Physics Contribution
Benchmarking Dosimetric Quality Assessment of Prostate Intensity-Modulated Radiotherapy

Presented in abstract form at the Royal Australian and New Zealand College of Radiologists Annual Scientific Meeting, Brisbane, QLD, Australia, October 2009.
https://doi.org/10.1016/j.ijrobp.2010.12.016Get rights and content

Purpose

To benchmark the dosimetric quality assessment of prostate intensity-modulated radiotherapy and determine whether the quality is influenced by disease or treatment factors.

Patients and Methods

We retrospectively analyzed the data from 155 consecutive men treated radically for prostate cancer using intensity-modulated radiotherapy to 78 Gy between January 2007 and March 2009 across six radiotherapy treatment centers. The plan quality was determined by the measures of coverage, homogeneity, and conformity. Tumor coverage was measured using the planning target volume (PTV) receiving 95% and 100% of the prescribed dose (V95% and V100%, respectively) and the clinical target volume (CTV) receiving 95% and 100% of the prescribed dose. Homogeneity was measured using the sigma index of the PTV and CTV. Conformity was measured using the lesion coverage factor, healthy tissue conformity index, and the conformity number. Multivariate regression models were created to determine the relationship between these and T stage, risk status, androgen deprivation therapy use, treatment center, planning system, and treatment date.

Results

The largest discriminatory measurements of coverage, homogeneity, and conformity were the PTV V95%, PTV sigma index, and conformity number. The mean PTV V95% was 92.5% (95% confidence interval, 91.3–93.7%). The mean PTV sigma index was 2.10 Gy (95% confidence interval, 1.90–2.20). The mean conformity number was 0.78 (95% confidence interval, 0.76–0.79). The treatment center independently influenced the coverage, homogeneity, and conformity (all p < .0001). The planning system independently influenced homogeneity (p = .038) and conformity (p = .021). The treatment date independently influenced the PTV V95% only, with it being better at the start (p = .013). Risk status, T stage, and the use of androgen deprivation therapy did not influence any aspect of plan quality.

Conclusion

Our study has benchmarked measures of coverage, homogeneity, and conformity for the treatment of prostate cancer using IMRT. The differences seen between centers and planning systems and the coverage deterioration over time highlight the need for every center to determine their own benchmarks and apply clinical vigilance with respect to maintaining these through quality assurance.

Introduction

In prostate cancer, three-dimensional conformal radiotherapy (RT) with a dose escalation from <70 Gy to 74–80 Gy has been shown to provide a biochemical control benefit of 15–20% 1, 2, 3, 4. Observational studies have suggested this improved biochemical control translates into distant metastasis-free survival 5, 6, 7 and cause-specific survival (6). However, these trials also showed an increase in late Radiation Therapy Oncology Group Grade 2 gastrointestinal toxicity from 8–11% to 14–19% 2, 3, 4. The narrowing therapeutic index with dose escalation, combined with the improving technology, has prompted the widespread use of intensity-modulated RT (IMRT).

The use of IMRT enabled dose escalation by achieving steeper dose gradients that spared organs at risk (OARs) (8); however, doing so, significantly increased the treatment planning, prescription, and reporting complexity. The inadequacy of the International Commission on Radiation Units and Measurements (ICRU) reporting standards developed for three-dimensional conformal RT 9, 10 for IMRT reporting has been shown. Considering 803 IMRT plans, Das et al. (11) showed that 46% of patients had received a maximal dose of >110% of the prescribed dose and 63% had received a minimal dose of <90% of the prescribed dose. This result was concerning, not only because it highlighted the existence of a potentially significant clinical issue, but also because it showed the current reporting of point doses made it impossible to quantify this issue and draw conclusions about the overall plan quality. This raised concerns about the validity of comparing the clinical outcomes from previous IMRT trial data, particularly given that RT quality has been correlated with overall survival (12), albeit in the head-and-neck setting. In a joint publication, the American Association of Physicists in Medicine and the American Society for Radiation Oncology (13) suggested a move to a “three-point dose–volume description,” similar to the recommendations made by the Radiation Therapy Oncology Group (RTOG) and National Cancer Institute, through the Advanced Technologies Consortium (14). Although a qualitative recommendation was made, the quantifying of it was left to the individual institutions and trial committees. An American Society for Radiation Oncology workgroup published reporting guidelines to qualitatively include both the clinical target volume (CTV) and the planning target volume (PTV) coverage indexes, again without quantitative recommendations (15). The stated aim was to provide the minimal requirement for future correlation with the clinical outcomes, rather than comprehensive descriptions of all aspects of IMRT plan quality. Recently, the ICRU largely supported these recommendations through report 83; however, they also added tertiary level recommendations that included descriptors of conformity, homogeneity, and biologic metrics (16).

The recommendations for IMRT reporting made through numerous sources have remained qualitative, and, to date, no publication has quantified a reasonable expectation for these reporting standards nor determined whether clinical factors could influence this expectation. The aim of this multicenter study was to benchmark the quality assessment of prostate IMRT and to determine whether quality could be influenced by disease or treatment factors. With plan quality quantified, it will facilitate future quality assurance and treatment benchmarking tailored to given clinical circumstances. From a research perspective, integrating this information into trail protocols will enable better interpretation and comparison between trials. Finally, and perhaps most importantly, benchmarking these results now will enable future correlation with outcomes data.

Section snippets

Patients and Methods

The present study was a retrospective analysis of 155 consecutive men, who had undergone RT for prostate cancer with radical intent IMRT to 78 Gy between January 2007 and March 2009 inclusive. Six treatment centers across Victoria, Australia contributed patients, including the Austin Health (Heidelberg), Epworth Healthcare (Richmond), and Peter MacCallum Cancer Center sites in East Melbourne, Boxhill, Moorabbin, and Bendigo. The human ethical review boards approved the study before we began.

All

Population

Between January 2007 and March 2009, 155 prostate cancer patients were treated radically using IMRT across six Victorian RT centers. The baseline characteristics of these patients are listed in Table 2. The median age was 70 years (range, 51–80), and the median disease factors were clinical Stage 2B, pretreatment prostate-specific antigen level of 11.2 ng/mL and Gleason score of 3 + 4 = 7. Although this represents an intermediate risk profile, 66% of patients had high-risk disease, confirming

Discussion

In the assessment of dosimetric quality using external beam RT, unlike permanent seed brachytherapy (24), no index has been correlated with the outcomes, consequently none have gained universal acceptance. Therefore, we believed it important to consider a wider perspective and, thus, assessed the coverage, homogeneity, and conformity separately. In keeping with the recommendations from the ICRU (16) and RTOG (15), multiple indexes were assessed; however, the PTV V95%, PTV sigma index, and CN

Conclusion

We have benchmarked the measures of coverage, homogeneity, and conformity for the treatment of prostate cancer using IMRT. The differences seen among the centers, planning systems, and coverage deterioration over time have highlighted the need for every center to determine their own benchmarks and apply clinical vigilance with respect to maintaining these through quality assurance. These differences also highlight the need to further explore the cause for the variations seen. This will

Acknowledgments

We wish to thank Mrs. Maureen Rolfo and Ms. Yolanda Aarons for their assistance with data collection.

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    Conflict of interest: none.

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