Conformality Study for Stereotactic Radiosurgery of the Lung
Article Outline
Abstract
The purpose of this study is to compare two techniques of developing highly conformal plans for stereotactic body radiation therapy (SBRT) that target a high ablative dose to the center of the tumor while dropping off rapidly in normal tissues to determine which technique produced a more desirable treatment plan. The techniques used for comparison are “field in field” (FIF) and “non field in field” (NFIF). Twelve case studies were used, all of which had been treated using the FIF technique. Each FIF plan was edited, maintaining the same geometry for each field but reducing the total number of fields to one half by deleting all of the fields that were inside another field; this edited plan was the NFIF plan. Normalization was made to the isodose line (NFIF-I) and to the target volume (NFIF-V) and both plans were compared with the standard FIF plan independently. Dose-ratio comparisons were made of the 80% and 50% isodose volumes, as well as maximum doses outside of the planning target volume, mean dose to the gross tumor volume (GTV), minimum dose coverage on the GTV, maximum dose to the spinal cord, and the dose to the volume of noninvolved lung receiving 2000 cGy (V20). The FIF plans resulted in the best sparing of normal tissue. The NFIF-I had the best target coverage but also resulted in the highest doses to normal tissues. The NFIF-V was not significantly different from the FIF in doses to normal tissue but had the lowest coverage to targets of any of the techniques. Overall, in our department, we have chosen to use the FIF technique for SBRT conformality to obtain optimal coverage while minimizing the dose to normal tissue.
Key Words: SBRT, Lung, Conformality
Introduction
Stereotactic radiosurgery (SRS) is defined by the American College of Radiology (ACR) as “radiation therapy delivered via stereotactic guidance with approximately 1-mm targeting accuracy to a cranial lesion in a single fraction.”1 Stereotactic body radiation therapy (SBRT) can be described as a technology that uses elements of 3D conformal therapy in addition to stereotactic targeting while incorporating systems for decreasing the effects of lung and other organ movements that would otherwise translate into target motion. SBRT allows for dramatic reduction of treatment volumes, facilitating hypofractionation with markedly increased daily doses and significantly reduced overall treatment time.2, 3, 4, 5, 6
Radiosurgery was primarily used for the treatment of intracranial conditions since its beginning in the 1950s. Over the past decade, advancements in radiation treatment delivery and portal visualizations have expanded the use of this technique to extracranial sites that were inaccessible or inadequate for surgery.2, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19
Stereotactic radiation is used to deliver a high ablative dose to a small volume, while simultaneously achieving acceptable toxicities to the surrounding normal tissues. It is desirable to deliver the conformally shaped prescription dose to the planning target volume (PTV), with the dose to surrounding tissues dropping off rapidly, while simultaneously delivering a concentrated higher boost in the tumor core, or gross tumor volume (GTV).
Various techniques can be used to achieve these goals, but limitations exist that are dependent in part on the treatment planning system (TPS). Intensity-modulated radiation therapy (IMRT) techniques may be used to achieve doses to the targets and organs at risk. However, early protocols prohibited the use of IMRT techniques, perhaps because the accuracy of the dose delivered with a multileaf collimator (MLC)–based IMRT technique due to target motion has been the subject of investigation.20, 21 The treatment techniques described in this paper do not include IMRT and the use of IMRT will not be discussed further.
The purpose of this study is to evaluate field-in-field (FIF) techniques compared with non field-in-field (NFIF) techniques to determine potential benefits in delivering a higher dose to the tumor core, and also for the purpose of limiting the dose to normal tissues.
Background
The Indiana University (IU) experience in treating extracranial lesions of the lung and liver began in 1997 using the 3D Render TPS and a non–MLC-based linear accelerator.2 From 1997 through 2008 more than 500 SBRT cases have been treated at IU, including metastatic and primary disease for both lung and liver, as well as vertebral bodies and pancreas. Dose-shaping compensators were designed to produce gradient dose distributions, with maximum dose at the center of the GTV and 80% of the dose conforming to the PTV. A library of cerrobend milled compensators designed for a range of field sizes was created, designed, and stored in the planning system for multiple use (Figs. 1 and 2). The number of beams usually ranged from 7–12, arranged three dimensionally so that no beams were parallel opposed and with a 0–1-mm margin around the PTV (Fig. 3). The dose was prescribed to the isodose line conforming to the PTV, typically 80%.
Fig. 1.
Diagram of bolus layers over target for compensator design.
Fig. 2.
Milled compensators from Render TPS design.
Fig. 3.
Multi-field non-coplanar beam arrangement typically used.
With the implementation of multileaf collimation and the desire to eliminate the use of the milled compensators to reduce total treatment time, a FIF technique was evaluated and put into practice. The FIF technique allowed for the higher concentration of dose over the tumor core and the PTV to be covered by the prescription isodose line. This method works well but results in twice the number of beams for each plan, which increases the total treatment time. In addition, because the prescription doses for a single treatment are considerably higher than typical single-fraction doses (∼20 Gy), reducing the total treatment time is always desirable.
Over time, two questions have arisen. Does the FIF technique produce isodose distributions superior to a NFIF technique such that it is worth the increased labor involved in the planning and in the treatment? Could the FIF technique be simplified and replaced by a NFIF technique without sacrificing results? Our study examines the correlation between normal tissue dose and target coverage with the two different delivery techniques.
Methods and Materials
The materials and equipment used to perform this study:Table 1. RTOG 0236 ratio table
Maximum PTV Dimension (cm) Ratio of Rx (80%) Isodose Volume to the PTV Ratio of 50% Rx Isodose Volume to the PTV, R50% Maximum Dose 2 cm from PTV in any Direction, D2cm (Gy) (PTV+2) Percent of Lung Receiving 20 Gy Total or More, V20 
(%)PTV Volume (cc) Deviation Deviation Deviation Deviation None Minor None Minor None Minor None Minor 2.0 <1.2 1.2–1.4 <3.9 3.9–4.1 <28.1 28.1–30.1 <10 10–15 1.8 2.5 <1.2 1.2–1.4 <3.9 3.9–4.1 <28.1 28.1–30.1 <10 10–15 3.8 3.0 <1.2 1.2–1.4 <3.9 3.9–4.1 <28.1 28.1–30.1 <10 10–15 7.4 3.5 <1.2 1.2–1.4 <3.9 3.9–4.1 <28.1 28.1–30.1 <10 10–15 13.2 4.0 <1.2 1.2–1.4 <3.8 3.8–4.0 <30.4 30.4–32.4 <10 10–15 21.9 4.5 <1.2 1.2–1.4 <3.7 3.7–3.9 <32.7 32.7–34.7 <10 10–15 33.8 5.0 <1.2 1.2–1.4 <3.6 3.6–3.8 <35.1 35.1–37.1 <10 10–15 49.6 5.5 <1.2 1.2–1.4 <3.5 3.5–3.7 <37.4 37.4–41.7 <10 10–15 69.9 6.0 <1.2 1.2–1.4 <3.3 3.3–3.5 <39.7 39.7–41.7 <10 10–15 95.1 6.5 <1.2 1.2–1.4 <3.1 3.1–3.3 <42.0 42.0–44.0 <10 10–15 125.8 7.0 <1.2 1.2–1.4 <2.9 2.9–3.1 <44.3 44.3–46.3 <10 10–15 162.6
For this study, the patients were randomly chosen from a list of stereotactic lung patients treated at IU between 2007 and 2008. They were not part of the previous RTOG protocol study 0236. Dose-limiting structures for lung patients include spinal cord, esophagus, heart, proximal bronchial tree, brachial plexus, rib and chest wall, and the skin.
For each patient, all critical structures were outlined on each slice of the planning CT. The standard PTV margin around the GTV of .5 cm axially and 1 cm in the cranial-caudal direction was created for each GTV. In a few instances, the PTV margin had been narrowed because of proximity to the heart. In 11 of the 12 patients both lungs were contoured as one structure for the purpose of the dose-volume histogram. In one case the patient had previously undergone a unilateral pneumonectomy so only the remaining lung was considered. In addition, structures called PTV + 1, PTV + 2 and PTV + 3 were created to evaluate the dose drop-off from the PTV (Fig. 4).
Fig. 4.
Displays target with PTV + 2 cm for evaluating dose fall off region.
All of the cases were initially planned with a FIF technique that included a non-coplanar beam arrangement of 10–12 different gantry/couch orientations, each angle with two beams, for a total of 20–24 beams. MLCs were used to shape two fields: one that covered the PTV with an axial margin of approximately 1 mm and a cranial-caudal margin of 3 mm along with a smaller field. The smaller field remained open in the cranial-caudal direction but other dimensions were reduced to obtain the desired dose differential from GTV to PTV. For each beam orientation approximately 75–85% of the beam weight was assigned to the larger fields with 15–25% of the beam weight assigned to the smaller fields. The beam arrangement and weighting were chosen to optimize the dose conformality to the targets and minimize critical structure doses. In all cases, at least 95% of the PTV volume was covered by the prescription isodose line.
For comparison and evaluation of the two techniques two additional plans were created for each of the patients on the study. The first was a NFIF technique (NFIF-I) which simply used the larger fields by adding the weight from each small beam to its larger counterpart. Doses were recalculated without any renormalization and the prescription remained the same as it had initially, i.e. 60Gy to the 80% line, etc. which simply shows the effect of removing the smaller fields but still calculating to the same isodose line.
The second NFIF plan (NFIF-V) was renormalized to match the prescribed dose volume from the FIF plan used for treatment. For example, if the dose was prescribed to the 80% isodose line for treatment and the 80% line covered 97% of the PTV volume then the dose for this NFIF plan was prescribed to the isodose line that covered 97% of the volume of the PTV on the NFIF-V plan.
The three different plans were evaluated according to the criteria set forth by the RTOG 0236 protocol even though these patients were not treated on RTOG 0236. (see Table 1).2
Additionally the maximum dose in all directions was determined for the PTV + 1 cm, the PTV + 2 cm, and the PTV + 3 cm. Also evaluated were the V20 of the lung, maximum cord dose, PTV mean dose, and the mean and maximum doses for the GTV.
In addition to one patient having had a pneumonectomy of one lung, another had a pacemaker so this particular plan was optimized to deliver negligible doses to the pacemaker.
Others had GTVs and PTVs in close proximity to the heart or proximal bronchial tree which created the necessity use alternate rather than standard beam arrangements. All patient plans on the study were calculated with heterogeneity corrections ON.
Results
To evaluate the 12 patients selected for our study, the RTOG chart values (Table 1) were used as a benchmark for comparison of the ratios of dose volume to PTV volumes. Charts were created that compared the ratios for the FIF, NFIF-I, and NFIF-V. The maximum PTV dimension was determined by measuring the x, y, and z at the central axis. In all but one case, the largest dimension was the z (cranial-caudal) axis. Structures were created from the 80% and 50% isodose volumes for each of the treatment alternatives for each case.
The ratio of the prescription dose volume, usually the 80% isovolume, to the PTV was evaluated to determine whether it met the acceptable criteria. To be considered acceptable for RTOG 0236 the 80% volume would be <1.2 times larger than the size of the PTV. Likewise, the 50% isodose volume of the prescription dose to the PTV was evaluated using the RTOG chart criteria. By RTOG 0236 standards, the 50% volume would be ≤4.1 times the volume of the PTV, and as the volume of the PTV increased the ratio decreased to 3.1. As would be expected mathematically, the FIF technique has more conformal dose ratios than the NFIF-I.
When evaluated according to the RTOG 0236 ratio chart, using the 80% volume criteria, 1 patient would have had a minor violation when treated with FIF. Eleven patients had no violation. When using the NFIF-I, 2 patients had no violation, 4 patients would have had a minor violation, and 6 patients would have been in major violation of the protocol. Evaluating the NFIF-V by prescription volume, 7 patients would have had no violation, 4 patients a minor violation, and 1 patient would still have been a major violation (Table 2).
Table 2. Ratio comparison of volume of prescription dose to volume of PTV for FIF, as treated to NFIF, prescribed to the same isodose a the FIF and to NFIF, prescribed to the same volume in cc a the FIF
| Volume (cc) | Dimensions at Central Axis | FIF 80% Isodose Volume to PTV Volume | NFIF 80% Isodose Volume to PTV Volume | NFIF Same Volume in cc as FinF to PTV Volume | FIF 50% Isodose Vol to PTV Volume | NFIF 50% Isodose Vol to PTV Volume | NFIF Same Volume in cc as FinF to PTV Volume |
|---|---|---|---|---|---|---|---|
| 20.7 | 3.4×2.8×3.8 | 1.15NON | 1.42MAJ | 1.17NON | 3.39NON | 3.82MIN | 3.51NON |
| 21.3 | 2.5×3.4×3.9 | 0.99NON | 1.14NON | 0.98NON | 3.11NON | 3.36NON | 3.11NON |
| 30.6 | 3.3×3.8×4.1 | 1.11NON | 1.38MIN | 1.15NON | 3.61NON | 4.02MIN | 3.78NON |
| 31.5 | 3.0×1.8×4.6 | 1.18NON | 1.58MAJ | 1.22MIN | 3.35NON | 3.94MAJ | 3.51NON |
| 34.8 | 3.6×3.8×4.1 | 1.05NON | 1.44MAJ | 1.04NON | 2.68NON | 3.23NON | 2.72NON |
| 38.1 | 3.6×3.8×4.3 | 1.10NON | 1.26MIN | 1.10NON | 2.69NON | 2.89NON | 2.70NON |
| 42.9 | 4.2×3.5×4.5 | 1.16NON | 1.43MAJ | 1.20MIN | 3.16NON | 3.56NON | 3.16NON |
| 50.6 | 5.0×4.1×4.0 | 1.27MIN | 1.43MAJ | 1.30MIN | 3.03NON | 3.29NON | 3.20NON |
| 53.3 | 4.0×4.6×5.1 | 1.19NON | 1.41MAJ | 1.22MIN | 2.91NON | 3.25NON | 2.99NON |
| 69.6 | 4.5×4.1×5.0 | 1.11NON | 1.36MIN | 1.09NON | 2.78NON | 3.17NON | 2.78NON |
| 90.2 | 5.1×5.8×6.5 | 1.13NON | 1.31MIN | 1.15NON | 2.83NON | 3.18NON | 2.95NON |
| 106.4 | 5.8×5.6×6.1 | 1.19NON⁎ | 1.24MIN⁎ | 1.17NON⁎ | 3.78MAJ⁎ | 3.90MAJ⁎ | 3.79MAJ⁎ |
| NON = no violation per protocol | 11 | 01 | 08 | 11 | 08 | 11 | |
| MIN = minor violation per protocol | 01 | 05 | 04 | 0 | 02 | 0 | |
| MAJ = major violation per protocol | 0 | 06 | 0 | 01 | 02 | 01 | |
⁎This case was treated with an 8-field plan, with only 4 of the 8 fields containing a smaller FIF. |
When evaluating the 50% volume ratio criteria, using the FIF 11 patients had no deviation, and 1 patient had a major deviation. The same result held true for the NFIF-V. With the NFIF-I, 8 patients had no deviation, 2 had a minor deviation, and 2 had major deviation (Table 2).
For each of the structures PTV+1, PTV+2, and PTV+3, the maximum dose was evaluated in percents of the prescribed dose to the PTV volume. For all three structures, the results for the FIF and NFIF-V were within ± 2% of each other. The NFIF-I always had the highest maximum dose to those structures. The average difference from NFIF-I to the FIF for the PTV+1 and PTV+2 was about 5%, and the average difference from NFIF-I to the FIF for PTV+3 was 4% (Table 3).
Table 3. Percentages of prescribed dose to the PTV volume: comparison of dose drop-off from PTV for FIF, as treated, to NFIF, prescribed to the same isodose as FIF, and to NFIF, prescribed to the sae volume in cc as FIF
| Volume (cc) | Dimensions at Central Axis | FIF PTV + 1 Max to Total Dose % | NFIF to 80% Isodose PTV + 1 Max to Total Dose % | Non FinF to Same Volume PTV + 1 Max to Total Dose % | FIF PTV + 2 Max to Total Dose % | NFIF to 80% Isodose PTV + 2 Max to Total Dose % | NFIF to Same Volume PTV + 2 Max to Total Dose % | FIF PTV + 3 Max to Total Dose % | NFIF to 80% Isodose PTV + 3 Max to Total Dose % | NFIF to Same Volume PTV + 3 Max to Total Dose % |
|---|---|---|---|---|---|---|---|---|---|---|
| 20.7 | 3.4×2.8×3.8 | 81.0 | 85.0 | 81.2 | 62.1 | 64.5 | 61.6 | 51.8 | 53.0 | 50.6 |
| 21.3 | 2.5×3.4×3.9 | 80.0 | 84.2 | 81.1 | 68.5 | 71.9 | 69.3 | 64.3 | 66.9 | 64.6 |
| 30.6 | 3.3×3.8×4.1 | 75.8 | 79.1 | 76.6 | 60.6 | 63.0 | 61.0 | 49.8 | 54.4 | 52.7 |
| 31.5 | 3.0×1.8×4.6 | 80.6 | 87.2 | 81.8 | 55.7 | 59.1 | 55.3 | 46.1 | 49.2 | 46.7 |
| 34.8 | 3.6×3.8×4.1 | 74.9 | 81.6 | 73.4 | 55.3 | 62.4 | 56.0 | 42.8 | 46.5 | 41.8 |
| 38.1 | 3.6×3.8×4.3 | 77.9 | 79.9 | 76.6 | 58.3 | 60.8 | 65.5 | 45.1 | 54.7 | 46.5 |
| 42.9 | 4.2×3.5×4.5 | 80.7 | 88.3 | 84.3 | 59.5 | 63.2 | 60.0 | 50.2 | 53.1 | 50.6 |
| 50.6 | 5.0×4.1×4.0 | 85.4 | 89.8 | 86.8 | 64.6 | 67.3 | 65.0 | 54.1 | 57.0 | 55.7 |
| 53.3 | 4.0×4.6×5.1 | 85.7 | 89.4 | 85.1 | 66.9 | 70.4 | 67.1 | 68.2 | 57.1 | 54.2 |
| 69.6 | 4.5×4.1×5.0 | 85.6 | 89.7 | 83.1 | 70.5 | 75.5 | 69.9 | 59.3 | 62.1 | 57.5 |
| 90.2 | 5.1×5.8×6.5 | 94.2 | 99.0 | 94.7 | 79.1 | 81.3 | 87.5 | 71.7 | 75.6 | 72.4 |
| 106.4 | 5.8×5.6×6.1 | 95.1 | 96.1 | 94.6 | 84.9 | 86.2 | 84.8 | 77.5 | 78.7 | 77.4 |
The comparison data for the Lung V20 (volume receiving 20 Gy is always the lowest for the FIF plan and always the highest when using the NFIF-I technique. In 4 of the 12 cases, the NFIF-V matches the lung volume from the FIF plan and the remaining 8 cases fall in between the other two plans.
The percentage cord maximum dose is generally lower for the FIF plan as well, although in several cases the numbers were not significantly different. The highest differential was 3.3% (Table 4a).
Table 4a. Comparison of structure dose for FIF, as treated, to NFIF, prescribed to the same isodose as FIF, and to NFIF, prescribed to the same volume in cc as FIF
| Volume (cc) | Dimensions at Central Axis | Lung V20 | Cord Max | ||||
|---|---|---|---|---|---|---|---|
| FIF % Vol | NFIF to 80% Isodose % Vol | NFIF to Same Volume % Vol | FIF % Presc Dose | NFIF to 80% Isodose % Presc Dose | NFIF to Same Volume % Presc Dose | ||
| 20.7 | 3.4×2.8×3.8 | 3.5 | 3.8 | 3.7 | 1.4 | 1.7 | 1.7 |
| 21.3 | 2.5×3.4×3.9 | 2.2 | 2.4 | 2.2 | 13.1 | 13.3 | 12.9 |
| 30.6 | 3.3×3.8×4.1 | 6.5 | 7.0 | 6.7 | 9.5 | 9.4 | 9.4 |
| 31.5 | 3.0×1.8×4.6 | 7.0⁎ | 8.0⁎ | 7.4⁎ | 10.7⁎ | 11.1⁎ | 10.4⁎ |
| 34.8 | 3.6×3.8×4.1 | 5.3 | 6.1 | 5.4 | 29.6 | 29.6 | 26.6 |
| 38.1 | 3.6×3.8×4.3 | 7.3 | 7.8 | 7.3 | 9.5 | 10.1 | 9.7 |
| 42.9 | 4.2×3.5×4.5 | 12.6 | 13.8 | 12.9 | 10.2 | 10.3 | 9.8 |
| 50.6 | 5.0×4.1×4.0 | 11.7 | 12.3 | 11.9 | 29.6 | 30.2 | 29.3 |
| 53.3 | 4.0×4.6×5.1 | 3.5 | 3.8 | 3.6 | 9.9 | 10.0 | 9.6 |
| 69.6 | 4.5×4.1×5.0 | 8.8 | 9.5 | 8.8 | 11.8 | 12.5 | 11.6 |
| 90.2 | 5.1×5.8×6.5 | 4.2 | 4.7 | 4.4 | 24.2 | 28.4 | 27.2 |
| 106.4 | 5.8×5.6×6.1 | 11.4 | 11.6 | 11.4 | 14.3 | 14.4 | 14.1 |
⁎This patient had only one lung. |
The mean doses to the PTV and GTV were the highest when using the NFIF-I. Using the NFIF-V resulted in a lower GTV mean dose that ranged from 1.9%–8.7%, with an average deviation of 5% and a lower PTV mean dose average of 2.9% with the difference ranging from 1.6%–4.8%. In one case, the GTV minimum was actually less than the prescribed dose to the PTV if using the NFIF-V technique (Table 4b).
Table 4b. Percentages of dose prescribed to PTV: comparison of target coverage
| PTV Mean | GTV Mean | GTV Min | ||||||
|---|---|---|---|---|---|---|---|---|
| FIF % | NFIF to 80% Isodose % | NFIF to Same Volume % | FIF % | NFIF to 80% Isodose % | NFIF to Same Volume % | FIF % | NFIF to 80% Isodose % | NFIF to Same Volume % |
| 112.6 | 115.0 | 109.9 | 122.0 | 122.0 | 116.7 | 111.4 | 112.4 | 107.5 |
| 109.3 | 111.6 | 107.6 | 119.5 | 120.0 | 115.9 | 107.1 | 109.7 | 105.9 |
| 110.2 | 112.2 | 108.6 | 118.8 | 119.4 | 115.6 | 107.3 | 109.4 | 105.9 |
| 112.3 | 116.1 | 108.8 | 120.5 | 121.6 | 114.0 | 109.7 | 111.9 | 104.8 |
| 114.1 | 119.8 | 107.7 | 122.4 | 124.0 | 113.7 | 105.7 | 117.0 | 105.3 |
| 113.4 | 115.6 | 110.7 | 121.1 | 121.6 | 116.5 | 109.6 | 111.9 | 107.2 |
| 112.6 | 115.7 | 110.4 | 121.2 | 122.0 | 116.3 | 107.5 | 111.9 | 106.7 |
| 116.9 | 118.5 | 114.6 | 123.3 | 123.6 | 119.6 | 110.9 | 113.7 | 110.1 |
| 114.4 | 117.1 | 111.6 | 121.3 | 122.1 | 116.3 | 106.0 | 109.1 | 103.9 |
| 115.0 | 119.2 | 110.2 | 123.1 | 124.4 | 115.2 | 104.4 | 112.7 | 104.3 |
| 115.8 | 117.7 | 112.7 | 123.3 | 123.7 | 118.3 | 102.2 | 104.4 | 99.9 |
| 111.9 | 112.5 | 110.6 | 117.0 | 117.0 | 115.1 | 100.5 | 103.7 | 102.1 |
The GTV minimum and mean doses generally followed the same trends as the PTV, but was not always consistent in one direction. That could be explained partly by the fact that there was always some variability in the creation of the smaller field when planning the FIF technique. Because we used the plans as treated, some of the MLCs had been adjusted to meet the desired criteria for that particular patient.
Conclusion
The data do reflect an improvement in the dose fall-off region when using the FIF technique versus a NFIF technique. In almost all cases, the dose to critical structures was lowest with the FIF plan, shown in Table 1. For every patient, the V20 of the lung was lowest with the FIF technique. However, the differences in the fall-off regions were minimal when comparing the FIF with the NFIF-V.
Although the results between the FIF and NFIF-V were comparable for dose fall-off, the same did not hold true for target coverage. The NFIF-V had the lowest mean doses to both the GTV and PTV targets. The best target coverage was obtained with the NFIF-I plan, with the PTV mean averaging +2.4% greater than the FIF and the GTV mean averaging .5% greater than the FIF.
In conclusion, using the FIF technique minimizes the dose to critical structures while providing optimal coverage of the target volumes. Although the NFIF-I had the best coverage of the targets, it also had the highest dose in normal tissue. NFIF-V resulted in a GTV mean dose that ranged from 1.9% to 8.7% lower and a PTV mean dose average of 2.9% lower.
The FIF beams are easily created by copying and pasting the original beam and then conforming the MLCs to a previously created “block” structure. Although there is a time factor for determining the weight ratios between large and small fields to obtain the desired DVH results, that factor is greatly reduced for the experienced planner. The extra time on the treatment machine is also minimal because there are no additional gantry positions, just movement of the MLC leaves. In our department we have chosen to routinely use the FIF technique, with one possible exception in the case of very small targets for which producing a FIF is impractical. Other exceptions are determined on a case-by-case basis.
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PII: S0958-3947(09)00123-X
doi:10.1016/j.meddos.2009.10.004
© 2011 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights reserved.
