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A quantitative study of IMRT delivery effects in commercial planning systems for the case of oesophagus and prostate tumours

Joint Department of Physics, Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT, UK

	Abstract

Top Abstract Introduction Method Results and discussion Conclusions References

 
This study focuses on understanding the impact of intensity-modulated radiotherapy (IMRT) delivery effects when applied to plans generated by commercial treatment-planning systems such as Pinnacle (ADAC Laboratories Inc.) and CadPlan/Helios (Varian Medical Systems). These commercial planning systems have had several version upgrades (with improvements in the optimization algorithm), but the IMRT delivery effects have not been incorporated into the optimization process. IMRT delivery effects include head-scatter fluence from IMRT fields, transmission through leaves and the effect of the rounded shape of the leaf ends. They are usually accounted for after optimization when leaf sequencing the "optimal" fluence profiles, to derive the delivered fluence profile. The study was divided into two main parts: (a) analysing the dose distribution within the planning-target volume (PTV), produced by each of the commercial treatment-planning systems, after the delivered fluence had been renormalized to deliver the correct dose to the PTV; and (b) studying the impact of the IMRT delivery technique on the surrounding critical organs such as the spinal cord, lungs, rectum, bladder etc. The study was performed for tumours of (i) the oesophagus and (ii) the prostate and pelvic nodes. An oesophagus case was planned with the Pinnacle planning system for IMRT delivery, via multiple-static fields (MSF) and compensators, using the Elekta SL25 with a multileaf collimator (MLC) component. A prostate and pelvic nodes IMRT plan was performed with the Cadplan/Helios system for a dynamic delivery (DMLC) using the Varian 120-leaf Millennium MLC. In these commercial planning systems, since IMRT delivery effects are not included into the optimization process, fluence renormalization is required such that the median delivered PTV dose equals the initial prescribed PTV dose. In preparing the optimum fluence profile for delivery, the PTV dose has been "smeared" by the IMRT delivery techniques. In the case of the oesophagus, the critical organ, spinal cord, received a greater dose than initially planned, due to the delivery effects. The increase in the spinal cord dose is of the order of 2–3 Gy. In the case of the prostate and pelvic nodes, the IMRT delivery effects led to an increase of approximately 2 Gy in the dose delivered to the secondary PTV, the pelvic nodes. In addition to this, the small bowel, rectum and bladder received an increased dose of the order of 2–3 Gy to 50% of their total volume. IMRT delivery techniques strongly influence the delivered dose distributions for the oesophagus and prostate/pelvic nodes tumour sites and these effects are not yet accounted for in the Pinnacle and the CadPlan/Helios planning systems. Currently, they must be taken into account during the optimization stage by altering the dose limits accepted during optimization so that the final (sequenced) dose is within the constraints.

	Introduction

Top Abstract Introduction Method Results and discussion Conclusions References

 
Inverse IMRT treatment-planning optimization is usually performed based on a set of physical and/or biological constraints, independent of the delivery method. Leaf sequencing then creates a set of MLC leaf positions/motions to generate an actual delivered fluence which matches the optimal inverse fluence as closely as possible. When converting the optimal plan, as calculated by the planning system, into a deliverable sequence of leaf positions (multiple-static fields, MSF or dynamic delivery, DMLC), the dose delivered to the planning target volume (PTV) and the organs at risk (OAR) will change depending on the MLC leaf characteristics taken into account during the leaf sequencing [1, 2]. In the present study, a quantitative analysis of the impact of IMRT delivery effects for 3 or 5 fields, uniformly distributed around the patient, for multiple-static fields (MSF), dynamic MLC or compensator delivery techniques has been investigated, for the case of two commercial planning systems and for two anatomical sites.

	Method

Top Abstract Introduction Method Results and discussion Conclusions References

 
Overview of IMRT delivery techniques: MSF, DMLC or Compensator
In converting the optimal fluence plan from the planning system into a deliverable sequence of leaf positions (MSF or DMLC) or compensator thickness, different sequencing algorithms are used. To calculate the leaf sequences for the MSF technique the ADAC (now Philips) Pinnacle planning system uses the K-means clustering algorithm [3]. This breaks the optimal fluence into smaller groups or clusters of equal value.

For the compensator, the density of the compensation material, width, height, resolution for the compensator and depth of the plane for the compensator shape optimization are defined. The energy fluence is then iteratively attenuated by the corresponding thickness of the modifier at each optimization fluence point. A "granular compensator" was selected with spatial resolution of 0.05 cm and material density of 4.9 g cm^–3 (approximate density of standard steel granulate compensator material). The source-to-compensator distance was 56.6 cm and the compensator was allowed a maximum thickness of 10 cm.

In the case of the DMLC technique, the optimal fluences are converted to the actual fluences (Varian terms) using the leaf motion calculator (LMC), which designs the leaf motion patterns. The LMC takes into account the various MLC parameters such as maximum leaf span (i.e. the physical length of the leaf), leaf speed, transmission, rounded-end effects and minimum leaf gaps. Since the X and Y jaws do not move during beam-on and hence cannot follow the trailing leaf, the maximum leaf span will determine how many MLC carriage positions (or static overlapping segments) will be required to deliver the fluence for a given field width (X jaws). The field is split into multiple overlapping fields of the appropriate number of carriage or jaw positions. Although the leaf motions are not fully synchronised, the time of travel across the field is the same for all leaf pairs, which helps to reduce the tongue-and-groove effect [4].

Patient setup and treatment objectives
Oesophagus tumour site and Pinnacle planning system
A patient with oesophageal carcinoma was planned with an Elekta SL25 linac using 3- or 5-field IMRT plans (gantry angles: 0° (anterior field), 120°, 240° and 0°, 72°, 144°, 216° and 288°), respectively, using P³IMRT (Pinnacle, ADAC version 6.0 g). In version 6.0 g, IMRT delivery effects have not been included in the optimization process.

The clinical target volumes (CTV), spinal cord and lung parenchyma were outlined on each image. The CTV region included both the oesophagus tumour and adjacent lymph nodes. The PTV region was generated by adding a three-dimensional margin of 15 mm to the CTV to account for movement and target definition uncertainties. The goal of the plan was to deliver 55 Gy to the PTV, while maintaining the spinal cord dose at less than 45 Gy and minimizing the dose to the lungs. The spinal cord dose constraint of 45 Gy is conservative. Martel et al [5] and Emami et al [6] have shown that, in head and neck cancers, the tolerance dose for the spinal cord is around 50 Gy, with a 5% chance of a complication occurring in 5 years. None of the patients treated by Martel et al [5] developed radiation myelitis, with the spinal cord receiving doses up to 50 Gy.

In addition to the spinal-cord dose constraint, the volume of lung irradiated to 18 Gy has been used as a planning constraint at the Royal Marsden NHS Foundation Trust. In the present study, no more than 20% of the lung could receive more than 18 Gy (V₁₈< 20%). The prediction of lung complications at the treatment-planning stage is not straightforward. There is no consensus on which dosimetric parameter should be used to reflect the clinical incidence of pneumonitis; however the volume of lung receiving 18 Gy was chosen [7]. Cardiac radiation toxicity for carcinoma of the oesophagus is not a major clinical concern because of the small number of long-term survivors. Therefore, the heart was not included as one of the OARs.

Prostate and pelvic nodes tumour site and Helios/CadPlan planning system
The patient with prostate carcinoma was planned with a 5-beam technique using CadPlan version 6.3.5, with the Helios inverse-planning module. Gantry angles of 180° (posterior), 270° (right lateral), 325° (right anterior oblique), 35° (left anterior oblique), 100° (left posterior oblique) were chosen such that the beams were approximately equispaced and were not opposing. These beam angles were found to provide good bowel sparing. The treatment was designed to deliver a dose of 70 Gy to the prostate and 50 Gy to the seminal vesicles and pelvic nodes. The prostate CTV was considered to be the entire visible prostate and was grown to a PTV with a 1 cm margin. However, if the overlap between the PTV and rectum was large, then the posterior margin was reduced to 8 mm. The nodal CTV was expanded to a PTV with a uniform 5 mm margin. The dose constraints used as goals for the prostate and pelvic node treatment were given by Clark et al [8].

For a prostate and pelvic node treatment with five gantry angles, typical beam lengths were 16–18 cm and beam widths were 10–18 cm. Typical monitor units (MUs) were 95 (for a section of the field) and 135 (for a maximum-width single field). The prescribed dose was 2.0 Gy per fraction to the median of the prostate PTV [8].

Clinical impact of IMRT delivery effects
An "optimum" IMRT plan was obtained from each of the commercial planning systems used. The optimum IMRT fluence profiles were then leaf-sequenced in order to generate the leaf positions or compensator thicknesses, to allow the delivery of the planned profile. A step-and-shoot K-means clustering algorithm from Pinnacle or the dynamic leaf motion calculators from CadPlan/Helios was used to leaf-sequence the IMRT profiles.

Each delivered IMRT plan would deliver a slightly different dose to the PTV, from that planned. The delivered IMRT fluence maps were then renormalized by the user such as to attain the prescribed dose to the median of the PTV, i.e. 55 Gy for the oesophagus tumour and 70 Gy for the prostate tumour. In the case of the oesophagus tumour site, the impact of clustering the IMRT fluence profiles was studied, where each fluence map was divided into equal fluence levels, using an error tolerance method equivalent to that described in Bär et al [9]. The Pinnacle sequencing software allows the error tolerance, i.e. the maximum difference between the optimal and sequenced fluence maps, to be specified. The effect of this was studied by specifying 2%, 5% and 10% tolerance levels.

In the case of the CadPlan/Helios planning system, only dose–volume values were compared before and after sequencing, for the secondary PTV and critical organs (rectum, bladder, etc.), for three prostate patients with nodal involvement because it was not possible to retrieve dose or fluence information before leaf sequencing, only dose–volume values. The dose–volume values compared correspond to the dose delivered to 90%, 75%, 50%, 25% and 10% of the volume of the bladder, rectum and small bowel and 95%, 75%, 50%, 25% and 5% of the volume of the PTV (prostate), right and left nodes.

	Results and discussion

Top Abstract Introduction Method Results and discussion Conclusions References

 
Oesophagus tumour site and Pinnacle planning system
Clinical impact of IMRT delivery effects on PTV
An "optimum" IMRT plan was obtained with the Pinnacle planning system for a prescribed dose to the PTV region of 55 Gy. The IMRT profiles were delivered using either the MSF or the compensator delivery techniques. In Figure 1, the DVH obtained for the delivered plans in case of 3 and 5 beams are compared with the optimal plan for the PTV (vertical axis starts at 45 Gy). In delivering the optimum fluence profile, the PTV dose has been "smeared" by both the MSF and the compensator techniques.

View larger version (12K): [in this window] [in a new window]   Figure 1. Dose–volume histograms (DVHs) of the optimum dose (OPT_PTV) or delivered dose to the planning target volume (PTV) by (a) 3 and (b) 5 intensity-modulated radiotherapy (IMRT) plans. The delivery techniques represented are the (i) multiple static fields (MSF) with 2%/compensator (red), 5% (blue) and 10% (green) error tolerance for the 3 beam and 5% (blue) and 10% (green) for the 5 beam and the (ii) compensator (orange) for the 5 beam case.

In the case of the 5-beam plan, the delivered plans for the MSF for 5% and 10% error tolerance and the compensator are shown to "smear" the dose distribution planned for the PTV. The 2% is not shown because the system would "crash", destroying any data previously calculated, possibly due to hardware and memory limitations. The compensator has produced the best delivered plan, of all the delivery methods represented for 5 beams.

In Figure 2, the dose distribution obtained on a CT slice of the patient for the 3-beam case, obtained for (a) the optimum plan, and MSF delivery with (b) 2% and (c) 5% error tolerance is shown. In Figure 2a, the 98% isodose curve conforms closely to the PTV region represented by the dark red circle. After delivery with the MSF technique, the high 98% isodose covers part of the adjacent lungs. In addition to this, the 80% isodose has significantly changed from the planned distribution (green curve in Figure 2a) to the delivered (green curve in Figure 2b,c). The lungs have received increased dose due to the inclusion of head-scatter and transmission. The spinal cord is also receiving an increased dose as may be observed comparing the 50% isodose (blue) curves in Figure 2.

View larger version (41K): [in this window] [in a new window]   Figure 2. A standard oesophageal dose distribution obtained from the(a) optimal and actual fluence delivered with MSF for (b) 2% and (c) 5% error tolerance. The 98%, 80%, 50% and 20% isodose levels are shown, respectively, as red, green, blue and yellow and the planning target volume (PTV) is thick red.

View larger version (14K): [in this window] [in a new window]   Figure 3. Dose–volume histograms (DVHs) obtained for the spinal cord in the case of (a) the 3 and (b) the 5 beam plans. The delivery techniques represented are the (i) multiple static fields (MSF) with 2% (red), 5% (blue) and 10% (green) error tolerance for the 3 beam and 5% (blue) and 10% (green) for the 5 beam and the (ii) compensator (orange) for the 5 beam case.

View larger version (27K): [in this window] [in a new window]   Figure 4. Dose–volume histograms (DVHs) obtained for the left and right lungs in the case of the 3 and 5 beams intensity-modulated radiotherapy (IMRT) plan.

Prostate and pelvic nodes and Helios planning system
An "optimum" IMRT plan was obtained with the Helios planning system with a prescribed dose to the PTV region of 70 Gy to the primary prostate tumour and 48 Gy to the nodes. The "prescribed" dose to the nodes was set at 48 Gy (not 50 Gy) at the planning stage, in order to compensate for the increase in the nodal dose after leaf-sequencing the optimum fluence. Therefore, the final dose delivered to the nodes will be approximately equal to 50 Gy. The optimum profiles were then leaf-sequenced in order to generate the leaf positions for the DMLC delivery. The plan was then calculated to allow the evaluation of the delivered profile and dose distribution. Each delivered IMRT plan would yield a different dose to the primary PTV (c.f. that determined by the optimization), with subsequent increase in dose to the secondary PTV (nodes) and OARs (bladder, small bowel and rectum).

The mean dose (over the group of three prostate patients) delivered to the volumes of interest "before" and "after" leaf-sequencing are presented (Table 1), for the various dose/volume points studied (the variation in the delivered dose, between the "after" and "before" case, is also given after each volume of interest in brackets). In addition to this, a typical DVH for a single patient is also presented in Figure 5 to illustrate the effect of leaf-sequencing on the final patient DVH. In the case of the OARs there is an increase of up to 3.32 Gy in the delivered dose to 50% of the total volume of the small bowel (c.f. Table 1 and Figure 5). The bladder and rectum are subject to 2.15 Gy and 3.03 Gy more dose, respectively, delivered to 50% of their total volume. The majority of the OARs volumes (bladder, small bowel and rectum) have received significantly more dose delivered to 90% of their volume. In the case of the rectum, this increase was the largest, being in the order of 5 Gy to 90% of its total volume. In the case of the bladder and small bowel, the increases in the dose delivered to 90% of the total volume were 3.78 Gy and 3.06 Gy, respectively. The greater increase in dose delivered to the OARs as compared with the increase to the PTVs is due to the extra transmission and scatter delivered to the OAR during the times when they are shielded by the MLCs. The proportion of time when the OARs are covered by the MLCs is greater than for the targets and therefore the increase in dose to the OARs is also greater.

View larger version (18K): [in this window] [in a new window]   Figure 5. A dose–volume histogram (DVH) of an example patient with prostate carcinoma and nodal involvement (with right (Node_RT) and left (Node_LT) nodes considered separately). The organs at risk considered are the bladder, small bowel and rectum. The dose values are presented for the two cases: (i) before (BEF) shown as points and (ii) after (continuous lines) leaf sequencing with the planning target volume (PTV) dose being normalized to 70 Gy. Arrows indicate the increase in the dose delivered to the volume of interest due to leaf-sequencing effects.

In Van Esch et al [10], Clark et al [11] and Hong et al [12] the transmission fluence for MLC leaves measured contributed an additional 1.7%, approximately, to the delivered fluence from Varian Linacs using the dynamic mode of delivery. The increase of approximately 2 Gy in the dose delivered to 50% volume of the pelvic nodes (Table 1) is mainly constituted of approximately 1.7% of transmission fluence (leading to an extra 0.85 Gy in deposited dose) and 2.3% of additional head scatter fluence (leading to an extra 1.15 Gy in deposited dose). The overall increase in the delivered dose is observed for all the volumes of interest: bladder, small bowel, rectum and nodes but not the primary PTV/prostate, to which the dose is renormalized "after" leaf-sequencing.

	Conclusions

Top Abstract Introduction Method Results and discussion Conclusions References

 
The impact of IMRT delivery effects on two commercial treatment-planning systems: Pinnacle planning system (ADAC Laboratories Inc.) and CadPlan/Helios (Varian Medical Systems) planning system, was evaluated. The study was performed separately for the oesophagus and the prostate (with pelvic nodes) tumour sites, where multiple static fields and compensator delivery techniques were evaluated for the first tumour site and DMLC was evaluated for the second tumour site.

In the case of the oesophagus tumour site and using the Pinnacle planning system, the IMRT delivery effects were shown to produce a "smearing" of the dose to the PTV (oesophagus) and an increase of 2–3 Gy in the dose delivered to the spinal cord. For the prostate (and pelvic nodes) tumour site planned with the CadPlan/Helios (Varian Medical Systems) system, an increase in delivered dose of approximately 3 Gy was observed for 50% of the total volume of the OARs: bladder, rectum and small bowel. In addition to this, an increase of approximately 2 Gy was observed in the dose delivered to the 50% of the total volume of the pelvic nodes, the secondary PTV. The results obtained show that if delivery effects are not accounted for at the planning/optimization stage, then an increase in delivered dose to the volumes of interest may be expected, after leaf sequencing the optimum IMRT profiles. Currently, they must be taken into account during the optimization stage by altering the dose limits accepted during optimization so that the final (sequenced) dose is within the desired constraints.

The work is supported by Cancer Research UK (under reference grant SP 2313|0201) and the Institute of Cancer Research.

Received for publication February 9, 2005. Accepted for publication September 7, 2005.

	References

Top Abstract Introduction Method Results and discussion Conclusions References

 

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