172 IS THERE AN ADVANTAGE TO IMRT FOR HARD TO TREAT PRIMARY BRAIN CANCER?

S54 minimize organ motion and improve reproducibility. The purpose of this study was to analyze bladder volumes of women having radiotherapy for GYN cancers and to relate variations in these volumes to potential influencing factors. Materials and Methods: This study reviewed bladder volumes from prospectively acquired on-line cone-beam CT (CBCT) and off-line magnetic resonance imaging (MRI) taken during external beam radiotherapy for GYN cancers at PMH between January 2003 and October 2008. Radiation Therapy Oncology Group (RTOG) acute GU toxicity scores were also reviewed. Bladder volumes were analyzed in relation to imaging modality, age, concurrent cisplatin chemotherapy and radiotherapy treatment fraction. Results: Eighty-four patients had CBCT or MRI imaging acquired during radiotherapy treatment. Fifty-one had > 5 weekly image sets. Bladder volumes from a total of 151 MRI and 126 CBCT images were analyzed. The average bladder volume at planning CT was 265.64cm3. The average volume during treatment was 209.89cm3 using CBCT and 91.98cm3 using MRI. Individual patients exhibited large variations in magnitude of bladder volume changes. Consistent bladder volumes were found to be more prevalent in women aged <52 years. Thirty patients had documentation of RTOG GU toxicity, which was incomplete in all cases. No correlation was found between bladder volume and acute GU toxicity, concurrent cisplatin chemotherapy or radiotherapy treatment fraction. Conclusion: Absolute bladder volumes measured during radiotherapy treatment for GYN cancers showed marked variation from the planned volume (although average volumes throughout treatment were quite consistent). Women of less than 52 years of age were found to have less overall variation in bladder volume than older women. Volumes from on-line CBCT were more closely correlated to the planning CT than those from off-line MRI, emphasizing the importance of on-line imaging. 170 DERIVING PROSTATE ALPHA-BETA RATIO USING CAREFULLY MATCHED GROUPS, LONG FOLLOW UP AND THE PHOENIX DEFINITION OF BIOCHEMICAL FAILURE R. Shaffer1, T. Pickles2, R. Lee2, V. Moiseenko2 1 Imperial College NHS Trust, London, UK 2 University of British Columbia, Vancouver, BC Purpose: Prior studies derived a low value of a/ß for prostate cancer (e.g. 1-2Gy) using outcome data from external beam radiotherapy (EBRT) and permanent prostate brachytherapy (PPB). However, these values are associated with wide confidence intervals and inaccuracies such as poorly matched groups, differing definitions of biochemical failure and insufficient follow up. Materials and Methods: Patients with Canadian Consensus Risk Group low- or low-tier intermediate risk prostate cancer, treated with either EBRT or PPB, were matched for PSA, Gleason score, T-stage, percentage of positive cores, androgen deprivation therapy duration and era, yielding 118 pairs. The Phoenix definition of biochemical failure was used. The best value for a/ß was found using maximum likelihood analysis, and 95% confidence intervals using the profile likelihood method. The linear quadratic formalism was applied with radiobiological parameters set at RBE = 1, Tpot = 45 days, and repair half-time = one hour. Sensitivity analysis was performed using extreme values of these parameters. Results: PPB and EBRT groups were well matched with respect to all known risk factors. Median follow up or time to failure was 60 months. Kaplan-Meier estimates of freedom from biochemical failure (bNED) showed superiority of PPB compared to EBRT (log-rank test p=0.001): Estimates of probability of bNED were 82% and 95% at 72 months for EBRT and PPB; and 63% and 95% at 90 months. The value of a/ß that best fitted the outcome data was >30 Gy, with a lower 95% confidence limit of 3.2Gy. This was confirmed

CARO 2009 on bootstrap analysis. Varying the parameters to extreme values yielded a best-fit a/ß of at least 3.0 Gy. Conclusion: Our result of >30 Gy as the best estimate of a/ß for low and low-intermediate risk prostate cancer directly contrasts with prior best estimates of 1-2 Gy. Obtained values of a/ß result from superior outcomes for PPB observed for long follow up time. If the true value of a/ß is not less than the rectal a/ß then radiation hypofractionation may not improve the therapeutic ratio. 171 PILOT STUDY OF PRE OPERATIVE INVOLVED FIELD RADIOTHERAPY FOR RECTAL CANCER D. Severin1, K. Joseph2, K. Tankel2, T. Nijjar2, J. Pedersen2, C. Small1, Nawaid Usmani2, Elizabeth Gaetz2, Chris de Gara2, Ron Hennig2, Alina Mihai3 1 Cross Cancer Institute, Edmonton, AB 2 University of Alberta, Edmonton, AB 3 Deacon Hospital, Dublin, Ireland Purpose: The treatment of rectal cancer includes surgery as the primary local modality. Radiation (RT) and chemotherapy are added to improve local control rates and survival. We hypothesize that pre-operative radiation to the rectal tumour with a margin to encompass the mesorectal tissue and adjacent lymph nodes (involved field) will lead to equivalent local control as standard radiotherapy volume but with less morbidity and improved quality of life. Materials and Methods: Twenty-two of 30 planned patients who are T3 or node positive have been accrued thus far to this Phase II clinical trial. All patients have received 45 Gy to the PTV which includes the GTV+ mesorectum + presacral space + lateral lymph nodes with a superior and inferior margin of 3.5 cm. A boost of 9 Gy to the tumour with a margin of 2 cm is given. Acute and late patient and physician reported toxicity, pathology data and local and distant recurrence data are collected. A dosimetric comparison of the standard plan with the treated involved field plan is being performed with dose to small bowels, femoral heads, anus and bladder reported. Results: Data from the first five patients shows acute RTOG GI toxicity of 0 to 2 and GU toxicity of 0 – 1. Late GI and GU toxicity at six months post-therapy is zero to two. The dosimetric comparison of dose to small bowel shows a greater mean dose in all standard plans compared to the involved field plan. The mean small bowel dose among all five patients for the IFRT plan is 963 cGy and for the standard plan is 1785 cGy. The mean bladder dose among all five patients for the IFRT plan is 3144 cGy and is 4018 cGy for the standard plan. The mean anal dose among all five patients for the IFRT plan is 3870 cGy and the mean anal dose for the standard plan is 3685 cGy. Conclusion: Involved field rectal treatment has been shown to be feasible. The dose to small bowel and to bladder is reduced. 172 IS THERE AN ADVANTAGE TO IMRT FOR HARD TO TREAT PRIMARY BRAIN CANCER? M. Follwell1, M. Davidson2, D. Moseley1, M. Tsao3, P. Davey3, N. Laperriere4, J. Perry3, L. Ma5, D. Larson5, A. Sahgal6 1 University of Toronto, Toronto, ON, 2 Odette Cancer Centre, Toronto, ON 3 University of Toronto, Odette Cancer Centre, Toronto, ON 4 University of Toronto, Princess Margaret Hospital, Toronto, ON 5 University of California, San Francisco, CA 6 University of Toronto, Princess Margaret Hospital, Odette Cancer Centre, Toronto, ON Purpose: We compare intensity modulated radiation therapy (IMRT) to 3DCRT for intracranial glioma adjacent to the optic chiasm and brainstem, as tumour coverage is often sacrificed

CARO 2009 in order to maintain dose thresholds. Single and two phase IMRT plans were also compared for high grade glioma (HGG). Materials and Methods: Ten retrospectively selected patients with brainstem glioma (BG, n=5) and HGG (n=5) were analyzed. Standard margins of a 1.5cm CTV and a 0.5cm PTV were applied to the GTV. For HGG, a two phase approach consisted of 50Gy/25 fractions to the PTV and a 10 Gy/5 fractions boost to the GTV plus 0.5cm (PTV60). Single phase IMRT consisted of 54 Gy/30 fractions to the PTV and a simultaneous 6 Gy boost to PTV60. BG were planned to 54 Gy/30 fractions with single phase IMRT and 3DCRT. Outcome measures include volume of PTV encompassed by 95% isodose (V95), equivalent uniform dose (EUD), conformity index (CI) and homogeneity index (HI). Maximum (dmax) and mean dose (dmean) to chiasm and brainstem, and the integral dose (ID) to normal brain as described by Hermanto et al 2007 were collected. Results: For HGG, two phase IMRT provided superior mean PTV V95, EUD, and CI for both phase one 50 Gy PTV and phase two PTV60 compared to 3DCRT. However, 3DCRT resulted in superior homogeneity for only phase one 50 Gy PTV. Single phase IMRT further improved target indices but at the expense of homogeneity to the phase one PTV. The ID, dmax and dmean to the chiasm and brainstem were significantly lower with single phase and two phase IMRT as compared to 3DCRT. BG IMRT resulted in superior PTV V95, CI, EUD as compared to 3DCRT, however, at the expense of homogeneity. Doses to the chiasm were greater with IMRT though well below accepted thresholds. Conclusion: IMRT has significant advantages over 3DCRT for glioma adjacent to, or involving, the optic chiasm and brainstem, at the expense of target homogeneity. Single phase IMRT is superior to two phase IMRT. The dogma of greater integral doses with IMRT is challenged with these results, and IMRT should be considered standard of care. Volmetric modulated arc therapy treatment planning is ongoing. 173 11C-METHIONINE POSITRON EMISSION TOMOGRAPHY TO ASSESS TREATMENT RESPONSE OF ACOUSTIC NEUROMAS TO STEREOTACTIC IRRADIATION F. Hsu1, M. Twiss2, K. Dinelle2, V. Sossi2, R. Ma1, T. Ruth2, R. Shaffer1, M. McKenzie1, A. Nichol1, M. Martin1, S. Reinsberg2 1 British Columbia Cancer Agency, Vancouver, BC 2 University of British Columbia, Vancouver, BC Introduction: Stereotactic irradiation is an effective treatment for acoustic neuromas. However, tumour shrinkage on conventional magnetic resonance imaging (MRI) takes years to manifest, thus requiring lengthy follow up with repeated imaging studies. Functional imaging may provide a physiologic assessment of treatment response before confirmation of anatomic changes. Purpose: A feasibility study to evaluate 11C-methionine (11CMET) as a radiotracer for positron emission tomography (PET) imaging to assess treatment response of acoustic neuromas to stereotactic irradiation. Materials and Methods: Eight patients with unilateral acoustic neuromas underwent baseline 11C-MET PET and gadoliniumenhanced MRI prior to irradiation. Thus far, four patients had sufficient follow up for repeat 11C-MET PET and MRI at six months post-treatment. Results: At baseline, all eight PET scans demonstrated increased 11C-MET uptake corresponding to the anatomic location of tumour on MRI. There was differential uptake within the tumour, with increased uptake in solid components and decreased uptake in large cystic components of tumour. At six months follow up, three out of the four tumours continued to demonstrate radiotracer uptake that was comparable to baseline. The fourth tumour no longer exhibited uptake while its size remained unchanged on MRI. Conclusion: 11C-MET may be a useful marker for assessing treatment response of acoustic neuromas to stereotactic

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irradiation. In our small study, most tumours continued to exhibit 11C-MET uptake six months after treatment, so continued observation with repeat PET will be necessary to determine when changes in uptake occur for most tumours and whether these changes correlate with clinical outcomes and other imaging measures. 174 EFFICACY OF HYPERBARIC OXYGEN THERAPY IN THE TREATMENT OF MEDICALLY REFRACTORY SOFT TISSUE NECROSIS AFTER PENILE BRACHYTHERAPY A. Gomez-Iturriaga, J. Crook, E.P. Saibishkumar, J. Jezioranski University of Toronto, Toronto, ON Introduction: Soft tissue necrosis (STN) is reported in up to 23% of patients undergoing radiotherapy for penile cancer being more common after brachytherapy than after external radiation. Common treatment options include local irrigation, wound debridment, antibiotics, local anti-inflammatories and analgesics. Refractory lesions are treated with partial penectomy. Hyperbaric Oxygen Therapy (HBO) has a welldefined role in the treatment of late radiation toxicities. We present our experience with HBO for medically refractory STN of the penis after penile brachytherapy. Materials and Methods: From November 2001 to January 2009, seven (16%) patients out of 43 treated with penile brachytherapy for squamous carcinoma developed medically refractory STN and were treated with HBO. All the patients had received a prescribed dose of 60Gy delivered with interstitial PDR brachytherapy using Paris system guidelines. All had failed more conservative medical therapies. Results: Median age of the seven patients was 60 years. Stage was T1 in five patients, T2 and T3 in one. Grade was moderate or poorly differentiated in three patients. The median number of needles was 6. Co-morbidities potentially effecting wound healing included hypertension in two patients, none had diabetes mellitus, or peripheral vascular disease. Five patients were current smokers and one former smoker. Median time between completion of brachytherapy and appearance of STN was 13 months (range 9-24). Median time between the onset of STN and starting HBO was six months (3-13). The median number of “dives” was 40 (30-44). All seven patients achieved excellent responses with complete healing of the necrosis and improvement of the symptoms (dysuria and pain). Two patients had recurrence of STN four months after the completion of HBO. Both of them underwent a second course of HBO with good response. No patient was submitted to penectomy because of the necrosis. Conclusion: HBO should be considered as a treatment option in patients with severe STN of the penis following brachytherapy, resistant to conventional treatments. 175 CHANGES IN THE USE OF PALLIATIVE RADIOTHERAPY FOR BONE METASTASES IN ONTARIO D. Sutton1, W. Mackillop2 1 Research Institute, Kingston, ON 2 Queen's University, Kingston, ON Purpose: Palliative Radiotherapy (PRT) plays an important role in the relief of bone pain in patients with bone metastases (mets) 1; however, little is known about the use of PRT on a population level 2.The purpose of this study was to describe temporal trends in the use of PRT for bone mets in Ontario. Methods: The Ontario Cancer Registry was used to gather information on all cancer deaths in Ontario between the years 1984-2004. The proportion of these cases receiving at least one course of PRT for bone mets within the last two years of life (PRT rate) was described over time and by disease site. Results: There were 435,055 cancer deaths in Ontario during the study period. Of these cases, 10.7% received one course of PRT for bone mets within the last two years of life. The rate of