Planning, computer optimization, and dosimetric verification of a segmented irradiation technique for prostate cancer

Evaluation of a combination tumor treatment using thermo-triggered liposomal drug delivery and carbon ion irradiation

The combination of radiotherapy with chemotherapy is one of the most promising strategies for cancer treatment. Here, a novel combination strategy utilizing carbon ion irradiation as a high-linear energy transfer (LET) radiotherapy and a thermo-triggered nanodevice is proposed, and drug accumulation in the tumor and treatment effects are evaluated using magnetic resonance imaging relaxometry and immunohistology (Ki-67, n = 15). The thermo-triggered liposomal anticancer nanodevice was administered into colon-26 tumor-grafted mice, and drug accumulation and efficacy was compared for 6 groups (n = 32) that received or did not receive the radiotherapy and thermo trigger. In vivo quantitative R₁ maps visually demonstrated that the multimodal thermosensitive polymer-modified liposomes (MTPLs) can accumulate in the tumor tissue regardless of whether the region was irradiated by carbon ions or not. The tumor volume after combination treatment with carbon ion irradiation and MTPLs with thermo-triggering was significantly smaller than all the control groups at 8 days after treatment. The proposed strategy of combining high-LET irradiation and the nanodevice provides an effective approach for minimally invasive cancer treatment.

A predictive model to guide management of the overlap region between target volume and organs at risk in prostate cancer volumetric modulated arc therapy

PURPOSE

The goal of this study is to determine whether the magnitude of overlap between planning target volume (PTV) and rectum (Rectumoverlap) or PTV and bladder (Bladderoverlap) in prostate cancer volumetric-modulated arc therapy (VMAT) is predictive of the dose-volume relationships achieved after optimization, and to identify predictive equations and cutoff values using these overlap volumes beyond which the Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) dose-volume constraints are unlikely to be met.

MATERIALS AND METHODS

Fifty-seven patients with prostate cancer underwent VMAT planning using identical optimization conditions and normalization. The PTV (for the 50.4 Gy primary plan and 30.6 Gy boost plan) included 5 to 10 mm margins around the prostate and seminal vesicles. Pearson correlations, linear regression analyses, and receiver operating characteristic (ROC) curves were used to correlate the percentage overlap with dose-volume parameters.

RESULTS

The percentage Rectumoverlap and Bladderoverlap correlated with sparing of that organ but minimally impacted other dose-volume parameters, predicted the primary plan rectum V45 and bladder V50 with R(2) = 0.78 and R(2) = 0.83, respectively, and predicted the boost plan rectum V30 and bladder V30 with R(2) = 0.53 and R(2) = 0.81, respectively. The optimal cutoff value of boost Rectumoverlap to predict rectum V75 >15% was 3.5% (sensitivity 100%, specificity 94%, p < 0.01), and the optimal cutoff value of boost Bladderoverlap to predict bladder V80 >10% was 5.0% (sensitivity 83%, specificity 100%, p < 0.01).

CONCLUSION

The degree of overlap between PTV and bladder or rectum can be used to accurately guide physicians on the use of interventions to limit the extent of the overlap region prior to optimization.

Fixed number of segments in unidirectional decompositions of fluence matrices for step-and-shoot IMRT

The decomposition of a fluence matrix in step-and-shoot mode for intensity-modulated radiation therapy (IMRT) usually yields a large number of segments (NS) and, consequently, treatment time is substantially increased. In this paper, we propose a method for reducing the original NS in multileaf collimator segmentations to a user-specified quantity. The proposed method clusters original segments into the same number of groups as desired NS, and computes for each group an equivalent segment and an associated weight. In order to avoid important changes in dose-volume histograms (DVHs), equivalent segments and weights are computed taking into account the original fluence matrix and preserving the highest fluence zones, thus staying as close as possible to the original planned radiation. The method is applicable to unidirectional segmentations, where there is no backtracking of leaves, since this property facilitates the grouping of segments. The experiments showed that treatment times can be considerably reduced, while maintaining similar DVHs and dosimetric indexes. Furthermore, the algorithm achieved an excellent reduction/dose-quality ratio since the final NS was close to that reported for direct step-and-shoot solutions.

A planning comparison of dose patterns in organs at risk and predicted risk for radiation induced malignancy in the contralateral breast following radiation therapy of primary breast using conventional, IMRT and volumetric modulated arc treatment techniques

PURPOSE

To investigate the impact of using different radiation therapy techniques on contra-lateral breast (CB) dose, and also dose to other involved organs at risk such as heart and lungs following radiation therapy of breast and regional lymph nodes. Furthermore, to predict the risk for induced malignancies in CB using linear and non linear models.

MATERIAL AND METHODS

Eight patients with stage II-III breast cancer were included in this analysis. It was focused on three treatment techniques; conventional radiotherapy technique forwardly planed, IMRT and volumetric modulated arc (RapidArc) techniques, inversely planed. The CC algorithm was employed to calculate the standard treatment plans whereas for the IMRT and RapidArc treatment plans AAA algorithm was adopted. The dose results based on mostly DVH analysis were compared. The excess relative risk (ERR) for cancer induction in CB, employed both linear and non-linear models, was estimated.

RESULTS

A better homogeneity and conformation in PTV was observed in the RapidArc plans. The highest minimum dose to PTV was observed in the conventional plans while no difference was observed for minimum significant doses D(98%) and D(99%) where D(X%) is the dose received by X% of the PTV volume. In terms of organ sparing, the IMRT and RapidArc plans spare ipsilateral-lung better, but a 40% lower mean dose in the contra-lateral lung in the conventional plans is observed. The mean dose to the contra-lateral breast was lowest for the RapidArc plans as well as the V(10Gy) and the maximum dose. The mean predicted ERR for the eight patients were lower for the conventional and RA plans than for the IMRT plans assuming a linear dose-risk relationship. The mean predicted ERR when using a non linear model was lower for all the three techniques (with lowest ERR for RapidArc plans).

CONCLUSIONS

From a clinical perspective, it should be concluded that all three solutions investigated in the study can offer high quality treatment of patients. Further comparative analysis of the two algorithms used in the present study, however, should be performed especially on the peripheral organ dose. The impact of CB exposure to a low-dose radiation on minimizing the risk of radiation induced malignancy in CB can be interpreted differently when using linear or non linear models to predict ERR. In general, no detriment was observed when using RapidArc compared to conventional treatments while a potentially higher risk could be associated to IMRT treatments with fixed gantry.

Arc-modulated radiation therapy (AMRT): a single-arc form of intensity-modulated arc therapy

Arc-modulated radiation therapy (AMRT) is a novel rotational intensity-modulated radiation therapy (IMRT) technique developed for a clinical linear accelerator that aims to deliver highly conformal radiation treatment using just one arc of gantry rotation. Compared to fixed-gantry IMRT and the multiple-arc intensity-modulated arc therapy (IMAT) techniques, AMRT promises the same treatment quality with a single-arc delivery. In this paper, we present a treatment planning scheme for AMRT, which addresses the challenges in inverse planning, leaf sequencing and dose calculation. The feasibility and performance of this AMRT treatment planning scheme have been verified with multiple clinical cases of various sites on Varian linear accelerators.

Leaf-sequencing for intensity-modulated arc therapy using graph algorithms

Intensity-modulated arc therapy (IMAT) is a rotational IMRT technique. It uses a set of overlapping or nonoverlapping arcs to create a prescribed dose distribution. Despite its numerous advantages, IMAT has not gained widespread clinical applications. This is mainly due to the lack of an effective IMAT leaf-sequencing algorithm that can convert the optimized intensity patterns for all beam directions into IMAT treatment arcs. To address this problem, we have developed an IMAT leaf-sequencing algorithm and software using graph algorithms in computer science. The input to our leaf-sequencing software includes (1) a set of (continuous) intensity patterns optimized by a treatment planning system at a sequence of equally spaced beam angles (typically 10 degrees apart), (2) a maximum leaf motion constraint, and (3) the number of desired arcs, k. The output is a set of treatment arcs that best approximates the set of optimized intensity patterns at all beam angles with guaranteed smooth delivery without violating the maximum leaf motion constraint. The new algorithm consists of the following key steps. First, the optimized intensity patterns are segmented into intensity profiles that are aligned with individual MLC leaf pairs. Then each intensity profile is segmented into k MLC leaf openings using a k-link shortest path algorithm. The leaf openings for all beam angles are subsequently connected together to form 1D IMAT arcs under the maximum leaf motion constraint using a shortest path algorithm. Finally, the 1D IMAT arcs are combined to form IMAT treatment arcs of MLC apertures. The performance of the implemented leaf-sequencing software has been tested for four treatment sites (prostate, breast, head and neck, and lung). In all cases, our leaf-sequencing algorithm produces efficient and highly conformal IMAT plans that rival their counterpart, the tomotherapy plans, and significantly improve the IMRT plans. Algorithm execution times ranging from a few seconds to 2 min are observed on a laptop computer equipped with a 2.0 GHz Pentium M processor.

Intensity modulated neutron radiotherapy for the treatment of adenocarcinoma of the prostate

PURPOSE

This study investigates the enhanced conformality of neutron dose distributions obtainable through the application of intensity modulated neutron radiotherapy (IMNRT) to the treatment of prostate adenocarcinoma.

METHODS AND MATERIALS

An in-house algorithm was used to optimize individual segments for IMNRT generated using an organ-at-risk (OAR) avoidance approach. A number of beam orientation schemes were investigated in an attempt to approach an optimum solution. The IMNRT plans were created retrospectively for 5 patients previously treated for prostate adenocarcinoma using fast neutron therapy (FNT), and a comparison of these plans is presented. Dose distributions and dose-volume histograms (DVHs) were analyzed and plans were evaluated based on percentage volumes of rectum and bladder receiving 95%, 80%, and 50% (V(95), V(80), V(50)) of the prescription dose, and on V(60) for both the femoral heads and GM(muscle) group.

RESULTS

Plans were normalized such that the IMNRT DVHs for prostate and seminal vesicles were nearly identical to those for conventional FNT plans. Use of IMNRT provided reductions in rectum V(95) and V(80) of 10% (2-27%) and 13% (5-28%), respectively, and reductions in bladder V(95) and V(80) of 12% (3-26%) and 4% (7-10%), respectively. The average decrease in V(60) for the femoral heads was 4.5% (1-18%), with no significant change in V(60) for the GM(muscle) group.

CONCLUSIONS

This study provides the first analysis of the application of intensity modulation to neutron radiotherapy. The IMNRT technique provides a substantial reduction in normal tissue dose in the treatment of prostate cancer. This reduction should result in a significant clinical advantage for this and other treatment sites.

Absolute and relative dose–surface and dose–volume histograms of the bladder: which one is the most representative for the actual treatment?

The purpose of this study was to quantify to what extent relative and absolute bladder dose-volume and dose-surface histograms of the planning CT scan were representative for the actual treatment. We used data of 17 patients, who each received 11 repeat CT scans and a planning CT scan. The repeat CT scans were matched on the planning CT scan by the bony anatomy. Clinical treatment plans were used to evaluate the impact of bladder filling changes on the four histogram types. The impact was quantified by calculating for this patient group the correlation coefficient between the planning histogram and the treatment histogram. We found that the absolute dose-surface histogram was the most representative one for the actual treatment.

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.

Additional dose constraints for analytical beam weighting optimization in IMRT

This work presents an improvement to an algorithm for analytical beam weighting optimization where a flexible objective function, which considers 'importance factors' for each anatomical region and 'allowed deviations' from the prescribed dose, is defined. This upgrading allows forcing the mean value of the dose distribution to be the desired value, by using Lagrange multipliers. A real case is presented to show the effect of this change.

Choix des contraintes et amélioration dosimétrique d’une radiothérapie conformationnelle du cancer de la prostate modulée en intensité pendant une partie du traitement

PURPOSE

Intensity modulated radiation therapy (IMRT) is based on a methodology called inverse planning. Starting from dosimetric objectives, constraints of optimization are fixed and given to the inverse planning system, which in turn calculates the modulated intensity to apply to each beam. Since the algorithms allow the constraints to be violated, the results of optimization may differ from the initial dosimetric objectives. Consequently, the user is compelled to adapt the choice of the constraints according to the type of modulation and until satisfactory results are found. The purpose of this work is to present our experience in the choice of these constraints for prostate cancer treatments, as we moved from conformal radiotherapy to IMRT. Treatments were performed with a Varian 23EX linac and calculations were realized with the Varian CadPlan-Helios planning system.

PATIENTS AND METHODS

The approach used for the first 12 patients treated at institut Curie with IMRT from June 2002 was analysed. The treatment always consisted of a combination of conformal radiotherapy with and without intensity modulation.

RESULTS AND CONCLUSION

Results showed that, a larger fraction of the treatment performed with IMRT induced a better sparing of the organs at risk for the same homogeneous dose distribution to the target volume. Apart from the dose-volume constraint for the rectum, a fixed set of constraints, slightly more restrictive than the dosimetric objectives, could be used for all patients. Compared with conformal radiotherapy, the conformation factor for IMRT increased up to 16%. A specific study was undertaken in view of treatments completely performed with IMRT. The optimal technique consisted in performing separated IMRT plans for the two target volumes, the prostate volume and the prostate plus seminal vesicles volume respectively. Another satisfactory possibility was to define new constraints on two separated planning target volumes, prostate and seminal vesicles. This last approach is now routinely implemented for our IMRT patients.

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.

Simplified intensity-modulated radiotherapy using pre-defined segments to reduce cardiac complications in left-sided breast cancer

BACKGROUND AND PURPOSE

Left-sided breast cancer patients pose a difficult clinical challenge when significant heart and contralateral breast irradiation are present, particularly with tangential uniform beams. The aims of the study are: (1) to design and evaluate a simplified intensity-modulated radiotherapy (IMRT) (SI) solution using pre-defined segments, (2) to compare the SI technique with a conformal (CN) and a full fluence IMRT (FI) approach using two sets of beam orientations, clinical (-C) and optimal (-O), and (3) to quantify the benefits of treatment technique and beam orientation.

PATIENTS AND METHODS

Nine left-sided breast cancer patients with a maximum heart distance of at least 2.0 cm were planned using three different techniques and two different beam orientations. All three techniques were planned using clinical orientations (i.e. CN-C, FI-C and SI-C). Two techniques were planned using more optimal orientations (i.e. FI-O and SI-O). Dose-volume histograms and radiobiologic modelling are used for plan evaluation.

RESULTS

The average mean planning target volume (PTV) doses are 91.6+/-4.5, 98.4+/-6.3, 102.0+/-8.7, 100.0+/-5.9 and 103.9+/-8.3% for the CN-C, FI-C, SI-C, FI-O and SI-O plans, respectively. The average normal tissue complication probabilities for late excess cardiac mortality are 2.1+/-0.6, 0.2+/-0.1, 0.2+/-0.1, 0.1+/-0.0 and 0.1+/-0.0%, respectively. For a given beam orientation, FI plans are the best and CN plans are the worst. The dose distributions for the SI-C and FI-C plans are almost identical with significant heart sparing but at a cost of some target underdosage. The dose distributions are better conformed around the PTV with more optimal beam orientations, resulting in better sparing of adjacent organs at risk. FI-C plans are inferior to SI-O plans.

CONCLUSIONS

For clinical uniform two-beam orientations, significant heart sparing is possible with the addition of intensity modulation but at the expense of worsening target coverage. Simplified IMRT can, for all intents, be substituted for full IMRT with clinical beam orientations. Applying more optimal non-uniform beam orientations improves PTV coverage while maintaining significant heart sparing but increases the PTV dose heterogeneity.

Quantification of local rectal wall displacements by virtual rectum unfolding

BACKGROUND AND PURPOSE

To develop a method to project surface elements of a bent tubular organ, e.g. the rectum, in order to create a two-dimensional (2D) map and to use this method to quantify on a local scale shape and position variations of the rectum.

PATIENTS AND METHODS

For this study we used data of 19 patients, who each received a planning CT scan and 9-13 repeat CT scans that were considered representative for the radiotherapy course. We combined maps from multiple CT scans of the same patient to quantify local rectal wall displacements. To make a map we first computed a central axis through the rectum and divided it into segments of equal length assuming that the length of these segments was invariant under rectum shape and position changes. Next, we constructed for each segment a planar cross section through the rectum, which was oriented orthogonally to that segment. The amount of rectal wall tissue was assumed to be constant in all orthogonal cross sections throughout the entire rectum. We unfolded the cross-sected rectal wall at the dorsal side and projected either the associated dose or the coordinates onto the map.

RESULTS

The largest variation in the position of the rectal wall during the treatment course occurred at the upper anterior, left and right side (1 SD=5-7 mm). Near the anus the variation was <3 mm (1 SD) and at the posterior side of the rectum <4 mm (1 SD). The anterior-posterior (AP) and left-right displacements between the rectum in the planning CT scan and the mean rectum shape during the treatment were localized between 40 and 80% of the central axis. At the upper anterior, left, and right side the displacements were 5-8 mm (1 SD). These rectal wall displacements correlated with the rectum volume in the planning CT scan. At the upper anterior side the correlation coefficient between the AP displacements and the planning rectum volume was 0.85.

CONCLUSIONS

We quantified variations in rectum shape and in dose in the rectal wall. The systematic error in rectal wall position was found to be larger than the random shape and position variations. We successfully developed a method to virtually unfold a rectum and to project the dose onto a 2D map. The spatial information of the dose distribution can be used in the analysis of rectum complications.

Computer optimization of class solutions designed on a beam segmentation basis

BACKGROUND AND PURPOSE

A method for analytically solving the optimization of beam weighting in radiotherapy treatments using beam segmentation is presented.

PATIENTS AND METHODS

A technique has been elaborated permitting the optimization of a flexible objective function which is defined by considering 'importance factors' for each anatomical region and 'allowed deviations' from the prescribed dose. As any change in these importance factors may lead to very different solutions, a statistical tool has been developed which varies the objective function automatically and iteratively to get the best possible results compatible with the chosen class solution. In addition, the spatial symmetry found in many anatomical sites is taken advantage of. Furthermore, freeware code has been written to run this optimization approach. The guidelines to design the beam segmentation used in our institution using organ avoidance criteria, and hence the suitable class solution for different anatomical sites, are given. A treatment-planning study for three anatomical sites is presented and, for two of them, the results obtained with both the suggested and the classical inverse approach are presented.

RESULTS

The work presented might be used for beam weighting optimization in any radiotherapy treatment and furthermore, the suggested procedure may successfully confront the intensity-modulated radiation therapy (IMRT) problem and the obtained dose distributions fit the clinical constraints for all anatomical sites studied. When comparing with the classical inverse approach, both results are comparable in terms of dose distribution, but the suggested technique reduces the integral dose as the total number of monitor units is lower.

CONCLUSIONS

The developed code performs the optimization with a very low time cost and in addition, this process can be carried out with a conventional treatment-planning system with no need of dedicated IMRT software.

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.

Impact of a micromultileaf collimator on stereotactic radiotherapy of uveal melanoma

PURPOSE

To evaluate the impact of a micro multileaf collimator (mMLC) on Linac-based stereotactic radiotherapy (SRT) of uveal melanoma by comparing circular arc with static conformal, dynamic arc, and intensity-modulated SRT.

MATERIALS AND METHODS

Forty uveal melanoma patients were selected from approximately 100 patients treated with SRT since 1996. For each patient, four treatment plans (BrainSCAN XL, V5.0) were made: conventional arc, static conformal, dynamic arc plan, and intensity-modulated radiotherapy (IMRT). The goal of treatment planning was to fully encompass the planning target volume (PTV) by the 80% isodose while minimizing doses to the optic nerve and lens. The following parameters were evaluated: target conformity; target homogeneity; ratio of the target volume and 50% isodose volume; normal tissue receiving doses >/=80%, >/=50%, and >/=20%; central nervous system volume irradiated to >/=20%; optical nerve volume irradiated >/=50%, D(max) of the lens; lens volume receiving >/=20%; and monitor units.

RESULTS

PTVs ranged from 0.68 to 4.90 cm(3) (mean 1.97 +/- 0.97 cm(3)). The average reduction of the prescription isodose volume was 1-1.5 cm(3) for conformal (range 2.6-0.3 cm(3)), dynamic arc (range 2.5-0.3 cm(3)), and IMRT plans (range 3.9-0.1 cm(3)), compared with conventional arc therapy. Central nervous system volumes irradiated to doses >/=20% were smallest for conventional or dynamic arc treatments. Average target dose homogeneity values were 1.74 +/- 0.50 for arc, 1.27 +/- 0.02 for static mMLC, 1.26 +/- 0.01 for dynamic arc, and 1.15 +/- 0.03 for IMRT plans. IMRT helped to reduce doses to the lens but did not provide an advantage for optical nerve sparing. When applying IMRT, the monitor units increased by approximately one-third compared with static mMLC-based SRT.

CONCLUSIONS

Conformal mMLC and dynamic arc SRT are the treatment options of choice for Linac-based SRT of uveal melanoma. They present dosimetric advantages, while being highly efficient in treatment planning and delivery.

Inverse and forward optimization of one- and two-dimensional intensity-modulated radiation therapy-based treatment of concave-shaped planning target volumes: the case of prostate cancer

BACKGROUND

Intensity-modulated radiation therapy (IMRT) was suggested as a suitable technique to protect the rectal wall, while maintaining a satisfactory planning target volume (PTV) irradiation in the case of high-dose radiotherapy of prostate cancer. However, up to now, few investigations tried to estimate the expected benefit with respect to conventional three-dimensional (3D) conformal radiotherapy (CRT).

PURPOSE

Estimating the expected clinical gain coming from both 1D and 2D IMRT against 3DCRT, in the case of prostate cancer by mean of radiobiological models. In order to enhance the impact of IMRT, the case of concave-shaped PTV including prostate and seminal vesicles (P+SV) was considered.

MATERIALS AND METHODS

Five patients with concave-shaped PTV including P+SV were selected. Two different sets of constraints were applied during planning: in the first one a quite large inhomogeneity of the dose distribution within the PTV was accepted (set (a)); in the other set (set (b)) a greater homogeneity was required. Tumor control probability (TCP) and normal tissue control probability (NTCP) indices were calculated through the Webb-Nahum and the Lyman-Kutcher models, respectively. Considering a dose interval from 64.8 to 100.8 Gy, the value giving a 5% NTCP for the rectum was found (D(NTCP(rectum)=5%)) using two different methods, and the corresponding TCP(NTCP(rectum)=5%) and NTCP(NTCP(rectum)=5%) for the other critical structures were derived. With the first method, the inverse optimization of the plans was performed just at a fixed 75.6 Gy ICRU dose; with the second method (applied to 2/5 patients) inverse treatment plannings were re-optimized at many dose levels (from 64.8 to 108 Gy with 3.6 Gy intervals). In this case, three different values of alpha/beta (10, 3, 1.5)were used for TCP calculation. The 3DCRT plan consisted of a 3-fields technique; in the IMRT plans, five equi-spaced beams were applied. The Helios Inverse Planning software from Varian was used for both the 2D IMRT and the 1D IMRT inverse optimization, the last one being performed fixing only one available pair of leaves for modulation. A previously proposed forward 1D IMRT 'class solution' technique was also considered, keeping the same irradiation geometry of the inversely optimized IMRT techniques.

RESULTS

With the first method, the average gains in TCP(NTCP(rectum)=5%) of the 2D IMRT technique, with respect 3DCRT, were 10.3 and 7.8%, depending on the choice of the DVHs constraints during the inverse optimization procedure (set (a) and set (b), respectively). The average gain (DeltaTCP(NTCP(rectum)=5%)) coming from the inverse 1D IMRT optimization was 5.0%, when fixing the set (b) DVHs constraints. Concerning the forward 1D IMRT optimization, the average gain in TCP(NTCP(rectum)=5%) was 4.5%. The gain was found to be correlated with the degree of overlapping between rectum and PTV. When comparing 2D IMRT and 1D IMRT, in the case of the more realistic set (b) constraints, DeltaTCP(NTCP(rectum)=5%) was always less than 3%, excepting one patient with a very large overlap region. Basing our choice on this result, the second method was applied to this patient and one of the remaining. Through the inverse re-optimization of the treatment plans at each dose level, the gain in TCP(NTCP(rectum)=5%) of the inverse 2D technique was significantly higher than the ones obtained by applying the first method (concerning the two patients: +6.1% and +2.4%), while no significant benefit was found for inverse 1D. The impact of changing the alpha/beta ratio was less evident in the patient with the lower gain in TCP(NTCP(rectum)=5%).

CONCLUSIONS

The expected benefit due to IMRT with respect to 3DCRT seems to be relevant when the overlap between PTV and rectum is high. Moreover, the difference between the inverse 2D and the simpler inverse or forward 1D IMRT techniques resulted in being relatively modest, with the exception of one patient, having a very large overlap between rectum and PTV. Optimizing the inverse planning at each dose level to find TCP(NTCP(rectum)=5%)e level to find TCP(NTCP(rectum)=5%) can improve the performances of inverse 2D IMRT, against a significant increase of the time for planning. These results suggest the importance of selecting the patients that could have significant benefit from the application of IMRT.

A DICOM-RT-based toolbox for the evaluation and verification of radiotherapy plans

The verification of radiotherapy plans is an essential step in the treatment planning process. This is especially important for highly conformal and IMRT plans which produce non-intuitive fluence maps and complex 3D dose distributions. In this work we present a DICOM (Digital Imaging and Communication in Medicine) based toolbox, developed for the evaluation and the verification of radiotherapy treatment plans. The toolbox offers the possibility of importing treatment plans generated with different calculation algorithms and/or different optimization engines and evaluating dose distributions on an independent platform. Furthermore the radiotherapy set-up can be exported to the BEAM Monte Carlo code system for dose verification. This can be done by simulating the irradiation of the patient CT dataset or the irradiation of a software-generated water phantom. We show the application of some of the functions implemented in this toolbox for the evaluation and verification of an IMRT treatment of the head and neck region.

Intensity modulated versus non-intensity modulated radiotherapy in the treatment of the left breast and upper internal mammary lymph node chain: a comparative planning study

BACKGROUND AND PURPOSE

To compare and evaluate intensity modulated (IMRT) and non-intensity modulated radiotherapy techniques in the treatment of the left breast and upper internal mammary lymph node chain.

MATERIALS AND METHODS

The breast, upper internal mammary chain (IMC), heart and lungs were delineated on a computed tomography (CT)-scan for 12 patients. Three different treatment plans were created: (1) tangential photon fields with oblique IMC electron-photon fields with manually optimized beam weights and wedges, (2) wide split tangential photon fields with a heart block and computer optimized wedge angles, and (3) IMRT tangential photon fields. For the IMRT technique, an inverse planning program (KonRad) generated the intensity profiles and a clinical three-dimensional treatment planning system (U-MPlan) optimized the segment weights. U-MPlan calculated the dose distribution for all three techniques. The normal tissue complication probabilities (NTCPs) for the organs at risk (ORs) were calculated for comparison.

RESULTS

The average root mean square deviation of the differential dose-volume histogram of the breast planning target volume was 4.6, 3.9 and 3.5% and the average mean dose to the IMC was 97.2, 108.0 and 99.6% for the oblique electron, wide split tangent and IMRT techniques, respectively. The average NTCP for the ORs (i.e. heart and lungs) were comparable between the oblique electron and IMRT techniques (<or=0.7%). The wide split tangent technique resulted in higher NTCP values (>or=2%) for the ORs.

CONCLUSIONS

The lowest NTCP values were found with the oblique electron and the IMRT techniques. The IMRT technique had the best breast and IMC target coverage.

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.

Intensity-modulated radiotherapy: current status and issues of interest

PURPOSE

To develop and disseminate a report aimed primarily at practicing radiation oncology physicians and medical physicists that describes the current state-of-the-art of intensity-modulated radiotherapy (IMRT). Those areas needing further research and development are identified by category and recommendations are given, which should also be of interest to IMRT equipment manufacturers and research funding agencies.

METHODS AND MATERIALS

The National Cancer Institute formed a Collaborative Working Group of experts in IMRT to develop consensus guidelines and recommendations for implementation of IMRT and for further research through a critical analysis of the published data supplemented by clinical experience. A glossary of the words and phrases currently used in IMRT is given in the. Recommendations for new terminology are given where clarification is needed.

RESULTS

IMRT, an advanced form of external beam irradiation, is a type of three-dimensional conformal radiotherapy (3D-CRT). It represents one of the most important technical advances in RT since the advent of the medical linear accelerator. 3D-CRT/IMRT is not just an add-on to the current radiation oncology process; it represents a radical change in practice, particularly for the radiation oncologist. For example, 3D-CRT/IMRT requires the use of 3D treatment planning capabilities, such as defining target volumes and organs at risk in three dimensions by drawing contours on cross-sectional images (i.e., CT, MRI) on a slice-by-slice basis as opposed to drawing beam portals on a simulator radiograph. In addition, IMRT requires that the physician clearly and quantitatively define the treatment objectives. Currently, most IMRT approaches will increase the time and effort required by physicians, medical physicists, dosimetrists, and radiation therapists, because IMRT planning and delivery systems are not yet robust enough to provide totally automated solutions for all disease sites. Considerable research is needed to model the clinical outcomes to allow truly automated solutions. Current IMRT delivery systems are essentially first-generation systems, and no single method stands out as the ultimate technique. The instrumentation and methods used for IMRT quality assurance procedures and testing are not yet well established. In addition, many fundamental questions regarding IMRT are still unanswered. For example, the radiobiologic consequences of altered time-dose fractionation are not completely understood. Also, because there may be a much greater ability to trade off dose heterogeneity in the target vs. avoidance of normal critical structures with IMRT compared with traditional RT techniques, conventional radiation oncology planning principles are challenged. All in all, this new process of planning and treatment delivery has significant potential for improving the therapeutic ratio and reducing toxicity. Also, although inefficient currently, it is expected that IMRT, when fully developed, will improve the overall efficiency with which external beam RT can be planned and delivered, and thus will potentially lower costs.

CONCLUSION

Recommendations in the areas pertinent to IMRT, including dose-calculation algorithms, acceptance testing, commissioning and quality assurance, facility planning and radiation safety, and target volume and dose specification, are presented. Several of the areas in which future research and development are needed are also indicated. These broad recommendations are intended to be both technical and advisory in nature, but the ultimate responsibility for clinical decisions pertaining to the implementation and use of IMRT rests with the radiation oncologist and radiation oncology physicist. This is an evolving field, and modifications of these recommendations are expected as new technology and data become available.