Proceedings of the 51st Annual ASTRO Meeting S703

second 6 months. For 2008, the average use is 66 hours per week. The treatment system was available 95% of the scheduled timeand 98% if unscheduled cyclotron downtime and power failures are excluded. Operational experience gained led to improvementsin the dose monitoring system and the rotating gantry floor. On two occasions the beam failed during patient treatment, leading toa partial dose delivery. After beam recovery and patient repositioning, the control systems re-started treatment at exactly the pointwhere the beam failed, leading to complete dose delivery as originally planned. Acute effects of treatment and early follow-up dataindicate clinical results compatible with the doses delivered.

Conclusions: The first proton treatment room in with uniform scanning proved reliable in all clinical operations.

Author Disclosure: M.M. Fitzek, None; A.F. Thornton, None; A. Chang, None; K. Shahnazi, None; M. Sullivan, None;M. Wolanski, None; C. Allgower, None; V. Anferov, None; D. Nichiporov, None; V. Derenchuk, None.

3160 Is There a Need for Daily Image Guidance in Proton Therapy of Prostate Cancer?

L. Zhao1, I. J. Das2, J. Kuros-Zolnierczuk1, P. A. Johnstone2

1Midwest Proton Radiotherapy Institute, Bloomington, IN, 2Department of Radiation Oncology, Indiana University School ofMedicine, Indianapolis, IN

Purposes/Objective(s): Inter - and intra-fractional prostate motion is an important challenge for radiation therapy and it is morecritical for proton therapy due to finite spread-out Bragg peak and range. Orthogonal kilovoltage images are acquired daily forprostate patient setup in most proton facilities to overcome interfractional setup error; this requires more treatment time comparedsimply to the beam time. Concerned about scarce proton resources and radiation exposures, we investigated the dosimetric effectsby simulation of interfraction motion in prostate proton treatment plans for a hypothetical situation without daily imaging guidance(IG). This study offers insights into the question of whether daily IG is needed for prostate treatment in proton therapy with suf-ficient margin.

Materials/Methods: A 2-field lateral treatment plan with a prescribed dose of 7920 cGy in 44 fractions was used with 2 conedownfields. In 28 fractions, 5040 cGy was delivered to prostate with 11 mm margin plus seminal vesicle with 5 mm margin, followed bya 2880 cGy boost to prostate with 6 mm margin excluding the anterior rectal wall. The CTV-to-PTV margin was 2 mm. A Gaussiandistribution with 5 mm as the standard deviation was used to represent random motion characteristics of the prostate. Prostate dis-placements in the AP direction were simulated in the treatment planning system by moving the beams and isocenter of the originalplan but by maintaining aperture and range compensator shapes fixed. The dose distribution was recalculated for each simulatedposition variability of 0, ± 2 mm, ± 5 mm, ± 7 mm, ± 10 mm, and ± 15 mm. The cumulative dose volume histograms (DVH) overall simulated positions, weighted by Gaussian probability density function, were recalculated for targets and OARs, and then com-pared with the original plan.

Results: Comparison of DVHs from the simulated plan and the original plan shows that the target dose coverage degraded min-imally (\3%), but was not clinically significant. In the presence of motion without daily IG, the GTV V95% was reduced by 0.5%(100% vs. 99.5%). The CTV1 and PTV1 receiving 95% of 5040 cGy were reduced by 0.2% (100% vs. 99.8%) and 0.6% (100% vs.99.4%). The CTV2 and PTV2 V95% were reduced by 1.8% (99.9% vs. 98.1%) and 2.2% (99.1% vs. 96.9%). The bladder volumereceiving 6000 cGy remained the same. The volume of anterior rectal wall receiving 7740 cGy was increased by 3.2% (10.1% vs.13.3%).

Conclusions: For a given prostate treatment plan with a sufficient margin around GTV to beam aperture and smearing to compen-sator, our results show that up to 15 mm random motion of the prostate has a relatively small effect on the dosimetry for targets andOARs. Recognizing this study did not include organ rotation and deformation, these results indicate that daily IG is not necessaryfor prostate treatment in proton therapy.

Author Disclosure: L. Zhao, None; I.J. Das, None; J. Kuros-Zolnierczuk, None; P.A. Johnstone, None.

3161 Feasibility of Hemi-prostate Dose Escalation to 91 Gy with 3D-conformal vs. Intensity-modulated Proton

Therapy

A. Trofimov1, P. L. Nguyen1,2, M. W. Lu1, J. Unkelbach1,3, J. Kang1, T. Bortfeld1, A. L. Zietman1

1Massachusetts General Hospital, Boston, MA, 2Harvard Radiation Oncology Program, Boston, MA, 3Dalle Molle Institute forArtificial Intelligence, Lugano, Switzerland

Purpose/Objective(s): Partial volume dose escalation targeting only diseased portions of the prostate gland may lead to similargains in disease control as whole-gland escalation, while minimizing the added risk of toxicity to nearby critical organs. We in-vestigated the feasibility of hemi-prostate dose boosting with urethral sparing in the treatment of prostate carcinoma with 3D-con-formal (3DCPT) and intensity-modulated proton therapy (IMPT) to a dose of 91Gy (which has been cited as the threshold dose forcost-effectiveness of prostate proton treatments).

Materials/Methods: Ten patients, who received 79.2 Gy to the whole prostate gland with 3DCPT using two parallel-op-posed beams, were selected for this treatment planning study. Based on the biopsy, either left or right hemi-prostate wasidentified as the target for dose escalation. The boost volume consisted of the hemi-prostate, excluding a 5-mm marginexpansion around the urethra (which was outlined on the sagittal CT), and a 5mm thickness at the anterior base. 3DCPTplans were designed as single-beam partial boost of 11.8 Gy (to the total of 91 Gy), following the clinical course. TheIMPT plans used two lateral opposed beams to deliver 79.2 Gy to the prostate, with the integrated partial boost to 91 Gy.IMPT optimization used the robust probabilistic method, which considered the uncertainties both in the patient set-upand in proton range in tissue*. Both types of uncertainties were assumed to obey the Gaussian distribution with the var-iance of 5 mm. No more than 15% of the rectum was to receive 75 Gy, and no more than 15% of the bladder to receive80 Gy.

Results: The prescription iso-dose coverage of over 95%, and the minimum dose of 88.3 Gy (97% of 91 Gy) for the boost volumewere required and achieved in all plans, with both delivery techniques. IMPT delivered a more inhomogeneous distribution, with