Influence of Dose Calculation Algorithm According to Dose Prescription Methods in Stereotactic Body Radiotherapy for Lung Cancer

Research Article

Austin J Radiat Oncol & Cancer. 2015; 1(2): 1010.

Influence of Dose Calculation Algorithm According to Dose Prescription Methods in Stereotactic Body Radiotherapy for Lung Cancer

Hamamoto Y*, Inata H, Tsuruoka S, Takeda H and Manabe T

Department of Radiology, Saiseikai-Imabari Hospital, Japan

*Corresponding author: Hamamoto Y, Department of Radiology, Saiseikai-Imabari Hospital, 7-1-6 Kitamura, Imabari-City, Ehime 799-1592, Japan

Received: May 07, 2015; Accepted: September 08, 2015; Published: September 30, 2015

Abstract

Purpose: Prescribed doses for stereotactic body radiotherapy (SBRT) of lung tumors are defined as the minimum dose received by 95% volume of the planning target volumes (D95 dose prescription method) or the dose at isocenter (isocentric dose prescription method). At present, both dose prescription methods and various calculation algorithm are used for SBRT. With regard to SBRT for lung tumors, impact of dose calculation algorithm according to dose prescription methods was evaluated.

Material and Methods: For eight patients with lung cancer, SBRT plans with 6-10 fixed beams of 6MV photon were prepared. In the isocentric dose prescription method, 12 Gy was delivered to the isocenter. In the D95 dose prescription method, 10 Gy or more was delivered to 95% volume of the planning target volumes. Monitor units (MUs) determined by the D95 dose prescription method and the isocentric dose prescription method were calculated by both pencil beam convolution algorithm with Batho Power Law (BPL) and heterogeneity corrected anisotropic analytical algorithm (AAA) in each patient.

Results: The differences between MUs calculated by BPL and AAA were 0.6-7.5% (mean, 2.7%) for the isocentric dose prescription method, and 2.7- 32.1% (mean, 12.2%) for the D95 dose prescription method (p=0.0176, paired t-test).

Conclusion: The differences of MUs according to calculation algorithm were larger in the D95 dose prescription method compared to the isocentric dose prescription method. In SBRT for lung cancer, differences of dose calculation algorithm should be noticed when the D95 dose prescription method is used.

Keywords: Stereotactic Body Radiotherapy; Dose Prescription; Calculation Algorithm

Background

Prescribed doses for stereotactic body radiotherapy (SBRT) were defined as the minimum dose received by 95% volume of the planning target volumes (D95 dose prescription method) or isocentric doses (isocentric dose prescription method). At present, dose prescription methods have not been unified. In the Radiation Therapy Oncology Group (RTOG) 0236 trial (A phase II trial of stereotactic body radiation therapy in the treatment of patients with medically inoperable stage I/II non-small cell lung cancer), 60 Gy in three fractions is delivered to 95% of the planning target volume. In Japan, isocentric dose prescription method was adopted in Japan clinical oncology (JCOG) 0403 phase II trial (a stereotactic body radiotherapy trial for stage I non-small cell lung cancer), while D95 dose prescription method was adopted in the JCOG 0702 phase I trial. Not only clinical trials but also clinical practice, both dose prescription methods are widely used in SBRT. Although Japanese Society for Radiation Oncology (JASTRO) guideline for SBRT recommends the isocentric dose prescription method in principle [1], there is a trend that dose prescription methods are changing from the isocentric dose prescription method to the D95 dose prescription method also in Japan.

In addition, a wide variety of algorithms for heterogeneity correction in dose calculation are used. Because of a variety of heterogeneity correction algorithms, it is difficult to compare outcomes among different institutions.

Monitor units (MUs) of radiation beams are calculated by variety of commercially-available calculation algorithm at present. Delivered MUs are affected by calculation algorithm. If differences of MUs calculated by different calculation algorithm are large, knowledge of optimal doses for tumors and normal tissue tolerance doses in SBRT will be confused. Because variety of calculation algorithm are commercially available at present, methods to decrease influences of calculation algorithm on MUs should be considered.

We examined the influences of calculation algorithm on MUs according to dose prescription methods in SBRT for lung cancer.

Material and Methods

For eight patients with lung cancer, SBRT plans with 6-10 fixed beams of 6MV photon were prepared. Delivered MUs were determined by the isocentric dose prescription method and the D95 dose prescription method. In the isocentric dose prescription method, 12 Gy was delivered to the isocenter in each fraction. In the D95 dose prescription method, 10 Gy or more was delivered to 95% volume of the planning target volumes in each fraction. Stereotactic body radiotherapy was performed with a linear accelerator (Clinac iX; Varian Medical Systems, Inc., Palo Alto, CA) and treatment planning was performed with treatment planning system of Eclipse (Varian Medical Systems, Inc., Palo Alto, CA). Monitor units (MUs) were calculated by the pencil beam convolution algorithm with Batho Power Law (BPL) and heterogeneity corrected anisotropic analytical algorithm (AAA) of Eclipse for both the isocentric dose prescription method and the D95 dose prescription method. Statistically significance of differences of averages was assessed by paired t-test.

Results

Differences between MUs calculated by BPL and AAA according to dose prescription methods. {(MUs calculated by AAA) / (MUs calculated by BPL) x 100} - 100 (%) were 0.6-7.5% (mean, 2.7%) for the isocentric dose prescription method, and 2.7-32.1% (mean, 12.2%) for the D95 dose prescription method (Table1,2). The differences of averages were statistically significant (p=0.0176).

Table 1: MUs determined by isocentric dose prescription method (Isocenter dose = 12 Gy).





  

    
case

    
MUs by BPL

    
MUs by AAA

    
differences of MUs

  

  

    
1

    
1638

    
1628

    
-0.60%

  

  

    
2

    
1685

    
1674

    
-0.70%

  

  

    
3

    
1646

    
1769

    
7.50%

  

  

    
4

    
1821

    
1794

    
-1.50%

  

  

    
5

    
1619

    
1674

    
3.40%

  

  

    
6

    
1617

    
1706

    
5.50%

  

  

    
7

    
1511

    
1523

    
0.80%

  

  

    
8

    
1564

    
1591

    
1.70%

  

  

    
mean of absolute values = 2.7%

  










Table 1:  MUs determined by isocentric dose prescription method (Isocenter
dose = 12 Gy).

Table 2: MUs determined by the D95 dose prescription method (D95 = 10 Gy).





  

    
case

    
MUs by BPL

    
MUs by AAA

    
differences of MUs

  

  

    
1

    
1409

    
1490

    
5.70%

  

  

    
2

    
1467

    
1506

    
2.70%

  

  

    
3

    
1425

    
1882

    
32.10%

  

  

    
4

    
1558

    
1667

    
7.00%

  

  

    
5

    
1417

    
1505

    
6.20%

  

  

    
6

    
1368

    
1695

    
23.90%

  

  

    
7

    
1329

    
1539

    
15.80%

  

  

    
8

    
1530

    
1591

    
4.00%

  

  

    
mean of    absolute values = 12.2%

  










Table 2:  MUs determined by the D95 dose prescription method (D95 = 10 Gy).

Discussion

Both the isocentric dose prescription method and the D95 dose prescription method are used in SBRT for lung tumors in clinical practice. In addition, delivered MUs are calculated by variety of calculation algorithm for heterogeneity correction. Based on our results, MUs determined by the D95 dose prescription method was significantly affected by calculation algorithm compared to the isocentric dose prescription method. In the D95 dose prescription method, differences between MUs calculated by the D95 sometimes reached more than 20-30% according to calculation algorithms.

Because the lung is histologically heterogeneous organs contains air and small amount of soft tissues, dose calculation using different heterogeneity corrections results in different dose distributions.

It had been reported that the dose calculation algorithm was the most significant factor responsible for inter-institutional variations in planning for SBRT for lung cancer [2]. It was reported thatdifferent heterogeneity corrections have a marked impact on the dose distributions around the targets [3-7]. When pencil-beam algorithms compared to convolution superposition-type algorithms or the Monte Carlo algorithm, doses at the periphery of the planning target volume was overestimated with depending on the field size and the energy of the beam.

Based on our results, isocentric dose prescription method seemed to be preferable for inter-institutional comparison of treatment outcomes. When the D95 dose prescription method is used for lung SBRT, differences of dose calculation algorithm should be emphasized.

Conclusion

The differences of calculation algorithm influenced on MUs of lung SBRT largely when the D95 dose prescription method was used compared to when the isocentric dose prescription method was used. In lung SBRT, differences of dose calculation algorithm should be emphasized when the D95 dose prescription method is used.

References

  1. Japanese Society for Radiation Oncology (JASTRO) guideline for stereotactic body radiotherapy.
  2. Matsuo Y, Takayama K, Nagata Y, Kunieda E, Tateoka K, Ishizuka N, et al. Interinstitutional variations in planning for stereotactic body radiation therapy for lung cancer. Int J Radiat Oncol Biol Phys 2007; 68:416–425.
  3. Narabayashi M, Mizowaki T, Matsuo Y, Nakamura M, Takayama K, Norihisa Y, et al. Dosimetric evaluation of the impacts of different heterogeneity correction algorithms on target doses in stereotactic body radiation therapy for lung tumors. J Radiat Res. 2012; 53:777–784.
  4. Ding GX, Duggan DM, Lu B, Hallahan DE, Cmelak A, Malcolm A, et al. Impact of inhomogeneity corrections on dose coverage in the treatment of lung cancer using stereotactic body radiation therapy. Med Phys. 2007;34:2985–2994.
  5. Traberg Hansen A, Petersen JB, Hψyer M, Christensen JJ. Comparison of two dose calculation methods applied to extracranial stereotactic radiotherapy treatment planning. Radiother Oncol 2005; 77:96–98.
  6. Panettieri V, Wennberg B, Gagliardi G, Duch MA, Ginjaume M, Lax I. SBRT of lung tumours: Monte Carlo simulation with PENELOPE of dose distributions including respiratory motion and comparison with different treatment planning systems. Phys Med Biol 2007;52:4265–4281.
  7. Haedinger U, Krieger T, Flentje M, Wulf J. Influence of calculation model on dose distribution in stereotactic radiotherapy for pulmonary targets. Int J Radiat Oncol Biol Phys 2005;61:239–249.

Citation: Hamamoto Y, Inata H, Tsuruoka S, Takeda H and Manabe T. Influence of Dose Calculation Algorithm According to Dose Prescription Methods in Stereotactic Body Radiotherapy for Lung Cancer. Austin J Radiat Oncol & Cancer. 2015; 1(2): 1010. ISSN:2471-0385