of late rectal toxicity. Constraints for doses as low as 30 Gy were statistically significant, further challenging the concept that the rectum is a serial structure where the maximum dose to the organ is the only consideration

Dose-volume constraints to reduce rectal side effects from prostate radiotherapy: evidence from MRC RT01 Trial ISRCTN 47772397.

PURPOSE: Radical radiotherapy for prostate cancer is effective but dose limited because of the proximity of normal tissues. Comprehensive dose-volume analysis of the incidence of clinically relevant late rectal toxicities could indicate how the dose to the rectum should be constrained. Previous emphasis has been on constraining the mid-to-high dose range (>/=50 Gy). Evidence is emerging that lower doses could also be important. METHODS AND MATERIALS: Data from a large multicenter randomized trial were used to investigate the correlation between seven clinically relevant rectal toxicity endpoints (including patient- and clinician-reported outcomes) and an absolute 5% increase in the volume of rectum receiving the specified doses. The results were quantified using odds ratios. Rectal dose-volume constraints were applied retrospectively to investigate the association of constraints with the incidence of late rectal toxicity. RESULTS: A statistically significant dose-volume response was observed for six of the seven endpoints for at least one of the dose levels tested in the range of 30-70 Gy. Statistically significant reductions in the incidence of these late rectal toxicities were observed for the group of patients whose treatment plans met specific proposed dose-volume constraints. The incidence of moderate/severe toxicity (any endpoint) decreased incrementally for patients whose treatment plans met increasing numbers of dose-volume constraints from the set of V3