Proceedings of the 49th Annual ASTRO Meeting S715

2916 Impact of Rectal Distension in IGRT for Prostate Cancer

P. J. Kim, A. Parthasarathy, J. Ho, B. Tam, H. Gee, J. W. Lee

California Pacific Medical Center, San Francisco, CA

Purpose/Objective(s): In obtaining CT scans for external beam radiotherapy planning for prostate cancer, various degrees ofrectal distension are observed. We investigated if the degree of daily isocenter shifts guided by intraprostatic fiducial gold seedscorrelated with the amount of rectal distension present at the time of CT simulation prior to treatment.

Materials/Methods: 41 consecutive patients with prostatic adenocarcinoma had intraprostatic gold seed-guided IGRT treatmentsin a community-hospital setting. Patients were instructed to empty bladder and rectum prior to simulation and daily treatments.Based on gold seed markers, daily shifts from the isocenter were performed with an onboard kV imager and recorded in X, Y,and Z dimensions. Standard deviations of shifts in all 3 dimensions were calculated. Patients with the five highest and five loweststandard deviations of shifts in each of the 3 dimensions were further analyzed by calculating individual rectal and bladder volumesat the time of CT simulation. In an effort to characterize rectal distension, rectal volumes were further divided into 3 parts: a) su-perior rectum, defined as located superior to the prostate up to the level of the recto-sigmoid junction, b) middle rectum, defined aslocated at the level of the prostate, and c) inferior rectum, defined as located inferior to the prostate up to 2 cm and may include analcanal. A single physician contoured all bladder, rectal, and prostate volumes for reproducibility. The correlation between degree ofdaily shifts throughout the treatment and contoured volumes were analyzed.

Results: Range of standard deviations of isocenter shifts in the X, Y, and Z directions were 0.4–1.28 cm, 0.24–0.82 cm, and 0.16–0.62 cm, respectively. Overall rectal volume did not correlate with standard deviation of shifts among all 3 dimensions (r =�0.148,p = 0.435, df = 28). Superior, middle, and inferior rectal volumes each did not correlate with standard deviation of shifts in all 3dimensions, nor did the ratio of combined superior and middle rectal volumes to inferior rectal volumes (r = �0.081, p = 0.672).Similarly, there was no correlation between standard deviation of shifts and greatest rectal width on axial images (r = �0.245,p = 0.192). Finally, other parameters such as overall bladder (r = 0.176, p = 0.352) and prostate (r = �0.259, p = 0.167) volumesdid not correlate with standard deviation of shifts among all 3 dimensions.

Conclusions: There was no correlation between parameters such as rectal or bladder distension at the time of simulation and IGRTshifts guided by gold seed fiducial markers. This lack of correlation may suggest that other factors are also influencing the degree ofisocenter shifts. Rectal or bladder distension at the time of CT simulation may reflect patient-specific properties that can be con-sistent throughout an external radiation treatment course. With the spectrum of isocenter shifts observed in this study, our dataunderscores the importance of daily localization of the prostate for accurate delivery of radiotherapy.

Author Disclosure: P.J. Kim, None; A. Parthasarathy, None; J. Ho, None; B. Tam, None; H. Gee, None; J.W. Lee, None.

2917 Assessment of IMRT Plans Optimized With Deliverable Step and Shoot (SS) and Dynamic

MLC (DMLC) Techniques

R. George, N. Dogan

Virginia Commonwealth University, Richmond, VA

Introduction: Traditional IMRT process consists of two steps (1) Estimation of ideal intensity distribution (2) Obtaining de-liverable intensity distribution using a leaf sequencing algorithm. An alternative technique consists of a single step in whichmultileaf collimator (MLC) delivery constraints are incorporated into the optimization process. This technique is called deliver-able optimization. The goal of this work was to determine if the IMRT plans obtained with dynamic MLC (DMLC) based andstep and shoot (SS) based deliverable optimization techniques were equivalent in terms of target coverage and critical structuresparing.

Materials/Methods: Four head and neck (HN) and four prostate patients were selected for this study. IMRT plans were cre-ated using (1) in-house DMLC based deliverable optimization software and (2) SS based deliverable software called directmachine parameter optimization (DMPO) available in Pinnacle3 system. While HN IMRT plans utilized nine coplanar beams,all prostate plans involved seven coplanar beams. DMLC plans are composed of large number segments. For DMPO plans,the maximum number of segments per beam was initially set to ten (e.g., for 9 split HN fields, 180 segments). To evaluate theeffect of fewer segments the number of segments (DMPOless) were reduced to 90 for HN and 70 for prostate and the planswere optimized with the same constraints. The dose-volume criteria evaluated for the target and critical structures are shown inTable 1.

Results: The results of this analysis are shown in Table 1. IMRT plans using DMLC were comparable to DMPO. The maximum %volume difference between the DMLC and DMPO plans were 0.71% in terms of target volume coverage and 2.33% in terms ofsparing of the critical structures over all cases. Similarly the maximum % volume difference between the DMLC and DMPOless

plans were 2.44% in terms of target volume coverage and 2.94% in terms of sparing of the critical structures. In addition the monitorunits for DMPO were seen to decrease by �29% for HN and �35% for prostate cases.

Conclusions: DMPO provides plans equivalent in quality to deliverable DMLC. Reducing the number of segments by half(DMPOless) did not significantly affect the quality of the plans. The number of monitor units significantly decreased when usingthe step and shoot deliverable optimization i.e. DMPO.

Future Work: Besides dose volume criteria evaluated in this study, the plan quality can be evaluated based on the hotspots in thetarget, dose volume histograms and isodose lines.

Monte Carlo based analysis and film measurements will be used to confirm the results.