MO-D-BRB-10: Modeling Inter-Patient Variation of Organ-At-Risk Sparing in IMRT Plans: An Evidence-Based Plan Quality Evaluation

PURPOSE

To develop a predictive model to assess the quality of critical organ dose sparing in IMRT plans by providing patient specific dose sparing references, based on an array of patient anatomical features and prior planning experience.

METHODS

Contributions of various patient anatomical features to the inter-patient OAR dose sparing variation in IMRT planning were systematically studied using machine learning method based on high quality prior plans. The dependence of anatomical factor on OAR dosimetric parameters is formulated into predictive models. The OAR dosimetric parameters generated by these predictive models represent the "best feasible" clinical outcomes based on past planning experiences. IMRT plans of 88 prostate, 106 head-and-neck (HN) and 21 spine SBRT treatments were used to train the models. The final models were tested by additional 24 prostate and 48 HN plans. The model for spine SBRT was tested by the leave-one-out method.

RESULTS

For HN and prostate planning, the significant patient anatomical features that affect OAR sparing are: the distance between OAR and PTV, the portion of OAR volume within an OAR specific distance range, the overlap volume between OAR and PTV, and the portion of OAR volume outside the primary treatment field. For spine SBRT planning, the most significant patient anatomical feature that affects cord sparing is the tightness of the geometric enclosure of PTV surrounding the cord and the homogeneity of PTV dose coverage. The dosimetric parameters predicted for the test patient cases using the models were in agreement with those from the clinical plans in more than 75% of the cases.

CONCLUSIONS

The developed predictive models indicated substantial correlation between some important patient anatomical features and OAR dose sparing based on expert experiences. These models can be used as effective tools for evaluating the quality of treatment plans customized to individual patient's anatomy. Partially supported by a master research agreement with Varian Medical System, Inc.

SU-E-J-209: A Simple Method to Minimize Uncertainty in ITV Delineation: Phantom Verification

PURPOSE

Irregular breathing causes variation in delineation of internal target volume (ITV), which is typically generated in the maximum intensity projection (MIP) images [1]. Previous studies have shown that MIP-based ITV can underestimate true tumor range [2]. This study examines a simple method to reduce such errors by combining the GTV of 3D-CT with the ITV of MIP.

METHODS

The Computerized Imaging Reference Systems (CIRS) Dynamic Thorax Phantom Model 008A (CIRS, Norfolk, VA) with CIRS motion control software was used to model 4 irregular patient respiratory profiles and one regular respiratory profile (sine wave). A 3 cm tumor insert was used as target. For each breathing profile, a 3D-CT and 3 repeated 4D-CT scans with random intervals within the breathing profile were performed on a 4-slice clinical scanner (Lightspeed, GE, WI). The RPM system (Varian, Palo Alto, CA) was used to track the respiratory profiles. GTV was contoured on 3D-CT, and ITV was contoured on each MIP (ITVMIP) using a consistent lung window by the same person. The new method of creating ITV was to combine the GTV and ITVMIP, namely ITVCOMB. To evaluate which ITV is more accurate, ITVCOMB and ITVMIP were compared to a 'ground truth' ITV (ITVGT) which was generated by combining the three ITVMIPs.

RESULTS

For the regular profile, both ITVMIP (27.25 cm³ ) and ITVCOMB (28.12 cm³ ) were comparable to ITVGT (27.25 cm³ ). For irregular profiles, the mean absolute difference between ITVCOMB and ITVGT (6.3%±4.9) was significantly (p-value=0.0078) smaller than that between ITVMIP and ITVGT (18.1%±12.3).

CONCLUSIONS

The results suggest that combining GTV of the 3D-CT with the ITV of the MIP is more accurate than the ITV of the MIP alone, and thus would be a simple method to reduce breathing irregularity induced errors in ITV delineation for treatment planning of lung cancer.

SU-E-T-626: Individualized Trade-Off of Dose Coverage and Sparing in IMRT Planning

PURPOSE

We present an evidence-based approach to quantifying the dose sparing trade-offs between different Organs-at-risk (OARs) and the trade-off between OAR dose sparing and target dose coverage in IMRT plans.

METHODS

The interaction between dose sparing of different OARs and between target dose coverage and OAR dose sparing are modeled using high-quality prior IMRT plans of prostate, head-and-neck (HN), and spine SBRT cases. The dose sparing for each OAR is modeled separately at first based on their own anatomical features and then the contributions from other OARs or PTV are added into the model to account for the possible trade-off effects in plans whose modeled OAR DVH deviates from that of actual plans.

RESULTS

The most significant OAR dose sparing trade-off is between bladder and rectum in prostate plans and between the left and the right parotids in HN plans. Trade-off is most apparent when the parotid on one side has very large overlap with the PTV. In most of these plans (83%), the planer chooses to loosen the dose constraint for that parotid in exchange for lower contra-lateral parotid median dose. The interaction between OAR dose sparing and PTV dose coverage is most significant in spine SBRT plans, (R2=0.37). Another important factor is the tightness of the geometric enclosure of PTV around OAR (R2=0.38).

CONCLUSIONS

The dosimetric trade-off between multiple OARs and between PTV and OAR in IMRT planning is quantified by studying prior plans. These findings will help physicians and planners obtain better OAR dose sparing by exploring different trade-off options for individual patient cases. Partially supported by a master research agreement with Varian Medical Systems, Inc.