Abstract
Purpose. To compare the different beam arrangement and delivery techniques for stereotactic body radiation therapy (SBRT) of lung lesions using the criteria of Radiation Therapy Oncology Group (RTOG) 0236 protocol. Material and methods. Thirty-seven medically inoperable lung cancers were evaluated with various planning techniques including multiple coplanar multiple static beams, multiple non-coplanar static beams and arc delivery. Twelve plans were evaluated for each case, including five plans using coplanar fixed beams, six plans using non-coplanar fixed beams and one plan using arc therapy. These plans were compared using the target prescription isodose coverage, high and low dose volumes, and critical organ dose-volume limits. Results. The prescription isodose coverage, high dose evaluation criteria and dose to critical organs were similar among treatment delivery techniques. However, there were differences in low dose criteria, especially in the ratio of the volume of 50% isodose of the prescription dose to the volume of planning treatment volume (R50%). The R50% in plans using non-coplanar static beams was lower than other plans in 30 of 37 cases (81%). Conclusion. Based on the dosimetric criteria outlined in RTOG 0236, the treatment technique using non-coplanar static beams showed the most preferable results for SBRT of lung lesions.
Surgical resection is the treatment of choice for patients with early or localized non-small cell lung cancer (NSCLC), and the five-year survival rate after surgery for stage I NSCLC is approximately 60–70%. However, some patients are inappropriate candidates for surgery due to poor respiratory function, cardiovascular disease or other medical conditions. For these medically inoperable patients, primary radiotherapy (RT) has been considered as a treatment option. A total dose of 45–66 Gy in 1.8–2.0 Gy daily fractions has been applied and the outcome of conventional RT is unsatisfactory, with a local relapse rate of 50–70% and a 5-year survival rate of only 10–30% [Citation1–5]. To improve the tumor control rate and survival rate, doses of primary RT should be increased but a higher dose given to the tumor without increasing complications is essentially impossible with conventional RT techniques.
With the introduction of 3-dimensional conformal radiotherapy (3-D CRT) and stereotactic body radiation therapy (SBRT), dose escalation using hypofractionation and reduction of the treatment volume is possible. Early phase I/II trials have reported high local control rates of 80–100% in stage I NSCLC [Citation6–9] and many studies have shown a dose-response relationship with various hypofractionation schedules [Citation6,Citation10–15]. With the increased use of hypofractionation RT to patients with lung tumors, radiation oncologists are choosing optimal SBRT techniques for different clinical situations. Recently, the Radiation Therapy Oncology Group (RTOG) performed a phase II study in the treatment of patients with medically inoperable stage I/II NSCLC. For successful treatment planning, the RTOG recommended treatment beam arrangements and provided some criteria to evaluate treatment plans [Citation16]. Although many institutions have reported their results with hypofractionation RT, a study comparing various beam arrangements has not been investigated to our knowledge. This present study is designed to compare different treatment delivery techniques and to determine the beam arrangement that is most optimal for SBRT in the treatment of patients with lung lesions according to the criteria of the RTOG 0236 protocol.
Material and methods
Treatment planning guideline from RTOG 0236
We briefly summarize the guidelines of RTOG 0236 for treatment planning. A minimum of seven static beams or 340 degrees of arc is recommended. The field aperture should correspond nearly identically to the beam's-eye-view projection of the planning treatment volume (PTV). The plan should be normalized to a defined point that corresponds closely to the center of the mass of the PTV (beam isocenter) and that receives 100% of the normalized dose. The prescription dose is 60 Gy in 3 fractions and there is no correction for tissue heterogeneity in dose calculation.
Simulation
All patients were immobilized using a body mask (3DLine Medical Systems, Milan, Italy) device and CT simulation was performed with a slice thickness of 0.3 cm. Two CT scans were obtained: free-breathing and 4D CT scan. The 4D CT scan was used to delineate the GTV on each CT images set corresponding to a phase of the respiratory cycle. The combination of the GTV on the CT images corresponding to each respiratory phase was the internal target volume (ITV). A uniform 5mm radial margin was added to the ITV to create the PTV. Critical organs were contoured according to the guidelines of the RTOG 0236 protocol.
Beam arrangements
Three types of the beam arrangements for SBRT of lung tumors were investigated in this work: (1) multiple coplanar static beams, (2) multiple non-coplanar static beams, and (3) arc therapy.
Multiple coplanar static beams. Treatment planning was performed with seven, nine, 11, 13, and 15 coplanar beams and two types of beam arrangements: equal-spaced beams and beam angles determined by the planner, were used. Beam angles for the latter approach were determined according to the location of tumor and critical organs to minimize dose to critical organs. For example, when the tumor was located in left lower lobe, more beams from the left direction were used and their beam weightings were higher than any other beams. Beam arrangements determined by the planner resulted in better dosimetric results compared with using equal-spaced beams and hence we only report on the results obtained with planner-selected beam arrangements.
Multiple non-coplanar static beams. We considered two types of non-coplanar beam arrangements (gantry movement was restricted from 330° and 30o) and two templates of non-coplanar beam arrangement were generated. The first arrangement used two non-coplanar beams (gantry angle of 30° and 330° with a 90° couch angle) and the second arrangement used four non-coplanar beams (gantry angle of 30° and 330° with a 45° couch angle, gantry angle of 30° and 330° with a 315° couch angle). By adding some coplanar static beams to each template of the non-coplanar static beam arrangement, the total number of beams was seven, nine, and 11 and six plans per each case were generated.
Arc therapy. Plans using rotational beams were generated by using one arc, consisted of 12 coplanar sub-arcs of 30° rotation each. For each beam angle in the sub-arc, the beam was shaped to the beam's-eye-view projection of the PTV.
Plan summary. In summary, a total of 12 plans—five plans with multiple coplanar static beams, six plans with multiple non-coplanar static beams, and one plan with arc therapy were generated for each patient and compared according to the RTOG 0236 criteria.
For dose calculation, we used the convolution-superposition algorithm available in the Pinnacle3 planning system (Philips Medical Systems, Cleveland, OH).
Criteria for the comparison of various treatment plans
Plans were compared using the following criteria:
(i) Prescription isodose surface coverage: For the PTV, it was intended that 95% of the volume received the prescription dose (i.e., 60 Gy) and 99% of the PTV received a minimum of 90% of the prescription dose (i.e., 54 Gy).
(ii) High dose volume: The cumulative volume of all tissue outside of the PTV receiving a dose greater than 105% of the prescription dose was intended to be ≤15% of the PTV volume. PTV dose conformality was evaluated as the ratio of the volume of the prescription isodose to the volume of the PTV. This ratio was intended to be less than 1.2.
(iii) Low dose volume: The maximum intended total dose to any point 2 cm or greater away from the PTV in any direction (D2cm) is defined. In addition, the intended maximum ratios of the volume of 50% of the prescription dose (i.e., 30 Gy) to the volume of the PTV (R50%) are defined according to RTOG 0236 protocol criteria [Citation16].
(iv) Dose-volume limits of the critical organ: The percent of the lung receiving 20 Gy or higher (V20) was intended to be less than 10% and dose limits for other critical organs are defined [Citation16].
Definition of deviation against the criteria
The minor deviation from intended protocol values is defined according to RTOG 0236 protocol [Citation16]. The protocol deviation greater than minor deviation is classified as major for protocol compliance. If calculated dose to the critical organ is greater than limited dose, it is considered as a deviation as well.
Results
Patient characteristics
The patient characteristics are shown in . The median age was 71 years and 73% of the patients were male patients. According to the tumor size, the longest diameter was less than 3 cm in 24 patients and 3 to 5 cm in 13 patients. One patient had a tumor with chest wall invasion.
Table I. Patients’ characteristics (n=37).
According to the tumor location, 18 cases were located in the right lung with nine, three and six cases in the upper, middle and lower lobe, respectively, and 19 cases were located in the left lung with seven in the upper and 12 in the lower lobe.
Dosimetric comparisons of the different plans
All of the prescription points were at the center of PTV, and the percentages of prescription dose covering the PTV were within 60–90%. At least one plan corresponding to every patient had a major or minor deviation (). However, if we chose the best plan for each patient, the minor or major deviations were only in the low dose volume criteria, especially for R50%. The total number of deviations in R50% was 26 (70%), of which nine (24%) were major and 17 (46%) were minor deviations.
Table II. Major and minor deviations according to RTOG 0236 criteria from all 12 plans per each patient (n=37).
R50% and plan comparisons for different beam arrangements
When all 12 treatment plans for each patient were compared against the criteria of R50%, 36 patients had at least one plan with a major or minor deviation (). For each case, the three best plans within a treatment technique (multiple coplanar static beams, multiple non-coplanar static beams and arc therapy) were compared for the R50% criteria. In 30 cases (81%), the numeric value of R50% was lowest in the plan using multiple non-coplanar static beams and in 20 cases (54%), the deviation was either minor or none with the use of multiple non-coplanar static beams. The lowest numeric value of R50% was obtained in three patients with the use of multiple coplanar static beams and arc therapy resulted in the highest value for R50%. This trend was consistent for all tumor locations.
Table III. Major and minor deviations according to R50% criteria from all 12 plans per each patient and from the best plan within each of three treatment techniques.
Discussion
The RTOG designed a phase II protocol with SBRT for the treatment of patients with medically inoperable stage I/II NSCLC. The RTOG recommended the use of some beam arrangements and provided criteria for deviations in treatment plan evaluation. There have been many studies reporting the results of hypofractionated irradiation for NSCLC and these studies have used various treatment techniques with different beam arrangements. The treatment beam arrangements can be classified into four types, including multiple coplanar static beams [Citation9,Citation17], multiple non-coplanar static beams [Citation6,Citation8,Citation9,Citation12,Citation13,Citation17], a single arc beam [Citation14], and multiple non-coplanar arc beams [Citation13,Citation18]. There have been few reports analyzing optimal beam arrangements to date.
Bo et al. reported that several non-coplanar arcs may be a reasonable technique for irradiation of tumors at the center of the right lung [Citation19]. Takayama et al. reported that although the role of non-coplanar beam directions may be limited in stereotactic radiotherapy, a homogenous target dose distribution, while avoiding high doses to normal tissues, was obtained with 5–10 non-coplanar and coplanar beams. In general, although a large number of beams make dose distributions more conformal as compared with a small number, simulation has revealed that there is little difference in the increase of the number of beams with more than 10 [Citation20].
In the present study, when all plans from the three types of beam arrangements were analyzed against the RTOG 0236 criteria, the violation of criteria was limited to the low dose volume, especially, R50%. Our findings indicate that treatment plans with multiple non-coplanar static beams show a steeper falloff gradient beyond the PTV compared with the other beam arrangements.
Considering that the major proportion of patients indicated for SBRT have pulmonary problems, a dose-volumetric evaluation for lung is very important and even a small difference in R50% may influence respiratory function. Some studies have described radiation-induced lung toxicity in conventional and hypofractionation RT. Takayama et al. showed with a total dose of 48 Gy with a 4-fraction schedule in 37 patients, there were three patients whose V20 exceeded 10% and only two patients (5%) had Grade 2 radiation pneumonitis based on the National Cancer Institute-Common Toxicity Criteria. In the RTOG 0236 protocol, while V20 is used for the evaluation of lung morbidity, two other components were considered as parameters for lung morbidity: D2cm and R50%. V20 should be less than 10% and the values of D2cm and R50% were defined according to the maximum PTV dimension or PTV volume [Citation16]. In the present study, all of the best plans for each case satisfied the criteria of V20 and D2cm without any deviation. Our results are based on calculations in which tissue inhomogeneity corrections were not considered. If instead tissue inhomogeneity corrections were considered, we expect V20 to remains largely unchanged, but we expect that the low dose volume (e.g., V5) may be slightly increased.
For the present study, we used multiple static beams in coplanar or non-coplanar arrangements. Many institutions use four to 15 beams for hypofractionation RT in lung tumors and the use of a larger number of beams in a treatment plan results in better overall PTV conformality and dose-falloff beyond the PTV. However, increasing the beam number beyond some threshold may not improve the dose gradient. According to Lie et al., the dose gradient improved with an increase in beam number from five to 15 for both coplanar and non-coplanar beam configurations. These investigators concluded that the optimal number of beams is 13 to 15 for SBRT using either coplanar or non-coplanar beam bouquets based on dosimetric criteria [Citation21]. As nearly all plans satisfied the RTOG 0236 criteria except R50% in this study, we tried to determine the optimal number of beams in each treatment beam arrangements using the criteria of R50%. In a coplanar static beam arrangement using seven, nine, 11, 13, and 15 beams, the numeric value of R50% decreased with an increase in the beam number, which agrees with the results reported by Lie et al. However, in a non-coplanar static beam arrangement, there was no difference in R50% between the beam numbers and the use of two non-coplanar beams was more effective than the use of four non-coplanar beams ().
Table IV. Mean and standard deviation (SD) values of R50% according to plans (n=37).
Conclusions
In conclusion, when the three different beam arrangement techniques for SBRT of lung lesions were compared according to the recommended criteria of RTOG 0236, nearly all treatment techniques satisfied the high dose volume constraints and critical organ constraints including the lung dose-volume limit. However, at the low dose level, there were deviations from the recommended criteria and the treatment technique using non-coplanar multiple static beams resulted in better numeric criteria compared with the other treatment beam arrangements.
Acknowledgements
This paper was presented as a poster viewing at the 50th ASTRO annual meeting. There is no conflict of interest in connection with this work.
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