Postmortem computed tomography (PMCT) and postmortem magnetic resonance (PMMR) imaging (PMMRI) have been applied to provide vital or additional information for conventional necropsy, along the pioneering virtopsy-driven cetacean stranding response program in Hong Kong waters. It is common for stranded carcasses to become badly degraded and susceptible to rapid cerebral autolysis and putrefaction. Necropsy on decomposed brains with limited sample analysis often defy a specific diagnosis. Studies on PMMR neuroimaging have focused on neuroanatomy and brain morphology in freshly deceased or preserved specimens. Moreover, the literature is devoid of any reference on the potential value of PMMRI examination of decomposed cetacean brains. To that end, this project evaluated the benefits of PMMR neuroimaging in situ in decomposed carcasses in comparison to PMCT. A total of 18 cetacean carcasses were studied by PMCT and PMMRI examinations. Anatomical brain structures and visible brain pathologies were evaluated and scored using Likert-scale rating. Intracranial gas accumulation was clearly depicted in all cases by all radiological techniques. Other features were more clearly depictable in PMMRI than in PMCT images. Results of this study indicated that superiority of PMMRI compared to PMCT increased with advanced putrefaction of the brain. The preservation of structural integrity was presented by PMMRI due to its superior capability to evaluate soft tissue. Brain PMMRI should be incorporated in postmortem investigation of decomposed stranded cetaceans.

Objectives: This study aimed to compare radiotherapy plan quality of coplanar VMAT (CO-VMAT) and non-coplanar VMAT (NC-VMAT) for postoperative primary brain tumour. Methods: A total of 16 patients who were treated for primary brain tumours were retrospectively selected for this study. For each patient, identical CT sets with structures were used for both CO-VMAT and NC-VMAT planning. For CO-VMAT, one full arc and two coplanar half arcs were used. For NC-VMAT, one full coplanar and two non-coplanar half arcs with couch rotation of 315° or 45°. Dose constraints were adhered to the RTOG0614 and 0933. Dose volumetric parameters were collected for statistical analysis. Results: There were no significant differences for the PTV, HI, CN and μ between the CO-VMAT and NC-VMAT. For the brainstem, Dmean of CO-VMAT and NC-VMAT were 6.04 ± 3.94 Gy and 4.69 ± 2.56 Gy respectively (p < 0.05). For the ipsilateral OARs including temporal lobe, TM joint and cochlear, Dmean of CO-VMAT and NC-VMAT were 31.80 ± 12.78 Gy and 25.51 ± 17.54 Gy (p < 0.01) ; 14.12 ± 8.6 Gy and 3.35 ± 4.12 Gy (p < 0.001); 11.96 ± 11.68 Gy and 6.62 ± 9.74 Gy (p < 0.01) respectively. For contralateral OARs including hippocampus, temporal lobe, TM joint, Optic nerve, lens, eyeball and cochlear, the Dmean of CO-VMAT and NC-VMAT were 6.16 ± 2.44 Gy and 4.49 ± 2.00 Gy (p < 0.01) ; 6.48 ± 2.76 Gy and 3.68 ± 1.76 Gy (p < 0.0001); 11.96 ± 11.68 Gy and 6.62 ± 9.74 Gy (p < 0.01) respectively. Conclusion: The proposed NC-VMAT showed more favourable plan quality than the CO-VMAT for primary brain tumours, in particular to OARs located to the contralateral side-of tumours. Advances in knowledge: For primary brain tumours RT, NC-VMAT can reduce doses to the brainstem, ipsilateral temporal lobe, TM joint and cochlear, as well as OARs located to the contralateral side-of tumours.

In prone breast radiation, as the medial tangential beam usually passes through the immobilization board and couch, it is necessary to quantify the attenuation effect and the potential skin dose enhancement from these external structures. The prone breast board studied consists of an insert on which the contralateral breast rests and a board base indexed to the couch. Two different Varian couch systems were also studied. Transmission factors (TF) of the board were measured using a Farmer chamber at 4 cm depth. Couch TFs were measured using a thimble chamber centered in a cylindrical phantom. A custom support model was created in the treatment planning system (TPS). TFs were then computed in the TPS for comparison. Selected clinical plans were recomputed in the TPS incorporating external structures for target coverage evaluation. The correction for the attenuation effect in the TPS was also demonstrated. Skin dose effects were evaluated using a Markus parallel plate chamber with a 1 mm buildup cap. Measured insert TFs ranged 0.976 to 0.983 for 6 MV and 0.990 to 0.999 for 23 MV. Board base TFs ranged 0.979 to 0.985 for 6 MV and 0.989 to 0.998 for 23 MV. TPS values agreed within 0.9% and 0.5% for the insert and board base, respectively. Assigned Hounsfield units (HUs) providing the best agreement were 20 0, −10 0, and −90 0 for the insert, the board "base shell" and "base inside," respectively. Varian Exact Couch and Exact IGRT Couch TFs varied with respect to couch angle, with minimum values of 0.837 and 0.956, respectively, for 6 MV. The clinical treatment volume (CTV) and whole breast receiving 95% of the prescription dose (CTV-V95 and WB-V95) of selected patients demonstrated reduced coverage due to attenuation of external structures. Close proximity to the base increased skin dose by up to 25% to 30%. Contacting the insert increased skin dose by 65% to 93% for 6 MV and 117% to 157% for 23 MV, respectively. Results have shown reduced coverage by attenuating external structures. Proper modeling of immobilization devices and couch structures in the TPS should be implemented for accurate dose calculation. Increased surface doses were observed due to direct contact to the insert or close proximity to the base. Further study is required to quantify such a skin dose enhancement effect and its correlation to clinically apparent skin effects and toxicity.