Page 91 - ARNM-2-4

Advances in Radiotherapy
& Nuclear Medicine Dose prescription and reporting in SRS

For the conformity index (CI), a general guideline The analysis and comparison of dose-volumetric
suggests maintaining values below 1.20 for SBRT/SRS parameters strongly depend on the definition of the
plans. Our findings revealed a mean CI of 1.17 (standard prescription dose. Some institutions use D , D , D , or
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100
80
95
deviation: 0.07), which aligns with this recommendation. D as the dose prescription for SBRT/SRS plans. However,
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these doses may be addressed to PTV or gross tumor
Further analysis examined the dependence of HI and CI
on the treatment planning approach used by each physicist, volume, and may also depend on the treatment site or the
dose calculation algorithm. These discrepancies make
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as illustrated in Figure 4. Statistical analysis (Kruskal– comparisons between different studies and trials nearly
Wallis test, P < 0.05) demonstrated that plans developed by impossible when using various dosimetric metrics. ICRU-
RMPs significantly differed in combined values of CI and 91 made noticeable progress in defining dose prescriptions
HI, suggesting variability in planning strategies among the and reporting for SBRT/SRS treatments. However, a
physicists.
worldwide consensus on SBRT/SRS planning, delivery, and
4. Discussion measurable metrics has yet to be achieved. Nevertheless,
particularly at our institution, we have standardized
In this single-institution retrospective study, we reported the approach for all treatments and sites, such that the
various treatment planning and reporting parameters prescription dose in the Record and Verify and Treatment
used for SBRT/SRS clinical plans. Treatment sites Planning systems is set at the 100% isodose level required
included the lung, brain, prostate, pelvis, liver, and bony for PTV coverage. Other standardized requirements include
regions. The median PTV volumes were in the range of the limitation of the hotspot dose to 130%, D99 > 95% as
20 – 45 cc for all cases, while 9.2 cc for brain targets and a mandatory criterion, and D95 > 95% as an acceptable
105.6 cc for prostate targets. The mean amount of MUs deviation. As shown in Figure 3, this standardization was
for all cases was about 3600, and its dependency on PTV clearly reflected in the consistency of dose-volumetric
volumes was analyzed. It is well-known that the target parameters across different treatment sites and regimes.
volume impacts the amount of MUs needed for a proper Notably, brain cases had a slightly lower mean hotspot,
VMAT treatment plan: smaller target volumes correlate likely due to the planner’s preference to avoid escalating
to a higher amount of MUs. However, as demonstrated the dose to brain tissue, especially in cases of tumor bed
in Figure 3, prostate cases formed a specific pattern, irradiation, whereas prostate cases were limited by a 110%
requiring a large amount of MU (mean value of 4600) for hotspot, as outlined in our specific prostate SBRT protocols.
clinical plan optimization. This can be due to at least two Like any standardization, our approach has a certain level
possible reasons: (1) prostate targets are deeply seated in of clinically acceptable variations. This work illustrates that
the body, requiring penetration through the femur heads, such variations were sufficient to create treatment plans that
and (2) high complexity of the treatment plan due to reflect an individual planner’s impact in terms of HI and CI.
high dose modulation close to OARs, such as the urethra, One notable limitation of this study is the exclusion
trigone, bladder, and rectum. of cases where the dose distribution of the PTV was
significantly compromised, particularly in situations
where PTV coverage was affected by the proximity of
OARs. In such cases, the dose distribution becomes more
challenging to describe and standardize. In addition, while
reirradiation cases present unique challenges in treatment
planning, such as accounting for prior radiation doses to
critical structures and the increased risk of normal tissue
toxicity, these cases were not included in our analysis, and
are usually discussed separately in the literature. 13,14 Since
reirradiation often requires more advanced dosimetric
planning and specialized dose distribution strategies,
excluding these cases limits the applicability of our findings
to patients undergoing retreatment.

5. Conclusion
Figure 4. Impact of radiation medical physicist’s (RMP) approach on
homogeneity index (HI) and conformity index (CI). This figure illustrates In this study, we collected and analyzed dosimetric data
the variability in HI and CI values based on the planning approach from 300 SBRT/SRS treatment plans. These data represent
employed by different RMPs. different treatment plan characteristics: D 0.1cc , D , D ,
95
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Volume 2 Issue 4 (2024) 4 doi: 10.36922/arnm.5450